A rally car must send engine torque to four tires that are constantly losing and regaining grip on gravel, mud, snow, and tarmac. Differentials make this possible by allowing connected wheels or axles to rotate at different speeds while still transmitting torque. In an all wheel drive rally drivetrain, the center differential manages front to rear torque, while the front and rear differentials manage left to right torque.
These parts strongly affect acceleration, cornering, stability, and how easily the driver can rotate the car.
Key Facts
- Wheel power is P = τω, where τ is torque and ω is angular speed.
- An open differential sends equal torque to both outputs, but total usable torque is limited by the tire with less grip.
- A center differential splits torque between front and rear axles, such as 50:50, 40:60, or actively variable ratios.
- A limited-slip differential creates locking torque to reduce speed difference between its two outputs.
- Torque bias ratio = maximum high-grip-side torque / low-grip-side torque.
- During a turn, outside wheels travel a larger radius, so they need higher rotational speed than inside wheels.
Vocabulary
- Differential
- A gear system that lets two outputs rotate at different speeds while receiving torque from one input.
- Center differential
- The differential that splits engine torque between the front and rear axles in an all wheel drive vehicle.
- Limited-slip differential
- A differential that resists excessive speed difference between its outputs to reduce wheelspin and improve traction.
- Torque bias ratio
- A measure of how much more torque a limited-slip differential can send to the tire or axle with better grip.
- Yaw
- The rotation of a vehicle about its vertical axis, which describes how the car turns left or right.
Common Mistakes to Avoid
- Thinking a differential always sends more torque to the wheel with more grip. An open differential sends equal torque to both sides, so a spinning low-grip tire can limit the whole axle.
- Ignoring the center differential when analyzing all wheel drive traction. The center differential controls front to rear torque flow, so it affects understeer, oversteer, and acceleration out of corners.
- Assuming a fully locked differential is always best. Locking can improve straight-line traction, but it can also cause tire scrub, push the car wide, and make the car harder to rotate.
- Confusing wheel speed difference with wasted power. Different wheel speeds are necessary in a turn because each tire follows a different path length.
Practice Questions
- 1 A center differential splits 600 N m of gearbox torque in a 40:60 front to rear ratio. How much torque goes to the front axle and how much goes to the rear axle?
- 2 An open front differential drives two front wheels. If the lower-grip wheel can support only 180 N m before spinning, what is the maximum torque the differential can deliver to each front wheel, and what is the total front axle torque?
- 3 A rally driver enters a loose gravel corner and wants the car to rotate without excessive inside rear wheelspin. Explain how the center, front, and rear differentials each influence the car's behavior.