A limited slip differential, or LSD, is a drivetrain device that helps a vehicle send useful torque to the rear wheel with more grip. A normal open differential lets the left and right wheels rotate at different speeds, which is essential for turning, but it can waste power when one tire slips. An LSD keeps some connection between the two axle shafts so the vehicle can still accelerate when traction is uneven.
This matters in racing, off-road driving, snow, rain, and any situation where one tire may lose grip before the other.
Key Facts
- An open differential allows different wheel speeds but sends the same torque to both axle shafts.
- Wheel torque is limited by traction: T_max = μNr, where μ is friction coefficient, N is normal force, and r is tire radius.
- A clutch-type LSD uses friction plates to resist large speed differences between the left and right axle shafts.
- Bias ratio describes torque sharing: bias ratio = higher wheel torque / lower wheel torque.
- If an LSD has a 3:1 bias ratio and the slipping wheel can support 100 N·m, the gripping wheel can receive up to 300 N·m.
- During a turn, an LSD must still allow some wheel speed difference so the outside tire can travel farther than the inside tire.
Vocabulary
- Differential
- A gear assembly that lets the left and right drive wheels rotate at different speeds while receiving power from the engine.
- Limited slip differential
- A differential that limits the speed difference between drive wheels so more torque can reach the wheel with better traction.
- Clutch pack
- A stack of friction plates inside some LSDs that presses together to transfer torque between axle shafts.
- Torque bias ratio
- The ratio of torque an LSD can send to the wheel with more grip compared with the wheel with less grip.
- Traction
- The frictional grip between a tire and the road surface that allows the tire to push the vehicle forward without slipping.
Common Mistakes to Avoid
- Thinking an LSD always locks both wheels together, which is wrong because it only limits slip and still allows different wheel speeds during turns.
- Assuming the wheel with grip gets all the engine torque, which is wrong because the amount depends on traction limits and the LSD torque bias ratio.
- Ignoring tire friction when predicting acceleration, which is wrong because no differential can create grip beyond the tire-road friction limit.
- Confusing an LSD with traction control, which is wrong because an LSD is mainly a mechanical torque-sharing device while traction control usually uses brakes and engine control.
Practice Questions
- 1 A slipping tire can support only 80 N·m of torque before spinning. If the LSD has a 2.5:1 torque bias ratio, what maximum torque can be sent to the tire with more grip?
- 2 A car turns a corner where the inside rear wheel rotates at 180 rpm and the outside rear wheel rotates at 220 rpm. What is the wheel speed difference, and why must a differential allow this difference?
- 3 A car with an open differential is stuck with one rear wheel on ice and one on dry pavement. Explain why a limited slip differential can help the car move even though it cannot increase the actual friction of the icy tire.