Differential Gears Explained: Open vs Limited Slip and Torque Distribution in Cars

Overview

This video explains how a differential gear train distributes power between the rear wheels in a car, with a focus on open differentials and the limited slip variant. It shows why different wheel speeds are needed during turning and how the differential prevents tire wear and handling issues.

It also introduces a hands-on model to visualize the gears, the main components, and how gear ratios influence performance. Viewers are invited to analyze gear rotation directions and discuss their answers in the comments.

Introduction and Context

The differential is typically located between the wheels of a car and splits engine power to the rear wheels. In straight-line driving, both rear wheels rotate at the same speed. When turning, the wheels follow paths of different lengths, so they must turn at different speeds to avoid tire slip and axle damage. The video uses a simplified rear-wheel-drive model to illustrate these ideas and sets up the contrast between open and limited slip differentials.

Open Differential Basics

In an open differential, power is transmitted from the drive pinion to the ring gear, and the spider gears connect to the two side gears that drive each axle. When the car goes straight, the forces are equal and the spider gears do not spin on their own axis; torque is evenly distributed, and wheels rotate in sync. When the car turns, the left and right wheels must rotate at different speeds. The left wheel speeds up and the spider gears rotate to slow the right wheel, allowing both wheels to push the car forward, but the speed difference is inversely proportional between the two sides.

Limitations of Open Differentials

On slippery surfaces such as ice, an open differential tends to send most power to the wheel with less traction, causing that wheel to spin and the car to get stuck. This is the primary drawback that led to the development of limited slip designs.

Limited Slip Differential (LSD) Basics

The LSD retains the same main components but adds clutch packs behind each side gear. These packs consist of alternating friction plates and steel discs. When the ring gear rotates, the clutch packs are driven by the side gears but remain interlocked only through friction. A spring applies a light pre-load, pressing the packs together. In normal straight-line driving the friction locks the side gears so they rotate together with the casing. When the car turns, the increased torque causes the clutch to slip, allowing the wheels to spin at different speeds while still providing traction overall. This improves grip on slick surfaces and reduces the likelihood of getting stuck.

Main Components and Gear Ratios

Beyond the differential itself there is the housing, a removable access plate, and bearings that support rotating parts. The drive shaft connects to the input gear, the pinion, which engages the much larger ring gear fixed to the differential casing. The video notes a typical example with 43 ring teeth and 11 pinion teeth, yielding a ratio around 3.9 to 1. Designers weigh the trade-offs: a lower ratio improves fuel efficiency and keeps the engine turning at moderate speeds, while a higher ratio boosts acceleration and torque.

Hands-On Model and Learning Takeaways

The presenter encourages building a model to see the concepts in action, arguing that hands-on exploration deepens understanding. The model demonstrates how the spider gears can rotate on their own axis and how speed differences arise as the car negotiates a turn. The video also positions the LSD as a solution to ice and other low-traction surfaces by distributing engine power more evenly between wheels.

Practical Considerations and Takeaway

Understanding differentials helps explain why some vehicles use LSDs to maintain traction in corners and on slippery roads. The key idea is that the differential must allow different wheel speeds while still delivering torque to both wheels as needed. The opening question at the start about which direction a gear would rotate reinforces the concept that an open differential allows the other gears to rotate even if one wheel is held in place, a property that LSDs modify through clutch engagement to balance stability and traction.

Conclusion and Next Steps

The video wraps up by inviting further exploration and discussion in the comments, and it points viewers toward additional learning resources about automotive engineering and gear trains.