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How would you run a lateral acceleration test in a vehicle on twisty roads at no more than 40mph?
How would you run a lateral acceleration test in a vehicle at slow speeds
from 20mph to 30mph on twisty roads and at no more than 40mph? The issue is discussed in this repair thread: o Clare - are smaller car tires easier to balance than SUV tires? <https://groups.google.com/forum/#!topic/alt.home.repair/So4om4fLtmI> Where I picked up a free graphical tool for testing acceleration: o Sensors Multitool, version 1.3.2, by Wered Software <https://play.google.com/store/apps/details?id=com.wered.sensorsmultitool> Where there are two types of acceleration graphed o acceleration o linear acceleration <https://i.postimg.cc/7hPhBY5b/accelerate01.jpg> Where I'm not sure yet how to construct the test experiment: <https://i.postimg.cc/qqZJjcfZ/accelerate02.jpg> Since what I want to measure is o lateral acceleration Any advice for how best to set up the measurement test? |
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How would you run a lateral acceleration test in a vehicle ontwisty roads at no more than 40mph?
On 22/6/19 4:56 am, Arlen G. Holder wrote:
> How would you run a lateral acceleration test in a vehicle at slow speeds > from 20mph to 30mph on twisty roads and at no more than 40mph? > > The issue is discussed in this repair thread: > o Clare - are smaller car tires easier to balance than SUV tires? > <https://groups.google.com/forum/#!topic/alt.home.repair/So4om4fLtmI> > > Where I picked up a free graphical tool for testing acceleration: > o Sensors Multitool, version 1.3.2, by Wered Software > <https://play.google.com/store/apps/details?id=com.wered.sensorsmultitool> > > Where there are two types of acceleration graphed > o acceleration > o linear acceleration > <https://i.postimg.cc/7hPhBY5b/accelerate01.jpg> > > Where I'm not sure yet how to construct the test experiment: > <https://i.postimg.cc/qqZJjcfZ/accelerate02.jpg> > > Since what I want to measure is > o lateral acceleration > > Any advice for how best to set up the measurement test? > https://www.academia.edu/people/sear...hicle+dynamics The above link should get you to this book; Thomas D. Gillespie-Fundamentals of Vehicle Dynamics Society of Automotive Engineers Inc (1992) You can then download the full PDF file and that should help you get started. I have the hardback copy of that book. The content therein should get you started in the right direction. -- Xeno Nothing astonishes Noddy so much as common sense and plain dealing. (with apologies to Ralph Waldo Emerson) |
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How would you run a lateral acceleration test in a vehicle on twisty roads at no more than 40mph?
On Sat, 22 Jun 2019 08:46:20 +1000, Xeno wrote:
> https://www.academia.edu/people/sear...hicle+dynamics > > The above link should get you to this book; > > Thomas D. Gillespie-Fundamentals of Vehicle Dynamics > Society of Automotive Engineers Inc (1992) > > You can then download the full PDF file and that should help you get > started. I have the hardback copy of that book. The content therein > should get you started in the right direction. Hi Xeno, Thanks for that starting point, where, as you know, the goal is to figure out first exactly what is going on, geometrically, with the outside feathering on long very windy (steering lock to steering lock) turns at slow speeds. Clare, as you know, had suggested testing lateral acceleration, where he was positing that the load on the inside tire, even at slow speeds (less than 40mph) on the downhill curves, could exceed the load range per tire. The download worked after I gave it a bogus email and password (8 characters was enforced) where I also had to give it a bogus university and department and matriculation status. Blue drawing number #8, on page 20 was particularly instructive in the book "Basic principles of vehicle dynamics", as was blue drawing #2 on page 16. |
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How would you run a lateral acceleration test in a vehicle ontwisty roads at no more than 40mph?
On 22/6/19 10:40 am, Arlen G. Holder wrote:
> On Sat, 22 Jun 2019 08:46:20 +1000, Xeno wrote: > >> https://www.academia.edu/people/sear...cle+dyna mics >> >> The above link should get you to this book; >> >> Thomas D. Gillespie-Fundamentals of Vehicle Dynamics >> Society of Automotive Engineers Inc (1992) >> >> You can then download the full PDF file and that should help you get >> started. I have the hardback copy of that book. The content therein >> should get you started in the right direction. > > Hi Xeno, > > Thanks for that starting point, where, as you know, the goal is to figure > out first exactly what is going on, geometrically, with the outside > feathering on long very windy (steering lock to steering lock) turns at > slow speeds. > > Clare, as you know, had suggested testing lateral acceleration, where he > was positing that the load on the inside tire, even at slow speeds (less > than 40mph) on the downhill curves, could exceed the load range per tire. Not a hope of it exceeding the load range of the tyre. If that were the case the tyres couldn't possibly grip at normal highways speeds where the lateral forces *and* weight transfer really come into play. Any lateral testing, whether at low speeds or high, will not take into account the effect of any weight transfer. It can be calculated and this is, IIRC, given in Fundamentals of Vehicle Dynamics by Gillespie. That said, at 20-30 mph, weight transfer is negligible in terms of the effect we are seeing on the tyre shoulder. It is the extra positive camber that is creating the effect and the longitudinal feathering is the result. > > The download worked after I gave it a bogus email and password (8 > characters was enforced) where I also had to give it a bogus university and > department and matriculation status. I used the college (www.kangan.edu.au) where I once taught even though I retired from there 18 years ago. It was happy enough with that. I was in the automotive department (www.aceauto.com.au). Interestingly, of the 43 teachers in my department - motor mechanics - when I first started there nearly 40 years ago, only 2 or 3 remain, all others have since retired or shuffled off the mortal coil. > > Blue drawing number #8, on page 20 was particularly instructive in the book > "Basic principles of vehicle dynamics", as was blue drawing #2 on page 16. > Yes, that was the point I was making with regard to tyre contact patch distortion. Thinking of the page 20 diagram, now imagine what the contact patch looks like with added positive camber. It'll be *curved* and the tread blocks will be distorting and *scrubbing* - more so at the outer extremity. BTW, I have that complete book from which that chapter is excerpted. Lots of really good info in there though it doesn't delve into the mathematics as much as does the *Fundamentals of Vehicle Dynamics*. -- Xeno Nothing astonishes Noddy so much as common sense and plain dealing. (with apologies to Ralph Waldo Emerson) |
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How would you run a lateral acceleration test in a vehicle on twisty roads at no more than 40mph?
