• Sajiv Shah

The Traction Problem and the Locking Differential

In the last article about open differentials, we briefly touched on the topic of the issues that come as a result of the mechanism design of an open differential. Below is an image that shows what the issue looks like.

image from learnengineering.com

The wheel on the slippery surface has less resistance or friction, so the differential gearbox will favor speed to that will and it will speed up, close to the rpm of the input axle. Meanwhile, the wheel on the surface with more traction receives little to no power and no torque. Therefore, the situation is analogous to one in which the engine was connected to only one wheel, the slippery wheel, which fails to address the major reason to switch away from that drivetrain.

So how do we ensure that both wheels get traction? Well, the first solution to the idea was fairly simple. We know that a locked differential, or direct drive axle, powers both wheels with the same torque, meaning that if one is on a slippery surface the other will still receive power and can get out of the situation. So, the simplest solution is to combine a locked differential with an open differential using a locking differential.

There are many different types of locking differentials, including electromagnetic collars. These systems can be automatically or driver actuated. For example, some cars will lock the axles at any speed under a certain limit, like 15mph, and unlock at higher speeds.

Although this system performs well, it requires a separate actuation and a non-mechanical link to solve the differential traction problem. In the next few posts, we will look at systems like limited-slip differential and Torsen differential to see how they use mechanisms that require no separate actuation to overcome the traction problem.