Safe Downgrade Descent Speed and Brake Load

The speed at which an engine brake alone holds a loaded truck on a grade, and how much heat the service brakes are absorbing above it. Going downhill, gravity does work on the truck at a rate proportional to weight times speed times grade, and that power has to be dissipated continuously or the truck accelerates. An engine brake absorbs a fixed amount of it -- a few hundred horsepower on a modern truck, much less on an older one or at low engine speed -- and whatever is left goes into the service brakes as heat. Service brakes have no steady-state capacity at all: they absorb heat, they do not reject it, so any sustained shortfall ends in fade. The balance speed is the useful output, because below it the truck is under control with the service brakes held in reserve for emergencies, and above it the service brakes are carrying the difference continuously. An 80,000 lb combination on a 6% grade with a 400 hp engine brake needs 320 hp at 25 mph, which the retarder covers with 80 hp to spare, and balances at 31.2 mph; at 45 mph the demand is 576 hp and 176 hp is going into the service brakes with nowhere to put it, and on a six-mile grade they will be gone before the bottom. Note how sharply the balance speed falls with weight: the same truck at 105,000 lb on a permit load balances at only 23.8 mph. This is the arithmetic behind the old rule about descending in a lower gear than you climbed in, and it makes the rule quantitative. A screen; the posted grade speed, the truck's own brake condition, and the driver govern.

descent_power_hp = gcw_lb x speed_fps x (grade_pct/100) / 550; balance_speed_fps = engine_brake_hp x 550 / (gcw_lb x grade); service_brake_hp = max(0, descent_power_hp - engine_brake_hp).

Descent power from the rate of gravitational work, P = W x v x grade / 550 in pounds, feet per second and horsepower, and the balance speed at which a retarder's rating exactly matches it, by name; public mechanics. A screen; the posted grade speed, the truck's own brake condition, and the driver govern.

The power relation is public mechanics. The weight, grade, speed, and retarder rating are the driver's own values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: gcw_lb, grade_pct, descent_speed_mph, engine_brake_hp, descent_power_hp, balance_speed_mph, service_brake_hp

• Power relation P = W x v x grade / 550 with W in lb and v in ft/spublic mechanics
• Service brakes absorb heat, do not reject it; there is no steady-state service-brake capacity, so a sustained shortfall ends in fadevehicle braking practice
• Retarder rating entered at the engine speed actually being held; a retarder's output falls sharply at low engine speedengine brake manufacturer data