The idea
Beginner explanation — switch to Technical for the engineering detail.
A tyre grips best while it is still turning, just a little slower than the road is passing underneath it. If you brake so hard that the wheel stops turning altogether, the tyre slides. A sliding tyre grips less than a rolling one, and — just as important — it cannot steer the car.
Anti-lock brakes (ABS) stop that from happening. A sensor at every wheel tells a small computer how fast that wheel is turning. If one wheel suddenly slows much faster than the car could, the computer knows it is about to lock. It briefly lets a little pressure out of that wheel’s brake so the wheel can speed up again, then puts the pressure back. It repeats this many times a second.
The result is a wheel that keeps turning while braking hard, close to the tyre’s best grip. On a wet or slippery road the car usually stops shorter than with locked wheels, and you can still steer around an obstacle.
In a car with ABS, an emergency stop is simple: press the pedal hard and keep pressing. A buzzing, pulsing pedal is ABS working — don’t lift off.
What you will learn
- Explain what wheel slip is and why a locked wheel brakes worse and cannot steer
- Describe how wheel-speed sensors let the ABS computer spot a wheel starting to lock
- Explain the hold, release and reapply valve states of an ABS modulator
- Compare a locked-wheel stop with an ABS stop on a low-grip road
- Know how to brake in an emergency in a car with ABS
Try it yourself
Press the pedal, watch hydraulic pressure clamp the pads, and see kinetic energy become heat — with a conceptual ABS demo. Open the full brakes lab
Step by step
Use “Show in 3D” to jump the animation to a step; the step currently playing is highlighted.
Step 1: The grip limit
On this wet road the driver presses harder than the tyre can grip. As the pressure rises, the brake asks for more than the road can give.
Step 2: Without ABS the wheel locks
With nothing to stop it, the wheel stops turning while the car keeps going. The tyre slides on the road — watch the sidewall mark freeze while the road keeps moving.
Step 3: A longer slide — and the tyre takes the heat
The locked car slides to a stop, further than it needed to. Because the disc wasn’t turning, most of the heat went into the sliding tyre, not the brake.
Step 4: Sensing that a lock is coming
With ABS on, the wheel-speed sensor counts the teeth of a ring turning with the hub. When the wheel slows much faster than the car could, the ABS computer knows trouble is starting.
Step 5: Keeping slip in the band
ABS now eases the brake off and on, again and again. The wheel never stops turning, so the tyre keeps gripping and the car can still be steered.
Step 6: Release: let pressure out
The ABS unit closes the way in and opens a way out, so some fluid leaves the caliper. The clamp eases and the wheel speeds back up.
Step 7: Hold and reapply
Once the wheel is turning properly again, ABS lets the pressure back in and squeezes harder, until the wheel starts to slow too quickly again. Then the cycle repeats.
Step 8: A shorter, steerable stop
With ABS the car stopped in a shorter distance and could have been steered all the way. The brake disc, not the tyre, took most of the heat.
Key components
Select a part to highlight it in the 3D scene.
How it flows
Read the diagram as text
- Wheel-speed sensor → ABS computer (speed signal)
- ABS computer → Modulator valves (commands)
- Modulator valves → Caliper pressure (pressure)
- Caliper pressure → Wheel speed (brake torque)
- Wheel speed → Wheel-speed sensor (feedback)
- Wheel speed → Tyre grip
- Wheel-speed sensor: Tone-ring pulses
- ABS computer: Slip & wheel decel.
- Modulator valves: Hold · release · reapply
- Caliper pressure: Rises and falls
- Wheel speed: Slip ≈ 10–20 %
- Tyre grip: Brake + steer
Go deeper
Optional. Open a panel for the engineering detail — or switch to Technical above.
The engineering explanation3
Longitudinal slip is s = (v − ωr) / v, where v is vehicle speed and ωr the tyre’s surface speed. Braking force rises with slip to a peak — roughly 10–20 % on many surfaces, the band this lab uses — and then falls towards the sliding value at s = 100 %. Lateral (cornering) force collapses as slip approaches lock, which is why a locked wheel cannot steer.
The ABS controller compares each wheel’s speed (from a tone ring and sensor) with an estimate of vehicle speed built from all the wheels, and also watches wheel deceleration. When slip or deceleration crosses a threshold it commands that wheel’s valves in the hydraulic modulator: HOLD (inlet valve closes, pressure trapped), RELEASE (outlet valve opens to a low-pressure accumulator, pressure falls, the wheel spins back up), then REAPPLY (outlet closes, inlet opens — often in pulses — to rebuild pressure). A return pump sends released fluid back to the master-cylinder side, which the driver feels as pedal pulsation.
