LOOK BEHIND YOU - Are your brakes correctly equipped to cope with the demands of 40/50kph tractors and fully laden trailers?
There is currently an industry-wide problem of "under-braked" trailed equipment being used. This leads to premature tractor brake wear or even failure. This page will help to highlight the relevant legislation and guide you through some of the available specs of trailed equipment.
The Effect of Speed and Vehicle Weight
When travelling, a tractor and trailer combination builds up kinetic energy. To slow down or stop, the kinetic energy must be converted to heat through friction in the tractor and trailed equipment braking systems. Both systems are designed to withstand and dissipate the heat generated at their rated operating weights and top road speeds.
Travelling faster or operating at greater weights increases the kinetic energy levels the braking systems have to cope with, but there is a sting in the tail in the form of forward speed.
Forward Speed
If, for example, the total (gross) weight of a tractor/trailer combination were increased by 50%, at any given speed its kinetic energy and the resulting braking system load would also increase by 50%. But if speed were increased by 50% (e.g. from 20 to 30mph), kinetic energy would increase by almost 150% (see graph above).
What may be thought to be an everyday speed increase actually places nearly 2 1/2 times more load on the tractor and trailer's braking systems. This is fine if the braking systems are designed to take these speed induced loads, but those fitted to many agricultural trailers are not. In these instances, braking while "over speeding" will subject the tractor and trailer braking system to heat cycles outside its design specification and cause accelerated wear. If not adjusted frequently, the responsibility for braking is progressively transferred to the tractor. This then causes rapid wear of the tractor's system and eventual long term damage to braking components.
Effect of gradient upon vehicle braking performance
A vehicle travelling down a gradient requires more braking effort to stop than a vehicle travelling at the same speed on level ground. This is because on a gradient, gravity will try to accelerate the vehicle downwards; the brakes have to overcome this effect before they can begin to stop the vehicle.
The best way to consider the effect of gradients is to think in terms of percentage gradient. A 1 in 10 gradient is a 10% slope. In this way the fraction of a vehicle's weight acting down the gradient is equal to the percentage slope of the gradient. So, to ensure that a vehicle can travel safely down a gradient, the total braking efficiency must be equal to, or greater than the gradient expressed as a percentage, plus the amount of braking efficiency necessary to stop the vehicle.
So what is braking efficiency?
Braking efficiency is the overall measure of vehicle braking performance (the terms braking rate or brake ratio are also sometimes used).
So the twin axles of a 14 tonne capacity trailer, each carrying 8 tonnes, would each need to produce 2 tonnes braking force to acheive 25% braking efficiency. This would increase to 4 tonnes braking force to meet the approx 50% braking efficiency requirement for above 20mph operation.
Getting Trailed Equipment braking systems to "stop their weight"
It is imperative that any trailed equipment has a braking system of the correct specification for the top speed travelled. It should also be maintained to ensure that it contributes appropriately to the overall braking effort of the combination, to prevent tractor brake overload.
Air Brakes
Currently, pneumatic brakes give quicker brake actuator reaction times, making them more sensitive and easier to control than conventional, simple hydraulic brakes. They are also cleaner and cheaper to operate.
Air brakes are always used on commercial lorry trailers and may be the way forward for agricultural equipment. However, upgraded hydraulic brakes can deliver comparable performance.
In Summary
Free-travel adjustment and shoe lining inspection. This will keep the brakes working at maximum efficiency and reduce response time to a minimum.
Increase brake actuator ram size to give greater braking force
Fit screw type brake lever arms; this encourages and assists regular brake adjustments
Increase hydraulic brake hose diameter to improve system response characteristics
Load sensing - essential when increasing braking performance for higher-speed operation, if wheel locking and excessive tyre wear are to be avoided
Air brakes give quicker reaction times with greater control than current hydraulic systems, but hydraulic brakes can be comparable if specified correctly.
To see an overview of the current UK agricultural vehicle braking legislation Click Here
If you would like more information on trailer braking and correct specifications for braking systems please call us on 01588 620545 or email
and ask for a copy of 'Look Behind You' 2nd edition.
  
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