Dylan,
You could always try driving the line that I did on turn 10 (or 3 going counterclockwise) at SOW to induce oversteer, although I would not suggest it. It was less than optimal and my rear came around quite nicely, then too much <img src="graemlins/c.gif" border="0" alt="[ouch]" /> I didn't even have to lift

Much like an autox, Streets of Willow is a place that you really want more oversteer (most of the time).

 

Martin,
Where on the 21mm bar would you drill the extra hole? Perhaps as close to the transition from flat to round as possible to get maximum stiffness? Thanks.
Dick

 

[quote] So, do you lower the rear pressures to reduce understeer?<hr></blockquote>

Dylan,

The last time I was on "Streets" I had Yoko street tires. My last three TimeTrials, though, I've been on the main track with the Victoracers and seemed to like hot pressures of 35-36F and 37-38R on 17" rims. (I tried lower and higher, and I agree that low 30s hot seemed a bit squirmy). To help reduce understeer further, you may want play with 2-3 pounds more in rear than the front. This stiffens the rear a bit more relative to the front. Keep in mind that this is a subtle adjustment and no where near as dramatic as a sway bar adjustment or tire size change. But if one is within the useful pressure range on the tires (i.e. not too low or too high for that make) I've found juggling a pound here and there helpful. Disclaimer: I am NOT a racer and am still on the learning curve trying to shave lap
time at TimeTrial events while realizing fully that my car is a daily driver and I can't afford to flog it too badly. So take my observations for what they're worth.

Edward

 

This thread has mutated into yet another discussion of air pressure, but over the weekend I remembered something about that drivers training exercise I mentioned above. Another helpful way to get the back end to come around is to pre-load the rear suspension. In the middle of the turn, hit the gas. This will cause the rear end to squat, compressing the rear springs. When you get to 5,000 RPM or so, lift. Two things will happen: the compressed rear springs will release and shift weight forward, and you’ll get the compression braking I mentioned above. This was easy to do during the exercise, as we could come into the turn really slow. We were way below the handling limit at the turn entry; otherwise, hitting the gas would have taken us over the limit. By the time we got to 5,000 RPM, we were close to the handling limit. This is harder to do on a track, as you tend to stay close to the handling limit throughout the turn.

Dylan is correct, though, on one very important point. If you’re in a second gear autocross turn, where the maximum speed keeps you below 3,000 or so, it’s very difficult to get the back end out, even when you’re at the handling limit.

Dylan also asked it you lower rear pressure to reduce understeer. This depends. Every tire has an optimum pressure for maximum traction. If you are above that pressure, increasing pressure will reduce traction, while lowering it will increase traction. If you want to reduce understeer, you need to get your front tires to their optimum pressure for maximum traction and you rear tires at something other than their optimum pressure. The key here is “optimum pressure.” This is best determined by tire temperatures, for which you need a pyrometer and some sort of test track. Or, your initials need to be M.S., in which case, you can probably deduce the proper temperatures by the feel of the car!

Thanks for not flaming me for telling people it’s OK to lift…

Tom
'95 993

 

[quote]Originally posted by tom_993:
<strong>Thanks for not flaming me for telling people it’s OK to lift…</strong><hr></blockquote>

Biting my tongue for all the beginners here...

 

[quote]Originally posted by E. J.:
<strong>

Biting my tongue for all the beginners here...

</strong><hr></blockquote>


E.J. is correct. I should reiterate what I said above. The standard advice is:

Whatever you do, don’t lift!

 

[quote] Dylan also asked it you lower rear pressure to reduce understeer. This depends. Every tire has an optimum pressure for maximum traction. If you are above that pressure, increasing pressure will reduce traction, while lowering it will increase traction. If you want to reduce understeer, you need to get your front tires to their optimum pressure for maximum traction and you rear tires at something other than their optimum pressure. The key here is “optimum pressure.” This is best determined by tire temperatures, for which you need a pyrometer and some sort of test track. <hr></blockquote>

Tom, yes, thanks for putting more clearly what I tried to say. One's tire pressures have to be in the "optimal" range when adjusting for subtlty, otherwise, it's just a stab in the dark. And the pyrometer is a must. I had someone at the track check mine at various tire pressures which is also how I derived my own recommendations, FWIW.

Edward