Brake Lock Differentials

Split-friction surfaces (where one wheel has grip and the other side of the vehicle is on a low-friction surface) or where a wheel is suspended off the ground and spins in the air are common off-road situations. An open differential by nature directs torque to the wheel that has the least amount of grip, and in these cases, the wheel spinning on the low-friction surface or the wheel spinning in the air, which is undesirable. While some vehicles use locking differentials to help overcome this situation and provide a solid connection between two wheels on a given axle, Brake Lock Differentials (BLDs) are used on this vehicle as an effective solution.

Vehicles equipped with the Off-Road Group (sales code AWL) include a separate set of BLD calibrations that are designed for Off-Road mode use while crawling over obstacles, during heavy articulation activity, split-friction surfaces and other conditions. These BLDs provide an alternative solution to locking differentials found on other vehicles by replacing the locking differential with the brake control hardware that is already in place on the vehicle. The same hardware used for Electronic Stability Control is used for Brake Lock Differentials.

Under normal conditions with the vehicle in Off-Road mode, for example, assume that the Electronically Controlled Coupling directs 148 ft. lbs. (200 N.m) of torque to the rear differential and each wheel receives an equal share, 74 ft. lbs. (100 N.m) of torque.

When wheel speeds are unequal across a given axle, the BLDs respond. In this example, the vehicle is crawling over rocky terrain and the vehicle experiences heavy axle articulation that suspends the right rear wheel in the air and the wheel spins. The open differential ports all 148 ft. lbs. (200 N.m) of torque to the wheel in the air.

The Antilock Brake Module (ABM) senses the right wheel is spinning faster than its mate on the opposite side of the rear axle and clamps the brake caliper on the spinning wheel to stop it from spinning and form a solid connection across the axle. If 74 ft. lbs. (100 N.m) of brake force is applied to stop the spinning wheel, an equal amount of torque is ported to the wheel with grip. This allows the vehicle to continue moving forward. The amount of torque the BLDs can send to the wheel with traction is dependent on the amount of torque the brake caliper can hold. The brakes are capable of holding the amount of torque used in this example.