AutoX brakes & tires

[Mike_Yi]

I just got back from my first Autocross. It was with the Chicago PCA at The Tire Rack in South Bend, IN. I won my class (Boxster & Boxster S) by 3 seconds! I had a lap a second faster than that, but I just grazed a cone and knocked it down. The guy working the corner swore that I didn't hit it, but it was laying down when I was done. I blame it on the wind.

Anyway, I have a couple of questions for those of you who autocross regularly.

1. Do you use race pads or street pads? I use Hawk Blues on the track. I know they aren't very efficient until they get hot. So it seems like they'd be worse for autocross than stock because they'd never get that hot.

2. What kind of tire pressures do you use when you autocross? I started with 38lbs all around. On the track I aim for 40 lb hot. However, after thinking about it, I began to think that you run higher pressure on the track for less rolling resistance on the straights. On an autocross, straight line speed isn't as important as cornering speed. When I lowered my pressures, my times came down and the car seemed to handle better. It definitely like less pressure in the fronts, otherwise it would understeer like mad. So what do you guys use?

[NickCats]

Mike,

Great job ! Here's my thoughts :

- I don't think there would be any advantage using race pads for autocross. The runs are too short for the pads to heat up enough to make a difference. Stick with street pads.

- I usually run 36 front / 38 rear ( set cold ) - seems to be a good combination for me. My buddy runs 34 front / 38 rear ( he felt dropping the front 2 lbs reduced his understeer ). Keep in mind he has an S and I have a non-S.

Hope this helps...

Nick

[cnavarro]

Mike,

Tammy's car has EBC Greenstuff pads. Seems to be the happy medium between a street and track pad. I use them year-round on all our daily drivers and have for years. They worked well at Blackhawk the week before for my wife. It's an experiment on the Boxster, as pretty much no one has any experience with them on the track. We'll see how they work at Road America this week!

I'm not sure I had the tire pressures quite right on the car, she wasn't putting that much heat into them, maybe 1 or 2 psi tops. The track temp wasn't very hot either, I measured it at 75F towards the end of the last session.

I'm just like you, trying to figure out what pressures I need to run. I want to say the pressure was 32 psi hot all around right when she came off the course with exception of passenger side rear which was closer to 33 psi and a bit hotter on the inside tread of the tire (measured with IR thermometer). Otherwise, all the tires had the same temp within a few degrees across the treat from inside to middle to outside at this pressure.

[Topless]

I run 29F 32R cold pressures for typical events. You are right. The brakes never get hot enough in AX to make best use of race pads. Nice runs!

[kabel]

I concur with much of the above.

Stock pads and rotors are more than adequate for a typical autocross situation.
I run the PS2's and always start out with cold pressures at 34 front 38 rear and adjust as needed throughout the day.

Oh, and I am running 225/45ZR17 up front.

[stephen wilson]

I just had a general comment, not Boxster specific. Pressures are adjusted to maximize grip, not reduce rolling resistance. You generally want an even temperature gradient across the tire contact patch, with the inside being hotter than the outside. In some instances higher pressures are required to prevent tire "roll-over" in heavier cars. The pressure also changes the spring rate of the tire, lowering pressures on a cold day can gain mechanical grip thru more tire compliance.
Steve

[Perfectlap]

PCA autocross around here isn't the most competitive. I've seen folks win by a second on occasion but that would never happen in SCCA or the other regional clubs. The difference there is as little as .001 in some of the classes. I remember one event where the difference was less than .3's down to 9th place.

I wouldn't make any equipment changes until you have at least 12 autox events under your belt. After that many events you'll see that autox has basically a limited number of type of turns to master. hairpin, slalom, "chicago box", six pin turn, four pin turn, hard right, hard left. Not a whole lot you can do in a confined space. Bascially, Grind what you have first.
I started on street tires and I was slip sliding all over and very tempted to go right into the r-comps. After I put in a ton of mistakes by deliberately pushing hard to test the limits my tires were shot for autox but fine for regualr driving. Then I ordered a set of 14 pound wheels with Azenis and minimized the brutalization and in the first even I was about a second and a half quicker with little punishmment to the stickier tires.

some tips I picked up which could be helpful:
Drive with your shoulders and arms and not your wrists.
keep your shoulder blades pinned, no rocking side to side.
put a piece of tape at the 12 o'clock position to keep you aware of wheel shuffle.
always have your foot on the gas or the brake. time loss comes from hesitation.
sketch a map of the course and number the braking points.

Do a search for Randy Probst the Porsche driver. He autocrosses, still, and had an interesting article about his a-ha momment that made him quicker. basically has to do with deliberately driving slow during a lap.