How Car Suspension Works: Springs, Dampers, Bars

A car's suspension connects its wheels to its body and lets each wheel move up and down over bumps while the tyres stay in contact with the road. Three kinds of part do most of the work: springs that absorb bumps, dampers that stop the springing from continuing, and anti-roll bars that limit body lean in corners. Every design is a trade-off between ride comfort and handling. This article covers what each part does, the main layouts and some history.

The job of the suspension

According to the How a Car Works guide, the suspension lets each wheel move on its springs and dampers while keeping the spacing between wheels, and the angle of the tyres to the road, nearly constant. It has extra demands at some wheels: the front wheels must pivot for steering, and driven wheels must keep turning with their drive shafts.

Springs

Springs take the impact when a wheel hits a bump. Wikipedia's article on vehicle suspension covers leaf springs, torsion bars and coil springs, along with alternatives such as rubber bushings, air springs and hydropneumatic systems. A spring stores energy and releases it, which is the problem: left alone, it keeps bouncing.

Dampers

A damper, commonly called a shock absorber, controls that bouncing. Wikipedia's article on shock absorbers explains that it turns the kinetic energy of the movement into heat, which is dissipated. In a hydraulic damper, a piston with small holes moves inside an oil-filled body. The oil is forced through the holes and through valves, and the resistance converts motion into heat. The valving can make the damper softer when compressing and stiffer when extending, to control rebound.

Two designs are described. A twin-tube damper has an inner working tube and an outer reserve tube holding extra fluid. A monotube damper has a single tube with a floating piston that separates the oil from high-pressure nitrogen gas, which the article puts at about 260 to 360 psi. The article says gas-charged twin-tube units account for most original-equipment installations, and that overheating can cause foaming that temporarily reduces damping.

Anti-roll bars

An anti-roll bar is a torsion bar, a bar that resists twisting, that runs across the car and pivots on the body, with each end linked to a wheel through rubber bushes. The How a Car Works guide says that when one wheel rises, the bar twists and pulls the opposite wheel up, keeping the car more level. Bars are usually fitted at the front, sometimes at the rear, or at both. Wikipedia adds that engineers use them to tune how much the body leans and how cornering load is shared between the front and rear.

Sprung and unsprung mass

Wikipedia separates the car's weight into two groups. The sprung mass is what the springs carry, mainly the body, chassis and cargo. The unsprung mass is the rest: wheels, tyres, hubs, brakes and part of the control arms. Lower unsprung weight generally improves ride and handling, which is one reason lighter aluminium wheels are valued.

Common layouts

Layouts divide into those where the wheels on an axle move together and those where each moves alone.

• Solid or live axle: both wheels are linked by one axle. Wikipedia calls it simple and suited to heavy loads, though the axle adds unsprung weight. A live axle carries the drive shafts and differential and is used on many rear-wheel-drive cars.
• Torsion beam: a semi-independent design where the wheels are linked by a beam that twists. The twist-beam rear suspension is the most common example, Wikipedia says.
• Double wishbone: two stacked arms keep the wheel upright. It gives engineers more control over geometry but costs more and takes more space.
• MacPherson strut: a telescopic strut carries the hub. It is very compact, which is why it is common on front-engined cars, and it is used at the front and rear.

The MacPherson strut

It is named for Earle S. MacPherson, an American engineer. Wikipedia's article on the design says he became chief engineer of Chevrolet's Light Car project in 1945, and that General Motors cancelled the Cadet, meant to be the first production car with struts, in 1947, the year patents were filed. The first production car with MacPherson struts was the British-built 1950 Ford Consul, according to the article, which says the French 1949 Ford Vedette is often cited but was developed before MacPherson and used wishbones.

Listed advantages are low manufacturing cost, simplicity and compact packaging that frees width in the engine bay for small front-wheel-drive cars. Listed disadvantages are some camber change or sideways movement as the wheel moves vertically, and less freedom to tune geometry, so handling is generally considered inferior to a double wishbone or multi-link design.

A short history

• Leaf springs began appearing on carriages around 1750.
• In 1901, Mors fitted an automobile with shock absorbers, and Henri Fournier won the Paris to Berlin race in it.
• Coil springs first appeared on a production vehicle in 1906, the Brush Runabout.
• Independent front suspension was pioneered on the Lancia Lambda in 1922.
• The 1948 Citroën 2CV was the first mass-production car with mechanical front-to-rear interconnected suspension.

Sources

• Suspension (vehicle), Wikipedia)
• Shock absorber, Wikipedia
• MacPherson strut, Wikipedia
• How car suspension works, How a Car Works