AutoX brakes & tires
 

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?

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

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.

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!

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.

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

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.

Mike, how many boxsters were there? I run the same pressure as I do on the street which is 33 front, 36 rear.

I think there were 7 Boxsters there. I don't know how many were Ss, though there was at least one. I know two of them had upgraded brakes and DOT legal race tires. It sounds like everybody's using pressures that are around where I ended up.

I was surprised that there weren't more cars there, and at the layout of the course. It's not anything like driving on a track. However, it only cost $35, and it's a hell of a lot of fun if you are at all competitive. I am, so in that respect, it's more fun that HPDE. I prefer HPDE, but it is great fun to compete and try to continually improve your time. I guess I could do that on the track if I invested in a lap counter. That's not a high priority now though.

The course at the tire rack was really spread out. Much more than any other autocross I've ever seen before. My only prior experience with autocross was those Mazda Rev-It-Up events where you get three runs on their autocross track in a Mazda 6 (or whatever model it was). Those courses were always much tighter than this one. It was fun as hell though.

it is refreshing to hear your positive opinion of autocross. All too often once someone has had a taste of track days, they have a hard time enjoying the autox format.

I start with 34 front/38 rear cold and adjust as needed. I run R-comps with 245 front/275 rear.

I'll be running at least one (if not two) events this weekend in Gary with the Northwest Indiana SCCA. They have a new site at one of the Casinos which is comparable to the Tire Rack events in South Bend or the bigger SCCA clubs in Chicago or Milwaukee. If interested PM me for details. It's a nice club with typically 50-80 cars (may be a little bigger with their new site) and some pretty fast Porsches.

I'm going out of town for a wedding this weekend.

I may look into some SCCA events later in the summer depending on my work schedule (I work a lot on weekends).

to anyone else considering autocross, check out my Kinko's race numbers.

I printed the number out on Word. They enlarged it and laminated for me.
I trimmed around the edges with skisors. Then I bought a package of these business card magnet things at Staples or Office Depot and they glued right to the back. Total cost was like $20. But just use blue painters tape underneath so that the magnet isn't rubbing on the paint.

http://memimage.cardomain.com/ride_i...057_medium.jpg

AutoX and brake pads...

I use different compounds to balance the car's stock braking system. I promise there is a HUGE difference in a Pagid Yellow/Orange/Black pad when used in AutoX or DE/TT/RR. Want more rear bite? Run Oranges in the rear and STOCK in the front. This helps the car rotate RIGHT NOW.

Think of it as a tractor. Tractors use "turning brakes" on the big rear wheels to maneuver. This is no different for a street car with sticky/aggressive rear pads and stock front pads. For years now I have used different compounds F+R to help people with NON ABS cars who locked up expensive slicks only to flat spot them :)



B

Guys with Boxster S... you don't run track or auto-x with the stock cross-drilled rear rotors because they crack easily right?

I beat up BoxsterS's at the track all the time on stock rotors :) You can run them until the cracks get to the outer edge (which would be about 6 track days total) depending on whether or not you are a newbie. Newbies are MUCH harder on the rotors :)


B

Interesting. I might try that.

BTW, you stole my number.

I hadn't even heard this was a problem. I use the OEM rotors. But I don't brake much. "To brake is to admit defeat!" :p :p :p

Kirk

Well correct me if I'm wrong but all cross-drilled rotors crack before they're worn down to the minimal thickness.

Most track guys go for slotted rotors, while x-drilled/slotted design is only for looks.

I just ordered a set of Performance Friction 2-piece dimpled rotors... we'll see how they work once they're on.

Rotors, like pads are normal replacement items for track days. I don't see anything wrong in running the stock drilled rotors. I don't know anyone who runs rotors to minimum thickness on track days anyway. It would be silly to start the day with thin rotors as they heat soak much more quickly.