Axle U-bolts are components used to secure the axles of vehicles to the leaf springs used for the suspension. The bolts are U-shaped and are secured into place using a top plate. Nuts screw onto the two ends of each U-bolt, as those ends are threaded to accept them. Other components are usually secured with the U-bolt as well, though most components deal with the axle or the suspension. Most vehicles will have several of these bolts rather than just one.
In most cases, the axle will rest beneath the leaf springs, which means the curved part of the axle U-bolts will be facing downward with the uprights facing up. The top plate will rest on top of the leaf springs, and the nuts will screw onto the uprights of the bolt that have slipped through the holes in the top plate. The axle itself will rest within the curved part of the axle U-bolts, and an axle seat will sit between the axle and the leaf springs to aid in stability.
In other configurations, the axle U-bolts may be flipped so the curved part is above the leaf springs and resting in a specially designed top plate. The nuts are screwed onto the bottom of the uprights, which are pointing downward, and secured in place with another plate. Very often the axle will be secured on either side, with two axle U-bolts being used instead of one. If the builder wants to drop the clearance of the vehicle, he or she can mount the leaf springs below the axle, in which case the bolts will be reversed to accommodate the new configuration.
Not all axle U-bolts feature a curve; some have flat sections instead of curves to accommodate different leaf spring designs. Semi-round bends may also be used for certain designs. The specific shape can change depending on the make and model of the vehicle, as well as the intended purpose of the vehicle. One of the primary functions of the U-bolt is to clamp the leaf springs to the desired tightness, which can affect the ride quality of the vehicle as well as its stability over certain terrain. It is not advisable to reuse old U-bolts, as the metal can fatigue after time and lead to failure.