On Wed, 19 Jun 2019 00:17:29 +1000, Xeno
wrote:

On 18/6/19 12:58 am, Clare Snyder wrote:
On Mon, 17 Jun 2019 15:01:47 +1000, Xeno
wrote:

On 17/6/19 8:48 am, Arlen G. Holder wrote:
On Sun, 16 Jun 2019 17:39:04 -0400, Clare Snyder wrote:

Just applying negative camber MAY be simpler and will accomplish the
same thing, without affecting lateral stability..

However, with the shoulder wear on BOTH edges, the most effective
change you can make is to "air up" the tires. Keep the carcass of the
tire rigid - prevent the tire from "rolling" out on the outer corner
and "rolling in" on the inner corner will cause the tread to wear more
evenly across the face of the tire - and tereby REDUCE the total wear.
I'd bet about 3/4 of the wear is caused on the downhill.

Hi Clare,
I appreciate your advice as you and Xeno know this stuff whereas the rest
just make it all up it seems.

If adding more negative camber is even better than reducing the positive
caster, then that's easier, as you noted, and likely better on high speed
straight-line driving (we don't do high speed cornering ever).

Adding more negative camber also increases SAI, one follows the other
and SAI is not separately adjustable since it is designed in. That means
your negative camber increase will increase SAI. As I have previously
stated, steering and suspension geometry is one huge compromise so you
need to think carefully about the unintended consequences of *any* non
standard setting you use. That's why I said to ensure your caster spec
was on the *low side* of the acceptable range. That will ensure the
least amount of unintended consequences as it will still be within
factory spec.


Adding negative camber will countderact the effects of SAI. Increased
SAI (or "included angle" in the spindle) causes more change in camber
as the wheel leaves the center position.. Changing camber DOES NOT
AFFECT SAI.

Are you sure about that? SAI is, after all, Steering Axis Inclination.


100%
KPI or SAI is also known as the "included angle" and is the angle
between the centerline of the steering axis and the centerline of the
wheel axis (or spindle) Without bending the pdindle assembly it
cannot be changed on modern vehicles. On older AMC and a few others
with bolt-on spindles (including SOME current FWD vehicles you CAN
change the SAI somewhat by inserting tapered shims between the knuckle
and the spindle. MOST RWD cars and a large number of FWD today do NOT
use a bolt-on spindle or a bolt-on bearing housing, making it
impossible. On the 2 part knuckles (separate knuckle and spindle) we
used to occaisionally shim them for" competition"

If we are talking about a *King Pin* in a beam axle arrangement, the KPI
angle is absolutely fixed and cannot be altered without recourse to
*bending* something.

What are you thinking of bending?

In terms of modern cars, however, king pins are obsolete. The last
vehicle I can recall working on that had king pins was mid 60s.

What we use now are, as you know, ball joints to anchor the steering
knuckle yet allow multi-degree articulation. Makes the kinematics a damn
sight easier to work out from the engineer's design viewpoint.

https://www.freeasestudyguides.com/g...clination..png

In the diagram above, there is a line referencing *true vertical*,
another referencing the SAI *angle*, and another referencing the tyre
camber angle. The *included angle* cannot be altered. It is designed in
and the only way to change that would be to *bend something*. In this
case, that something would be the steering knuckle. Definitely not
recommended but something that used to be done in vehicles of the 50s
and 60s when you ran out of adjustment. I was fortunate I never needed
to do that.


I never bent a spindle, but the flamboprough hobby stock car guys did
sometimes. It was easier to use an AMC knuckle and just shim it - no
worrying about if you got the heat treat right -- - -
Looking at the linked diagram, if I feel the camber angle is too great,
what options do I have. I have to move one of the ball joints in
relation to the other. Assume the adjustment for camber is shims at the
inner end of the top control arm. So I set to and remove sufficient
shims to move upper control arm, including the upper ball joint, inwards
and change the *camber angle*, say, 1 degree to the negative. That means
I have now *reduced* by 1 degree the angle between the true vertical and
centreline of the tyre. The tyre is now more vertical than it was.
But, if I cannot change the *included angle*. I must also have also
*increased* the angle between the true vertical and the *steering axis*
through the ball joints. That 1 degree of angle didn't just disappear,
it simply moved to the other side of that line that represents true
vertical. That means I have *increased* the steering axis *inclination*
from the true vertical by 1 degree. If SAI was, say, 9 degrees before,
it will now be 10 degrees.

No - SAI IS the included angle.
Camber IS the deviation from true vertical
SAI is measured from the spindle centerline to the steering axis and
does NOT change, even if the car is laying on it's side. You get
CAMBER CHANGE with roll and squat - but you do NOT get SAI change.