On Sat, 22 Jun 2019 22:03:18 +1000, Xeno wrote:
> It isn't described well at all. In most cases it is simply inferred. I > think that's because everyone focuses on the *outside* wheel whereas the > actual camber scrub is happening on the *inside* wheel because it is > perched up on its outer edge because of the positive camber and is more > lightly loaded, hence the tread blocks tend to slip rather than grip and > flex. That sideways flexing and scrubbing is what gives them that > characteristic feathered edge. Hi Xeno, I much appreciate that you UNDERSTAND the complex geometric changes that occur during camber scrub low-speed lock-to-lock conditions, where we both agree the specific phenomenon I am troubleshooting is a specific very mountainous situation which isn't covered well on the net, but which does have pragmatic workarounds, as you & Clave have discussed. Here's a shot from today with the vehicle parked at one of the curves. o Passenger tire at (static) steering-wheel lock, heading uphill: <https://i.postimg.cc/T1HkcsX5/mount31.jpg> o Driver side tire at that same wheel lock situation: <https://i.postimg.cc/KYXHVC3n/mount32.jpg> It's not easy to tell, but that inner tire (which is the one wearing the most in these slow speed lock-to-lock turns) should be taking on a more positive camber, while the outer tire should be taking on a more negative camber. Even though the outside tire is taking on more of the force, the wear is happening more so to the outside shoulder of the inside tire (the tire with the more positive camber). <https://i.postimg.cc/YqHVb5gY/mount33.jpg> As you've explained it prior, at _slow_ speeds (30 mph nominally), there isn't as much weight transfer to the outside wheel, and yet the inside wheel is at a tighter lock than the outside wheel due to Ackerman Angle effects, where the more positive camber on that inside wheel causes the outside tread area to longitudinally feather unidirectionally more so than the outside wheel, which takes on a lesser more positive camber. Since we're effectively riding on the outside tread blocks of the inside tire, those outside blocks are forced to break traction and slide, which is what's causing the longitudinal unidirectional feathering, particularly when traveling downhill. Note that the passenger tire of this rather heavy bimmer SUV is the original tire of only about a year and a half old (about 15K miles or so), where the outside edge counter rotational longitudinal feathering is almost worn away, but the driver's side tire had to be replaced a few months ago, where the counter rotational longitudinal feathering is easily felt on the outside few inches of the tread. Am I correct that these are the possible ameliorations, bearing in mind that every change made has an effect somewhere else in alignment and that each change has to be made in the standard caster/camber/toe order? 1. First, increase tire pressure (to decrease tread squirm) 2. Second, potentially decrease positive caster (to the low end of spec) (where the goal is to change how SAI affects the camber angle under turns) 3. Third, possibly (increase?) static negative camber (within spec) (although increasing negative static camber "may" also decrease the SAI) 4. Set toe to spec last. [If I got anything wrong, please let me know as it's confusing!] Obviously this is a compromise, as weight shift, self centering and steering forces may correspondingly change at speed, as you're well aware from this video clip you prior suggested <https://youtu.be/VbReLNi2JP4?t=831> |
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How would you run a lateral acceleration test in a vehicle ontwisty roads at no more than 40mph?
On 23/6/19 8:45 am, Arlen G. Holder wrote:
> On Sat, 22 Jun 2019 22:03:18 +1000, Xeno wrote: > >> It isn't described well at all. In most cases it is simply inferred. I >> think that's because everyone focuses on the *outside* wheel whereas the >> actual camber scrub is happening on the *inside* wheel because it is >> perched up on its outer edge because of the positive camber and is more >> lightly loaded, hence the tread blocks tend to slip rather than grip and >> flex. That sideways flexing and scrubbing is what gives them that >> characteristic feathered edge. > > Hi Xeno, > I much appreciate that you UNDERSTAND the complex geometric changes that > occur during camber scrub low-speed lock-to-lock conditions, where we both Sorry about the delay in this response; I have been giving the issue lots of thought. Far more thought than I have ever previously been required to put into it. I understand the specific phenomenon, up to a point. A lot of the heavy maths involved are beyond my pay grade. ;-) > agree the specific phenomenon I am troubleshooting is a specific very > mountainous situation which isn't covered well on the net, but which does > have pragmatic workarounds, as you & Clave have discussed. Yes, start with caster setting reductions as they are the safest to dick with. Also, tolerances can be leveraged as well. For instance, if you have a caster range of 1-3 degrees positive with a half degree tolerance, you could arguably drop the caster to half a degree positive. You will have reduced steering return but SAI and pneumatic trail should still work for you as designed. Steering will be lighter, that is one benefit, but may be a little less stable at higher road speeds. It's a case of trying it out and seeing the effect on the highway. > > Here's a shot from today with the vehicle parked at one of the curves. > o Passenger tire at (static) steering-wheel lock, heading uphill: > <https://i.postimg.cc/T1HkcsX5/mount31.jpg> That's quite a bit of positive camber there. > o Driver side tire at that same wheel lock situation: > <https://i.postimg.cc/KYXHVC3n/mount32.jpg> Man, does that ever look quite negatively cambered. I'd have expected it to be closer to vertical. It could of course be camera angles. Have you tried dropping a plumb bob next to the tyre to get an idea of what the camber angle actually is on both wheels? > > It's not easy to tell, but that inner tire (which is the one wearing the > most in these slow speed lock-to-lock turns) should be taking on a more > positive camber, while the outer tire should be taking on a more negative > camber. Yes but, due to the direction of forces at the contact patch, the one with positive camber is getting the same force direction as the outer negatively cambered tyre. That's an important point which your link makes clear and, I must say, is the only place I have ever seen that *in print*. I had already deduced that much from observation but it is nice to see that others see it too. > > Even though the outside tire is taking on more of the force, the wear is > happening more so to the outside shoulder of the inside tire (the tire with > the more positive camber). <https://i.postimg.cc/YqHVb5gY/mount33.jpg> > > As you've explained it prior, at _slow_ speeds (30 mph nominally), there > isn't as much weight transfer to the outside wheel, and yet the inside > wheel is at a tighter lock than the outside wheel due to Ackerman Angle > effects, where the more positive camber on that inside wheel causes the > outside tread area to longitudinally feather unidirectionally more so than > the outside wheel, which takes on a lesser more positive camber. > > Since we're effectively riding on the outside tread blocks of the inside > tire, those outside blocks are forced to break traction and slide, which is > what's causing the longitudinal unidirectional feathering, particularly > when traveling downhill. > > Note that the passenger tire of this rather heavy bimmer SUV is the > original tire of only about a year and a half old (about 15K miles or so), > where the outside edge counter rotational longitudinal feathering is almost > worn away, but the driver's side tire had to be replaced a few months ago, > where the counter rotational longitudinal feathering is easily felt on the > outside few inches of the tread. > > Am I correct that these are the possible ameliorations, bearing in mind > that every change made has an effect somewhere else in alignment and that > each change has to be made in the standard caster/camber/toe order? > 1. First, increase tire pressure (to decrease tread squirm) By making the tyre stiffer with increased pressure, you may be reducing tread squirm. A little bit of *tyre* flex to help the tread blocks maintain contact as they pass through the contact patch would, I suspect, be a good thing. I wouldn't make overpressuring the tyres the first stage. > 2. Second, potentially decrease positive caster (to the low end of spec) > (where the goal is to change how SAI affects the camber angle under turns) Note my point re tolerances. > 3. Third, possibly (increase?) static negative camber (within spec) > (although increasing negative static camber "may" also decrease the SAI) Increasing negative camber will *increase* SAI. Remember, camber and SAI (inclination) are locked together because