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A custom axle is an axle that has been altered from its original design to conform to another use and dimension. Found most commonly in racing and performance applications, a custom axle is often created to fit a narrowed rear-axle housing. By narrowing the axle housing, the axle is required to be cut to a shortened length to fit the new housing's dimensions. Whether this is accomplished by simply cutting the original axle and re-splining the axle shaft to fit within the carrier or manufacturing new axles to the altered dimensions, a custom axle can be readily purchased from several auto parts vendors and machine shops around the globe.
In a performance vehicle such as a drag racing vehicle, it is often necessary to cut and narrow a rear axle housing to allow for the fitment of larger and wider tires to provide added traction to the vehicle. When this is done, a custom axle must be machined for each side of the axle housing. Many low- to moderately-powered race vehicles are able to re-use the stock axle once it has been shortened and machined to fit the altered axle housing. When machining a stock axle to create a custom axle, the axle must be shortened by cutting the axle shaft to the proper length. This is typically accomplished through the use of a carbide cutoff wheel.
The shortened custom axle must then have new splines machined into the end of it to allow the axle to fit inside of the axle carrier. The number of splines machined into the axle is determined by the number of splines that are in the carrier. The strength of the axle is determined by the number of splines it has. Where a 33-spline axle is stronger than a 28-spline axle, a 40-spline offers superior strength when compared to the 33-spline version. The typical street vehicle is equipped with 28-spline axles. It is often necessary to re-drill the axle flange to enable it to accept another manufacturer's bolt pattern and wheels.
One of the most common custom axle conversions involves the uses of the Ford Motor Company rear axle. Many of the most knowledgeable racing engineers agree that the Ford axle is the strongest axle available. When the Ford axle is swapped into another manufacturer's vehicle, the axle flange is often re-drilled to the vehicle manufacturer's original wheel bolt pattern. This allows the original style wheels to be placed on the custom axle.