How Car Differentials Work: Open vs Limited Slip and a DIY Differential Model

Learn how the differential gear train distributes power between the two rear wheels in a car and why they must rotate at different speeds when the vehicle turns. The video explains open differentials that can lose traction on slippery surfaces, and limited slip designs that improve grip. It breaks down the key components—drive pinion, ring gear, spider gears, side gears, and axles—and shows how a gear ratio like 3.9 to 1 affects acceleration and fuel efficiency. You’ll also see a hands-on model you can build from downloadable design files, with options for free 3D printing. This session merges conceptual explanations with practical demonstrations to illuminate how differentials work in real cars.

Overview of the Differential Gear Train

The differential gear train sits between the drive shaft and the wheels, most commonly located between the rear axles on a rear-wheel-drive car. Its purpose is to distribute engine power to the rear wheels while allowing them to rotate at different speeds when the car turns. When driving straight, all wheels share torque evenly and rotate at the same rate. When turning, the inner wheel travels a shorter path and the outer wheel a longer path, so the wheels must spin at different speeds to avoid tire scrubbing and excessive wear.

Open versus Limited Slip Differentials

An open differential permits the two rear wheels to rotate at different speeds, but if one wheel encounters slick surface, little torque may be transferred to the other wheel, causing the car to become stuck. A limited slip differential adds friction elements that temporarily lock the wheels together under higher torque, improving traction on slippery surfaces by distributing power more effectively between wheels.

Main Internal Components

The differential consists of a main housing, drive pinion, ring gear, spider gears, side gears, and axles. In a limited slip design, each side gear is backed by a clutch pack made of alternating friction plates and discs. The ring gear drives the casing and the spider gears rotate with the casing but can rotate on their own axis to accommodate different wheel speeds. The clutch packs provide a preload that keeps the side gears locked to the discs during straight-line driving, while allowing slip when turning or under high torque.

How Torque is Distributed

When the ring gear turns, the spider gears are carried around the same axis and interlock with the side gears to transfer torque to the axles. In an open differential, the spider gears rotate with the casing and do not develop a large axial difference in speed. In a limited slip differential, the clutch packs engage under light preload to lock the gears together, enabling equal rotation under straight-line load, and releasing the joint to allow different wheel speeds when needed.

Turning, Speed Differences, and Gear Ratios

During a turn, the left and right wheels must rotate at different speeds to follow paths of different lengths. For example, the left wheel might run faster than the right to accommodate the outer wheel’s longer arc. The difference in wheel speeds is inversely related to the other wheel’s speed, so increasing one wheel’s speed reduces the other’s relative speed. The gear ratio between ring and pinion (for example, 43:11 ≈ 3.9:1) influences acceleration and fuel efficiency: a higher ratio improves acceleration but reduces fuel economy, while a lower ratio does the opposite.

Hands-on Learning and Model Availability

To help viewers visualize the concept, a DIY differential model is provided with downloadable design files. The creator encourages building and experimenting with the model to see the differential in action. The video showcases a comparison between a self-made 3D printed version and a professionally produced version printed through a sponsor’s service, illustrating how hands-on exploration can deepen understanding of complex mechanical systems.

Takeaways and Practical Implications

The differential is essential for smooth handling and tire longevity, enabling different wheel speeds during cornering. Open differentials can lead to loss of traction on slick surfaces, while limited slip designs mitigate this problem by distributing torque more effectively. Designers trade off fuel efficiency, acceleration, and traction by selecting appropriate gear ratios and differential types. The video combines theory with practical demonstrations and invites readers to build their own model to gain intuition about differential behavior.