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Tire Pressure


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I've been riding track for a year now, and still don't know quite how it works. I keep the pressure up, and deflate it to what the tech inspector recommends. In the heat (Arizona heat), when it's warmer, they tell me to run at 30/30, and in the winter they tell me to run 28-30/28. I thought it would be the other way around due to increased expansion in the heat, but that's what I consistently get.

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It really depneds on what tires you are running. I know that Hubbard has Pilot Powers on his bike so the tire pressures between that and the Dunlop Ntec slicks I am running are very different. Roger at Arroyo Seco raceway recommended 24 psi in the front for me and 26 in the rear when hot. I hear that if you use nitrogen to inflate your tires you get alot less heat expansion too.

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On our student bikes, Dunlop Qualifiers, we run 30 front, 28 rear. On the older style slicks (not the new low pressure N-tech), we run 29 rear, 31 front.

 

We commonly see tire presssures way off--either too high or too low (mostly too high). If it's too high, it will just reduce the contact patch. The tires on the sides of the tire are for 2 up, loaded to the max, hot day pressures.

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I've been riding track for a year now, and still don't know quite how it works. I keep the pressure up, and deflate it to what the tech inspector recommends. In the heat (Arizona heat), when it's warmer, they tell me to run at 30/30, and in the winter they tell me to run 28-30/28. I thought it would be the other way around due to increased expansion in the heat, but that's what I consistently get.

 

Howdy Hub (and Shannon),

 

Expansion is caused by heating due to the tire carcass flexing. The degree of expansion will be proportional to the difference between the ambient temperature ("cold" /pre-ride) and terminal tire temp ("hot" /post-ride). If you start with lower pressures, the tire flexes more. So, more flexion equals more heat equals more expansion and pressure.

 

Although having your tire pressure set too high can reduce the size of the contact patch, and pressure set too low can cause the front tire to squirm under braking, the most critical factor is the terminal temperature of the compound. On a hot day, you won't need to change the temperature of the tire as much as on a cold day to reach optimum tire temp. By the same token, you need more relative tire temp increase on a cold day. Hence, on a cold day, you need to set the pre-ride pressure a bit lower to create more flexion to get the tire compound up to optimum temp. And vice versa on a hot day.

 

Back in the "old days" of bias ply race rubber, the rule of thumb was to look for about a 15% rise in pressure. And 10% was still a fair ballpark figure for the "new" radial race rubber, but, not really accurate. If memory serves, the rule of thumb is/was about 180 F. (Hence why non-adjustable tire warmers were pre-set for that range.) Your tire rep should be able to provide you with the optimum temperature (and pressures) for your brand and model for best traction and wear. A tire temp probe should be available from a race supply store on-line or at the track.

 

Cheers,

racer

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