Causes of Rear End Chatter

When making a turn, the outer wheels of a vehicle must travel faster than the inner wheels. It is up to the differential to act as a balance arm, establishing constant equilibrium of torques and forces between the left-hand and right-hand driven wheels.

In standard (or open) differentials, power is directed to the wheel offering the least resistance. This works well when making a turn, but in a situation where one wheel is lifted off the ground, it causes the suspended wheel to turn while the other remains motionless. A typical example is when watching a typical rear wheel drive vehicle take off from a standing start abruptly. One tire will spin leaving a black strip of rubber on the roadway while the other leaves none.

Limited-slip (or positraction) and locking differentials are traction-increasing devices. Locking differentials transfer all available torque to both wheels. The teeth of the spider assembly engage with those on the locker clutch, causing both axle shafts to operate as one.

Locking differentials don't allow one drive wheel to spin while the other remains stationary. Instead, they keep both wheels in drive mode, but unlock to permit different wheel speeds when turning. The engaging and disengaging of the teeth in automatic locking differentials often causes a "banging and clunking sound". Manual locking differentials usually use an electric motor or vacuum diaphragm to engage the locker, reducing this noise.

Limited-slip differentials allow different wheel speeds when turning, but when one wheel loses traction, greater torque is transferred to the wheel offering more resistance. Limited-slip differentials make use of friction between clutch plates, as opposed to the meshing teeth employed by locking differentials. Transferring less torque per side, limited-slip differentials are generally smoother and quieter than locking differentials, but sometimes "chatter" when transferring torque to the wheel with traction.

This "chatter" is due to stick-slip. While it is annoying, typically there is no mechanical damage done to the differential.

Stick-slip is essentially the buildup and release of energy between the differential clutch packs during operation. This phenomenon oftentimes occurs when torque is transferred between the clutch packs or when rotational speeds change. During the stick-slip phase, energy is built up to a point where contact between the clutch plates changes between static friction (stick) and dynamic friction (slip), resulting in noticeable vibrations. It is this vibration that causes an audible chatter or ratcheting to be heard within the differential.

In an effort to control this chatter, friction modifiers are often used. These additives result in a smoother transition between the two states of friction; static and dynamic. Static friction occurs when the clutches physically lock together, while dynamic friction occurs as the clutches are slipping.