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A solid axle is a device used to propel a vehicle by converting the circular power of a drive shaft into linear motion through the use of a pinion, ring gear and differential. Commonly found in rear-wheel drive automobile and truck applications, a solid axle is a common rear-end configuration, however, it is not used in an independent, rear-axle suspension. Commonly considered to be stronger than an independent suspension or split-axle design, a solid axle is commonly swapped into the front-axle position of four-wheel drive trucks. While it is true that most axles are of a solid design, this reference commonly means the axle housing be of solid and unjointed construction.
An independent front axle or split-axle configuration provides a typically softer and smoother ride than that of a comparable solid axle. This is the reason that most auto manufacturers use this type of axle in the production of most light-duty, four-wheel-drive trucks. The problem with the design is that the axle uses several joints in the construction of the unit, providing several potentially weak areas that commonly fail under severe load. A common axle swap in the light-duty trucks is completed by swapping an early solid axle in place of the split axle.
A split axle is typically suspended by coil springs or torsion bar suspension, while a solid axle commonly uses leaf springs. The most common solid axle swap involves installing leaf springs under the front of the chassis. In order to accomplish this feat, all of the suspension and steering components need to be removed from the chassis. This is often done with the aid of a cutting torch and a grinder. Once the frame is clear of all mounting brackets and ground smooth, the leaf spring brackets can be welded in place to suspend the axle.
By making the conversion from a split axle to that of a solid unit, the front axle of a four-wheel drive truck is greatly strengthened. The axle is limited to a single joint in each end at the steering knuckle. For the ultimate in solid axle strength, the common universal joint is often changed to a planetary joint or a constant velocity joint. The use of these stronger axle joints can prevent joint breakage that will typically strand a newly converted, solid-axle-equipped vehicle along the side of a trail or in the clutches of a swampy mud bog.