of design and the proof is in the *included angle*. Let's establish a few reference points here; Study the following diagram; https://encrypted-tbn0.gstatic.com/i...sn-_-So5HKf2GM The salient point; SAI + Camber = Included Angle. Note that the included angle is designed into the steering knuckle and cannot be changed. That means the relationship between the camber angle and the steering axis *inclination* is fixed. Now observe this one; https://slideplayer.com/slide/463401...suspension.jpg It's the same thing with an SLA suspension. This one is more relevant to us. A little aside here. If I want to change the camber to the negative, I would have to move the upper ball joint inwards. That is usually done with shims at the upper control arm inner pivot. If the adjustment is at the inner end of the lower control arm, I'd have to move the lower ball joint outwards to get camber more negative. In this case the adjustment likely would be eccentric plates or bolts on the lower control arm inner pivots. Since I cannot alter the *included angle* as it is designed in to the steering knuckle, what do you think is happening to the tilt of that pivot axis (line between ball joint centres) as I move the upper ball joint inwards while the lower ball joint remains where it is? You have to be making the steering axis tilt *away* from the vertical reference Try to imagine you're confronted with a vehicle where the static camber setting is 0 and the included angle is, say, 10 degrees. Given this; SAI + Camber = Included Angle - therefore 0 + 10 = 10. If I now want to change my camber's current 0 degree setting to the negative, say, by 2 degrees, and given my included angle is currently 10, my camber change has to increase the inclination of the steering axis by 2 degrees. It will be leaning further inwards at the top ball joint by 2 degrees from its original 10. It will now be inclined by 12 degrees with respect to the vertical reference point. We know the prime cause of the camber gain on the inside wheel of a turn is because of the SAI angle. We also know that positive caster merely worsens the effect. So, by moving the camber by any degree to the negative will worsen our SAI status by the same amount. This is the conundrum, one of many, that face the suspension designer. The increased SAI, in and of itself, will increase camber gain - worsening the very problem we are trying to solve. The manufacturer believes, with the current settings they specify, that they have the middle ground covered. The difficulty is that your situation is at the extreme boundary of that middle ground. Have a look under the car in question and see what the camber adjustment actually does. If it changes the relative *lateral* location of one ball joint to the other, you are effectively screwed. That means the included angle cannot be changed without some bending or a different steering knuckle with the appropriate included angle. > 4. Set toe to spec last. > [If I got anything wrong, please let me know as it's confusing!] You are definitely getting there. As I said, it really does one's head in trying to visualise all these steering kinematics. It gets a bit more messy with MacPherson struts, especially when you move to superstruts. Given the distance between the two pivot points on the steering axis, getting any decent degree of camber and caster change is rendered more difficult. That said, depending on the manufacturer's method of attaching the steering knuckle to the strut, there are ways to adjust camber without *adversely* affecting SAI. For that purpose you use what are known in the trade as camber bolts. These are eccentric bolts that allow about ~1.5 degrees of camber adjustment. You can get camber kits that *extend* the limited factory adjustment as this video shows; https://www.youtube.com/watch?v=Qe9AS99o3o8 There may be some SAI effect depending on the system used but, compared to the *limited* options (bending, replacement knuckle) available to you on SLA suspensions, there is hope. Note, I think that camber kit as shown in the video will have some effect on SAI but not as much, in terms of degree change, as it has on the camber itself. Confused now? > > Obviously this is a compromise, as weight shift, self centering and > steering forces may correspondingly change at speed, as you're well aware > from this video clip you prior suggested > <https://youtu.be/VbReLNi2JP4?t=831> > Steering and suspension design is one huge mass of compromises. -- Xeno Nothing astonishes Noddy so much as common sense and plain dealing. (with apologies to Ralph Waldo Emerson) |
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How would you run a lateral acceleration test in a vehicle on twisty roads at no more than 40mph?
On Mon, 24 Jun 2019 15:44:03 +1000, Xeno wrote:
> Sorry about the delay in this response; I have been giving the issue > lots of thought. Far more thought than I have ever previously been > required to put into it. I understand the specific phenomenon, up to a > point. A lot of the heavy maths involved are beyond my pay grade. ;-) Hi Xeno, I appreciate the time, effort, and care for accuracy, as very few people would understand my common statement that the main reason most people can't do alignments at home isn't that they can't measure or tweak, but that the math would make most people's head explode. As you and Clare are well aware, it's a LOT more than just trig! o Suspension geometry inter-relationships are engineeringly complex! > Yes, start with caster setting reductions as they are the safest to dick > with. Thanks for confirming that the first step (whether or not I choose to go by airing up the tires), is to lower positive caster to the lowest angle within spec. > Also, tolerances can be leveraged as well. For instance, if you > have a caster range of 1-3 degrees positive with a half degree > tolerance, you could arguably drop the caster to half a degree positive. I agree with the tactic of using the tolerances, inevitable in any suspension geometry result, which is why the spec is always a range. The good news is that it's possible to measure with sufficient accuracy, I believe, at home, using the home measuring equipment we've already stated, all of which, as always with tools, end up being free: 1. Camber bubble gauge & wheel clamp jig 2. Toe plates (usually an extension to the camber wheel clamp jig) 3. Tape measure and trigonometric calculator In addition, we've discussed these "niceties" in terms of free tools 4. Steering wheel lock (which can be rednecked) 5. Turn plates (which can be rednecked) Since the home alignment tools are free, the problem is simply that the math involved makes most people's head explode; which is why I very much appreciate your astute step-by-step advice. > You will have reduced steering return but SAI and pneumatic trail should > still work for you as designed. Steering will be lighter, that is one > benefit, but may be a little less stable at higher road speeds. It's a > case of trying it out and seeing the effect on the highway. I agree with all your statements that I need to realize alignment spec ranges are a mix of compromises, where, luckily, I drive like a little old lady on the highway, so "at speed" isn't more than about 75mph or 80mph at most, even on Highway 5 which has a 70mph speed limit, as I recall. >> Here's a shot from today with the vehicle parked at one of the curves. >> o Passenger tire at (static) steering-wheel lock, heading uphill: >> <https://i.postimg.cc/T1HkcsX5/mount31.jpg> > > That's quite a bit of positive camber there. I thank you for cluing me in to LOOK for the high positive camber at steering wheel lock, which is, in reality, the REASON for the contact patch unidirectional feathering on ONLY the outside edge of the INSIDE tire! <https://i.postimg.cc/hGvsXBjK/mount34.jpg> Given that front tire is clearly worn ONLY on the outside edge, that positive caster of the inside front tire at steering wheel lock says it (almost) all, does it not? >> o Driver side tire at that same wheel lock situation: >> <https://i.postimg.cc/KYXHVC3n/mount32.jpg> > > Man, does that ever look quite negatively cambered. I'd have expected it > to be closer to vertical. It could of course be camera angles. Have you > tried dropping a plumb bob next to the tyre to get an idea of what the > camber angle actually is on both wheels? Thanks for pointing out that this outside tire, at static steering wheel lock, shows what appears to be negative camber, but where the tire, which has a few thousand miles on it, is STILL worn more on the outside edge, with the unidirectional feathering easily felt when running my hand over the tire in both directions. My main plan for measurement is to obtain the camber gauge and wheel jig first, and then I can check all sorts of things for a DIY alignment o Camber at tires pointed straight ahead o Camber with tires at plus and minus 15 degrees (to calculate caster) o Camber at full steering wheel lock (to assess effects of camber