How Your Car Transmits Power To The Road

Getting power from your car’s engine—which is fixed to the body, naturally!—to your wheels, which move up and down and, in some cases, side to side—requires an intricate system of shafts with special joints built in. In the latest episode of David Dissects, I’ll show you how they work.

Believe it or not, trying to smoothly rotate a wheel that can translate when it hits bumps and yaw when the driver turns isn’t really straightforward. To be sure, on solid axle vehicles, it’s not rocket science: the transmission or transfer case sends power to the differential via a driveshaft that can plunge to absorb bumps (thanks to splines) and bend (thanks to U-joints, which are basically just steel crosses with caps that rotate on a series of needle bearings).

The diff then sends power through two solid shafts in the rear, and two halfshafts in the front with U-joints built in to accommodate the steering angle. Here’s a photo of that front u-joint style halfshaft:

A u-joint style axle shaft.
A u-joint style axle shaft.

Vehicles with independent suspension are a bit more complex. Rear-wheel drive vehicles have driveshafts that don’t have to bend or plunge to accommodate bumps since the rear differential is fixed to the body. Instead, the axle shafts, which stick straight out of the rear diff, are equipped with joints that take up the bumps.

On front-wheel drive cars with independent suspension, it’s a similar—the axle shafts exiting the transaxle (which is fixed to the body) do all the bending and plunging, except unlike on rear-drive vehicles, there are no driveshafts, and the axle shafts have to to account for side to side motion during steering as well extension during bumps.

Both rear drive and front drive vehicles with independent suspension use CV-style axles. These types of halfshafts come out of the diff and have two types of joints on them: on front-drive vehicles, the inner tends to be a tripod joint (which uses little needle bearings to allow the shaft to extend and contract with bumps), while the outer one is usually a standard ball type CV joint, which can bend at severe angles during bump or tight turns. On rear-drive cars, both joints are usually tripods.

A u-joint style axle shaft.
A u-joint style axle shaft.

CV joints have become ubiquitous in vehicles, and not just in independent suspension setups. Some solid-axle vehicles even use CV joints because this style joint tend to transfer power in a smoother manner than U-joints (hence why they’re called constant velocity joints). In fact, CV joints are even found in driveshafts, replacing the typical dual U-Joint “double cardan” style driveshaft.

Now you know. Tune in next time for more lessons from the junkyard.



Nicely done, sir.

Suggestion you didn’t ask for: If you want to make the information useful for budding mechanics, add something about how to identify a failed one, either by symptoms the car experiences on failure or, when it’s disassembled, how to check for failure.

When I was learning about mechanics, that was my biggest issue - understanding how to interpret a fault, then test the components that were likely the culprit. I work with software, but the things I learned troubleshooting old cars and motorcycles still informs my testing methodologies when I have issues.

Really good. Thank you.