ABS and ESP

These are the oldest assistance systems, and precisely because of that a stubborn misconception surrounds them: ABS is not there to stop you sooner, but to let you steer while braking.

How does it work?

ABS, the anti-lock braking system, detects when a wheel is about to lock during hard braking. When it does, the system briefly releases brake pressure on that wheel many times per second. A rolling wheel keeps grip in the direction it points; a locked wheel slides straight on, however hard you steer. You feel that pulsing in the pedal. That is not a fault, that is the system working.

ESP, the electronic stability programme, compares the direction you are steering with the direction the car is actually going. If those diverge too far, ESP brakes one or more wheels individually to push the car back on track. Both systems have been mandatory on new cars for years and cannot practically be switched off.

What the CBR asks about this

• You know what these systems do and do not do. They keep you steerable; they do not make any speed safe.
• In an emergency stop you keep the brake pedal pressed firmly and steer around the obstacle. That is what ABS is for.
• You adapt your speed to road and weather. Counting on ESP instead of on your own choice of speed is negative behavioural adaptation.

This system falls under these guidelines from the Rijprocedure B:

Limitations: This is where the exam questions are

ABS does not always shorten your braking distance

On normal dry tarmac ABS usually gains a little. On snow, gravel or loose sand the braking distance can get longer, because locked wheels build a wedge of material in front of them that adds braking. So you trade braking distance for steerability.

ESP cannot beat physics

Enter a bend far too fast and there is simply no grip left to distribute. ESP delays the moment it goes wrong, but does not prevent it.

Grip comes from your tyres, not the electronics

Both systems can only work with the grip your tyres provide. Worn or under-inflated tyres leave ABS and ESP largely powerless.

The pulsing feels like a fault

Many drivers are startled by the vibrating pedal and lift off. That is exactly when the protection disappears. In an emergency stop you stay hard on the pedal.

Three practice questions

Open one for the answer and the explanation.

You are on a snow-covered road and have to make an emergency stop. What is the effect of ABS on your braking distance?

• The braking distance can get longer, but you can keep steering
• The braking distance gets shorter because the wheels do not lock
• The braking distance does not change, but the car stays steerable
• The braking distance gets shorter and you can keep steering

Correct answer: The braking distance can get longer, but you can keep steering

Explanation: On snow or gravel, locked wheels build up a braking wedge. ABS prevents that and trades braking distance for steering control.

You feel the brake pedal pulsing hard during an emergency stop. What do you do?

• Stay hard on the pedal; the pulsing is ABS working
• Release the pedal and press again
• Brake more gently until the pulsing stops
• Use the handbrake for extra braking

Correct answer: Stay hard on the pedal; the pulsing is ABS working

Explanation: The pulsing is not a fault but the system regulating brake pressure. Lifting off switches the protection away.

You enter a bend far too fast. What does ESP do?

• It tries to correct, but cannot always prevent a crash
• It keeps the car on the road no matter what
• It brakes the car to a complete stop
• It switches itself off above a certain speed

Correct answer: It tries to correct, but cannot always prevent a crash

Explanation: ESP distributes the available grip more cleverly, but cannot create grip that is not there. Your choice of speed stays decisive.

Written on the basis of the official CBR Rijprocedure B. These are not real exam questions.