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)