99 A6Q 2.8. My front tires are showing excessive wear on the outside
edges of the tread. In fact, I feel the need to replace them, even
though the center of the tire still has about one-third of the useable
tread left. Air pressure, cold, is ~32 psi. The dealer says this wear
is "normal". I can't help but feel that something is wrong in the front
Any thoughts or comments?
Its the byproduct of being able to turn your wheel while stopped, try and
get into the habit of pulling away or at least start moving before you turn
the wheel, this will elliminate most of the tyre wear
Absolutely. The OP's pressures of 32 psi (approx 2,3 kPa) are similar to what
used earlier on, and I got same problem. Handling also had that distinctly
squishy North American feel, too.
On 205/55-16 tyres (for an A4 weighing about 1500 kg), I've used 2,9 kPa
(just over 40 psi) and found it too harsh, but tread wear was nice and even.
On this car I now have the tyres to 2,7 kPa (38 psi) and it's a good
Make the appropriate translation for your tyre dimensions, car weight, and
and you'll probably still find that you need to increase your air pressure by
20% compared with what you have today, to get even tyre wear (and good
Your statement is complete rubbish. That's not the way radial tyres work.
If what you said were true then one would expect much more wear in the
middle of the tread, compared with the edges, once values over ca.
2,5 kPa are used. You have to go *way* over that to get any symptoms
like the ones you describe.
Very doubtful you've ever tested your theory.
... but subsequently corrected that to 16.66667% less rubber on the
I didn't know that I'd described any symptoms. I merely pointed out
that increasing pressure reduces contact area
My theory? No, I've never set up any experiments to test it. It's
based on simple physics/mechanics. If you take the weight carried by a
single wheel (in pounds), and divide the weight by the tyre's inflation
pressure (in pounds per sq. inch), this should give you the area of
rubber in contact with the ground (in sq. inches). As inflation
pressure increases, I would expect the contact patch to become shorter,
but not to vary significantly in width (within the range of pressures
which would be sensible).
Peter Bell - firstname.lastname@example.org
Beginning to see what you're driving at - you are right, that a tyre will
its contact patch area reduced if more air is added to it (you are of
course indicating the contact patch measured longitudinally - I thought
you meant the contact patch measured laterally)
But I doubt that the relationship between contact patch area
and pressure is linear.
A tyre being loaded by side forces (turning) will deform to a greater or
degree, depending on the tyre pressure. At lower pressures, the deformation
will cause the outer tyre's sidewall to contact the pavement more, causing
in that area, which is what the OP had commented.
At higher pressures, this problem is reduced, though at a small price to
straight-line stopping characteristics. But with today's much wider tyres,
this is usually not a problem.
A car moving in a straight line will always deform the tyre ever so
to the toe-in employed to improve handling characteristics. A higher tyre
pressure should reduce the amount of deformation and thus the amount of
wear on the outside of the tread.
----8<----- cutting ----
Quite right. The width of the contact patch will not decrease
to the cords running along the circumference of the tyre.
Would you agree that higher pressures might cure the OP's problem? (once
he gets new tyres, that is ...)
You are quite right of course. Elegantly put. The dimensions of pressure
are (in SI terms) kg/m2. Thanks for the insight, I now understand what my
6th form physics teacher was trying to tell me - 27 years later ;-)
Don't agree there, regardless of what theory may tell us. My empiria showed
me that a higher tyre pressure fixes the problem of outer edge tread wear,
without sacrificing braking effectiveness, and gave me better cornering
as a bonus.
Anybody else reading this try more air to fix uneven tread wear? What
results did you get?
You must be younger than I am, then - I left 6th form in 1972. I must
say that the relationship between inflation pressure and contact area
only struck me in recent years. This then put paid to the idea of
'getting more rubber on the road' by fitting wider wheels/tyres -
unless the tyre pressure is reduced with the wider tyre, all that
happens is that the contact patch changes shape.
Yes, the essential cause of outside edge wear is the tyre squirming
sideways during hard cornering such that the shoulder of the tyre rolls
under and makes heavy contact with the road. Of course, it is the
outside edge of the more heavily-loaded wheel which bears the brunt.
Increasing pressure stiffens the sidewall, reducing the squirm.
Whether this can entirely eliminate uneven wear across the tyre will
depend on other factors such as suspension geometry and roll-stiffness.
Note the 'won't necessarily' in my previous post!
Yes, I have, and there's no doubt that it will improve matters. Don't
forget that increasing pressure reduces rolling resistance and, within
reasonable limits, reduces tyre wear anyway.
Peter Bell - email@example.com
So what -- sideforce is not just a matter of contact patch. As you
incresase the pressure (within reason) the extra stiffness in the side wall
increases cornering force, despite the (slightly) smaller contact patch.
Ask anybody who races on street tires.
You can carry this too far, however. Above about 45 lb (most street tires)
cornering force will start to drop off, as the sidewalls are now too stiff
to allow the tread to stay flat on the road.
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