Differential

Our last post, about the transmission, considered the first component after the engine in a vehicle’s drive system. In this post, we will look at the differential, which, depending on the type of drive system, may be the next component. With front-wheel drive or all-wheel drive, a differential is likely the next component after the transmission – with these systems, it is actually, generally, part of the transmission (or transaxle – more on this later).

In the context of machinery, Dictionary.com defines a differential as “an epicyclic train of gears designed to permit two or more shafts to rotate at different speeds, as a set of gears in an automobile permitting the rear wheels to be driven at different speeds when the car is turning.” Not specifically referring to its usage as part of an automobile, the word came into usage between 1640 and 1650, from Medieval Latin “differentiālis.”

As noted in the definition, the differential is important when a vehicle is turning. If a vehicle was not able to have the inner and outer tire rotate at different speeds when turning, the inside wheel would need to slip. Generally, we don’t want our tires to slip – we want them to maintain traction.

In a front-wheel drive, or rear-wheel drive vehicle, the differential goes between the two drive wheels, allowing the the outside wheel to rotate at a greater speed than the inside wheel while cornering. The other two wheels (we’ll assume that we’re talking about 4-wheeled vehicles) are “just along for the ride.”

In an all-wheel drive vehicle, there is a differential between each pair of drive wheels, the front pair and the rear pair. Traditionally, there is also a differential between the two pairs of drive wheels. That is a front-to-back differential that allows the front wheels to rotate at a different speed than the rear wheels. This can be useful as the vehicle navigates a road where traction conditions change linearly, such as might be found in wet or winter driving.

In a four-wheel drive vehicle, there isn’t usually a center differential, so the front and rear pairs of drive wheels are somewhat forced to rotate at the same speed. Rather, than a center differential, a four-wheel drive vehicle will have a transfer case, which we’ll examine more closely in a future post. There are some transfer cases with an all-wheel drive mode which does introduce a center differential – it is likely to also have a traditional 4WD mode which would lock the drive wheel pairs into rotating at the same speed.

Our family Subaru has AWD with three differentials: one between the front wheels, one between the rear wheels, and one between the two pairs. Our Toyota has 4WD with two differentials: one in the front and one in the back, and a transfer case in the middle. When not in 4WD, it is, functionally, a rear-wheel drive vehicle, with all of the power being sent through the transmission, through the transfer case, through a drive shaft, to the rear differential before being distributed to the two rear wheels.

With transmissions, we’ve developed a solid preference for automatics. When it comes to AWD versus 4WD, we don’t have a clear preference. AWD takes no human intervention to activate, but 4WD gives the feeling of control. Either way we’re certainly glad to have a differential, or two or three, for routine driving conditions.

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