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Most vehicles have drive shafts to transmit engine and transmission torque to the wheels. Usually, people refer to the longitudinal shaft — running front to back— as the “drive shaft,” and the transverse — from the differential to the wheel — as an “axle shaft.” Front-wheel-drive vehicles have two axle shafts, while four-wheel-drive vehicles may have two drive shafts and four axle shafts, for a total of six.

Short axle shafts are typically solid steel, while longer drive shafts are usually tubular steel, aluminum or carbon fiber, to reduce weight. Because the shaft is a spinning drivetrain member, it is dynamically balanced to eliminate vibrations. Sliding joints and universal (U) joints allow for variations in suspension height; giubo, “rag” joints or flex discs allow for shock absorption; and inboard tripod constant velocity (CV) joints and outboard Rzeppa joints allow for wheel turning on axle shafts. Some automakers use Rzeppa joints on all shafts. Finally, long shafts may be split, with a central carrier bearing and additional U-joints. These parts don’t “wear out” like most moving components, but they can be damaged. Here are five common indicators of problems.

1. Vibration

If you feel unusual vibrations, particularly those that seem to come and go at certain speeds, the drive shaft may be off-balance. This can happen if it is bent from an impact — off-roading lately? — or if it loses a counterweight. Unbalanced shafts can accelerate wear on joints, bearings and seals.

2. Clunking

Clunking when putting the transmission in gear, changing forward to reverse, or vice versa may indicate excessive differential clearance or worn U-joints. Worn joints might also lead to other noises and vibrations. If a joint breaks, the drive shaft could fly free of the vehicle, causing collateral damage and possible loss of vehicle control.

3. Slapping

Road debris may wrap around the drive shaft or axle shaft. This might result in slapping noises, rubbing noises, off-balance vibrations and collateral damage. Fuel leaks, short circuits and even fire could result.

4. Bucking

There are two sliding joint types. Sliding yokes depend on the transmission for lubrication, and so aren’t prone to failure, but slider drive shafts feature a sliding joint —usually splined and booted — in the shaft itself. If the lubricant dries out or leaks out, it may bind, leading to a bucking sensation when coming to a stop or when the suspension settles.

5. Leaking

Drive shafts don’t “leak,” but their supporting joints can. This is especially true of CV boots. Over time or due to damage, CV boots may crack or break, releasing protective grease and allowing water or debris to contaminate the joint.

Regular maintenance, such as lubricating all Zerk fittings or replacing broken CV boots, will keep your drive shaft running true. If you feel unusual vibrations or hear unfamiliar noises, have it inspected and corrected before it damages anything else or becomes a safety hazard.

Check out all the drivetrain parts available on NAPA Online or trust one of our 16,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on drive shaft operation and troubleshooting, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

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