The Flaw - You Must Pay Attention
By Ron Grable
Having just picked up this beautiful orange 4-wheel-drive, turbocharged, intercooled, glove-leathered Audi Quattro, how could I not have been seduced by the world's most perfect-looking onramp? It beckoned in the dark, and this wonderful car and I simply could not resist. So we flew in there at eight or nine tenths, and everything would have been great except for The Flaw. We learned about The Flaw together, Quattro and I, and almost bashed the cement ramp railing for our commencement exercise.
The scenario goes like this. The onramp was constant radius until very near the end, where it tightened up. But the reduced-radius section was not visible until the car was almost in it, so the car was moving at a speed reasonable for the constant-radius section. As I recognized the decreasing radius and lifted off the throttle to slow the car slightly, it dove to the inside of the corner, trying its best to collect the railing. Why?
It has to do with front-wheel drive and, in this case, 4-wheel drive. All the forces that accelerate, decelerate, and drive a vehicle around a corner are reacted through tire contact patches (disregarding aerodynamic considerations). In the case of rear-wheel drive the rear wheels thrust against the road to accelerate the car, and the front wheels steer by developing slip angles. In the case of front-wheel drive, the front tires are asked to develop driving (accelerating) thrusts as well as cornering thrusts. This is where the trouble begins.
The tire contact patch has a finite tractive force available, and it is the sum of all the forces acting on it. This means that if you increase one of the forces, the other must decrease by the same amount. In the situation described earlier, the car was being maintained on the constant-radius section of the corner with approximately 3/4 throttle. Since a portion of this thrust was being delivered to the ground by the front tires, they were forced to run at a larger slip angle than the rears (by virtue of their dual requirements of driving and cornering). Larger slip angle meant the fronts were pointed toward the inside of the corner. When the throttle was closed, the thrust requirement was removed and the cornering force suddenly got bigger. Because the front wheels were aimed at the inside of the corner, that is exactly where the Audi tried to go. Instant adrenalin.
You're probably thinking I got exactly what I deserved, roaring around onramps in the middle of the night; my initial reaction was similar. However, I think the conditions required to reveal The Flaw are not difficult to duplicate. The seventy of the reaction depends on a number of factors relating to the dynamics of the vehicle at the instant the throttle is closed, of course, and under normal operating conditions there shouldn't be a problem. This is not a normal vehicle, however, and some of its owners--most, in all probability-will want to explore its limits. These limits are very high, which means the consequences of an error can be even higher.
Please, you must pay attention.
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