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#11
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Clare - are smaller car tires easier to balance than SUV tires?
On 16/6/19 4:31 pm, Arlen G. Holder wrote:
> On Sat, 15 Jun 2019 14:14:27 +1000, Xeno wrote: > >> You won't. Information on it is scarce since it is an undesired effect. >> However, if you look at what the wheel is doing vis a vis camber during >> high angle (note - not high speed) cornering, then you can visualise the >> issue. Note too that tread blocks have a limited degree of flexibility >> and, as such, are likely to exceed that during high angle cornering >> resulting in tread scrubbing. > > Hi Xeno, > > While I don't race, our conditions are "extreme" enough, in that constant > incessant repeated nearly full back-to-back wheel locks are causing > "something" to scrape away rubber, so this "camber scrub" is intriguing. That is also considered *extreme* even though you aren't doing it fast. > > I snapped this photo of tires that I mounted about a month ago, which only > have about 1000 miles on them, where they clearly show this pattern which > "might" be what you've been referring to as "camber scrub". > <https://i.postimg.cc/zvvyL2tq/mount24.jpg> > > Does _that_ feathering look like what you're referring to as "camber scrub"? > It does indeed! Same kind of feathering I'm getting on my Toyota's front tyres - a feathering that you can easily feel in the early stages as you run your hands for and aft along that section of tread area. Now try to imagine what those tread blocks are doing as they roll around in a tight circle with heaps of camber gain. It's not pretty and, worse, there is SFA you can do about it. BTW, positive caster will accentuate the camber scrub. Caster is generally not a tyre wearing angle. However, the more caster your steering has, the more camber *change* you will get when turning the steering. Positive caster will give you a beneficial gain in terms of handling. You will get more camber gain (more +ve) on the inside wheel but the outside wheel will experience camber *loss* and become more vertical or even negative. Since the more vertical tyre is on the outside, the tread will get more grip with reduced slip angle aided by weight transfer. This is great for cornering at speed. However when travelling at slow speeds, weight transfer is not as significant and the camber angle on the inside wheel, the one at the tightest lock, heads towards positive extremes. It is, in effect, riding heavily on the outer edge of the tread and this is where, and why, the damage is being done. The tread blocks have only so much flex before they are forced to break contact with the road and slide. You've seen the evidence of what that does. Caster specifications are usually given as a range, say between 1 and 2 degrees with a side to side variation limit. All you can really do to mitigate the effect is to set your caster to the low end of the specified range. -- Xeno Nothing astonishes Noddy so much as common sense and plain dealing. (with apologies to Ralph Waldo Emerson) |
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#12
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Clare - are smaller car tires easier to balance than SUV tires?
On Sun, 16 Jun 2019 20:40:34 +1000, Xeno wrote:
> It does indeed! Same kind of feathering I'm getting on my Toyota's front > tyres - a feathering that you can easily feel in the early stages as you > run your hands for and aft along that section of tread area. Thanks for that information, where the one correction I need to make is that you can only feel this feathering running your hand "*backward*" (clockwise) over the outside quarter of the tread pattern. <https://i.postimg.cc/zvvyL2tq/mount24.jpg> If you run your hand toward the front (counterclockwise), you can't feel the feathering because each "lip" is downward. <https://i.postimg.cc/vTZLmZrN/mount25.jpg> When you run your hand toward the rear of the vehicle, each lip is upward. <https://i.postimg.cc/X7hcV3ps/mount26.jpg> That's the oddity. The feathering is only one way. <https://i.postimg.cc/KYhPMN7L/mount27.jpg> It's reproducible for years - so it's always the same. That one-way lip feathering should be diagnostic, should it not? <https://i.postimg.cc/Wzyrb6bd/mount28.jpg> > BTW, positive caster will accentuate the camber scrub. Caster is > generally not a tyre wearing angle. However, the more caster your > steering has, the more camber *change* you will get when turning the > steering. Hmmmmmmmm.... maybe I can consider lessening positive caster a teeny bit? > Positive caster will give you a beneficial gain in terms of > handling. You will get more camber gain (more +ve) on the inside wheel > but the outside wheel will experience camber *loss* and become more > vertical or even negative. Since the more vertical tyre is on the > outside, the tread will get more grip with reduced slip angle aided by > weight transfer. This is great for cornering at speed. However when > travelling at slow speeds, weight transfer is not as significant and the > camber angle on the inside wheel, the one at the tightest lock, heads > towards positive extremes. This is very useful information, as all our lock-to-lock cornering is at 30mph to 40mph ... never faster because I ran a test last week where anything over 40mph is impossible to do even remotely safely, as all the turns are blind turns and the file miles of 9% twisty road can't even be twenty feet wide at the maximum (I should measure it but it's something like that, as it's too narrow for the county to