Re: boge & tire pressures, alignment

[phile@swamp.stpaul.gov (Philip J Ethier)]

Eliot Lim writes > 
> On Fri, 16 Dec 1994, Philip J Ethier wrote > >
> > 
> > Toe in or out?  I assume out, as the front is driven, and the theory is
> > the the suspension is "pulled" towards zero toe at speed.  0.7 degrees
> > overall?  On a 23" tire (guessing on tire size) that is 0.281".  A full
> > quarter-inch toe out?!  No wonder you have tire wear on the inside!  This
> > number must be wrong.
> 
> please accept my apologies.. it is 7 minutes of toe in.. not 0.7
> degrees.  i did say i was tired, didn't i.. :)

OK.  On a 23" tire that's 3/64" toe in.  Certainly not a radical setting. 
I would shoot for zero or slightly out on a powered axle, due to the
tendency of the wheels to pull towards toe-in under power, but 3/64" toe
in is pretty close.

> so, should i reduce the front tire pressures then?  

I would.  I don't see the advantage you are gaining.  You are not
apparently rolling under the tire in cornering.  You are reducing your
traction, because the the higher pressure gives you a smaller contact patch.
This is why some VW GTi autocrossers put ultra-high pressures in the rear
tires to induce oversteer by making the contact patch smaller.

> if the inside wear
> is not caused by excessive toe, is it the pressures?  note that these
> new settings were set just a couple of days ago.. before that the car
> was running strictly stock.. (except for the tire pressures)
> so whatever it was that was causing the inside wear it was not the
> current settings.

Perhaps the alignment was incorrectly toed out too much before the
alignment.  Did you get a pre-adjustment printout of the settings when you
first brought the car into the shop?

In an ideal world, with a wheel running exactly true with dynamic toe and
camber at zero, and too much tire pressure, the tire will wear in a stripe
right on the tire centerline.  Something caused that stripe to move to the
inside on your tires.  

Was there any "feathering"?  Inspect the tread closely, looking at the
edges of the tread blocks in the worn area.  If the inside edge of each
tread block is rounded, and the outside of each tread block is sharp, or
has a little "tail" hanging from it, then you can conclude that the
wearing action on that stripe proceeded from the inside of the tire to the
outside.  This implies that the tire was run with too much pressure
(stripe of wear) and too much toe-out (stripe on inside, and with feathering)

If there is no evidence of feathering, it may be due to excessive negative
camber.  This seems less likely, as I assume you asked for more negative
camber when you brought the car in, and I'd expect more than one degree
would have been noticeable before.

In any case, "that was then, this is now".  Were I you, I would run about
35 psi in the front and drive in my usual style, keeping an eye on the
tread wear.  Feathering will show up rather quickly, if it is going to.  I
expect your present settings should work out just fine.

Of course, there are two kinds of wear, straight line and cornering.  On
my Lotus, the cornering wear is dominant, as I autocross it a lot, and
drive few miles overall.  My father's Caddy is never cornered hard and
sees a lot of long trips, so the straight-line wear-pattern rules apply.
The Caddy should therefore have essentially zero static camber to maximize
tire life.

You have the complication of both wear types, and have to balance the wear
from both causes.  One degree negative sounds reasonable.

phil.ethier@stpaul.gov