911 Understeer Issues at Auto-Cross (long)

[Hotrod911T]

Evan,

I was working Turn 4 (at the end of the mid-course chicane) during your second practice session. I enjoyed seeing you lay down about 100 feet of rubber and a lot of smoke from front wheel lock-up on 2 or three of your runs. It was quite a show until my co-driver (read wife) did the same thing in our car, then it wasn't so much fun thinking about how much our front v710's would be flat spotted for my timed runs.

Both you and your dad did a great job of driving your car. With a little more suspension setup on the car and the way you drive, you might get down into Mr. Palmquist's territory. My advice is to just stay out of the FI class and my excuse is that I had too much birthday cake and got a sugar fix which affected my timed runs.

See you on the course.

Rod

[Evan Fullerton]

Ya we will put brake modulation in the category of something I really need to work on. The problem I have is I can't feel that they are locked up until I go to turn in . The car still has the same pads on it though as when we pulled it out of a field so no doubt with some pads that offer better feel that could be improved upon.

The good thing though is the Nitto Nt-01 we had on at the time were on their last legs anyways and didn't seem to suffer too badly from the experience. When we put the A6s on later, the braking grip was one of the big things I noticed.

[vntgspd]

Originally Posted by Evan Fullerton

The problem I have is I can't feel that they are locked up until I go to turn in . The car still has the same pads on it though as when we pulled it out of a field so no doubt with some pads that offer better feel that could be improved upon.

Hi guys.

Interesting that I have the same issue with brake feel. I'm thinking that a fresh set of pads might help as well. That, and slowing down a bit sooner.

[Evan Fullerton]

Ya but if I am ever going to catch you, slowing down isn't a viable option .

[rfloz]

Well, at least I seem to be in good company with the front brake lockup issue.

Please feel free to post your auto-cross brake pad recommendations.

Evan-

My car is a little heavier (as is the driver), but not a lot. It is most unsquare with 9/11.5 rims and 245/315 710s. Still, the 24/27 pressures seem to be working better than the 28/32.

I have a probe type tire pyrometer, but forgot to bring it Sunday. With 28/32 I was good across the tread front and rear, but the front was a lot cooler - 20+ degrees (see way back at the start of this thread for my questions about this). Anyway, I will check the temps next event.

FWIW, I had a 61.0 in practice and a 61.4 in timed runs (actually it was 61.104 so I only lost a tenth). I suspect the car has a lot more in it. More driver tuneup needed.

[TRE Cup]

Have any of you tried to use different pad compounds front and rear? A call to Porterfield in costa mesa, could yield you the right combination to help reduce front lock up. Pads are less expensive and less time consuming to work with than installing adjustable bias valves.

also make sure your flex lines are fresh. they can expand quite a bit under hard use yielding a longer pedal.

personally on the use of larger master cylinders- i feel as long as it is sized correctly to work with your caliper install, there is a disadvantage in autocross going with too large a unit. you lose a lot of sensitivity as the pedal is soo hard to actuate the calipers with

[berettafan]

Am i not the only person that has had trouble feeling the front lock up w/ V710's?

What were temps when you guys have experienced this? Only have one day on my V710's but I had no idea i'd locked up the tires until i saw the video. Am hoping warmer temps (was maybe 65 that day IIRC) will help.

Different brake pads is an interesting idea too.

Did NOT have an understeering issue however. In fact it was an entirely new experience having to fight the steering wheel through long sweepers.

Evan, a 2400lb 993?!!! Do you have a thread on this car somewhere? wow!

[Randy W]

Hi Dave, I've used different pads front and rear for autocross and it helps. The only issue is how much of a difference can you afford to run if you also take your 911 to the track. Because the front tires on a 911 rarely get as warm as the rears in an autocross run, your ideal set up will be different at the track. Also, the rear tires in a 911 will gain significantly more grip in each successive autocross run, so it's nice to be able to dial the brake bias to compensate for this. Throw in the usual torque bias limited slip found in autocross 911s and you introduce yet another factor to consider at the track. As far as large master cylinders go, your experience mirrors mine. On my "stock" '72S, going to the larger M/C reduced brake "feel" for the S calipers but reduced brake fade at the track. Brake pad choice, brake pad condition, heat in tires, tire condition, brake bias, limited slip, rain/no rain - It's always a compromise when you use the same 911 for both autocross and track. That's why I use a brake proportioning valve and constantly adjust it for the conditions.

[J P Stein]

Did you ever notice that 25-35 year old cars lock up their brakes at AX. Golly, none of them newer cars do that....those guys must really be good at threshold braking, eh? I wonder why/how they manage that while running over surfaces that are normally used as parking lots, race car pits & flight lines and are at best
semi-lumpy. We have a problem in an ex lumber/log storage yard. Go figure. ......

[berettafan]

Well JP in my 40 year old car I can feel street tires locking up but the V710s are a different matter.

[J P Stein]

Originally Posted by berettafan

Well JP in my 40 year old car I can feel street tires locking up but the V710s are a different matter.

When them slicks get hot it's like the hand of God reaching out to slow you down, at that it can still be overdone......even without the odd patch of oil, antifreeze, Elk ****, ....whatever.

I guess my sarcasm was lost. Without ABS it' not unusual at AX. I have lots of pics with smoke....

[berettafan]

No no appreciated the sarcasm! Us antique drivers do it the old fashioned way!

[kwikt 911]

Originally Posted by berettafan

Well JP in my 40 year old car I can feel street tires locking up but the V710s are a different matter.

Try autoxing with the A6's. You don't notice flat spotting them until it's too late. I had better luck not flat spotting them with the 710's.

[berettafan]

That was my fear with the Kumho's. I suppose rotating fronts to rear every other event would help with this.

[J P Stein]

Flat spotting has never been an issue for me at autocross. I pay a lot of attention to my tires and have seen smallish flatspots on occasion. Generally, speeds are low so damage is minimal and un-noticable while driving. Also, where there is smoke does not mean fire.....if the tire is still turning, no flat spot.

[Tom Oxner]

We have a 72 911 with a 3.2 (stock w/Motronic). The car weighs 2270 w/o driver. We are running 22mm front torsion bars and 29mm rear torsion bars. We use 15X9 wheels on the rear with 275/35 A6's and 15 X 7 wheels on the front with 225/45 a6's. Since we run SCCA rules we cannot change the brakes (other than upgrading to Carerra calipers). We run 21 mm sway bars on the front and rear.
I am the tire warmer for my 16 year old son. We have managed to get some pretty good times out of the car. The biggest difference in autocross handling that we have experienced was putting the large sway bar on the rear. We set the bar as stiff as we can get it and still keep the rear end under control in the slalom. The car turns in like crazy! In an autocross situation, oversteer seems to be a good thing.
The 275/35 tires also lower the gearing a great deal, which is good for shorter autocross courses.
I still drive the car to work from time to time. After our last event, I changed tires and forgot about resetting the rear bar. "Spirited" street driving would get the tail out! I reset the rear bar that evening.
Good luck-and more rear bar!

Tom