Proportional Braking for Large Trucks in Mountain Mine Service-Some Aspects of Design

Large rear end dump trucks are used for downhaul operations in mountain mines. On these trucks mechanical service brakes are provided on both front and rear wheel sets. They may be either disc or drum brakes actuated by a hydraulic/pneumatic system. On some trucks provision is made for the brake effort on the front wheel sets to be reduced at the discretion of the driver. This proportional braking is intended to minimise the possibility of front wheel skidding on icy roads with consequent loss of steering. However, this voluntary reduction in front wheel braking seriously impairs the braking capacity of the truck.

A study was made of the braking requirements of heavy vehicles on inclined surfaces. It was found for typical loaded truck geometries in present use that the rear wheels will always slip first. Therefore, with present front and rear brake proportions any reduction in front wheel braking appears unjustified if the objective is to eliminate the possibility of front wheel slip before the rear wheel starts to slip.

Design charts are included for critical slip conditions for road grades up to 20 percent for different road/tire interface coefficients of friction for a range of vehicle deceleration rates.