Drivetrain and Transmission

The engine makes torque over a fairly narrow useful speed range. The drivetrain's job is to take that and give the wheels what they need, which changes constantly.

The layouts

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A transfer case is the part that distinguishes four-wheel drive. It sits behind the transmission and sends power both forward and back. If a question describes a component splitting power between two axles, that is the transfer case.

Front-wheel drive frees up cabin space because no shaft runs down the middle, and it puts the engine's weight over the driven wheels, which helps traction in slippery conditions.

The parts in order

Engine, clutch or torque converter, transmission, driveshaft, differential, axles, wheels. That order is asked in the same way the fuel path order is asked.

A universal joint exists because the axle moves up and down while the transmission does not. The shaft between them must change angle as it turns, and a U-joint is what permits that.

Gear ratios

This is the gear lesson applied. A larger driven gear turns more slowly and with more torque than the smaller gear driving it, and the ratio is the trade.

Low gear: large ratio, high torque, low speed. First gear moves a stationary vehicle because it multiplies engine torque heavily.

High gear: small ratio, low torque, high speed. Top gear lets the engine turn slowly at highway speed, which saves fuel.

Nothing is gained for free. A gear that multiplies torque by three turns the output at a third of the speed, which is the work-in-equals-work-out rule from the mechanical advantage lesson.

The final drive ratio is in the differential, and it multiplies whatever the transmission handed it. A numerically higher final drive gives stronger acceleration and a lower top speed - a truck for towing rather than a car for cruising.

The differential

In a corner, the outside wheel travels a longer path than the inside one, so it has to turn faster. A solid axle would force them to turn together, which means one tire has to scrub.

The differential lets them turn at different speeds while both receive drive. That is its purpose, and it is what a question wants.

It also turns the drive through 90 degrees on a rear-drive vehicle, from a shaft running front to back into axles running side to side.

The weakness is traction. An open differential sends torque to the wheel with the least grip, which is why one wheel spins on ice while the other sits still. A limited-slip differential or a locking differential exists to defeat that.

What you can skip

Across the 59 questions on this topic in our bank:

• Automatic transmission internals. Planetary gearsets come up twice; the torque converter four times. Know what the torque converter does, not how the gearsets shift.
• Constant-velocity joints never appear; the universal joint does.
• Limited-slip differentials come up once. The ordinary differential and why it lets the wheels turn at different speeds is what recurs.

Where people lose points

Saying a low gear gives more speed. It gives more torque.

Thinking the differential makes the wheels turn together. It lets them differ.

Calling a torque converter a clutch. It is a fluid coupling.

Attributing four-wheel drive to the differential. The transfer case splits power between axles.

Thinking gearing creates power. It trades speed for torque.

Forgetting the universal joint's reason. The shaft's angle changes as the suspension moves.

Work one in under a minute

A rear-wheel drive vehicle is stuck with one rear wheel on ice. That wheel spins freely while the other does not turn at all. Which component explains this, and what would prevent it?

The open differential.

It splits drive between the two wheels and allows them to turn at different speeds, which is exactly what it is for in a corner.

The consequence is that it follows the path of least resistance. The wheel on ice needs almost no torque to turn, so that is where the torque goes, and the wheel with grip gets very little.

A limited-slip or locking differential prevents it by restricting how far the two sides can differ.

The same property that makes the differential necessary is what makes it fail here, which is why the question is worth more than a definition.

Where this leads

Every ratio in this lesson is the gear lesson's rule, and the traction the differential mishandles is the friction lesson's subject.