Vehicle suspension has three main jobs: smooth out bumps, keep tires touching the road and control vehicle stability. You can read more about the basics in How Car Suspensions Work, but let's do a quick review. The spring absorbs the energy of forces acting on your wheels. The two most common types of springs are coil springs and leaf springs. Both types absorb energy well, but require shock absorbers to decrease or dampen their vibration after hitting a bump.
Each vehicle wheel gets a spring and a shock absorber, but the front suspension isn't the same as the rear suspension. That's because the front two wheels must turn right and left. The front suspension also absorbs more braking torque. Accordingly, front suspensions tend to be more complex.
The front suspensions of off-road vehicles tend to be even more complicated for two reasons: The wheels are usually bigger, and the terrain offers nastier bumps. The front suspension systems of four-wheel-drive vehicles and SUVs most often feature a solid axle with leaf springs. Such a suspension is called a dependent system because the wheels are connected laterally so that they move together as a unit. Leaf springs attach to the frame of the vehicle and then, via U-bolts, to the axle housing. A sway bar mounted to each side of the axle controls body roll.
Other 4x4/SUV front suspensions include the following:
- Solid axle with coil springs, another dependent system that replaces multiple-leaf springs with coil springs. Many off-road drivers prefer coil springs because they have a compact design and deliver a quieter, smoother ride.
- Independent front suspension (IFS), a type of suspension in which the two front wheels move independently. In an IFS system, an upper and a lower control arm attach to the wheel on one side, to the frame on the other side. Springing is accomplished either with torsion bars, which act like straightened-out coil springs, or coil struts, which combine a coil spring and a shock absorber into a single unit.
- Twin-traction beam (TTB), or twin I-beam, a suspension designed by Ford to combine the best of dependent and independent systems. A TTB system has two beams at the front of the vehicle. Each beam mounts on a pivot on one end, on the wheel on the other. The beams overlap quite a bit, so they act in essence like long control arms. A U-joint in the center allows for independent movement of both beams.
On the rear of most 4x4 vehicles, you'll almost always find a solid axle with leaf or coil springs. Off-road drivers often mount shock absorbers on opposing sides of the axle -- one in front and one behind the axle -- to reduce axle tramp, a rapid up-and-down motion of the rear axle caused by sudden acceleration.