On 13/07/2017 1:03 PM, micky wrote:
In rec.autos.tech, on Sun, 9 Jul 2017 13:03:26 +0000 (UTC), Chaya Eve
wrote:

On Sun, 9 Jul 2017 19:56:29 +1000, Xeno wrote:

The car steering geometry specs will have been designed to make your car
safe to drive in all circumstances.

The specs are almost always a *range* so there's room to be at one end or
the other, isn't there?

I think the answer is no, not always.

If caster has a range of 2 to 4 and camber 5 to 7, that doesn't mean
that you can put caster at 2 and camber at 7 and everything is even
moderately acceptable, aiui.

Camber is a tyre wearing angle and at 7 degrees would definitely lead to
tyres wearing on one side. For road going cars, 1 degree
positive/negative would normally be the maximum you would aim for.
Competition cars can go to 2 or even 3 degrees.

Caster, on the other hand, has a good deal more leeway since it isn't a
tyre wearing angle. It does create self aligning torque so a decrease
will affect the car's ability to straighten up out of a turn. An
increase, on the other hand, will make the steering heavier.

Toe in changes can affect the understeering/oversteering characteristics
of the car and, therefore, affect the handling.

You need to have an understanding of suspension kinematics before you
arbitrarily play around with manufacturers specifications. Simply
lowering a car will upset the kinematics and, as a consequence, the
handling.

The range refers to one value at a time, that if the caster has a range
of 2 to 4 and you have it set at 1.9, that's bad in itself, regardless
of what the other values are. (Maybe in some special situations a
skilled aligner can safely go outside the range, I don't know one way or
the other, but my point is made in the paragraph above, which I think
you think would be acceptable.)

Alignment is a compromise and any particular setting of camber, for
example, may well limit the acceptable setting of caster to a smaller
range than the range given in the spec. Competent pros know this and
allow for it.

Corrections?

I hope you don't intend to tell the shop what settings you want, and
will instead tell them your problem** and what you think woudl help,
without ordering them to do that.

**I'm not sure why some feathering is worth all this trouble in the
first place. Will your tires wear out sooner? How much sooner? How
much is that in dollars?

It generally isn't. It is an effect of the design compromises in
steering kinematics versus vehicle usage. The type of usage described by
the OP is the perfect example of an extreme that is outside the
manufacturers design expectations. The penalty, if it can be called
that, is increased tyre wear. It's no different to expecting increased
tyre wear if every launch is like that of a jackrabbit.

--

Xeno