The lab shows this as a CONCEPTUAL, stylised cycle: in its illustrative wet-road case the demand of 1.2 g exceeds the 0.7 g grip. Without ABS the wheel locks and the quarter-car slides on μ_slide = 0.5, stopping from 80 km/h in about 51 m; with the stylised cycle averaging about 92 % of peak grip it stops in about 41 m. Real controllers are far more sophisticated, adapting thresholds to the surface, load, cornering and to stability-control demands; on loose gravel or deep snow a locked wheel can sometimes stop shorter by building a wedge of material, which modern systems allow for.
Key concepts5
- Wheel slip
- How much slower the tyre’s surface is moving than the road beneath it, as a fraction of vehicle speed. 0 % is free rolling; 100 % is a locked, sliding wheel.
s = (v − ωr) / v- Peak versus sliding grip
- A braking tyre grips best at a small amount of slip and less once it slides, so a locked wheel decelerates the car less than one held near the peak.
a_lock = μ_slide g < μ_peak g- Valve states
- Each wheel circuit in the modulator has an inlet and an outlet valve. Hold traps the pressure, release lets some out, reapply lets it back in.
hold → release → hold → reapply- Steerability
- A tyre can only produce cornering force while it is rolling. Keeping the wheels turning is what lets an ABS-equipped car steer during an emergency stop.
- Stopping distance at the grip limit
- Once braking is limited by grip, the stopping distance depends on the friction the tyres achieve, so holding the tyres near peak grip stops the car shorter than sliding.
d ≈ v₀² / (2 μ g)
Common misconceptions4
MythABS always makes the car stop in a shorter distance.
RealityIts main job is to keep the car steerable. On most paved surfaces it also shortens the stop, but on loose gravel or deep snow a locked wheel can sometimes stop shorter by ploughing up material — and ABS cannot add grip that isn’t there, so on ice the stop is still long.
MythWhen the pedal starts pulsing and buzzing, something has gone wrong — ease off.
RealityPulsing and noise mean ABS is working. Keep pressing hard and steer where you want to go; lifting off makes the stop longer.
MythYou should pump the brake pedal in an emergency, even with ABS.
RealityPumping was a technique for cars without ABS. With ABS, the system modulates far faster and per wheel; your job is to press firmly and keep pressing.
MythWith ABS you can brake as late as you like.
RealityABS cannot create grip. Stopping distance is still set by speed and the road surface — it grows with the square of speed and is several times longer on ice.
In the real world5
- In an emergency stop in a car with ABS: press the brake hard, keep it pressed through the pulsing, and steer around the hazard if you need to.
- An amber ABS warning light means the anti-lock function is off; normal braking usually still works, but have it checked soon and leave more distance meanwhile.
- A faulty wheel-speed sensor (often from dirt, damage or a broken wire) is a common cause of an ABS warning light.
- On a car without ABS, a skidding stop leaves dark rubber marks on the road; with ABS the marks, if any, are faint and broken.
- Mismatched or badly worn tyres reduce the grip ABS has to work with — the system can only hold the tyre near the peak it actually has.
Common questions
Short answers to the questions people most often ask about this topic.
How does ABS work?
Sensors measure every wheel's speed. If a wheel starts slowing much faster than the car, a sign it is about to lock, the ABS unit briefly releases, holds and reapplies that wheel's brake pressure, many times a second, keeping the tyre just short of locking.
Why does the brake pedal judder when ABS works?
The ABS valves are rapidly changing the brake pressure, and those pulses are felt through the pedal, often with a buzzing noise. It is normal: keep pressing firmly and steer where you want to go.
Does ABS make a car stop shorter?
On most surfaces it stops at least as short, but its main benefit is that you can still steer while braking hard. On loose gravel or deep snow a locked wheel can build up a wedge of material and sometimes stop a little shorter.
Check your understanding
5 questions. Your best score is saved in this browser.
0 of 5 answered
Question 1 of 5
The car is doing 20 m/s and the tyre’s surface speed (ωr) is 17 m/s. What is the wheel slip?
The story behind it
Who worked this out, and what they were up against. Optional.
1920
Gabriel Voisin
Experimenter with aircraft braking
Tried to stop wheels locking up, on aircraft, in 1920.
In 1920 Gabriel Voisin experimented with modulating hydraulic brake pressure on aircraft brakes, using a flywheel and a valve to reduce tyre slippage; the same idea underlies anti-lock braking today. Karl Wässel patented a system in 1928 but never produced a working product, Dunlop's Maxaret was in widespread UK aviation use by the early 1950s, and Bosch's electronic four-wheel ABS became an option on the Mercedes-Benz W116 from 1978.
Worth knowing: Purely mechanical systems on cars (the Jensen FF, Ferguson P99 and an experimental Ford Zodiac) saw limited use in the 1960s and proved expensive and unreliable.
Sources
Retold in our own words from the sources above.
Almost there
- Overview — Read the introduction to the end.
- See it work — 8 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.
Sources & attribution
Further reading
- How ABS 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.
- BrakingHow 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.