scrub) etc. >> It's not easy to tell, but that inner tire (which is the one wearing the >> most in these slow speed lock-to-lock turns) should be taking on a more >> positive camber, while the outer tire should be taking on a more negative >> camber. > Yes but, due to the direction of forces at the contact patch, the one > with positive camber is getting the same force direction as the outer > negatively cambered tyre. Aurrgh. This suspension stuff is so confusing it makes my head explode! I "thought" that the outside front tire in a slow speed tight turn downhill carries most of the weight, but, that outside tire has a flatter contact patch due to the effect of decreasing positive camber, so it's the inside front tire that wears most because, while it's not carrying most of the weight in the steep tight turn, it's far more tilted onto it's outside edge, such that the weight it does carry is disproportionate carried by a small portion of the outside tread of that inside tire. > That's an important point which your link > makes clear and, I must say, is the only place I have ever seen that *in > print*. I had already deduced that much from observation but it is nice > to see that others see it too. Hmmmm... I'm confused so can you clarify which link you're referring to? Since it's an important point that both of us know better than almost anyone on this thread, which is that what we're seeking to explain is not "in print" on the net in very many places, if any - is _this_ the link? <https://i.postimg.cc/YqHVb5gY/mount33.jpg> If so, that came from this link, for reference purposes: <https://www.quora.com/When-turning-I-see-there-is-a-plus-camber-in-a-vehicle-Why> Which, I quote here for reference: "The combination of these various angles affects the wheel camber when steered. The inner wheel in a turn takes on positive camber because the steering pivot is angled. By the same token, the outer wheel takes on more negative camber. But actually both of these things are desirable. The purpose of negative camber is to increase grip in a turn. As a car turns, the tyre has to resist the force which is causing the car to turn. The outer wheel does most of the work, but without negative camber, the tyre would tend to roll away under the wheel and reduce contact with the road. The camber counteracts this effect so that the tendency of the tyre to roll under actually increases the contact, and hence grip, just when it¢s needed. But think about the inner wheel in the same turn. While it carries less of the turning force, what appears to be positive camber is actually negative camber with respect to the turning force. So that tyre is also gripping harder than it would do without camber. *So it only looks like positive camber - in fact it¢s really negative* *camber, because it¢s the inner edge of the wheel that is leading in a* *turn*" Aurrgh. This constantly changing camber stuff is making my head explode! For example, I've re-read this sentence a hundred times, and I still can't make any sense out of the camber mechanism ... can you? "But think about the inner wheel in the same turn. While it carries less of the turning force, what appears to be positive camber is actually negative camber with respect ot the turning force" <https://www.quora.com/When-turning-I-see-there-is-a-plus-camber-in-a-vehicle-Why> What does _that_ mean? How can positive camber be negative camber at the same time? (I realize force vector diagrams are involved - but it's hard to fathom.) > By making the tyre stiffer with increased pressure, you may be reducing > tread squirm. A little bit of *tyre* flex to help the tread blocks > maintain contact as they pass through the contact patch would, I > suspect, be a good thing. I wouldn't make overpressuring the tyres the > first stage. Interesting ... ok, so reducing positive caster to the low end of spec can be the first step. Thanks for that advice. > >> 2. Second, potentially decrease positive caster (to the low end of spec) >> (where the goal is to change how SAI affects the camber angle under turns) > > Note my point re tolerances. Yup. It can actually be below spec, if I could measure to better than tolerance, given the spec includes worst case tolerance already. Thanks for that observation. >> 3. Third, possibly (increase?) static negative camber (within spec) >> (although increasing negative static camber "may" also decrease the SAI) > > Increasing negative camber will *increase* SAI. Remember, camber and SAI > (inclination) are locked together because of design and the proof is in > the *included angle*. Let's establish a few reference points here; Study > the following diagram; > https://encrypted-tbn0.gstatic.com/i...sn-_-So5HKf2GM > The salient point; SAI + Camber = Included Angle. That's a nice diagram that shows the relationship between o Steering axis inclination o Camber Resulting in o Included angle > Note that the included angle is designed into the steering knuckle and > cannot be changed. That means the relationship between the camber angle > and the steering axis *inclination* is fixed. > > Now observe this one; > https://slideplayer.com/slide/463401...suspension.jpg It looks the same to me. o Steering axis inclination o Camber Resulting in o Included angle > It's the same thing with an SLA suspension. This one is more relevant to > us. A little aside here. If I want to change the camber to the negative, > I would have to move the upper ball joint inwards. That is usually done > with shims at the upper control arm inner pivot. If the adjustment is at > the inner end of the lower control arm, I'd have to move the lower ball > joint outwards to get camber more negative. In this case the adjustment > likely would be eccentric plates or bolts on the lower control arm inner > pivots. Since I cannot alter the *included angle* as it is designed in > to the steering knuckle, what do you think is happening to the tilt of > that pivot axis (line between ball joint centres) as I move the upper > ball joint inwards while the lower ball joint remains where it is? You > have to be making the steering axis tilt *away* from the vertical reference > > Try to imagine you're confronted with a vehicle where the static camber > setting is 0 and the included angle is, say, 10 degrees. > Given this; SAI + Camber = Included Angle - therefore 0 + 10 = 10. If I > now want to change my camber's current 0 degree setting to the negative, > say, by 2 degrees, and given my included angle is currently 10, my > camber change has to increase the inclination of the steering axis by 2 > degrees. It will be leaning further inwards at the top ball joint by 2 > degrees from its original 10. It will now be inclined by 12 degrees with > respect to the vertical reference point. As always, you're way ahead of me, so I'm reading (and re-reading what you wrote), where it all makes sense while I read it, but I have to ABSORB it to make sure it makes sense intuitively to me (sort of like how I have to constantly shift my mind when thinking of gravity as a curvature in spacetime as opposed to a simple force). > We know the prime cause of the camber gain on the inside wheel of a turn > is because of the SAI angle. This is the key point, I think, is it not? > We also know that positive caster merely > worsens the effect. Yup. This need to lessen positive caster is something you've finally worked into my brain so that it's now "intuitive", much like how I'm trying to work the fact that there are, in reality, something like 10 dimensions to the universe, which takes a while before it becomes intuitive. > So, by moving the camber by any degree to the > negative will worsen our SAI status by the same amount. Ah. This is important! It means I don't want any more negative camber than the spec (plus tolerance) allows for, most likely (at least for this one purpose). > This is the > conundrum, one of many, that face the suspension designer. The increased > SAI, in and of itself, will increase camber gain - worsening the very > problem we are trying to solve. The manufacturer believes, with the > current settings they specify, that they have the middle ground covered. > The difficulty is that your situation is at the extreme boundary of that > middle ground. Yup. Luckily I don't care much for high speed stability, if they define high speed as over about 80mph, which I almost never go. I care about tire wear on the outside edge of the front tires mostly. > Have a look under the car in question and see what the camber adjustment > actually does. If it changes the relative *lateral* location of one ball > joint to the other, you are effectively screwed. That means the included > angle cannot be changed without some bending or a different steering > knuckle with the appropriate included angle. Each vehicle is different, where my bimmer sedan, for example, only has camber adjustments in the rear, and where aftermarket camber plates are needed on the front struts, which is going too far, IMHO. I'll stick to the low end of