Brakes act to prevent the wheels from turning, generating braking forces to slow the vehicle (red arrows in illustration).
However, if the trailer is heavier, the brakes have to work harder as they have to generate more braking force to slow it down at the same rate as before.
Forces which act on a trailer during braking
How is braking efficiency calculated?
Braking Efficiency (%) = Vehicle Braking Force over Vehicle Weight x 100
For example, see illustration:
Trailer Braking Efficiency = 10 tonnes (red arrows), 20 tonnes (blue arrows) x 100 = 50%
Important: Notice how the trailer braking efficiency calculation only uses the weight carried by the vehicle’s wheels? (e.g. 20 tonnes in illustration). Many trailers also transfer a vertical load onto the tractor via the drawbar. This increases the tractor’s weight and adds to its own braking load. When towing a trailer, the tractor’s brakes have to work harder than when travelling solo, even if the trailer’s brakes are working perfectly.
