PR Log (Press Release) – Feb 15, 2010 – There are a number of factors which need to be taken into account when brake testing if the conclusions which are reached are to be accurate. The more important of these are commented on:

1. Reaction Time
When brake efficiency is determined by measuring braking force or deceleration, reaction time is not involved. when either stopping time or distance is measured, depending on the method used, reaction time may influence the measurement.

A typical minimum reaction time with an alert driver can be as low as 0.5 sec. If this were included with the actual stopping time, it would influence considerable the estimate of brake efficiency being made. It is important to include reaction time when, for road safety purposes, estimates are being made of stopping distances as in the Highway Code but it must not be allowed to influence tests of the brakes themselves.

2. Braking on Gradients
Although it is more usual to conduct brake tests which are carried out on the road on a level surface, equally accurate results can be obtained on a constant incline, the means of making allowance being very simple. The severity of a gradient can be expressed as a decimal by calculating the sine of the angle of the slope which will be a number between 0 and 1.

The significance of this result is that it gives the force acting to push the vehicle down the slope as a proportion of the gross weight. For example if a vehicle is standing facing down a 1 in 8 slope, the gradient may be described as 1/8, 0.125 or as 12.5% and the force acting down the slope is 1/8 of the vehicles gross weight.

If then the braking efficiency is determined by measuring either deceleration, stopping time or stopping distance, the result will be 0.125 too low and can be corrected to level road conditions by adding 0.125 or 12.5%.Similarly, a rising gradient helps a vehicle stop and the result obtained must be corrected by deducting from it the measure of the gradient.

3. Weight Transfer
Weight transfer during braking varies the axle loading and so affects the adhesion available. It also affects the reading of decelerometers of all types very slightly if the suspension is such that it allows the body of the vehicle to tip forward significantly when transfer takes place. For most vehicles this error may be ignored.

4. Wheel Locking
If one of more wheels lock, the overall efficiency recorded will be less than that which would have been indicated if locking had just been avoided. Since, as has already been noted, brake tests should only be made under suitable conditions, this state of affairs should only arise at high decelerations and brakes should be released immediately to avoid unnecessary tyre wear.

5. The Effect of Speed
Any effect is very small and the results achieved may be assumed to be independent of the test speed used over the range 0-40 mile/hr (0-64 km/h).

6. Brake Fade
True fade is a loss of brake output due to overheating of the brake linings. Modern drum brake linings are little affected by heat until operation temperatures exceed 350-440 C while disc brake linings are more heat resistant.

To exceed these temperatures a vehicle must be driven very hard and even then the onset of fade is very slow. Brake linings also lose their friction if they become soaked in either hydraulic fluid of lubricating oil, or if linings get wet. Recovery from immersion in water is usually fairly rapid but if linings have become oily they must be replaced and the discs/drums cleaned.
For further details refer to www.tapley.org.uk