center stripe it legally). > It is, in effect, riding heavily on the outer > edge of the tread and this is where, and why, the damage is being done. I need to study more - where your conclusion is spot on perfect but where I don't get the individual steps only because I think of alignment as being 'static' so to speak. I know it changes - but my brain doesn't know 'how' it changes under those slow speed lock-to-lock downhill (or uphill) turns. > The tread blocks have only so much flex before they are forced to break > contact with the road and slide. You've seen the evidence of what that does. Yes. That's for sure. The outer tread blocks "feather" such that you can feel it, and barely see it, after about 1000 miles. The only thing I can do, is change the alignment or rotate every 1000 miles (but even rotation won't stop it - it just evens it out with the rears). > Caster specifications are usually given as a range, say between 1 and 2 > degrees with a side to side variation limit. All you can really do to > mitigate the effect is to set your caster to the low end of the > specified range. That's EXACTLY what I'll do! I have to admit I need to read, and re-read and re-read again what you wrote above, as my brain needs to work in step-by-step fashion. You didn't skip a step but I don't quite "believe" in my brain all the steps, if you know what I mean. It's not that I don't believe you, but that my brain has to understand EACH step before moving to the next step when it comes to UNDERSTANDING why this happens. (It's kind of like a series of math equations where I need to understand every step.) On the other hand, once there is a conclusion, I can EXPERIMENT easily, which is how a lot of cars get fixed (by throwing parts at the problems without understanding them). So I will change the caster. I have an alignment shop which runs a sale for $30 off to drop the $160 price to $130 who lessened my bimmer's rear camber from negative 2 degrees to almost 0 degrees - where if I go to him - I can ask for the least caster in the spec. Better yet, I need to buy the tools to do that caster change myself - but that's a topic for a different thread since I have to MEASURE it first. |
#13
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Clare - are smaller car tires easier to balance than SUV tires?
Arlen G. Holder wrote: "maybe I can consider lessening positive caster a teeny bit? "
At the risk of slightly less self-centering. If anything, I'd accept slightly more edge wear and max the caster, within specs. Most cars I've driven since 1980 have video-game steering feel - not safe in my estimation! |
#14
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Clare - are smaller car tires easier to balance than SUV tires?
On 17/6/19 6:41 am, Arlen G. Holder wrote:
> On Sun, 16 Jun 2019 20:40:34 +1000, Xeno wrote: > >> It does indeed! Same kind of feathering I'm getting on my Toyota's front >> tyres - a feathering that you can easily feel in the early stages as you >> run your hands for and aft along that section of tread area. > > Thanks for that information, where the one correction I need to make is > that you can only feel this feathering running your hand "*backward*" > (clockwise) over the outside quarter of the tread pattern. > <https://i.postimg.cc/zvvyL2tq/mount24.jpg> > > If you run your hand toward the front (counterclockwise), you can't feel > the feathering because each "lip" is downward. > <https://i.postimg.cc/vTZLmZrN/mount25.jpg> > > When you run your hand toward the rear of the vehicle, each lip is upward. > <https://i.postimg.cc/X7hcV3ps/mount26.jpg> > > That's the oddity. The feathering is only one way. > <https://i.postimg.cc/KYhPMN7L/mount27.jpg> Not really. You are only driving in the one direction so the feathering will only be in one direction. If it were in both directions, it wouldn't be feathering, it would be *scalloping*, a diagnosis for a different cause. > > It's reproducible for years - so it's always the same. > That one-way lip feathering should be diagnostic, should it not? > <https://i.postimg.cc/Wzyrb6bd/mount28.jpg> > >> BTW, positive caster will accentuate the camber scrub. Caster is >> generally not a tyre wearing angle. However, the more caster your >> steering has, the more camber *change* you will get when turning the >> steering. > > Hmmmmmmmm.... maybe I can consider lessening positive caster a teeny bit? You could experiment with it. After all, caster is not the only driver of steering returnability but you need to be very judicious in doing so noting that steering will be less precise, possibly more vague. Regardless, any caster reduction will be only part of the story since the primary cause, SAI, is well out of your control as it is a designed in feature. > >> Positive caster will give you a beneficial gain in terms of >> handling. You will get more camber gain (more +ve) on the inside wheel >> but the outside wheel will experience camber *loss* and become more >> vertical or even negative. Since the more vertical tyre is on the >> outside, the tread will get more grip with reduced slip angle aided by >> weight transfer. This is great for cornering at speed. However when >> travelling at slow speeds, weight transfer is not as significant and the >> camber angle on the inside wheel, the one at the tightest lock, heads >> towards positive extremes. > > This is very useful information, as all our lock-to-lock cornering is at > 30mph to 40mph ... never faster because I ran a test last week where > anything over 40mph is impossible to do even remotely safely, as all the > turns are blind