In a rear-wheel-drive drift car, the rear tires must lose grip in a controlled way while the driver keeps the car pointed through the corner. An open differential lets the left and right rear wheels spin at different speeds, which is useful for normal turning but can make drifting unstable. A limited-slip differential, or LSD, limits that speed difference so both rear wheels receive usable torque.
This helps both tires approach and exceed the traction limit together, creating a smoother and more predictable slide.
Inside an LSD, clutches, gears, or viscous fluid create resistance when one wheel tries to spin much faster than the other. During throttle application, this resistance sends more torque to the wheel with better grip instead of letting only the unloaded inside tire spin. In a drift, the LSD helps maintain balanced rear tire slip, so the car can hold angle while still accelerating through the corner.
The driver then controls the drift with throttle, steering angle, weight transfer, and tire grip.
Key Facts
- Wheel torque is limited by tire grip: maximum usable drive force is Fmax = μN.
- An open differential sends equal torque to both axle shafts, but total drive is limited by the wheel with less grip.
- A limited-slip differential resists excessive speed difference between left and right rear wheels.
- Locking effect helps both rear tires spin together, making rear slip more symmetric and predictable.
- Power at the wheels is P = τω, where τ is torque and ω is angular speed.
- Yaw moment helps rotate the car: τyaw = rF, where r is lever arm from the center of mass to the tire force.
Vocabulary
- Limited-slip differential
- A differential that allows some wheel speed difference while resisting large differences to share torque more effectively between drive wheels.
- Open differential
- A standard differential that allows the left and right wheels to rotate at different speeds but can send little useful drive when one tire has low grip.
- Traction limit
- The maximum friction force a tire can produce before it begins to slide.
- Slip angle
- The angle between the direction a tire is pointing and the direction it is actually moving.
- Yaw
- The rotation of a car around its vertical axis, which changes the direction the nose of the car points.
Common Mistakes to Avoid
- Thinking an LSD makes unlimited grip, which is wrong because tire force is still limited by Fmax = μN and the road surface.
- Confusing an LSD with a fully locked axle, which is wrong because an LSD still allows some speed difference for turning and stability.
- Assuming only more throttle creates a good drift, which is wrong because throttle must work with steering, weight transfer, and available tire grip.
- Ignoring normal force on each rear tire, which is wrong because load transfer changes N and therefore changes the maximum friction force each tire can produce.
Practice Questions
- 1 A rear tire has a normal force of 3500 N and the tire-road coefficient of friction is 0.75. What is the maximum friction force that tire can provide before sliding?
- 2 A car sends 400 N·m of torque to a rear wheel rotating at 80 rad/s. What power is delivered at that wheel in watts?
- 3 Explain why an open differential can make a drift less stable than a limited-slip differential when the inside rear tire becomes lightly loaded during cornering.