This note requests insight on a problem I've had since I started competing in
CMOD (in the San Francisco/Sacramento, CA area) and suggestions for correcting
it. For four seasons I've been driving a 1985/86 Citation Formula Ford with
the original rocker suspension at both ends and Fox single-adjustable shocks.
As I've learned how to drive the car and how to set up its suspension, I've
improved my PAX placement appreciably, but I still have a problem of picking up
significant amounts of other people's rubber under some circumstances. This
year, for example, during the season the competition location changed from a
high-grip, smooth asphalt surface to one with only modest grip and several
bumps. After the change occurred my tires went from being fairly clean at the
end of an event to having significant pickup. On days when MOD cars ran after
the high-horsepower BSPVintage Corvettes I could sometimes scrape off enough
rubber from my tires to form a ball 2-3 inches in diameter. I use Hoosier
tires and find that the pickup at the new site becomes noticeable whenever the
tire temperature as measured with a penetration pyrometer gets much over 110F
even though I spray the tires to cool their surfaces. I would appreciate
learning whether others recognize this problem and can suggest solutions to it.
The following paragraph contains some more information on how the car is set
up in case that helps you give me useful feedback.
The front/rear motion ratios are 0.43/0.47, and the present front/rear spring
rates are 550/800 lb/in. Thus, the front/rear wheel rates are 102/177 lb/in.
I've also used other spring sets: 350/500, 500/700, and 700/800. I find that
500/700 and 550/800 work best. 350/500 lets the car scrape too often, and
700/800 results in objectionable understeer. I typically run the rear antiroll
bar at its softest setting and adjust the front one (and the shocks) to tweak
the balance. I've used both Hoosier and Goodyear tires and tried various front
tire widths; the present tires are Hoosiers with the 7-inch front tires. My
wife and I co-drive the car, and we often get ten runs between the two drivers.
On a typical hot day the tires reach a final temperature of about 130F, and
pickup becomes noticeable at the present site after about 6 runs. Thus, the
pickup and consequent loss of grip often affect our last several runs.
Thanks for your help.
George Fisk = gafisk@aol.com
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