Towards more accurate brake testing

Classically, all brake designers have used vehicle and dynamometer testing to evaluate new designs of brake. Often there is no vehicle to fully test the design, and when it does arrive, the brake design is well established and changes are more expensive to implement and redesign quickly. There are often considerable differences encountered between the initial dynamometer and the actual vehicle performance. Designers are aware of this and invariably learn over years of experience which of the dynamometers they own give the most realistic results and add a margin of safety or they over-engineer the design so that it will enjoy a smoother passage through realistic trials. On the other hand vehicle testing is subject to the variance of the weather, the state of surface and not least the driver. Neither dynamometer nor vehicle methods therefore are ideal. Confusingly enough, the poorer the perceived performance of the dynamometer the better the correlation with reality in many cases. For instance, the dynamometer with poor main bearings and gearboxes, giving low run-down times is probably more realistically following the losses in a vehicle than a perfect dynamometer. This was particularly brought to the forefront of consideration when in 1993 the "Friction Free" dynamometer with real time compensations for losses was introduced. Many users complained that although the stopping distances and times had to be measured in milliseconds and millimetres to detect errors, the brakes required excessive levels of cooling flow to keep the temperature rise during braking similar to the vehicle. By incorporating precision measuring equipment and software it is possible to overcome the limitations of dynamometer testing by driving/retarding the main motor during testing to add or subtract energy from the braking application making them more accurately repeat the true vehicle test. In addition the interface between tyre and road can have a significant effect on the work done in decelerating the vehicle. This can also be simulated by the latest equipment.