Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairHard
An ABS control module uses data from the wheel speed sensors to determine if a wheel is locking up. What is the approximate percentage of wheel slip that the ABS system typically tries to maintain during hard braking to achieve optimal stopping power?
- A10-20%.
- B50-60%.
- C90-100% (full lock-up).
- D0% (no slip at all).
Show answer & explanationAnswer & explanation
Correct answer: A. 10-20%.
Optimal braking force is achieved when there is a small amount of wheel slip, typically between 10-20%. This allows the tires to generate maximum grip (adhesion) with the road surface. ABS systems are designed to modulate brake pressure rapidly to keep the wheels in this optimal slip range, preventing full lock-up (which would be 100% slip) and maintaining steerability.
Why the other options are wrong
- B. 50-60% slip would indicate significant wheel spin or lock-up, leading to reduced braking efficiency and control.
- C. 90-100% slip means the wheel is almost or completely locked up, resulting in skidding and loss of steering control, which ABS aims to prevent.
- D. 0% slip means the wheel is rolling freely without any braking force, which would not stop the vehicle effectively.
Optimal ABS Wheel Slip
The ideal percentage of wheel slip that an ABS system aims to maintain during braking for maximum stopping power and control.
- Typically ranges from 10% to 20% slip.
- Achieves the highest coefficient of friction between tire and road.
- Prevents full wheel lock-up, maintaining steerability during hard braking.
Memory trick: For the best stop, a little slip is the top crop (10-20 percent, not a flop)!