How an Open Rear Differential Works: gears, spider gears, and torque distribution

The Engineering Mindset explains the open rear differential used in many cars. The video starts by detailing the main housing and a removable access plate that protects the gears from debris and retains oil for lubrication. When the housing is removed, the drive pinion and ring gear are revealed, along with bearings and the spider gears connected to the side gears. The gears are spiral-cut to increase contact surface, and the video illustrates a gear ratio example of 43 ring teeth to 11 pinion teeth, about 3.9 to 1, and discusses how lower or higher ratios affect fuel efficiency and acceleration. The differential distributes torque to both wheels while allowing different speeds, essential for turning. In straight motion, torque is equal; in a turn, the left wheel speeds up and the right slows down, with the speeds inversely related. The video also connects drivetrain context and references related content on the channel.

Overview

This video provides a detailed look at the open rear differential used in many vehicles, focusing on how the main housing protects internals, retains oil, and enables access through a removable plate. It then delves into the gear train that transfers power from the engine to the wheels, emphasizing the function of the drive pinion, ring gear, spider gears, and side gears.

Key Components and Gear Train

The differential’s main housing surrounds all internal components and is crucial for debris protection and lubrication. After removing the housing, you can see the gears attached to the axles, supported by bearings for smooth rotation. The engine powers the system through the input gear, the drive pinion, which engages the larger ring gear. The gears are spiral-cut to maximize contact surface area, improving strength and efficiency. A specific example in the video notes 43 ring teeth and 11 pinion teeth, yielding a ratio of about 3.9 to 1. A lower ratio favors fuel efficiency, while a higher ratio enhances acceleration, illustrating a key design trade-off for car designers.

Spider Gears, Side Gears, and Torque Distribution

Inside the differential, the ring gear drives two spider gears that are often mounted on a small shaft. The spider gears can rotate on their own axis and interact with two side gears, one connected to each rear wheel axle. The arrangement allows torque to be transferred to both wheels while permitting different wheel speeds, which is essential when the car turns. The spider gears distribute forces evenly between the two wheels via the side gears, coordinating with the axles to propel the vehicle.

How the Open Differential Works in Practice

Looking inside the differential during operation, the drive shaft turns the pinion gear, which turns the ring gear. As the ring gear rotates, the differential casing (to which the spider gears are attached) also rotates. If the car travels in a straight line, the forces are equal on both rear wheels, so the spider gears remain aligned with the casing and no differential action occurs. When turning, the wheels must rotate at different speeds—usually the inner wheel slows down while the outer wheel speeds up. The video demonstrates this with a numerical example, such as straight-line speeds around 300 RPM, then turning where the left wheel might reach 360 RPM and the right 240 RPM. This inverse relationship between wheel speeds is the core function of an open differential, enabling smooth steering while transferring torque to the wheels.

Context within the Drivetrain

The differential is part of the broader drivetrain sequence: engine power moves pistons, drives the camshaft, engages the clutch, and passes through the transmission to the driveshaft, which then enters the rear differential. The video notes that it has covered transmission concepts before and points viewers to related content for a deeper understanding of how power flows from engine to wheels.

Takeaways

Open differentials provide a reliable method for distributing torque while allowing wheel speed differences, which is essential for turning. The gear-cut type, gear ratio, and the arrangement of spider and side gears all influence performance characteristics such as fuel efficiency and acceleration. The video invites viewers to explore more about automotive engineering through linked content and social media channels.