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ABS und Bremsschlupf

About this app

μ-Schlupf-Kurve, ABS-Regelzyklus mit Einlass- und Auslassventil und Bremsweg mit und ohne ABS auf fünf Fahrbahnen, inklusive Ausweichen um ein Hindernis.

With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.

Brake slip and friction

A tyre only transmits braking force when it turns slightly slower than the vehicle travels. This difference is called brake slip: λ = (v_veh − v_wheel) / v_veh. A freely rolling wheel has 0% slip, a locked one 100%. The longitudinal friction coefficient rises steeply with slip and peaks at small slip, typically around 10 to 20% on dry asphalt. Beyond that it falls towards the locked wheel. Lateral guidance drops sharply as slip increases: a locked wheel transmits no lateral force, and the vehicle can no longer be steered.

What the app shows

Three cards show the same emergency stop. The first shows the μ-slip curves for dry, wet, snow, ice and gravel with the friction peak, the sliding coefficient and the lateral guidance curve at a 5° slip angle. The second shows vehicle and wheel speed over time, and below it the brake pressure with the control phases build-up, hold and release. The third is a top view with an obstacle, skid marks and the braking distances with and without ABS. A diagram shows inlet valve, outlet valve, accumulator and return pump in each phase. The model is a quarter car with example parameters.

Things to try

Switch ABS off and start the emergency stop: the wheel locks within a fraction of a second, the operating point jumps to 100%, and the car slides straight into the obstacle although the driver steers. With ABS the slip oscillates around the peak and the car drives past the obstacle. Freeze the stop with the time slider and follow the valve positions. Then switch to gravel: there the braking distance with ABS is longer, because the locked wheel pushes a wedge of material ahead of it. Still, only with ABS does the car stay steerable.

Frequently asked questions

Why does a locked wheel brake less?
Friction between tyre and road is highest at small slip. When the wheel locks, the tyre slides, and the sliding coefficient is lower than the peak coefficient. In addition, a locked wheel transmits no lateral force, so the vehicle no longer follows the steering.
Does ABS always shorten the braking distance?
No. On a firm road the braking distance with ABS is usually shorter. On loose ground such as gravel or deep snow, a locked wheel pushes a wedge of material ahead of it that adds braking force, so the braking distance with ABS can be longer there. The main benefit of ABS is that the vehicle stays steerable and stable while braking.
What do the inlet and outlet valves in the ABS hydraulic unit do?
The inlet valve is open when de-energised and lets pressure pass from the master cylinder to the wheel brake. To hold pressure it closes. To release pressure, the outlet valve, which is closed when de-energised, opens as well; brake fluid flows into the accumulator, and the return pump sends it back to the master cylinder. The driver feels this as pedal pulsation.
How accurate is the app?
It uses a quarter-car model with 400 kg per wheel, friction curves following the Burckhardt approach with example parameters and a simplified control around the friction peak. Load transfer and air drag are neglected, and lateral motion is simplified. The figures are model values, not measurements of a real vehicle.

Subject: Automotive engineering

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