astock165

I know this is often a topic of discussion but hopefully I will be indulged. I used to run the stock staggered wheel/tire setup and then I went to a square setup with 9.5" wheels. For both of those applications I used chaulk on the tire shoulders to raise or lower pressure as necessary. It was good enough for me at the time ...

But now I'm running 11" ZR1 wheels on all four corners with 315s and am not sure if the chaulk thing gives me enough info. So my questions are:

1) I know a pyrometer is the best way to set tire pressure but is that statement true for autocross? Given that the runs are shorter AND the layout of most courses require you to go back to the grid, resulting in at least a minute or more of cool down time before you'll get temps?

TIA

2) Is there a significant difference between optimum tire pressures for say a 275 tire and a 315 tire of the same make? More tread width, smaller side wall ratio requires more/less pressure?

XPC5R

You are correct on the pyrometer checks, it work best when you can run the car on a skidpad, and stop after a few laps and check.

Coming off course, if you have negative camber, the inner edges of the tire tend to read warmer. One thing you can do is make sure you stick the probe needle in pretty deep into the carcass, as the base rubber does not lose heat as quick. Big things to check for are really hot ouside edges (more than 10-15 degrees more - needs more camber) and hot or cool centers (needs less or more air).

I would start with the pressures you had before. The bigger tires are supported by a larger wheel, so if anything they may require less air to limit rollover.

KCHOTBOAT

How do you like the 11's all the way around?

RacePro Engineering

AStock,

In our "Prep your 911 for the Track" seminars, we devote significant time to READING YOUR TIRES. You are already used to some of this from your comments about chalk. We speak of it in 2 phases: Immediate and Historic.

The pyrometer, used correctly as XPC5R indicates, gives immediate feedback - this track, this day, this ambient temperature, this alignment, this exact air pressure. We spend considerable time discussing how to interpret the pyrometer readings, and how to "correct" for things which are out of range. XPC5R refers to extra high heat on the outside of each tire. As he points out, this usually indicates a need for more (NEGATIVE!) camber; but it can also point to too much toe IN. PM to us if you want more details.

Less scientific, but equally important, is simply examining your tires at the end of an event, and even using chalk as an indication of sidewall roll-over. Now we are talking about history. Are the tires wearing (scrubbing) evenly across the contact patch? Too much on the outside? the inside? cupped in the center?

And to complicate matters even further, autocrossers in stock classes regularly use tire air pressures to fine tune "spring" rate and wheel rate, effectively making their car more stiffly sprung by running high pressures. So, in practice, the "fast" setup on a particular day at a particular track may well be a compromise between maximum contact patch on the pavement vs. better grip through a higher wheel rate.

Our advice: run a pressure which puts as much contact patch WIDTH on the track as possible (indicated by "scrub"), on each corner. Then raise air pressures to see if grip improves.

Hope this helps.
Ed LoPresti

astock165

Thanks for all the feedback, it was great. The little tips were great too. I guess I can say I've got a pretty good understanding of what to do, now I just have to get off my duff and do it!

Thanks again.

astock165

In a phrase "I like them". I have only done a few hundred street miles and two a/x events (one with co-driver) so keep that in mind:

1) I used the 36 mm ZR1 offset to get more inside clearance. I can go to full lock without rubbing (it is very close to fender well & sway bar but doesn't rub).

2) driving them to and from events can be a handful. They follow EVERY rut in the road and if you change lanes on a rutty multilane road ... be ready.

3) There is no doubt that they provide more grip. I am running Victoracers for the first time as well so this is a big step in both wheels & tires (I have run V710s before but only on 9.5" wheels).

4) My impression so far is that tire grip only goes so far however. My car does have more grip and does handle better but I can still feel the effects of body roll, and sometimes it does feel like it overcomes the grip. My car is a 93 with (base) FE1 suspension so the springs & sways are stock. I think to get the most out of it I will upgrade those components next.

5) I wanted to do both springs/bars & wide tires but only had the $$$ for one and chose the tires as it was quicker, less work and more immediate gratification. At this point I will save the springs/sways for a winter project and can't wait to run her when it's done.

jwt1603

Not trying to hijack the thread but I have a related question.

Did you find any difference in turn in response time? I am thinking of going to the 11 inch wheels all around but I have heard it makes a C4 turn in slower.

astock165

Not that I've noticed but again I've only done two events on them.

My thought would be that even if turn in is slowed by the different geometry, the added grip should help even it out. Also, toe out can be used to improve the turn in as well.

I ran an event Sunday with the (4) 11s, Victoracers and 0 front toe and I thought she turned in pretty well. I felt like she was loose more than she pushed. And I know what it feels like when she pushes.