Even though friction brakes are very necessary, they have certain limitations as to heat capacity that is especially noticeable on hills, but is also a factor in congested fast moving traffic. A 45,000 lb tractor and semi will generate about 300 hp going down an 8% grade at about 45 mph and less than one third of this if taken up by windage, rolling, and engine friction. The brakes must get rid of the rest as heat.
This is where a supplementary brake, or retarder, comes in. An eddy current retarder will not stop or hold a vehicle but it can dissipate a lot of heat and so save the wheel brakes for the emergency, and also eliminate “engine braking” and so reduce wear and tear on engine and transmission. Even with retarders, however, it is advisable to distribute the force over the principally loaded axles.