Market pressures around the world are pushing for more fuel efficient vehicles which produce lower vehicle emissions. The result is an engineering desire to improve vehicle efficiency and to reduce any sources of residual drag in the driveline. The objective of this paper is to show some quantification of the effects brake running clearance in commercial vehicle disk brakes and a positive pad retraction might have on the residual drag experienced at the wheel.

This study is based on dynamometer testing to quantify the residual drag and how it is affected by brake running clearance. Multiple tests are conducted at different running clearance values with different rotor temperatures to quantify the benefits introduced by increasing the running clearance of the brake. A repeat investigation is conducted looking at the effect of positive pad retraction on residual drag torques.

The investigations described above are currently limited to quantifying effects under controlled laboratory conditions on a brake dynamometer without the consideration of road excitation, steering effects and lateral forces and their effects on the running clearance and residual drag.

This paper shows that unlike on passenger cars, where the pistons have a knock back capability, thermal effects (pad swell, rotor expansion and rotor coning) on commercial vehicles can have a much greater influence on residual drag. Ultimately this requests different design considerations in the attempt to reduce residual drag.

Under controlled conditions on a dynamometer, increasing the running clearance gives a measurable reduction in residual drag torque. This is valid up to a limit figure which is defined primarily by the amount of coning of the rotor. Rotor expansion and pad swell are secondary contributors. Advanced designs should pick up on increased running clearances and less coning rotor designs. A positive pad retraction on top would be beneficial.

Refaat El-Malki, Peter Gibbens - Meritor (HVBS)