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Tire pressures and "Pressure Rise"


Let's go back to the "old days" when we didn't have a way to measure tire temperatures (short of seeing if wax would melt on the tire surface).

Under hard use track use we used:

5-6 psi rise from cooled in shade after practice to right after pulling off the track after practice.
example:  cold was 30 psi before practice and was 35 psi when you pulled off the track. Good.

3-4 psi for the front
example:  cold was 30 psi before practice and was 32 psi when you pulled off the track. Bad.

Too much/ high of a rise = too low tire initial pressure = tire gets too hot during the race">


logo2.gif (417 bytes)

 

Tire pressures and "Pressure Rise"


Let's go back to the "old days" when we didn't have a way to measure tire temperatures (short of seeing if wax would melt on the tire surface).

Under hard use track use we used:

5-6 psi rise from cooled in shade after practice to right after pulling off the track after practice.
example:  cold was 30 psi before practice and was 35 psi when you pulled off the track. Good.

3-4 psi for the front
example:  cold was 30 psi before practice and was 32 psi when you pulled off the track. Bad.

Too much/ high of a rise = too low tire initial pressure = tire gets too hot during the race, overheats and becomes "greasy".
Too little / low of a rise = too high initial tire pressure = tire doesn't get hot enough to reach tire manufacturer's desired maximum adhesiveness.


   Tire flex creates heat  

More flex / with lower pressure = more heat developed.

Too much pressure = less heat developed. (yes, if you spin the tire more, you will create some heat, too)

Tires are manufactured with a rubber compound that delivers maximum traction at a small temperature band. Some rubber compounds have wider ranges than others. We need to be there, in that temp. range during the race.

The starting temperature is measured after the bike has been used and the tire has been sitting in the shade for 15 to 20 minutes. Hot temperature is as the bike was exiting the track.

Different tracks required different starting temps. A track with a lot of tire "spinning" and flex requires a higher cold temp and a track with good, constant traction allows the use of lower pressures, which let the tire reach the desired operating temp during the race. Lower tire pressures also allow the tire to flex and conform to track surface irregularities for better traction.

If cold rear tire pressure is too high and the tire doesn't reach best temp during your race or perhaps even never during your race - you lost the advantage of traction. If you run too low a cold pressure, the tire will flex too much and reach too high of a temperature during the race and get "greasy" - requiring that you back off for a lap to let the tire cool down a bit.

At Sears Point Raceway, on a rear tire, with 8 lap races, if you were too low of a pressure by 1 psi, you would get a +7 psi rise and the tire would get greasy" on the 6th lap. If your tire pressure was too high by 1 psi, you'd get a +4 psi rise and the tire wouldn't "come in" to best traction for 2 laps (instead of 1.3 laps) and never reach best traction during the race unless you "spun" the heck out of the rear tire when exiting corners and still the traction would have been better with lower pressure.

Different riders with different riding styles used different cold tire temps, but we all settled on the same pressure rise as optimum - even on different tires. So, pressure rise from cold to hot was a good constant setup concept. Example: Gennady Liubimsky, on his RD350 with Dunlop KR91 rear tire started with a bit more pressure than I did (using Goodyear slicks), but we both ended up deriving the same cold vs. hot pressure rise amount.

It still works. There is NO blanket, perfect cold tire pressure.

Tire guys, by giving you one, are trying to make it easy for non technical people to deal with tire pressure.

There IS a best temperature. There is no doubt.

You still need to, for best traction, need to practice "Pressure Rise" to get to best tire temperature during a race weekend.

The lower limit, for us, is when there was not enough rigidity in the tire carcass to allow stability.
We ran (and I don't suggest anybody try these low pressures!!!!) from 25psi on some extremely short, 1/16th mile roadrace tracks (Bakersfield) to 32 psi (high speed tracks) on a front tire and 26psi to 33psi on the rear tire.

If we had to lower the pressure to the extremes, we tried  to get a softer tire - but, we ran what we had available and did the best we could at that time.
Privateers (aka: test dummies) used to be able to get 2-3 different compounds and several different tire cross sectional profiles - but there wasn't any trackside support during the Ice Age.

Disclaimer.....
This is all anecdotal information. It's just here to make you think. Don't do what's written here. This is a fiction. We made it up. Don't call us up and say that "I crashed and it's your fault". We told you not to do it! Do only what your OEM bike manufacturer and tire manufacturer said to do. What the heck do I know? Anybody can write anything on the Internet. 

We seemed to have lost the Art of Tuning and Testing over the last 20 years.........

I guess I thought that I should write it down before all the old guys completely forget!!

Marc

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