turns and the file miles of 9% twisty road can't even be > twenty feet wide at the maximum (I should measure it but it's something > like that, as it's too narrow for the county to center stripe it legally). I was going to suggest you run wide arcs around the bends but it seems that's not even possible. > >> It is, in effect, riding heavily on the outer >> edge of the tread and this is where, and why, the damage is being done. > > I need to study more - where your conclusion is spot on perfect but where I > don't get the individual steps only because I think of alignment as being > 'static' so to speak. I know it changes - but my brain doesn't know 'how' > it changes under those slow speed lock-to-lock downhill (or uphill) turns. It's what the tread is doing at the contact patch that is the critical issue here and that is damn difficult to visualise. Start with the forces acting on a tyre contact patch and you will see what I mean; http://racingcardynamics.com/racing-...lateral-force/ In particular, this diagram; http://racingcardynamics.com/wp-cont...10/Figure3.jpg In your case, that contact patch centre of pressure will be off to the outside and not even. See the curved path? No small wonder that tread block deformation occurs. Once I started to understand the forces creating slip angles, then I began to get the bigger picture on tyre wear and, more importantly, handling. https://encrypted-tbn0.gstatic.com/i...7M7U1oj7bpK3ow Again note the curved path through the contact patch while cornering. That is the start point to understanding the concept as that is a result of the combination of forces acting at that pint.. > >> The tread blocks have only so much flex before they are forced to break >> contact with the road and slide. You've seen the evidence of what that does. > > Yes. That's for sure. The outer tread blocks "feather" such that you can > feel it, and barely see it, after about 1000 miles. The only thing I can > do, is change the alignment or rotate every 1000 miles (but even rotation > won't stop it - it just evens it out with the rears). I am presented with that same fait acompli with my car. My case is not as bad as yours as I get ~60,000 kilometres per set of tyres anyway, even with camber scrub issues. > >> Caster specifications are usually given as a range, say between 1 and 2 >> degrees with a side to side variation limit. All you can really do to >> mitigate the effect is to set your caster to the low end of the >> specified range. > > That's EXACTLY what I'll do! > I have to admit I need to read, and re-read and re-read again what you > wrote above, as my brain needs to work in step-by-step fashion. You aren't Robinson Crusoe in that respect. It has taken me years to come to the level of understanding of steering I now have. I have a greater understanding of steering and handling now than had when I was teaching the topic at a technical college. I have found traditional texts on the topic aren't sufficient to give one the depth of understanding required - seek instead engineering texts on the topic and those devoted to motor racing. Those who work at the extremes of handling seem to have a better idea of what's happening at that contact patch. > > You didn't skip a step but I don't quite "believe" in my brain all the I probably did. There is a whole section there on what actually happens when that tread passes through the contact patch. I cannot find a suitable diagram that provides, in and of itself, a decent explanation save for those on slip angle forces linked above because, as you have noted, it is difficult to picture in your mind what is going on. > steps, if you know what I mean. It's not that I don't believe you, but that > my brain has to understand EACH step before moving to the next step when it > comes to UNDERSTANDING why this happens. (It's kind of like a series of > math equations where I need to understand every step.) I know what you mean. It has taken me a long time and a lot of reading about steering geometry before I had enough of an understanding how it all works. > > On the other hand, once there is a conclusion, I can EXPERIMENT easily, > which is how a lot of cars get fixed (by throwing parts at the problems > without understanding them). So I will change the caster. A lot of cars have *money* wasted on them following that process. Far better to understand system operation and then experiment around the causes of the problem in order to effect a better *compromise* that mitigates your issue. As has been noted, you will not likely be able to cure your issue since it is a result of steering geometry compromise favouring highway operation but you now have a more precise start point. > > I have an alignment shop which runs a sale for $30 off to drop the $160 > price to $130 who lessened my bimmer's rear camber from negative 2 degrees > to almost 0 degrees - where if I go to him - I can ask for the least caster > in the spec. You might want to discuss with him your issue and why you want to change the spec. Get him onside. > > Better yet, I need to buy the tools to do that caster change myself - but > that's a topic for a different thread since I have to MEASURE it first. > -- Xeno Nothing astonishes Noddy so much as common sense and plain dealing. (with apologies to Ralph Waldo Emerson) |
#15
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Clare - are smaller car tires easier to balance than SUV tires?
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#16
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Clare - are smaller car tires easier to balance than SUV tires?
