Motor vehicles usually move forward by rolling on two or more tires. A front axle assembly is the unit that connects two wheels at the front of a vehicle, and allows those wheels to rotate freely and sometimes independently. Very simple front axle assembly models may not include much beyond the axle, axle housing, and bearings to allow the axle itself to rotate, while other assemblies that are more complex may include steering and suspension components. The primary purpose of the axle in most cases is to support the weight of the vehicle, but this is not always the case.
If the front axle assembly is a drive axle, that unit will help deliver driving torque to the wheels. This means the transmission and engine will produce the energy and transfer it to the axle, which will have gears that rotate the axle shaft. Front wheel drive cars will feature such a front axle assembly, while rear wheel drive vehicles will not. On a rear wheel drive vehicle, the front axle may simply be a load-bearing and suspension component. It may still also feature steering components as well.
The specific size, shape, and function of a front axle assembly can vary significantly according to the type of vehicle to which it is mounted. A portal axle, for example, will be mounted in such a way that it is not directly in line with the centers of the wheels. It will instead be above the center; this axle design is useful for off-road vehicles that require extra clearance over rough terrain. The hubs of such axle systems often feature their own drive systems, which means a portal axle is likely to have more moving parts, and also more weight than a typical axle. The cost of such assemblies can also be much higher than a standard system.
Larger vehicles may feature more than one axle. A dead axle, or one that simply rotates freely and does not feature any steering systems, may be mounted in front of a drive axle to support the weight of the vehicle, effectively taking this strain off the drive axle itself. This dead axle may be mounted in front of or behind the drive axle, depending on the specific design of the vehicle. If it is located in front of the drive assembly, it is usually known as a pusher axle, and it is known as a tag axle if it is mounted behind the drive assembly.