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Q Given that load-proportioning valves
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are fitted to an articulated outfit, can any benefit be derived from separate application of the trailer brakes before the tractor brakes are applied?
A In the interests of stability, it is
desirable that the load on the tractor axle should not be reduced during the braking operation as this can cause jackknifing.
When the tractor brakes are applied, load transfer reduces the weight imposed on the rear axle, whereas application of the trailer brakes increases the load on this axle. If the tractor brakes are applied before the trailer brakes, the load on the tractor rear axle is reduced during the delay period and the danger of jack-knifing is increased.
This unstable transient condition is eliminated if the proportioning valves are sufficiently sensitive to dynamic load changes, but some doubt exists as to whether this advantage is provided by a typical system and separate application of the trailer-brakes can be recommended as a play-safe measure.