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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, 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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