spec, within tolerances, as you suggested. > >> 4. Set toe to spec last. >> [If I got anything wrong, please let me know as it's confusing!] > > You are definitely getting there. As I said, it really does one's head > in trying to visualise all these steering kinematics. Yup. Of the half dozen jobs I want to do that most people NEVER do in their entire lives, the alignment is singularly different not because it's hard to measure at home (it's not), nor because it's hard to adjust (that's easy) - but - because the knowledge needed is of an amount that makes most people's brains explode. I think the MAIN takeaway is to reduce caster to the low end of spec. > It gets a bit more messy with MacPherson struts, especially when you > move to superstruts. Given the distance between the two pivot points on > the steering axis, getting any decent degree of camber and caster change > is rendered more difficult. That said, depending on the manufacturer's > method of attaching the steering knuckle to the strut, there are ways to > adjust camber without *adversely* affecting SAI. For that purpose you > use what are known in the trade as camber bolts. These are eccentric > bolts that allow about ~1.5 degrees of camber adjustment. You can get > camber kits that *extend* the limited factory adjustment as this video > shows; > https://www.youtube.com/watch?v=Qe9AS99o3o8 Some, but not all, the vehicles I'm seeking to align at home have no front camber adjustment anyway, where I've made a conscious decision to stay within OEM specs. > There may be some SAI effect depending on the system used but, compared > to the *limited* options (bending, replacement knuckle) available to you > on SLA suspensions, there is hope. Note, I think that camber kit as > shown in the video will have some effect on SAI but not as much, in > terms of degree change, as it has on the camber itself. > > Confused now? Yup. I think I have my battle plan though, although no battle plan survives intact after contact with the enemy... > Steering and suspension design is one huge mass of compromises. Yup. One thing is whether we can accurately measure camber "well enough" with a cellphone, where I use a $100 level but a cellphone "might" be accurate enough by now... do you think? |
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How would you run a lateral acceleration test in a vehicle on twisty roads at no more than 40mph?
On Mon, 24 Jun 2019 18:31:52 -0000 (UTC), Arlen G. Holder wrote:
> I appreciate the time, effort, and care for accuracy, as very few people > would understand my common statement that the main reason most people can't > do alignments at home isn't that they can't measure or tweak, but that the > math would make most people's head explode. Hi Xeno, BTW, one statement I can't yet wrap my head around is this one, which says that the high positive camber on the inside tire on turns is actually a NEGATIVE camber with respect to the force vectors. <https://i.postimg.cc/YqHVb5gY/mount33.jpg> What the heck does _that_ mean? |
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How would you run a lateral acceleration test in a vehicle ontwisty roads at no more than 40mph?
On 25/6/19 8:03 am, Arlen G. Holder wrote:
> On Mon, 24 Jun 2019 18:31:52 -0000 (UTC), Arlen G. Holder wrote: > >> I appreciate the time, effort, and care for accuracy, as very few people >> would understand my common statement that the main reason most people can't >> do alignments at home isn't that they can't measure or tweak, but that the >> math would make most people's head explode. > > Hi Xeno, > BTW, one statement I can't yet wrap my head around is this one, which says > that the high positive camber on the inside tire on turns is actually a > NEGATIVE camber with respect to the force vectors. > > <https://i.postimg.cc/YqHVb5gY/mount33.jpg> > > > What the heck does _that_ mean? > I thought that one would stump you. ;-) The clue is in the wording - *with respect to the force vectors*. That diagram you linked to above, I'll use that in an attempt to explain what is meant. For a start, dismiss the middle wheel, ignore it. Now imagine the left hand wheel is on the *outside* of the turn. You can see the effect of the lateral forces acting on the tread. The arrow indicates the *force vector* direction. Lets now deal with the other wheel. Imagine it is the wheel on the other side of the car and the axle is coming out of the left side rather than the right. It has positive camber with respect to its position on the car but, naturally, it is leaning the other way. The lateral forces, however, are acting in the same direction as the outside wheel but the effect on the inner wheel is different. *To these forces*, the inside wheel looks to have negative camber because they aren't referencing the vehicle as being left or right. The forces see two wheels trying to turn in the same direction but are leaning in opposite directions. This picture will assist you in imagining the scenario. http://www.conceptcarz.com/images/Bu...-10-MH-010.jpg One wheel, the outer, is showing a positive camber to the lateral force. The inner, on the other hand, is showing a negative camber lean with respect to the lateral forces. It will look like this; https://www.gmforum.com/attachments/...ivecamber1.jpg The above pic shows what the inner wheel likely looks like on the road in a sharp turn. Just imagine that red arrow is the turning force at the contact patch. It is not hitting the outer tread edge *first* like the opposite wheel is. The outer tread is not touching the road so the force is acting on the outer half, or less, of the tread. That will cause some distortion of the sidewalls as the mass of the car tries to pull against the contact patch and flatten the tread somewhat. But, and it is a big but, modern radials don't have a lot of ability to flatten the tread, steel belted radials even less so. Overinflated tyres really aren't going to mitigate that very much. The pic really does show what you are up against. Have you looked under your tyres whilst parked on a flat surface? I think you would be enlightened. Whilst on the topic, have a look at this book; https://manualzz.com/doc/31232614/th...ing-principles I was browsing through my dead tree library here and came across the following. Go to page 181, section 3.5.3. If you don't get the meaning of the text, Fig. 3.50 should make it clear. The kinematics of the double wishbone suspension (the SLA) make it clear that the KPI (same as SAI) *angle* alters on bump and rebound due to the different radii in the arc of each arm. That same angle alteration of KPI angle also affects camber. It is in the text description of the diagram; Construction determination of the kingpin inclination alteration angle on double wishbones which is *equal* to the camber alteration. The highlight is mine but what that diagram is telling you is that as you go into *bump*, your KPI increases and your camber moves commensurately to the negative. The same applies to McPherson Struts as can be seen in Fig 3.51 This backs up what I was saying with regard to the camber angle change on an SLA suspension following the SAI/KPI change, and vice versa. If you alter the camber, you have also altered the SAI/KPI. So, by moving camber to the negative you are increasing the SAI angle, with all the possibilities, both negative and positive, that entails. What does this mean to us when cornering? Well, weight transfer will force the *outer* wheel into bump and give you a more negative camber on the *outer* wheel. The problem *for you* is that weight transfer works best *at speed*. At low speed, the outer wheel is probably retaining some positive camber. Have a look at your outer wheel when your steering is at the full lock. I'm guessing it may well still be at a positive camber angle. Not as much as the inner wheel but it too could be contributing to the edge wear. Anyway, the first pic on this thread led me to this article which I think you will find interesting. https://www.gmforum.com/buick-172/to...cement-305196/ It relates to him switching from a strut with no camber adjustment provision to one that did have that provision. If you recall, I said that a strut suspension, depending on where the adjuster is, can have a camber adjuster that doesn't affect SAI much, if at all. In summary, if you want to understand the kinematics and elastokinematics of suspension systems, this book will do it for you; https://manualzz.com/doc/31232614/th...ing-principles I linked to it previously. I have dead tree versions of the first (English) edition 1996 and the second (English) edition 2001. I'll respond to your other post shortly. Right now I've done my head in again cogitating over all this stuff. ;-) -- Xeno Nothing astonishes Noddy so much as common sense and plain dealing. (with apologies to Ralph Waldo Emerson) |
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How would you run a lateral acceleration test in a vehicle ontwisty roads at no more than 40mph?