What can we see from this? The wheel will be more vertical, no doubt,
since we have reduced camber. But, because the steering axis inclination
has increased, we will likely experience heavier steering. That may or
may not be noticeable. The vehicle will have more steering self
centering.

How?

Take a good look at the Turnolgy site for some good explanations.

https://www.turnology.com/tech-stori...ke-you-faster/

Also, we are likely to get more of a camber change on the
inner wheel when the steering turns to either lock. Remember, positive
caster is playing a role in this camber change too. Will it matter? I
can't say because I have always worked within manufacturers
specifications and I can't readily visualise the kinematics of it.

The one thing you will almost certainly have done is changed the slip
angles at the front wheels because that is what camber changes to the
negative at the front wheels do. That will have, in turn, altered the
oversteer/understeer balance between the front and rear of the vehicle.
Vehicles are given positive camber at the front wheels and zero or
negative camber at the rear. That promotes understeer, a safer situation
for the average driver. By reducing caster to the minimum spec, you will
achieve a similar outcome with regard to tyre wear but will not be
dicking with the handling balance.

When turning in a downhill thrusting turn the outer tire
will have a negative shift in camber while the inner wheel will have a
positive shift. The more positive caster, the more pronounced this
"tilt" or "corner carving" tendancy. If the tires are not stiff
enough or are underinflated, the tires will wear excessively on the
side facing the outside of the turn. If the tires ARE stiff enough or
inflated hard, the wear will tend to move towards the side of the tire
on the INSIDE of the turn - tending to even the wear across the face
of the tire.

That's why my FIRST recommendation is to air up the tires, and to
reduce the wear on the outside of the turn, possibly increase the
negative camber. Setting the caster to the high side of the positive
spec will give you extra negative camber on turns without any possible
negative effects of negative camber on the straight and level. It's
all a compromize. Dive and roll come into play along with road
banking.

a good article on camber change is he
https://www.hotrod.com/articles/camb...tire-traction/.
It is slanted towards circle track racing, but covers all the basics.

Yes, but it covers all the basics with a distinct *bias*. For someone
who wants to learn about the basics, it is best they look at what is
done for *road going cars* first. It is a different world with regard to
handling. Sure, progress to racing cars but do get the basics down pat
first.

This IS a "handling" issue To be wearing the tires the way they are
being worn the vehicle is operating at the ragged edge of it's
handling capability on those carving downhills.

For a person who is an engineer, this book is a better start point.
https://www.springer.com/gp/book/9783319054483

As a followup, this one;
https://www.springer.com/gp/book/9783834809940

I have both the above and they make for very interesting reading.

For several years my road-going car was a 1972 Vauxhall Firenza. It
had several degrees of negative caster on an unequal length control
arm suspension, and it handled like it was on rails . With the wide
Radial TA tires I had on it there was virtually NO tire wear over
almost 50,000 miles (between my driving and the friend I sold it to)

Vehicles with strut suspension require MORE negative static camber to
compensate effectively for cornering camber change




I will take your advice on the multi-side shoulder wear, by bringing the
front tires of this RWD vehicle to 40psi or so, which I understand and
where I appreciate that advice.

Since these tires were religiously rotated every 5K miles using a pattern I
devised myself of H-X-H-X-H-X, etc., the tires ended up wearing
"relatively" evenly overall, which is shown in this shot of the rears:
https://i.postimg.cc/63Kc80x9/mount29.jpg

I was just about to mount & balance these rear tires as we type where you
can see they're worn sort of kind of evenly except in the inside edge,
where these were mostly worn when on the front axle.

Just refresh my memory here, was it the *inside* or the *outside* of
your tread that was wearing more and with the longitudinal feathering?
Camber scrub tends to affect the *outside* edge of the tyre. For it to
affect the *inside edge*, the camber on the tyre would need to be going
well into the negative. It really can't be doing that. As I have stated
previously, the camber of the tyre on the outside of the turn loses
camber and goes more vertical whilst the tyre on the inside of the turn
goes from slightly positive to heavily positive. The tyre on the outside
of the turn could be heading slightly into negative territory but that
would depend on static camber settings and would be minimal. Certainly
the tyre at the high positive camber will be doing most of the shoulder
wearing in those sharp slow speed turns.

I likely should flip them on the wheel at the 10,000 mile mark after the
first two X-H rotations have been done, which will move the inside edge to
the outside edge. These darn tires have a whitewall stripe, which I hate,
so that's why I didn't flip them on the rims prior.