In our rally car we check the pressure constantly. We start out each morning
at 30 lbs. (Michelin X radials). Tire warm up is part of the morning run,
then the speedo calibration follows, usually 15-30 miles, the longer the
better. The pressure rises to 33 lbs. and stays there (as long as the tire is
warm).
In Great Race, the Rallymaster establishes the ideal time based on how he
runs the course. The racers are all comparing their times to that 'ideal'
time. We see speedo calibrations change unexplainably from day to day, and
some cars appear more affected by it than others: indeed, while some are
adding seconds, other are taking them off! Since most of us are using the
same electronic speedo, the variable has to be the tires.
The time of day or weather may make a change all the cars see (for example, a
rain shower cools the tire, but only part of the field runs into it, throwing
them off if they continue to apply the same correction). Tires also expand
with altitude, as the surrounding pressure decreases. So, are we seeing
variations based on different types of tires with different expansion rates,
run at different times of the day, at different altitudes and on different
surfaces? And which factors are the most influential?
It appears that with some tires, once a temperature threshold is reached (and
not exceeded) the pressure seems to remain fairly constant. In short, between
cold and hot is warm, and warm seems to have a range where the tire stays at
about at the same pressure. That's our experience, anyway.
Great Racers often run nitrogen, particularly in the big old tall skinny
tires, where a percentage change is magnified because of the size. That
theory goes that bigger (taller) tires change more than smaller ones. Cars
required to run period tires (bias plies) often push the pressures way over
normal, like to 50 lbs., to further reduce the amount of change. To maintain
contact patch, sometimes the tires are first pumped up to the desired
pressure, then shaved.
Some old automotive theory books I have talk about the heat being generated
by the 'rubber bulge' that develops with the tread compressing in front of
(and laterally to) the contact patch. The other big operating theory is that
radials change less than bias plies, because their lower rolling resistance
runs cooler. It goes on to say that the main tread grooves at the
circumference are there primarily to reduce the heat of the rubber being
compressed: in short, the rubber squeezes sideways as well as to the front,
and without grooves in thick tread, the heat build up would be excessive.
This is all old style tire stuff, but racing tires usually have much less
rubber without tread than street tires, so I think some of that theory
crosses over. Less rubber in the tread, the cooler the tire runs, and
therefore the less pressure change.
In the old days of vintage racing, where we were required to use period
street tires, I found that shaving the tread right down to 1/16th (minimum
allowed) radically cooled the tire, and I could maintain consistent lap
times: before that, as the tire warmed up it felt just like someone pouring
oil on them. Conversely, as we know, modern racing tires (stickys) need a
minimum threshold heat in the tread to soften the rubber and create adhesion.
So, since the temperature of the tire is a factor in maintaining optimum lap
or leg times, the thickness of the rubber, as well as the gas used to inflate
it: both seem to be big factors, and they are directly related. Thick rubber
with nitrogen or thin rubber with ambient air might have the same net result.
Taken to an extreme conclusion, small diameter tires with thin tread and
filled with nitrogen have the least amount of change? Any thoughts?
Steve & Janet Hedke
"Moss Motors Team Scrappy"
Great Race #45, 1957 Triumph TR3
britpac@aol.com
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