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Axle design can vary significantly depending on what type of vehicle the axle is on. Car axles will be different from truck axles, which will be different from motorcycle and bicycle axles. Bicycles often have the simplest axle design; the spindle is often steel or aluminum, and it is mounted between sets of bearings. Car axles, however, are more complex, especially if they feature other components, such as a drive system or steering components. Other types of vehicle axles are quite simple; known as dead axles, these components simply allow the wheels to rotate and do not have any components related to steering or propulsion.
One important type of axle design is the drive axle. This series of components is mounted at the rear of a vehicle if it is a rear wheel drive car or truck, or at the front of the vehicle for front wheel drive systems. When it is mounted at the rear, the axle design includes a system known as the rear differential. This unit transfers torque from the drive shaft to the axle itself; within the differential housing, a gear attached to the drive shaft will meet with two gears from the broken rear axle. One axle shaft will go from the left wheel to the differential, and the other will go from the right wheel to the differential. In most cases, the differential is located at the center of the axle.
A portal axle works differently than most other designs. Instead of being mounted at the center point of the wheels, the housing is above the centerline. This means gears will be necessary at each end of the axle, usually very close to the wheels, to transfer torque from the axle to the wheels. Portal systems are usually one of the more expensive axle design types, but they are useful for heavy-duty machines as well as off-road vehicles that will benefit from the higher ground clearance.
A simpler axle design is the dead axle. This system is usually used primarily for two functions: to allow the wheels to rotate freely, and to support the weight of the vehicle. In many cases, the dead axle is attached to the suspension system — usually leaf springs, especially on heavy duty vehicles such as pickup trucks or tractor trailers — and it bears most, if not all, of the weight of the vehicle rather than allowing that weight to be transferred to a drive axle.