The idea
Beginner explanation — switch to Technical for the engineering detail.
Roads are never perfectly flat. Without suspension every bump would jolt straight into the car, and the wheels would bounce off the surface. The suspension sits between each wheel and the body so that the wheel can move up and down by itself while the body carries on smoothly.
Each corner has three kinds of part. Links — here an upper and a lower control arm, often called wishbones — guide the wheel along a controlled path and hold it in place against braking and cornering forces. A spring carries the weight of the car and gives way when the wheel is pushed up. A damper (shock absorber) slows the movement down so the car does not keep bouncing after the bump.
Engineers split the car into two parts. Everything above the springs — body, engine, passengers — is the sprung mass. The wheel, tyre, hub and knuckle that move with the road are the unsprung mass. Good suspension keeps the sprung mass calm and the unsprung mass following the road, because a tyre can only grip while it is pressed onto the surface.
What you will learn
- Explain the suspension's two jobs: isolating the body from the road and keeping the tyre loaded on the road
- Name the parts of a double-wishbone corner and say what each one does
- Tell sprung mass from unsprung mass and describe suspension travel in bump and rebound
- Describe what happens to the wheel, spring, damper and body as a car drives over a bump
- Explain why a tyre can leave the road in a pothole and why that matters for steering and braking
Try it yourself
Drive over bumps and see the spring, damper and control arms keep the tyre on the road. Open the full suspension lab
- Phase
- Approach
- Body
- +1 mm
- Wheel
- 0 mm
- Travel
- at ride height
- Tyre load
- 0.99 × static
- Body
- Wheel
- Travel
- Tyre load
6× slower than real time · illustrative
Step by step
Use “Show in 3D” to jump the animation to a step; the step currently playing is highlighted.
Step 1: Cruising: the car rests on its springs
Now playing in the 3D sceneOn a smooth road, once the last bump's bouncing has died away, nothing moves. The coil spring holds the body up at its ride height and the tyre carries a steady share of the car's weight. Everything above the spring is 'sprung'; the wheel, hub and knuckle below it are 'unsprung'.
Step 2: Bump: the road pushes the wheel up
As the tyre climbs the 60 mm hump, the road shoves the wheel upward. Both control arms swing up on their pivots, the spring gets shorter and the damper rod slides into its body. Because the spring gives way, the body hardly moves at first.
Step 3: Full bump: the most compressed moment
At the top of the hump the wheel is about 55 mm closer to the body than at rest. Look at the spring: the gaps between its coils have closed up. The top of the wheel also leans in a little, because the upper arm is shorter than the lower one.
Step 4: Rebound: the spring pushes back
Past the crest the road falls away and the squeezed spring pushes the wheel back down and the body up. The damper slows this rebound, so the body rises gently instead of leaping off its springs.
Step 5: Settling back to ride height
With a well-matched damper the body makes one gentle swing up, a smaller one down, and settles. The energy the bump put in has been turned into a little heat in the damper oil.
Step 6: Pothole: the tyre leaves the road
In a 50 mm pothole the road drops away faster than the spring can push the wheel down after it. For a split second the tyre is in the air, and while it is, it cannot steer or brake. Light wheels and well-chosen springs and dampers keep that moment short.
Step 7: Ripples: the wheel works, the body rests
Small, closely spaced ripples make the wheel move up and down quickly, but the heavy body hardly follows: the spring isolates it. Here the tyre and the light wheel assembly do almost all the moving.
Key components
Select a part to highlight it in the 3D scene.
How it flows
Read the diagram as text
- Road surface → Tyre (vertical input)
- Tyre → Wheel, hub & knuckle
- Wheel, hub & knuckle → Control arms (ball joints)
- Control arms → Coil spring (motion ratio)
- Control arms → Damper
- Coil spring → Body
- Damper → Body
- Tyre → Grip (tyre load)
- Road surface: Bumps, potholes, ripples
- Tyre: Stiff air spring k_t
- Wheel, hub & knuckle: Unsprung mass m_u
- Control arms: Guide the wheel's path
- Coil spring: Carries weight, stores energy
- Damper: Turns motion into heat
- Body: Sprung mass m_s
- Grip: Needs a steady tyre load
Go deeper
Optional. Open a panel for the engineering detail — or switch to Technical above.
The engineering explanation3
A suspension has two partly competing tasks. Ride: isolate the sprung mass m_s from road inputs, which calls for a low body natural frequency (soft springs) and enough damping to stop it oscillating. Road holding: keep the vertical tyre load F_t as steady as possible, because the tyre's available longitudinal and lateral force scales with it. The classic way to study both is the quarter-car model: m_s on the suspension spring k and damper c, above the unsprung mass m_u, which sits on the tyre spring k_t. Displacements z_s and z_u are measured from static equilibrium, and the suspension travel is s = z_u − z_s (positive in bump/compression, negative in rebound/extension).
The linkage sets the path of the wheel. In a double wishbone the two arms pivot on body-mounted bushes and carry ball joints on a rigid knuckle; together they form a four-bar linkage in front view. A shorter upper arm (short–long arm, SLA) makes the knuckle tilt inward as it rises, giving negative camber gain that offsets body roll in corners; arm lengths and angles also set the roll-centre height and keep the lateral scrub of the tyre small. The coil-over usually acts on the lower arm, so it moves less than the wheel: with motion ratio MR = Δspring/Δwheel ≈ 0.55 here, the wheel rate is k_wheel = k_spring × MR², so the spring itself must be about 3.3× stiffer than the wheel rate it provides.
