half shaft

A half shaft is a rigid steel shaft that transmits rotational power from the differential gear to a single wheel hub. In a car with a rear-wheel-drive or all-wheel-drive layout, two half shafts extend outward from the differential case, one to each rear wheel. In front-wheel-drive vehicles, half shafts run from the transaxle (which combines gearbox and differential in one unit) to the front wheels. Each shaft must accommodate both the torque load and the angular motion of the wheel as it bounces, steers, or encounters uneven ground.

The typical half shaft is a hollow or solid steel tube, 20 to 40 mm in diameter, with a splined connection at one end (to the differential or transaxle) and a flanged or splined connection at the other (to the wheel hub). The splines are helical grooves that transmit torque while allowing for slight axial and angular misalignment. Many half shafts are coated or plated to resist corrosion from road salt and moisture. The length varies widely: a rear half shaft on a sedan might run 1.2 to 1.5 meters, while a front half shaft on a compact hatchback could be much shorter due to the compact engine bay.

In independent rear suspensions, each half shaft must flex and articulate as the suspension moves. This real-world motion puts bending and torsional stress on the shaft far beyond static calculations. Fatigue cracking typically begins at stress concentrations, such as the spline root or a change in diameter. The most common failure point is near the differential end, where the spline fillet radius is often the tightest. Corrosion pitting or prior impact damage can accelerate crack initiation. Once a crack grows through the wall, sudden separation can occur, leaving a wheel unpowered or, in severe cases, unbraked.

Variants and load paths

Some performance vehicles use carbon-fiber or aluminum half shafts to reduce unsprung mass, though steel remains the standard due to its cost and proven fatigue performance. All-wheel-drive vehicles may have shorter half shafts at each end because the transfer case or center differential sits closer to the wheels. In locking differentials or limited-slip units, half shaft loads can spike when one wheel is on low-grip surfaces, making shaft choice critical in off-road or racing applications. Front half shafts in front-wheel-drive cars must also serve as steering pivots, so they are often called axles or CV shafts when they include constant-velocity joints at each end to handle the angle changes of steering and suspension together.

Inspection requires checking for visible cracks, corrosion, looseness at the spline, or unusual vibration during acceleration or cornering. A worn or damaged half shaft can cause vibration, clunking under load, or subtle handling changes that are easy to misdiagnose as suspension wear. Because replacement often involves removing the wheel, brake assembly, and suspension arm, labor costs can rival or exceed parts cost. The half shaft name derives from its role as one of a pair that share the work of the full drive shaft in earlier rigid-axle designs, a historical distinction that persists in the vocabulary even though modern half shafts are structural members in their own right.