On 25/6/19 4:31 am, Arlen G. Holder wrote:
> On Mon, 24 Jun 2019 15:44:03 +1000, Xeno wrote: > >> Sorry about the delay in this response; I have been giving the issue >> lots of thought. Far more thought than I have ever previously been >> required to put into it. I understand the specific phenomenon, up to a >> point. A lot of the heavy maths involved are beyond my pay grade. ;-) > > Hi Xeno, > > I appreciate the time, effort, and care for accuracy, as very few people > would understand my common statement that the main reason most people can't > do alignments at home isn't that they can't measure or tweak, but that the > math would make most people's head explode. > > As you and Clare are well aware, it's a LOT more than just trig! > o Suspension geometry inter-relationships are engineeringly complex! It is when you get into elastokinematics without which many suspension designs wouldn't work. It isn't just for *insulation* that rubber bushes are used. Some designs wouldn't work without that bit of give that the bushings provide. > >> Yes, start with caster setting reductions as they are the safest to dick >> with. > > Thanks for confirming that the first step (whether or not I choose to go by > airing up the tires), is to lower positive caster to the lowest angle > within spec. As I mentioned in a previous post, airing up the tyres will put more pressure on the *centre* of the tread and reduce the tyre's ability to conform to the road surface under lateral deflection. Camber and the reduction thereof, on the other hand, has to reduce the effect given that it is *one* of the causes, albeit not a major one. > >> Also, tolerances can be leveraged as well. For instance, if you >> have a caster range of 1-3 degrees positive with a half degree >> tolerance, you could arguably drop the caster to half a degree positive. > > I agree with the tactic of using the tolerances, inevitable in any > suspension geometry result, which is why the spec is always a range. > > The good news is that it's possible to measure with sufficient accuracy, I > believe, at home, using the home measuring equipment we've already stated, > all of which, as always with tools, end up being free: Are you using *free* meaning the tools pay for themselves in the long, or short, term. > 1. Camber bubble gauge & wheel clamp jig > 2. Toe plates (usually an extension to the camber wheel clamp jig) > 3. Tape measure and trigonometric calculator > > In addition, we've discussed these "niceties" in terms of free tools > 4. Steering wheel lock (which can be rednecked) > 5. Turn plates (which can be rednecked) > > Since the home alignment tools are free, the problem is simply that the > math involved makes most people's head explode; which is why I very much > appreciate your astute step-by-step advice. > >> You will have reduced steering return but SAI and pneumatic trail should >> still work for you as designed. Steering will be lighter, that is one >> benefit, but may be a little less stable at higher road speeds. It's a >> case of trying it out and seeing the effect on the highway. > > I agree with all your statements that I need to realize alignment spec > ranges are a mix of compromises, where, luckily, I drive like a little old > lady on the highway, so "at speed" isn't more than about 75mph or 80mph at > most, even on Highway 5 which has a 70mph speed limit, as I recall. > >>> Here's a shot from today with the vehicle parked at one of the curves. >>> o Passenger tire at (static) steering-wheel lock, heading uphill: >>> <https://i.postimg.cc/T1HkcsX5/mount31.jpg> >> >> That's quite a bit of positive camber there. > > I thank you for cluing me in to LOOK for the high positive camber at > steering wheel lock, which is, in reality, the REASON for the contact patch > unidirectional feathering on ONLY the outside edge of the INSIDE tire! > <https://i.postimg.cc/hGvsXBjK/mount34.jpg> > > Given that front tire is clearly worn ONLY on the outside edge, that > positive caster of the inside front tire at steering wheel lock says it > (almost) all, does it not? Pretty much. > >>> o Driver side tire at that same wheel lock situation: >>> <https://i.postimg.cc/KYXHVC3n/mount32.jpg> >> >> Man, does that ever look quite negatively cambered. I'd have expected it >> to be closer to vertical. It could of course be camera angles. Have you >> tried dropping a plumb bob next to the tyre to get an idea of what the >> camber angle actually is on both wheels? > > Thanks for pointing out that this outside tire, at static steering wheel > lock, shows what appears to be negative camber, but where the tire, which > has a few thousand miles on it, is STILL worn more on the outside edge, > with the unidirectional feathering easily felt when running my hand over > the tire in both directions. Don't forget, you turn left *and* right. For 50% of the time each wheel has an inside running to a high camber turn. > > My main plan for measurement is to obtain the camber gauge and wheel jig > first, and then I can check all sorts of things for a DIY alignment > o Camber at tires pointed straight ahead > o Camber with tires at plus and minus 15 degrees (to calculate caster) > o Camber at full steering wheel lock (to assess effects of camber scrub) > etc. You really are keen on this, aren't you? > >>> It's not easy to tell, but that inner tire (which is the one wearing the >>> most in these slow speed lock-to-lock turns) should be taking on a more >>> positive camber, while the outer tire should be taking on a more negative >>> camber. >> Yes but, due to the direction of forces at the contact patch, the one >> with positive camber is getting the same force direction as the outer >> negatively cambered tyre. > > Aurrgh. This suspension stuff is so confusing it makes my head explode! You were warned! ;-) > > I "thought" that the outside front tire in a slow speed tight turn downhill > carries most of the weight, but, that outside tire has a flatter contact > patch due to the effect of decreasing positive camber, so it's the inside > front tire that wears most because, while it's not carrying most of the > weight in the steep tight turn, it's far more tilted onto it's outside > edge, such that the weight it does carry is disproportionate carried by a > small portion of the outside tread of that inside tire. Yes, you have the hang of it. > >> That's an important point which your link >> makes clear and, I must say, is the only place I have ever seen that *in >> print*. I had already deduced that much from observation but it is nice >> to see that others see it too. > > Hmmmm... I'm confused so can you clarify which link you're referring to? > > Since it's an important point that both of us know better than almost > anyone on this thread, which is that what we're seeking to explain is not > "in print" on the net in very many places, if any - is _this_ the link? > <https://i.postimg.cc/YqHVb5gY/mount33.jpg> That's the very one. I did *attempt* to explain it in my other post. I hope I was successful in that little endeavour. > > If so, that came from this link, for reference purposes: > <https://www.quora.com/When-turning-I-see-there-is-a-plus-camber-in-a-vehicle-Why> > > Which, I quote here for reference: > "The combination of these various angles affects the wheel camber when > steered. The inner wheel in a turn takes on positive camber because the > steering pivot is angled. By the same token, the outer wheel takes on more > negative camber. > > But actually both of these things are desirable. The purpose of negative > camber is to increase grip in a turn. As a car turns, the tyre has to > resist the force which is causing the car to turn. The outer wheel does > most of the work, but without negative camber, the tyre would tend to roll > away under the wheel and reduce contact with the road. The camber > counteracts this effect so that the tendency of the tyre to roll under > actually increases the contact, and hence