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      01-21-2013, 04:17 PM   #43
KoenG
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Drives: i4 eDrive40 & Cupra Leon 300
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Quote:
Originally Posted by ovekvam View Post
You seem to assume that there is always a longitudinal component of the force on the contact patch. That is not the case. If you maximize the transverse component, there is no grip left for forward traction. If you choose to sacrifice the transverse force by adding forward traction on an AWD car, the overall force will not be any higher. The more forward force you add, the more transverse force you have to take away. If both components are the same, they will both be around 70 percent of the maximum transverse force. The vector sum is still not higher.
OK, you're an intelligent guy (not that I didn't know)... assume the max attainable grip line is a perfect circle, like you take in your reasoning. Then I follow you: vectorial sum is the same size. So the AWD has the liberty to let you shape the force in the front-tire: or 100% transversal or 70/70% split or even further biased towards longitudinal traction or any combination in between.

Take one step back for now, you enter the corner and you're before apex. During these conditions you're either braking or approaching at constant speed. It's during this phase that understeer is felt most and a real joy killer. AWD and RWD are identical in this phase, except the extra weight and inertion and lower placed center of gravity. But that difference is not noticable (by an amateur like me at least). During this phase you want the max. attainable transversal grip.

The second phase after APEX, you want to pick up speed while "opening" up the turn and gradually reduce steering angle. Here the AWD will send torque to the front tires, and by consequence have a higher tendency to understeer. Because of the mechanism you describe, the max attainable transversal grip is smaller, but in this phase the max is rather never exploited, or you have done something wrong and need to recover. On top, a higher tendency to understeer is not felt as a joy killer in the second half of the turn since the car is already anticipating on reducing steering angle via the accelerator even before you actually steer out.

I believe that this is what intelligent AWD systems tend to do: before apex, don't engage Front wheels to avoid understeer in this critical phase and don't compromise max attainable transversal grip, after apex, engage the Front wheels to help straightening the car and max acceleration.
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