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.