Re: Tyres again - obvious really!

A couple interesting sites to check: http://n4dl.thecomputernews.com/setup/tires.shtml http://www.howstuffworks.com/nascar5.htm 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. "Richard S Beckett" <[email hidden]> wrote in message news:agf17p$340$[email hidden]... > 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. > >

they should use helium, might lighten those fat barges up a bit....<G> Chris H. <[email hidden]> wrote in message news:AEFW8.643160$%[email hidden]... > A couple interesting sites to check: > http://n4dl.thecomputernews.com/setup/tires.shtml > http://www.howstuffworks.com/nascar5.htm > > 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 poundsper > square inch to 80. Safety regulations also require that they run two > tires - outer and inner - on each rim. 8-) > -- > Chris H. > > "Richard S Beckett" <[email hidden]> wrotein

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> message news:agf17p$340$[email hidden]... > > 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

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> > 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, thetyre

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> > 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. > > > > > >

"Richard S Beckett" <[email hidden]> wrote in message news:agf17p$340$[email hidden]... > 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 theface > by an obvious thought... > > Some people say that if you want a tyre to run hotter increase thepressure, > and if you want it to run cooler, reduce it. 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.

Well, that's true to a point. Increasing the pressure will increase the temperature at the -center- of a tire. Overall, it decreases the temperature, because of lower rolling resistance. 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 hard on 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 "Richard S Beckett" <[email hidden]> wrote in message news:agf17p$340$[email hidden]... > 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 theface > by an obvious thought... > > Some people say that if you want a tyre to run hotter increase thepressure, > 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 pressurewill

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> 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 pointof

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> 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. > >

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>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 ;-) >

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 rubber 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 rubber itself (nothing to do with the road/tire contact here, strictly rubber/rubber friction in the tire rubber 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 rubber through the patch there's some friction caused by the rubber elements flexing/rubbing against each other. This friction *is* the heat that "goes into" the tire. The more flexing, the more rubber/rubber 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 rubber friction. And of course, more friction translates into more heat. Tire heating.... Interesting thing... Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com My little car sim screenshots: http://performancesimulations.com/scnshot4.htm