grip, just when it¢s needed. But > think about the inner wheel in the same turn. While it carries less of the > turning force, what appears to be positive camber is actually negative > camber with respect to the turning force. So that tyre is also gripping > harder than it would do without camber. > > *So it only looks like positive camber - in fact it¢s really negative* > *camber, because it¢s the inner edge of the wheel that is leading in a* > *turn*" > > Aurrgh. This constantly changing camber stuff is making my head explode! It takes time, and patience, lots and lots of patience. > > For example, I've re-read this sentence a hundred times, and I still can't > make any sense out of the camber mechanism ... can you? Yes, I can, but I have been working at it - a lot! > "But think about the inner wheel in the same turn. While it carries less > of the turning force, what appears to be positive camber is actually > negative camber with respect ot the turning force" > <https://www.quora.com/When-turning-I-see-there-is-a-plus-camber-in-a-vehicle-Why> > > What does _that_ mean? When you look at a vehicle, you only see camber as it is with respect to the vehicle centreline, not with respect to the forces acting on it. Positive camber is the wheel leaning out at the top, *away from* the vehicle centreline. That locks you into a specific way of thinking. I was fortunate that I have worked on all sorts of equipment in my time. There have been times where I have had to think *differently*. Like when I was working on these; https://encrypted-tbn0.gstatic.com/i...GhEgjwO8f0jPWg Not a lot different to your car really, isn't it? Note that camber gain on that inside turning wheel. But graders have more force vectors to worry about than just cornering forces. The blade hanging underneath creates a whole different set of force vectors and the *steering* needs to be able to cope with that. To that end, grader steering has a few tricks up its sleeve. The one that concerns us however is this; https://cdn.forconstructionpros.com/...e2c6fd4f. jpg In the case above, we now do not have camber angles referencing the grader centreline. Instead we have the camber angles referencing the *load*. In this case the load is the *reaction force* from the grader blade trying to push the grader to our right *as we view it*. That reaction force is being resisted by the forces at the contact patch of those wheels which is in the opposite direction - to our left. The wheels now are presenting themselves as *negative camber* to the ground force. The extreme angle allows the tyres to bite in to the ground and increase the grip. This is precisely what your front tyres are doing, though much less exaggerated, not the result of a hydraulic system and the force is inertial because of cornering. The fundamentals are there however. > > How can positive camber be negative camber at the same time? > (I realize force vector diagrams are involved - but it's hard to fathom.) I hope my *grader* makes it more clear. Forget the vehicle centreline and focus on tyre angles with respect to the lateral forces and you'll see it all. > >> By making the tyre stiffer with increased pressure, you may be reducing >> tread squirm. A little bit of *tyre* flex to help the tread blocks >> maintain contact as they pass through the contact patch would, I >> suspect, be a good thing. I wouldn't make overpressuring the tyres the >> first stage. > > Interesting ... ok, so reducing positive caster to the low end of spec can > be the first step. Thanks for that advice. It is the way I would go. > >> >>> 2. Second, potentially decrease positive caster (to the low end of spec) >>> (where the goal is to change how SAI affects the camber angle under turns) >> >> Note my point re tolerances. > > Yup. It can actually be below spec, if I could measure to better than > tolerance, given the spec includes worst case tolerance already. > > Thanks for that observation. > >>> 3. Third, possibly (increase?) static negative camber (within spec) >>> (although increasing negative static camber "may" also decrease the SAI) >> >> Increasing negative camber will *increase* SAI. Remember, camber and SAI >> (inclination) are locked together because of design and the proof is in >> the *included angle*. Let's establish a few reference points here; Study >> the following diagram; >> https://encrypted-tbn0.gstatic.com/i...sn-_-So5HKf2GM >> The salient point; SAI + Camber = Included Angle. > > That's a nice diagram that shows the relationship between > o Steering axis inclination > o Camber > Resulting in > o Included angle Yes, we are all visual thinkers. > >> Note that the included angle is designed into the steering knuckle and >> cannot be changed. That means the relationship between the camber angle >> and the steering axis *inclination* is fixed. >> >> Now observe this one; >> https://slideplayer.com/slide/463401...suspension.jpg > > It looks the same to me. > o Steering axis inclination > o Camber > Resulting in > o Included angle Fundamentally it is but the difference is that SAI change is a lot more difficult because the two pivot points are far apart. > >> It's the same thing with an SLA suspension. This one is more relevant to >> us. A little aside here. If I want to change the camber to the negative, >> I would have to move the upper ball joint inwards. That is usually done >> with shims at the upper control arm inner pivot. If the adjustment is at >> the inner end of the lower control arm, I'd have to move the lower ball >> joint outwards to get camber more negative. In this case the adjustment >> likely would be eccentric plates or bolts on the lower control arm inner >> pivots. Since I cannot alter the *included angle* as it is designed in >> to the steering knuckle, what do you think is happening to the tilt of >> that pivot axis (line between ball joint centres) as I move the upper >> ball joint inwards while the lower ball joint remains where it is? You >> have to be making the steering axis tilt *away* from the vertical reference >> >> Try to imagine you're confronted with a vehicle where the static camber >> setting is 0 and the included angle is, say, 10 degrees. >> Given this; SAI + Camber = Included Angle - therefore 0 + 10 = 10. If I >> now want to change my camber's current 0 degree setting to the negative, >> say, by 2 degrees, and given my included angle is currently 10, my >> camber change has to increase the inclination of the steering axis by 2 >> degrees. It will be leaning further inwards at the top ball joint by 2 >> degrees from its original 10. It will now be inclined by 12 degrees with >> respect to the vertical reference point. > > As always, you're way ahead of me, so I'm reading (and re-reading what you > wrote), where it all makes sense while I read it, but I have to ABSORB it > to make sure it makes sense intuitively to me (sort of like how I have to > constantly shift my mind when thinking of gravity as a curvature in > spacetime as opposed to a simple force). > >> We know the prime cause of the camber gain on the inside wheel of a turn >> is because of the SAI angle. > > This is the key point, I think, is it not? That is indeed key. > >> We also know that positive caster merely >> worsens the effect. > > Yup. This need to lessen positive caster is something you've finally worked > into my brain so that it's now "intuitive", much like how I'm trying to > work the fact that there are, in reality, something like 10 dimensions to > the universe, which takes a while before it becomes intuitive. Universe dimensions, I never grasped that bit of intuition. Have enough hassles dealing with real world issues, like suspension kinematics. > >> So, by moving the camber by any degree to the >> negative will worsen our SAI status by the same amount. > > Ah. This is important! It is indeed a salient point. The other point, which I made in my other post, is that body roll will, on SLA suspensions, increase