The point of AWD is two-fold:
1. To distribute torque more evenly across all 4 wheels so that wheel spin is less likely in the first place;
2. In case of wheel spin, re-direct torque to wheels with traction.
Haldex-based systems that do 95/5 nominal split do not do a good job for #1. Considering that under hard acceleration, the car's weight shifts to the rear, the optimal nominal torque split is around 40/60. This is achieved by Audi's new TORSEN quattro, as well as Mercedes' 4Matic. Subaru's (and Audi's older TORSEN quattro) use 50/50 which is slightly sub-optimal. BMW's new xDrive is nominally 0/100, their older xi system was around 40/60.
Different approaches to #2 also affect response time in case of wheel slip. Viscous coupling systems are generally the worst. Next up is Mercedes' 4Matic based on 3 open differentials and electronic braking. Slightly better is a multi-plate clutch like in Haldex, Porsche and Lamborghini. Systems with a TORSEN center differential are generally considererd the best here.
In recent years, there is a growing trend towards a 3rd point for AWD:
3. To enhance handling on curvy roads.
Examples are Acura's SH-AWD and Saab's new system. They add active torque distribution ("vectoring") to the rear differential. Since these cars start with something like 95/5 nominal split, they inherently feel like FWD cars, and the new technology only partially compensates for the disadvantage. You never get the same level of handling as a RWD car. On the other hand, it's rumored that Audi will add rear torque vectoring to their 40/60 TORSEN system very soon (next RS4/RS5). That will be interesting.
1. To distribute torque more evenly across all 4 wheels so that wheel spin is less likely in the first place;
2. In case of wheel spin, re-direct torque to wheels with traction.
Haldex-based systems that do 95/5 nominal split do not do a good job for #1. Considering that under hard acceleration, the car's weight shifts to the rear, the optimal nominal torque split is around 40/60. This is achieved by Audi's new TORSEN quattro, as well as Mercedes' 4Matic. Subaru's (and Audi's older TORSEN quattro) use 50/50 which is slightly sub-optimal. BMW's new xDrive is nominally 0/100, their older xi system was around 40/60.
Different approaches to #2 also affect response time in case of wheel slip. Viscous coupling systems are generally the worst. Next up is Mercedes' 4Matic based on 3 open differentials and electronic braking. Slightly better is a multi-plate clutch like in Haldex, Porsche and Lamborghini. Systems with a TORSEN center differential are generally considererd the best here.
In recent years, there is a growing trend towards a 3rd point for AWD:
3. To enhance handling on curvy roads.
Examples are Acura's SH-AWD and Saab's new system. They add active torque distribution ("vectoring") to the rear differential. Since these cars start with something like 95/5 nominal split, they inherently feel like FWD cars, and the new technology only partially compensates for the disadvantage. You never get the same level of handling as a RWD car. On the other hand, it's rumored that Audi will add rear torque vectoring to their 40/60 TORSEN system very soon (next RS4/RS5). That will be interesting.
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