Overrun brake

Overrun brake

An overrun brake is a brake system commonly used on small trailers, where the motion of the trailer with respect to the towing vehicle is used to actuate the brake.

A spring enables the drawbar to move back and forth slightly relative to the trailer chassis. During towing vehicle braking, the inertia of the trailer pushes on this spring, compressing it, and thus the inertia of the trailer provides the force to apply the brakes.

Issues

The performance issues with this system are considerable, and an insufficiently well designed and implemented system can be nonfunctional and undrivable. The problem with these systems happens as follows:

  1. As the towing vehicle brakes, the trailer travels forward, compressing the spring, for a distance before trailer braking begins.
  2. the speed difference & inertia built up by now results in the trailer brakes being rapidly & progressively applied ever harder, until trailer speed drops to below towing vehicle speed
  3. due to this, the brakes are now soon entirely released
  4. since the towing vehicle is still slowing down, the whole cycle repeats itself. The result is not only severe vibration, but the lack of effectiveness of the trailer brakes leaves the towing vehicle to do nearly all the braking. The front vehicle thus also fails to brake effectively, and lateral stability is also compromised, risking a jackknife situation.

The problem can be eliminated by minimising slack spring travel, and by reducing the amount of trailer braking force applied for a given drawbar force. However this results in the towing vehicle bearing some of the trailer braking load, something the towing vehicle is often not designed to do. The result is compromised braking.

Because of its issues, all first world countries limit the maximum road legal towable weight using such a braking system.


See also

External references



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