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)