A beam axle, rigid axle or solid axle is a dependent suspension design in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once commonly used at the rear wheels of a vehicle, but historically they have also been used as front axles in four-wheel-drive vehicles. In most automobiles, beam axles have been replaced with front and rear independent suspensions.
A beam axle is a solid style of axle also called a live axle and is typically used in the rear of a rear-wheel drive vehicle. This axle is also often found on trailers and in both the front and rear of four-wheel drive trucks. Primarily used due to the low cost required to manufacture the axle, a beam axle does not produce as smooth of a ride as other types of axles due to the very heavy unsprung weight it produces. Typically suspended by leaf-spring-type suspension, the beam axle can also be suspended by coil springs and will occasionally use a panhard bar to maintain the axle's position between the chassis' frame rails.
The beam axle has been used primarily in every type of automobile and truck ever produced. By placing axle tubes rigidly mounted to a differential, the beam axle creates a strong platform in which to design a vehicle. While not as smooth and not producing the ride quality of an independent rear axle, the beam axle is able to be brought up to high quality ride standards through careful spring and shock-absorber selection. This will aid in off-setting the unsprung weight generated by the rear of the vehicle bouncing when a bump is encountered.
While the beam axle is able to produce very good straight line speed and handling, it falls far short of an independent axle in cornering and bump strikes. When an independent axle encounters a bump, the tire rides straight up and over the bump while the spring absorbs the shock, and the shock absorber softly places the tire back on the road. When the same bump is encountered by a beam axle, the entire axle tips to the side as one side of the axle is sent up and over the bump. This causes both tires to ride on the sidewall, thereby limiting traction and causing an unsteady handling characteristic.
Some of the earliest vehicle designs used a beam axle on the front of the vehicle. This is partially due to the vehicle's manufacturers being mostly former wagon manufacturers and to the solid front axle being used on horse-drawn wagons. The axle design continues to be used on semi trucks as well as other large trucks due to the strength of the axle. Other versions of the straight axle have experimented with softer, spring-type steel in the housing to work as a type of anti-roll bar and axle housing in one package.