stock performance at the track

[emayer]

bobt993 wrote:

Eric,

Here is the short of ABS and how it is used:

The modern cars have greatly increased their ability to stay composed with increased slip angle. The 911 in particular has a very advanced rear suspension taming what in the past was a tail happy car. The weight bias is to the rear of course. During maximum braking (let's say 1g for simple math), the front is loaded with additional weight /lean from the decell of the car. The added load increases the traction patch on the front tires while reducing that of the rear. Think of a teeter-totter that has the same weight on each end and one person scoots further towards the end tilting the other side upward slightly. ABS actively monitors the front to rear wheel rotations calculating wheel lockup. Should the back wheels begin to lock, then the brake pressure is modulated. Same if the front should do so. This is done much faster than the human foot can respond.

Next is more where the system really works. Corner entry. If you are trail braking into the corner (car is now leaning and turning towards the apex) the front tires continue to grip, but at different levels. Traction has now moved to the front outside wheel and is leaving the inside wheel. As this happens the inside wheel can easily "lock". ABS now considers the difference from left to right not just front to rear. This is why we mention 4channel ABS. The ability to keep the inside wheel rolling while braking/turning avoids having to slow down the car. Without four pedals in the car, the human foot cannot accomplish this especially at the speed required.

Standing on the brakes until the pedal pushes back is not using ABS , but relying on it to slow the car down by using proper braking technique along with a smooth transition of trail braking maximumizes use of the available traction as it moves from the front two tire (threshold), front outside tire (trailing, turn-in), front outside and rear outside (midcorner to apex), back around to rear traction and acceleration at exit.

I hope that answers some of what your asking. If I am in the passenger seat and feel the ABS working, then your abusing the system. If the sensation is felt in the steering wheel, then your using it much more effectively. Factory Porsche driver Patrick Long explains ABS as a tool that should be used as described above. It is not the easiest skill to learn, but very rewarding when accomplished.

Thanks for the great explanation. If I understand correctly, the goal here is to exploit ABS by allowing it to primarily trigger on the inside wheel during trail braking rather than completely activating on approach. This makes sense intuitively both for maximizing speed and leaving extra braking capability if needed, though it must take plenty of practice to get it right. I need more track time!.......