Bill,
I wasn't saying that an autocross setup (camber/caster etc) will work
perfectly for road racing. I was just pointing out a way to get to a
starting point for finding good tire temps.
If you'd like, you can imagine an autocross as a 40 to 70 second run at a
figure 8 skid pad. The tires are being placed under their maximum sidewall
loading. And while autocross is generally a low speed event, speeds of 50
or 60 mph are not unknown. In a big parking lot.
In autocross the goal in setting tire pressures is to keep the tire from
rolling too much onto the sidewalls. Basically, to keep the car riding on
the tread portion of the tire.
Cheers,
Jim
Dallas TX
'76 Spitfire autocrosser
-----Original Message-----
From: Bill Babcock [mailto:BillB@bnj.com]
Sent: Thursday, July 15, 2004 12:01 AM
To: 'Jim Gambony'; fot@autox.team.net
Subject: RE: Yoko tire pressures
I know next to nothing about autocross, but I suspect setups that work for a
race that is over in 20 seconds with a max speed of 40 MPH are not going to
be the same as one that lasts 20 minutes with a max speed of 130. In fact I
can't see how tire temps would be all that meaningful--I don't trust the
temps I take at the track because the straights are too long--the outside
edges get cooled off. That's why I use a skidpad.
I'm not saying that low pressure is better than high. My comment was that if
you raise the tire pressure more than you need to for the weight of the car
that the center of the tire will get hotter than the edges. Generally for
racing you want as even a tire temp as you can get, because that means the
tire is flat on the road from edge to edge during extreme cornering--which
is when you need the best contact patch. But there are a lot of variables
that can make the readings spurious. You could have perfect camber for
extreme turns, but when the car is going straight the contact patch will be
at the outer edges, with the inner edges hardly touching. That's a good
thing--less friction in a straight line and generally higher stability. But
the tire temperatures will be meaningless.
I dropped a second off Peyote's time last weekend by just optimizing this
stuff--and Peyote would be competitive in club racing, never mind vintage. I
think with the right gearing and an extra tug on my shoulder belts I could
get it below 1:30 at Portland International Raceway with the chicane. Mid
pack in the sports racer/formula Atlantic pack is 1:28. Not bad for a forty
year old Triumph in drag.
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