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The very important job of a suspension system is to smooth out the ride while maintaining excellent control. This may sound like a simple job, but with acceleration comes force, and force translates into raw energy. The system is made up of several parts that can vary depending on the type of suspension, but typically include the chassis or frame, coil springs, leaf springs, dampeners including struts and shock absorbers, and anti-sway or torsion bars. Various combinations of these might form a particular system.
When a vehicle accelerates down a road, bumps cause forward energy to be converted into vertical energy, which travels through the frame of the vehicle. Without coil and leaf springs to absorb this, the vertical energy would cause the vehicle to jump up off the road, reducing tire friction and control. The car would then come bounding back down with even greater force, making for a very uncomfortable and dangerous ride.
Coil springs and leaf springs are designed to absorb up/down forces to keep tires firmly planted on the ground. Passenger cars commonly have coil springs in front and leaf springs in the rear, while many 4-wheel drive utility vehicles, sports utility vehicles, and trucks have coil springs in the front and rear, or independent front/rear suspension. Dampeners, namely struts and shock absorbers, dissipate the energy absorbed by coil springs, so up/down motion is quickly quieted to zero. If the dampeners are in proper working order, the passenger cabin should remain fairly unaffected by traveling over dips or bumps in the road. When shock absorbers are old or faulty, once the car starts bouncing, it continues bouncing for elongated periods of time.
Some vehicles benefit from torsion bars, also called anti-sway or anti-roll bars. These bars span the frame, helping to level out side-to-side motions while cornering. Torsion bars are an important feature on high profile vehicles that are considered top-heavy.
While passenger and luxury vehicles have systems designed for maximum comfort, stiff suspension systems are typical of utility vehicles designed for carrying heavy loads. Vehicles that have been lifted for a higher profile, often for off-roading, replace key parts of the system. Lift kits are designed for a particular model to compensate for the shift of weight and how it will affect the steering and suspension.
A vehicle that does not sit level or has excessive bouncing or poor handling is demonstrating problems associated with the suspension system. This greatly affects the vehicle's safety, so it should be repaired before driving.