In this lab the corner drives over a looping test road at 18 km/h. The quarter-car equations are integrated with a fixed-step fourth-order Runge–Kutta scheme until the response repeats every lap, and the 3D linkage is solved for the resulting travel at every frame, so the arms, ball joints, coil-over, tie rod and anti-roll-bar link always close exactly. All masses, rates and hard points are illustrative teaching values, not data for a real car.
Key concepts6
- Sprung and unsprung mass
- Sprung mass is everything carried by the springs (body, engine, occupants). Unsprung mass moves with the road: wheel, tyre, hub, knuckle and brakes, plus part of the arms and springs.
- Suspension travel
- How far the wheel has moved relative to the body. Positive is bump (compression), negative is rebound (extension).
s = z_u − z_s- Motion ratio and wheel rate
- When the spring is mounted part-way along an arm it moves less than the wheel, so the stiffness felt at the wheel is lower than the spring's own rate.
k_wheel = k_spring × MR²- Body (ride) frequency
- How fast the body bounces on its springs. Passenger cars are usually tuned to roughly 1–1.5 Hz.
f_body ≈ (1 / 2π) × √(k_ride / m_s), k_ride = k·k_t / (k + k_t)- Camber gain
- The change in the wheel's lean as it moves up and down. A shorter upper arm tilts the top of the wheel inward in bump, keeping the tyre flatter on the road when the body rolls.
- Tyre load
- The vertical force between tyre and road. The tyre behaves as a stiff spring that can push but not pull, so its load drops to zero when the wheel lifts off.
F_t = k_t × (z_r − z_u + δ_static), F_t ≥ 0
Common misconceptions3
MythSuspension is only there to make the ride comfortable.
RealityComfort is half the job. The other half is keeping each tyre pressed onto the road with a steady load, because a tyre can only steer, brake and accelerate the car while it is in firm contact.
MythThe shock absorbers absorb the bump.
RealityThe springs take the bump by storing its energy. The dampers ('shock absorbers') then control how that energy is released, turning the motion into heat so the car does not keep bouncing.
MythStiffer suspension always means more grip.
RealityOn a smooth track stiffer settings reduce body movement, but on a bumpy road an over-stiff suspension cannot let the wheel follow the surface, so tyre load swings wildly and grip falls. The lab's 'too stiff' damper shows the tyre skipping off a bump.
In the real world4
- Crossing a speed hump at about 18 km/h — the lab's single-bump road — takes the wheel through about 55 mm of travel while a well-damped body moves less than the hump is high and settles within one bounce.
- Hitting a deep pothole briefly lifts the tyre off the road. Braking or steering hard at that instant has nothing to work with, which is one reason to slow down before a pothole rather than in it.
- A heavily loaded car sits lower because each spring compresses further under the extra weight; the suspension then has less bump travel left before it reaches the bump stops.
- After a control arm, ball joint or tie-rod end is replaced, the wheel alignment (camber and toe) is checked, because those parts set the wheel's angles.
Common questions
Short answers to the questions people most often ask about this topic.
What does car suspension do?
It lets each wheel move up and down over bumps while the body stays comparatively level, and it keeps the tyres pressed firmly onto the road so they can grip for steering, braking and acceleration. It also controls the wheels' angles as they move.
What is the difference between a MacPherson strut and double wishbones?
A MacPherson strut combines the spring and damper in one tall unit that also locates the top of the wheel, with a single lower arm: compact and cheap. Double wishbones use an upper and a lower arm, which give engineers more control over the wheel's angles as the suspension moves.
What does unsprung mass mean?
It is everything not carried by the springs: wheels, tyres, brakes, hubs and part of the suspension arms. The lighter it is, the more easily the wheel can follow bumps, which helps both grip and ride comfort.
Check your understanding
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0 of 5 answered
Question 1 of 5
Which of these is part of the unsprung mass?
The story behind it
Who worked this out, and what they were up against. Optional.
1947–1950
Earle S. MacPherson
American automotive engineer
Packaged a front suspension into one cheap, compact strut.
MacPherson became chief engineer of Chevrolet's Light Car project in 1945, but GM cancelled the Cadet in 1947, largely over forecast profit margins, and he moved to Ford. Patents followed: 1947 for GM and 1949 for Ford. The first production car with MacPherson struts was the British-built 1950 Ford Consul, followed by the Zephyr. Dropping the upper control arm leaves more room in the engine bay, and fewer joints means less to wear out.
Worth knowing: The French 1949 Vedette is commonly named as the first strut car, but the source says that is a misidentification.
Sources
Retold in our own words from the sources above.
Almost there
- Overview — Read the introduction to the end.
- See it work — 7 steps left to read; try the lab (play, scrub or select a part)
- Go deeper (optional) — Optional: open a deeper panel or switch to Technical.
- Check — Answer the quiz questions.
- Story (optional) — Optional: who worked this out?
Progress is stored only in this browser — there is no account.
Earlier in this route you skipped Brake Hydraulics, Disc Brakes, Anti-lock Braking (ABS), Steering Systems.
Sources & attribution
Further reading
- How car suspension worksHow a Car Works · consulted 2026-10-21(external site, opens in a new tab)
Further reading on an external site. Linked only; AutoVerse lesson text is written independently.
- Licence
- AutoVerse original content
This lesson is original AutoVerse writing. The links above are suggestions for further reading, not the source of this text. Numbers shown in the 3D simulation are illustrative, not measured engineering data.