Hi guys,

I'll take my C4S autocrossing for the second time this week-end and I was wondering if there is anything I should be watchful of or take advantage of versus the front wheel drive A3 I use to take A-X.

Obviously they are very different cars, how will the AWD specifically change things for me?

Erik

 

This is sort of generic AWD tips & tricks, if anyone wants to be 911 specific please correct me.

At the lower speeds of autocross nearly every car exhibits understeer. That is, your front tires give up before the rears - and you "push" outside of your requested line. This is normal, the trick is to manage it.

In a RWD car, the front tires handle steering duty, the rear tires handle acceleration, and all four handle braking. With a FWD, and to lesser extent, an AWD car, you ask even more of the already taxed front tires.

A tire only has so much to give. The limits of adhesion are sometimes called the "friction circle," which is a little bit of a misnomer because it's actually more irregular oval, but I digress, it's easier to picture a circle:



The graph shows that if you ask for acceleration or braking while cornering, you have to give up some cornering demand to stay in the friction circle.

From here, consider mid corner where you are using 100% of your available front grip - the tires will be lightly squealing (not howling) and the steering wheel will still be firm. If you try to feed in power here, you will ask more of the fronts, go outside the friction circle, and the tires will begin to slide. It's important to note that sliding tires give less grip than static tires. Think of moving a couch, once you get it sliding, it takes less force to keep it moving. Not good, you want stick.

What does this all mean? Proper technique for all cars still applies - brake hard late in a straight line, release brakes before turning in, enter wide and slower than you think, turn in late to make a late apex, so you can power on earlier. Feed in power as you feed out steering input once you reach your late apex. That's the same as RWD. Faster is coming out of corners faster, NOT going into corners faster.

Mid corner, AWD rules are similar to FWD: Find the limit, more gas and you'll overwhelm the fronts and push. Lift, you'll transfer weight to the front and tighten up. If you're picking your entry speeds close to right, that's all you need to know. Things going pear shaped? Stab the gas hard, and you may overwhelm the rears and induce oversteer. Lift hard, and you may transfer too much off the rear, and also induce oversteer.

The trick is to really get in tune with what the front tires are communicating. Ask for every last drop of what they can do, but not a shade more. It's a touchy feely thing that takes a long time to master, but try to err on the "ask less" side and work your way up.

Good luck, have fun learning, and don't be afraid to try things - always try to keep thinking of what you are doing, why, and what were the results.

 

Very interesting, I'll need to reread a few times (and yes, this is for a 911 so the big *** might have further impact in the oversteer?).

 

Just go and have fun. Seat time is the key. Since its AX (and if not SCCA) you will likely get plenty of runs to try new things.

Yes the weight's in the back, but by the C4S, things had been worked pretty well to tame the rear weight tendancies. That said, still best not to "lift" as you enter a corner unless you are prepared for some oversteering fun. "Slow in, Fast out" works well when beginning.

 

I've done 5 AX over the past two years with a 96 C4. Times are similar to the C2's but the C4 seems to be easier to pull out of a 4 wheel slide. Tire pressure seems to make a big difference in keeping the front planted.