stephens@no-fixed-abode.com

On Tue, 05 Aug 2003 11:01:45 +0100, Peter Bell <peter@bellfamily.org.uk>
wrote:

> In message <3F2F668D.67A3D83E@tripxxxnet.se>
> Robert Brown <robxxxert.broxxxwn@tripxxxnet.se> wrote:
> > Make the appropriate translation for your tyre dimensions, car weight,
> > and load, and you'll probably still find that you need to increase your
> > air pressure by about 20% compared with what you have today, to get even
> > tyre wear (and good handling).
>
> Be aware that increasing pressures by 20% will reduce the amount of
> rubber on the road by 20%!

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.

Peter Bell

In message <3F3002E5.A2ED378B@tripxxxnet.se>
Robert Brown <robxxxert.broxxxwn@tripxxxnet.se> wrote:

> Peter Bell wrote:
> > Be aware that increasing pressures by 20% will reduce the amount of
> > rubber on the road by 20%!

.... but subsequently corrected that to 16.66667% less rubber on the
road.

>
> 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.

Why?

> You have to go *way* over that to get any symptoms like the ones you
> describe.

I didn't know that I'd described any symptoms. I merely pointed out
that increasing pressure reduces contact area

> Very doubtful you've ever tested your theory.

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 - peter@bellfamily.org.uk

Doug Ramage

"Howard Delman" <delman@blueneptune.com> wrote in message
news:vivtk1ggi0b16b@corp.supernews.com...
> Doug Ramage wrote:
> > "Howard Delman" <delman@blueneptune.com> wrote in message
> > news:vit89lln5g83e6@corp.supernews.com...
> >
> >>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
> >>end.
> >>
> >>Any thoughts or comments?
> >>
> >>Howard Delman
> >>
> >
> >
> > How many miles have they done?
> > --
> > Doug Ramage
> >
> >
>
> I'm not sure, because I bought the car used last year. The car had 30K
> miles when I bought it, and I don't think the tires are original. They
> had significant tread at that point. In fact, the center of the tread
> still has 6/32" left, 15K miles later. (The car now has 45K miles on
> it.) The outer inch is completely bald. My concern is that this wear
> pattern seems new. I don't recall noticing any abnormal wear during my
> pre-purchase inspection. I am going to replace the tires, and I don't
> want the new ones to wear out prematurely.
>
> Howard
>

Have you had the alignment/geometry checked recently - using proper laser
equipment? I used to have mine done about every 12-18 months (15,000 -20,000
miles).
--
Doug Ramage

Robert Brown

Peter Bell wrote:

> In message <3F3002E5.A2ED378B@tripxxxnet.se>
> Robert Brown <robxxxert.broxxxwn@tripxxxnet.se> wrote:
>
> > Peter Bell wrote:
> > > Be aware that increasing pressures by 20% will reduce the amount of
> > > rubber on the road by 20%!
>
> ... but subsequently corrected that to 16.66667% less rubber on the
> road.
>
> >
> > 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.
>
> Why?

Beginning to see what you're driving at - you are right, that a tyre will
have
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
lesser
degree, depending on the tyre pressure. At lower pressures, the deformation

will cause the outer tyre's sidewall to contact the pavement more, causing
wear
in that area, which is what the OP had commented.

At higher pressures, this problem is reduced, though at a small price to
the
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
slightly due
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 ----

> 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).

Quite right. The width of the contact patch will not decrease
significantly, due
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 ...)

/Robert

Robert Brown

Peter Bell wrote:

---8<---cut

> > But I doubt that the relationship between contact patch area
> > and pressure is linear.
>
> No, it's an inverse relationship. The point I am making is that it is
> the air which holds the car up (the tyre merely constrains the air,
> and has little capability to support the car in its own right - look
> what happens when you let the air out, the tyre can barely support
> the weight of the wheel!). The reaction of the air pressure against the
> ground must be equal to the weight which is being supported through the
> wheel. Hence:-
> contact area = weight supported / inflation pressure

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

---8<----cut

> > Would you agree that higher pressures might cure the OP's problem?
> > (once he gets new tyres, that is ...)
>
> Well, it may move the high wear area away from the very edge of the
> tread, but it won't necessarily cure the basic tendency to wear the
> outside more than the inside.

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?

/Robert

Peter Bell

In message <3F32627F.6D81D8FC@tripxxxnet.se>
Robert Brown <robxxxert.broxxxwn@tripxxxnet.se> wrote:

> Peter Bell wrote:

[Snip]

> The reaction of the air pressure against the
> > ground must be equal to the weight which is being supported through the
> > wheel. Hence:-
> > contact area = weight supported / inflation pressure
>
> 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

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.

>
> ---8<----cut
>
> > > Would you agree that higher pressures might cure the OP's problem?
> > > (once he gets new tyres, that is ...)
> >
> > Well, it may move the high wear area away from the very edge of the
> > tread, but it won't necessarily cure the basic tendency to wear the
> > outside more than the inside.
>
> 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.

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!

> Anybody else reading this try more air to fix uneven tread wear? What
> results did you get?

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 - peter@bellfamily.org.uk