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A spindle is a part of a vehicle's suspension. Tying the wheel and tire into the steering system, the spindle is in the front of the vehicle in most cases; however, some front-wheel drive vehicles also have rear spindles. The spindle pivots between the upper and lower A-frames or on the strut. Both the inner and outer wheel bearings ride on the spindle and the retaining nut on the end secures the wheel into position. On most vehicles, the front brake caliper is also mounted on the spindle.
Acting like a short axle, a spindle is used to attach a wheel assembly to a vehicle. Typically manufactured from a forged piece of steel, spindles must be extremely strong and durable to support the weight of the vehicle. While most spindles utilize a wheel hub to mount the wheel and tire in place, some wheels are mounted directly onto the spindle without the use of a hub. These wheels are known as spindle mount wheels.
Often, trailers and farm implements use spindles in place of solid, full-length axles. These spindles are subjected to incredible loads and occasionally break. When this happens, it is seldom possible to weld the spindles in place and repair them. New spindles are used to replace broken ones and are typically changed as a complete unit. The use of spindles instead of axles on machinery allows for better articulation and movement of the tires and suspension from side to side.
A vehicle's spindles rarely require specialized maintenance. Greasing the inner and outer wheel bearings and ensuring that a cotter key is in place in the castle nut are usually all that is required. Most spindles fail due to heat from a dry or over-tightened wheel bearing or stress from turning a very sharp corner. When the unit is turned too sharp, it causes the tires to skid and be dragged around the turn. This dragging produces a tremendous amount of stress and load on the spindles, often cracking and breaking them.
In order to prevent breakage, trailers often use several spindle-mounted wheel assemblies paired with an air lift system to raise select wheels up and off of the road when making sharp turns. Once the rig has completed its turn, the tag axles or spindles are once again lowered to the road. This arrangement allows multiple wheels to evenly disperse the load's weight to prevent damage to the spindles.