A car differential is a gear system that sends engine torque to the drive wheels while allowing the left and right wheels to spin at different speeds. This matters most when a vehicle turns, because the outside wheel travels a longer path than the inside wheel in the same amount of time. Without a differential, the tires would scrub, the vehicle would be harder to steer, and drivetrain parts would experience extra stress.
In a typical rear axle, the differential sits inside the axle housing between the two drive wheels.
Key Facts
- A differential lets the left and right drive wheels rotate at different angular speeds while still receiving torque.
- For an open differential, average wheel speed equals carrier speed: carrier speed = (left wheel speed + right wheel speed) / 2.
- During a turn, the outside wheel must rotate faster because it follows a larger radius path.
- Torque is delivered through the ring gear, differential case, spider gears, side gears, and axle shafts.
- Wheel linear speed is related to rotation by v = rω, where v is linear speed, r is tire radius, and ω is angular speed.
- In an open differential, both drive wheels receive approximately equal torque, but the wheel with less traction can limit usable drive force.
Vocabulary
- Differential
- A gear assembly that allows two drive wheels on the same axle to rotate at different speeds while receiving torque.
- Ring gear
- The large gear attached to the differential case that is driven by the pinion gear from the driveshaft.
- Spider gears
- Small gears inside the differential that allow the side gears and axle shafts to rotate at different speeds.
- Side gears
- Gears connected to the axle shafts that send torque from the differential to the left and right wheels.
- Open differential
- A common type of differential that allows free speed difference between wheels but can lose traction if one wheel slips.
Common Mistakes to Avoid
- Assuming both wheels always spin at the same speed, which is wrong because the outside wheel must rotate faster than the inside wheel during a turn.
- Thinking the differential creates extra power, which is wrong because it only divides and redirects torque that already comes from the engine.
- Forgetting that an open differential sends approximately equal torque to both wheels, which is why one slipping wheel can limit the force available at the other wheel.
- Confusing torque with speed, which is wrong because the differential can let wheel speeds differ while the torque sharing is controlled by gear contact and traction.
Practice Questions
- 1 A car turns in a circle. The inside drive wheel travels 20 m while the outside drive wheel travels 25 m in the same time. If the inside wheel makes 10 rotations, how many rotations does the outside wheel make, assuming both tires have the same size?
- 2 In an open differential, the carrier speed is 300 rpm and the left wheel speed is 240 rpm. Using carrier speed = (left wheel speed + right wheel speed) / 2, find the right wheel speed.
- 3 Explain why a car with a solid locked axle is harder to turn smoothly on pavement than a car with an open differential.