NASCAR doesn't use air in the tires, it is Nitrogen, and the pressure
depends on the track and grip they seek, ranging anywhere from 8 pounds per
square inch to 80. Safety regulations also require that they run two
tires - outer and inner - on each rim. 8-)
--
Chris H.
> NASCAR doesn't use air in the tires, it is Nitrogen, and the pressure
> depends on the track and grip they seek, ranging anywhere from 8 pounds
per
> square inch to 80. Safety regulations also require that they run two
> tires - outer and inner - on each rim. 8-)
> --
> Chris H.
in
> > x-no-archive: yes
> > I've been trying to get my head round the conflicting advice that people
> > give about racing tyres and their pressures, when I was slapped in the
> face
> > by an obvious thought...
> > Some people say that if you want a tyre to run hotter increase the
> pressure,
> > and if you want it to run cooler, reduce it.
> > This _must_ be bolox, because:
> > When a tyre heats up, it's pressure increases.
> > If this increase in pressure causes it to run hotter, then the pressure
> will
> > increase more.
> > The tyre will soon explode.
> > I have come to the conclusion that _lowering_ the pressure will cause
the
> > tyre to run hotter, because:
> > Low pressure makes the tyre heat up. As it heats up, the pressure
> increases,
> > so the rate at which it heats up will decrease, until it reaches a point
> of
> > equilibrium.
> > I.e. the "lowness" of the pressure generates heat, the tyre pressure
goes
> > up, the higher pressure makes the tyre run cooler, the tyre cools, the
> > pressure drops, the "lowness" of the pressure generates heat, the tyre
> > pressure goes up, the higher pressure makes the tyre run cooler, the
tyre
> > cools, the pressure drops...
> > Based on the simpler physical laws, the latter scenario must be right,
> > otherwise tyres would be an open loop system.
> > Feel free to pick out holes ;-)
> > R.
Yes, this is wrong, as you assumed.
Low pressure means the tyre moves about more on the rim (creating heat)
because it's less rigid than with a higher pressure.
Another factor is that the higher the pressure, the more weight is put on
the centre of the contact patch, and with a low pressure, more weight goes
to the inside & outside of the contact patch. It's easier to picture if you
imagine a wide tyre.
The ideal pressure, to put it simply, will balance the temps at all three
points.
This is why your car handbook recommends increasing the pressure in your
rear tyres if you are going to carry a heavy load.
Also, lower pressure gives more grip, because the contact patch 'moulds'
itself around uneven ground (eg 4x4 off-roading) and higher pressure gives
less rolling resistance (increasing your mpg on motorways etc). Just don't
go to extremes either way ;-)
hth, Malc.
Extremes either way, of course, cause problems. Extremely low tires will
eventually heat up to the point of explosion. Extremely high tires will
wear down the center, and I guess could be somewhat at risk to explosion if
one was driving *** them on a very hot day (such as right now where I
live...of course my A/C is broken in my truck...blah)
Milhouse
AFAIK, Bingo :-D
Just to add a bit to this in case everyone doesn't know already, the primary
reason that a low pressure tire heats up more quickly is because it's deforming
more. The contact patch is longer and the center of the wheel is lower at the
same time, causing all that *** to flex out of shape on the way through the
patch more on a low pressure tire than a high pressure one. The tread shears
one way on the way into the patch up to about the middle point, then shears the
other way on the way out.
This actually causes a forward force on the wheel in the front half of the
contact patch, from my understanding, and a rearward force in the last half of
the patch. If there was no friction in the *** itself (nothing to do with
the road/tire contact here, strictly ***/*** friction in the tire ***
itself), these forces would exactly cancel each other out and there'd be no
such thing as rolling resistance in an automobile tire.
Of course, this doesn't really happen as we all know! During the
compression/expansion of the *** through the patch there's some friction
caused by the *** elements flexing/rubbing against each other.
This friction *is* the heat that "goes into" the tire. The more flexing, the
more ***/*** friction, so the more heat, which causes the pressure to
rise, which reduces flexing and therefore heating rate. At some point
(hopefully), things balance out much like Richard Beckett imagined. This
phenomenon is called hysteresis, if I'm not mistaken and am having a good
spelling day :-P
This hysteresis is also is in effect when a tire generates slip angles,
probably much more so than just the pure rolling deformation just mentioned.
When a tire's pressure is low, the tread/sidewalls will generally flex more to
produce any given force (including the peak force you're using hopefully all
the time while racing). More flexing = more internal *** friction. And of
course, more friction translates into more heat.
Tire heating.... Interesting thing...
Todd Wasson
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