Cooling Rates of Brake Drums and Discs

Abstract

An investigation has been made of the rates of cooling of vehicle brake drums and discs. Thermocouples were inserted in the drums and discs and in the wheel hubs and their outputs fed via slip ring units to meters mounted inside the vehicle. The drums or discs were heated to a uniform temperature of 300°-400°C by drag braking and the rate at which they cooled measured while the vehicle was driven at constant speed. Measurements were made at various speeds in the range 0 to 90 mile/h.

From the log (temperature) against time plot a cooling coefficient b_v was determined. It is shown that at a vehicle speed v the quantity b_v can be expressed in the form b_v = b₀ +Kv^0·8 where b₀ represents the loss of heat to the hub and K is a constant depending on the size and shape of the drum or disc. Values of these constants have been determined on a variety of cars having discs and drums varying from 7 in to 11 in diameter and on a lorry fitted with 16·75 in diameter drums. Cooling rates are shown to depend on shape and size of the disc or drum. Results show that the cooling rates of front brakes are about 20 per cent higher than the rear brakes and that front discs cool about 25 per cent more quickly than the corresponding drum size recommended for the same vehicle. The cooling rate of front discs did not change when wire wheels were fitted instead of solid wheels. Ventilated discs and solid discs were also compared. The effect of fitting dust shields on disc brakes is shown to reduce the cooling rates by about 30 per cent. The effect of otherwise disturbing the air flow was studied.