negative camber on the outer wheel which will also increase the SAI angle. By going beyond spec with the static negative camber, you could go beyond optimal negative camber on that outer wheel during hard cornering and adversely affect the size and/or shape of the contact patch. That could impact slip angles. Remember, because of weight transfer, the outside front wheel carries way more than its fair share of vehicle mass. You really don't want to do anything that will impact that wheel's grip on the road. A negative camber change could easily alter (reduce) the slip angle at that front wheel and turn your car into an oversteerer. > > It means I don't want any more negative camber than the spec (plus > tolerance) allows for, most likely (at least for this one purpose). At this point in time, yes. > >> This is the >> conundrum, one of many, that face the suspension designer. The increased >> SAI, in and of itself, will increase camber gain - worsening the very >> problem we are trying to solve. The manufacturer believes, with the >> current settings they specify, that they have the middle ground covered. >> The difficulty is that your situation is at the extreme boundary of that >> middle ground. > > Yup. Luckily I don't care much for high speed stability, if they define > high speed as over about 80mph, which I almost never go. High speed is *highway speed* which is generally 60mph+ > > I care about tire wear on the outside edge of the front tires mostly. > >> Have a look under the car in question and see what the camber adjustment >> actually does. If it changes the relative *lateral* location of one ball >> joint to the other, you are effectively screwed. That means the included >> angle cannot be changed without some bending or a different steering >> knuckle with the appropriate included angle. > > Each vehicle is different, where my bimmer sedan, for example, only has > camber adjustments in the rear, and where aftermarket camber plates are > needed on the front struts, which is going too far, IMHO > > I'll stick to the low end of spec, within tolerances, as you suggested. > >> >>> 4. Set toe to spec last. >>> [If I got anything wrong, please let me know as it's confusing!] >> >> You are definitely getting there. As I said, it really does one's head >> in trying to visualise all these steering kinematics. > > Yup. Of the half dozen jobs I want to do that most people NEVER do in their > entire lives, the alignment is singularly different not because it's hard > to measure at home (it's not), nor because it's hard to adjust (that's > easy) - but - because the knowledge needed is of an amount that makes most > people's brains explode. You got it. With minimal knowledge of steering and suspension, I can have you doing wheel alignments in no time. You just need to understand the fundamentals of SAI, caster, camber, toe, toe out on turns and a few others and you can do the job. The trick though is diagnosis and that's where a much greater depth of knowledge is required. > > I think the MAIN takeaway is to reduce caster to the low end of spec. > >> It gets a bit more messy with MacPherson struts, especially when you >> move to superstruts. Given the distance between the two pivot points on >> the steering axis, getting any decent degree of camber and caster change >> is rendered more difficult. That said, depending on the manufacturer's >> method of attaching the steering knuckle to the strut, there are ways to >> adjust camber without *adversely* affecting SAI. For that purpose you >> use what are known in the trade as camber bolts. These are eccentric >> bolts that allow about ~1.5 degrees of camber adjustment. You can get >> camber kits that *extend* the limited factory adjustment as this video >> shows; >> https://www.youtube.com/watch?v=Qe9AS99o3o8 > > Some, but not all, the vehicles I'm seeking to align at home have no front > camber adjustment anyway, where I've made a conscious decision to stay > within OEM specs. Wise. I remember one chap who wanted *more* steering lock so he dicked with the lock stops (they were external and on the steering arm). It all went well for him until one day, whilst engaged in a bit of *spirited driving*, including some opposite locking, his steering box jammed at the end of its travel. It didn't end well for him. Those lock stops had a *specification* for a very good reason, a hard lesson he learnt well. > >> There may be some SAI effect depending on the system used but, compared >> to the *limited* options (bending, replacement knuckle) available to you >> on SLA suspensions, there is hope. Note, I think that camber kit as >> shown in the video will have some effect on SAI but not as much, in >> terms of degree change, as it has on the camber itself. >> >> Confused now? > > Yup. > I think I have my battle plan though, although no battle plan survives > intact after contact with the enemy... Consider it a learning exercise, the outcome of which will influence your next *strategy*. > >> Steering and suspension design is one huge mass of compromises. > Yup. > > One thing is whether we can accurately measure camber "well enough" with a > cellphone, where I use a $100 level but a cellphone "might" be accurate > enough by now... do you think? > You can but try it. A straight edge against the rim and the phone on that. You would need to check wheel bearings and ball joints for play as that will influence your readings. Ball joints, ah, there is another misunderstood item in the car industry. But let's not go there. BTW, you are not alone; http://www.peachparts.com/shopforum/...tire-wear.html Apart from him getting neg and pos caster mixed up, there are a few points made in this brief forum discussion that you might want to take note of. Also, not all the pros understand the camber scrub issue. A perfect example here; http://www.mcgrathauto.com.au/tyre-wear-2/ They say it's cornering on roundabouts that does damage and only to the passenger side. *but* the scrubbing can be on *both sides* depending on the direction you are turning. Roundabouts, because you always travel the same way, would give you a bias to one side. Camber scrub should occur on the driver's side if the roundabout is *tight* and a lot here are. But, when you think about it, leaving roundabouts aside, a turn to the passenger side on any *intersection* crossroad will always be *sharper* than a turn to the drivers side. In your case, LHD, when you turn right from the RHS side of the road, you turn *tight* in order to keep on the RHS of the exit road after you complete your turn. On the other hand, when you turn left, you enter the intersection from the RHS of the road, you swing *wide* so you exit the intersection on the RHS of the exit road. Right turn is a *small* arc, left turn is a *large* arc and, as we know, it's the small arc turns that do the damage and, in this case, it's the passenger side tyre that suffers. For us here in Australia, the reverse is true since we are RHD. In summary, the inside of your tightest turns will be when you turn to the passenger side. Since that tyre is the inside runner on the tightest turns, there is likely to be greater camber scrub on the passenger side. In your case, you have tight turns *both ways*. Is there a bias in the wear pattern however? FWD cars have a greater issue with camber scrub since they generally have high SAI angles, not to mention a greater forward weight bias. The general consensus is that the passenger side will wear more on the outside of the tread than the driver's side. There are a great number of reasons given for this, including the *camber of the road* but, when you sit down and think deeply about it, you wonder if camber scrub is playing a devilish little role here also. Have I messed with your mind again?? ;-) -- Xeno Nothing astonishes Noddy so much as common sense and plain dealing. (with apologies to Ralph Waldo Emerson) |
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