Tractor-trailer incompatibility could be a reason for poor braking

When it comes to apportioning blame for the poor braking ability of many tractor and trailer set-ups on British roads, it’s generally the trailer maker or the farmer that gets the finger pointed at them.

But in some cases it could be the design of the tractor’s brakes that is causing some of the problems, says the head of one of Britain’s most respected trailer makers.

James Stewart is a director of Aberdeenshire trailer manufacturer Stewart Agricultural, which makes a range of trailers reckoned to be some of the safest on the market.

“It’s true that there’s a lot of trailed equipment out there with poor brakes,” he says. “Equally, a lot of trailers get very poor maintenance or often none at all.”

But some aspects of the fundamental design of tractor brakes are flawed, he points out. “Some of the brakes fitted in tractors, though fine for 40kph, don’t appear to be adequate for the 50kph speeds now commonly used, as it is possible to burn out a set of tractor brakes in four hours,” he says.

“Yet we have a Volvo truck that has done 800,000km and is still running on its original discs. That’s normal in the truck industry.”

It’s instructive to compare farm tractors and trailers with trucks, he adds. “Truck brakes allow the pressure to be applied progressively to the trailer while tractor brakes often apply 2bar of pressure the moment you touch the pedal. Only 0.1bar is required to activate the trailer brakes so the result is a severe and dangerous overbraking of the trailer.”

A serious problem is the lack of compatibility between tractor and trailer, he says. “On a properly set up system the tractor brakes should stop the tractor and the trailer brakes stop the trailer at the same time.

“The trailer brakes will only work to their best ability when the tractor systems improve. We will not see the end of brake problems until the compatibility issue is addressed by the tractor manufacturers.”

One of the fundamental problems is that tractor drivers often drive their machines like a car, coming up fast to junctions and roundabouts and then standing on the brakes. “The brakes on the tractor need to be able to cope with that, but the fully-enclosed, oil-immersed systems used generate a lot of heat that has nowhere to go, resulting in premature brake failure.”

But tractor makers insist their brakes are up to scratch. “An agricultural tractor and trailer has similar characteristics to a commercial HGV tractor unit and trailer,” says New Holland’s Richard Hollins. “Try stopping an HGV rig without any trailer brakes and you’ll wear out the tractor unit brakes in a very short time, it’s no different from an agricultural tractor.”

“50kph tractors tend to have a higher braking capacity with the addition of front axle brakes – we have well over 60% braking efficiency. We’ve also added extra brake cooling and also now have the option of a commercial style exhaust brake.

“What we have not seen until recently is any additional braking capacity on the trailers to cope with the higher speeds. Decelerating a load down from 50kph requires twice as much energy to be absorbed by the brakes as slowing down from 40 kph. It’s a massive difference.”

Driving style does require some attention, he agrees, as does maintenance. “HGV trailers undergo a recorded maintenance schedule. It doesn’t just happen once a year before harvest, or not at all.”