Xeno wrote:
" It really is a case of trying it and seeing. Most cars now have power steering and loss of feel is because of that in many cases. I have to say that road feel with power steering has been vastly improved over those vehicles from the 80s and before. The engineers have a much better understanding of the feedback mechanisms involved. I have come to accept more edge wear with my car. As I said in a previous post, I still get 60,000 kilometres out of a set of tyres with regular rotation. " Actually, two developments have led to a slight *decrease* in road feel and steering heft: (1) The adoption of EPS(electric power steering. During the first five years of the spread of EPS, engineers inadvertantly built so much boost into the new motors that some cars had trouble with returnability of the front wheels! I also made the mistake of buying the car that, at least in the U.S., had the WORST EPS experience ever - the 2011-14 Hyundai Sonata. It, and the 2011-14 Elantra, were Korea's first foray into electric steering, and it was, to put it best, a DISASTER. The cars would steer themselves! Some customers successfully had dealers plug into the cars' OBD, access the steering menus, and select a 'harder' steeering mode(less assist boost) which helped matter somewhat, unless, like my Sonata trim level, they had... (2)Wider, lower profile tires. This is the other primary reason for vague/light steering feel, even if more road feel. Wider tires in general react quicker to driver steering inputs, and sometimes also react to road imperfections(tugging, tramlining). Unfortunately, wide tires with 'wagon-wheel' rims are now the fad, so the only ways to overcome light steering are A, less steering assist, or B, higher directional angles (SAI and/or Caster). |
#17
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Clare - are smaller car tires easier to balance than SUV tires?
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#18
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Clare - are smaller car tires easier to balance than SUV tires?
Xeno wrote: "
Wider tyres actually have less road feel. That is because of the reduced effects of *pneumatic trail*. Wider tyres, while they have higher grip, reach their adhesion limits much more suddenly. Skinny tyres with a longitudinal contact patch are much more forgiving and give the alert driver plenty of advance warning of exceeding the limits of the slip angle. " So basically I knew it all along: Wide low profile tires serve no practical - read: consumer! - value off the race track. Except for aesthetics - show as much wheel/rim and as little sidewall as possible. <facepalm> As for your earlier Toyota experience with EPS, it echoes the same experience I had with my 2005 Chevy Malibu(Epsilon chassis). The steering in that thing had next to no weight off center, and no connection to the road. Even with relatively 'higher' profile 65-series 15 tires. Again, the common denominator behind vague/numb steering complaints with electric power steering is just too damn much BOOST. Pound per pound, given the same vehicle, an EPS motor will provide more steering assist than conventional (hydraulic) PS. My current car thankfully has three EPS modes, including sport, which turns down the amount of assist the motor gives the driver. Normal is, normal, and 'Comfort' makes it easy enough to blow on the steering wheel to turn it. |
#19
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Clare - are smaller car tires easier to balance than SUV tires?
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#20
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Clare - are smaller car tires easier to balance than SUV tires?
Xeno wrote: "
At a given tyre pressure and vehicle mass, the wide and narrow tyres will have the same contact patch area. So no gain there. What does happen is that the contact patch becomes wider. A wide contact patch provides adequate cornering force at smaller slip angles but they reduce self aligning torque because of the reduced pneumatic trail. Therefore natural stbility and steering feel is commensurately reduced. There is a practical width of tyre for every vehicle but *cosmetic appearance* trumps this so people want the look of wider low profile tyres. There is also a section profile that is optimum for both ride comfort and handling allowing the tyre to form part of the *springing*. IIRC, this was about 70. at a section of 65, the ride gets more harsh and continues to do so the lower the section profile gets. " Agreed, and makes perfect sense! I'd say the lowest profile I'd go for is 65-series. Even if the contact patch for, say, a 70 and a 40 series tire is approx. the same area, that patch should be parallel, not perpendicular, to direction of travel. That should be common sense, and flies in the face of all the aftermarket modders who want to throw 40-series bling 20" rims on a '85 Caprice or some such that was specifically engineered to work with 70-75- series tires and a specific rim diameter(typically 15 or 16") and width. By the way, the Sonata I drove that 'steered itself' exacerbated matters due to its 55-series tires on 17" rims - a Limited. Thing had a reasonable steering heft, yet somehow felt like driving on ICE, even on a dry highway on a sunny day. The steering wheel would slowly start tuggin to one side, and I'd start applying counter forcd, and then the steering wheel would snap quickly too far thr other way, causing the wander. It was a 'sticky' feeling, sort of. I wish I'd kept the car, and just had gone to the dealer to minus-size down to 60 or 65series tires on 16 or 15" rims. Combined with the reduced-assist Sport mode, the 'ghost' in the EPS would have been even less of an issue. |
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