Modifications
Drivetrain
Drexler limited slip differential
Limited slip differentials have been exclusive to BMW M-cars since the late nineties. The Alpina Roadster S is based on the Z4 3.0i and comes with a regular open diffferential. An open differential supplies equal torque to each of the rear wheels, decreasing overall torque when one of the wheels reaches the limit of traction. A limited slip differential allows extra torque to be sent to the wheel with high resistance when the limit of friction is reached at one of the driven wheels. The added torque improves cornering speeds when the inside wheel loses traction and improves acceleration on surfaces where the rear wheels encounter uneven levels of traction.
The Drexler limited slip differential was one of the first mods I fitted to the Roadster S. Drexler Motorsport is a manufacturer of high performance clutch type LSDs for use in motor sport and limited production cars. The company is also supplying LSDs to Alpina for the B5 S and B6 S. The Drexler LSD is more aggressive than the more commonly used Quaife LSD, featuring quick response times, strong locking performance and allowing lockup on both acceleration and deceleration. The LSD was set up with a ramp angle of 50 degrees under acceleration and 40 degrees under deceleration.
After upgrading to the Drexler LSD, the effect of the limited slip differential was immediately noticeable in high speed corners and on poorly surfaced roads. The LSD promotes oversteer in high speed corners and slippery conditions, making the car feel more lively. There were no issues with the onboard traction and stability control software when the car was still running on the stock Michelin Pilot Sport 2 tyres. There is some disagreement between the LSD's dynamic distribution of torque and the DTC/DSC when running on Cup tyres. In high speed corners, the stability control comes in way too early, aggressively braking the wheels on one side of the car, thereby decreasing stability. The problem only happens with Michelin Pilot Sport Cup tyres and can be easily avoided by completely turning off the traction and stability controls.
The Drexler LSD comes at a very steep price tag. Since the part is built for motor racing it also comes with no warranty. For road use the Quaife LSD delivers better value for money.
Comments