Brake force is the force the brake pads and calliper exert onto the brake disk and therefore onto the wheel.
The maximum brake force any wheel can exert onto the vehicle is depending on the force (weight) with which that wheel is 'pushed' onto the road or roller surface and the grip between the surface and the tyre.
The simplest way to get your head around this is:
Think of an empty cardboard box on a concrete surface. It is relatively easy to push this box around. The force (like brake force) needed to make the box 'loose grip' on the gripping surface is relatively light.
Now think of the same cardboard box but now full with for example steel parts. It takes a lot more force (brake force) to make the box slide (loose grip).
Now think of the same box with the same weight on a layer of smooth ice (low friction coefficient). It takes less effort to make the box loose grip than on a surface with plenty of grip (high friction coefficient).
Full understandings of the weight and friction coefficient are important when gauging the results shown on any brake tester.
The friction coefficient (f) on the VTEQ rollers is approx. 0.85
Roughly the following calculation can be made (in reality various small compensation factors are in the calculation):
The vehicle's axle weight (G) transferred into Kilo Newton (KN) times the friction coefficient (f) equals the brake force (F) in KN
G x f = F
- an axle weight of 1100 Kg equals 550 kg per wheel.
- Divide 550 Kg's by +/- 100 (gravity), equals 5.5 KN
- 5.5KN wheel downward force times the friction coefficient (f) of 0.85 equals a brake force of 4.68 KN per wheel.
It should be perfectly clear that if the roller surface is worn smooth the friction coefficient lowers, think about the cardboard box on ice.
The brakes can not be tested on worn rollers as the maximum brake force is not achieved anymore, the wheels slip on the roller surface with too little effort.
The VTEQ rollers are rated to maintain their 'f' for 50,000 tests which is far higher than other types of surfaces.
Standard VTEQ traction surface
The VTEQ tester can also optionally be equipped with 'Nordic' specification rollers which are un rated as there are no worn rollers yet.
The Nordic rollers are high carbon steel to withstand steel spike tyres.