Be road safe: Improving the efficiency of trailer brakes Q&A


It’s a fair bet that most tractor owners wouldn’t have a clue about the braking efficiency of their trailers or other trailed equipment.

But as road speeds increase, and greater loads are towed, many trailers are disasters waiting to happen.

An initiative was introduced in 2010 by industry, manufacturers and the HSE aiming to offer certificated brake tests through local dealers. It gives owners the chance to not only establish brake efficiency but also to get advice on how to upgrade their existing machines to meet the standards required.

There’s an industry-wide problem of under-braked trailed equipment, says the HSE. According to recent joint research carried out by the HSE, the Department for Transport and the industry, most farm trailers tested failed to meet basic safety requirements and fell far short of the required efficiency levels.

Of the trailers tested, both with hydraulic and air systems, many took longer to stop than they should and in some cases suffered complete loss of control.

Quick Links

What is braking efficiency?
What does a typical braking system consist of?
But the tractor stops it – so why do I need to worry?
What about trailed equipment?
But I’ve got air brakes…
What do I need if I want to travel faster than 20mph?
I’ve got 10-stud wheels, does that mean I’ve got a high-speed axle?
What difference does speed make to braking?
Who polices trailer braking?
Why carry out a BAGMA brake performance test?
What does the test involve?
What are the main reasons for poor brake performance?
What can be done to bring trailers up to spec?
How can I make adjustment easier?
Where can I get a test carried out?
What’s the cost of a test?


What is braking efficiency?

It’s the overall measure of vehicle braking performance (also referred to as braking rate or brake ratio).

On the road, the minimum legal requirement for trailer brakes is 25% braking efficiency at a maximum 20mph (32kph).

Any trailer intended for use faster than 20mph must have a failsafe (ie dual-line hydraulic or air) with at least 45% braking efficiency.

Braking efficiency equals the braking force exerted by the wheels divided by the vehicle weight multiplied by 100.

For a 14t capacity trailer with twin axles each carrying 8t, the braking system would need to produce 2t braking force to achieve 25% braking efficiency.

For speeds of more than 20mph, this would need to be increased to 4t to meet the 45% efficiency requirement.

What does a typical braking system consist of?

Trailer braking performance depends on brake size and the force at which they are applied. The brake drum and shoes form the foundation of the braking system, and are usually defined by drum diameter and shoe width.

High-speed trailers require larger foundation (main) brakes. A bigger surface area will result in better braking, but if you haven’t got the clamping force necessary (ie the right actuating ram) this can reduce overall braking performance, says BAGMA’s technical and training manager, Adam Wyatt.

“Having insufficient rams can also result in brakes glazing up without actually working, as before enough pressure is put on the drum the tractor brakes have already stopped both tractor and trailer.”

But the tractor stops it – so why do I need to worry?

As weight and speed increases, braking tends to be progressively transferred to the tractor, which means rapid wear of tractor brakes and in some cases, long-term damage.

Manufacturers are waking up to this. According to the HSE, the UK and Ireland have the highest number of warranty claims for brakes in the world and many manufacturers won’t cover brakes under warranty.

Brakes should last in the region of 5000-6000 hours, however some dealers are reporting tractors of one year old coming in with worn brakes.

At best, this just involves the cost of replacing brake pads. At worst, if brake disc residue has contaminated the transmission oil it will mean a back-end rebuild, costing as much as £4000.

What about trailed equipment?

Legally, any trailed equipment like balers and cultivators under 14t doesn’t need to be braked. However, on the Continent, most trailed kit comes as standard with brakes and in Germany it’s obligatory. It’s often the case that equipment is de-specced when sold to the UK market.

But I’ve got air brakes…

Many drivers assume that, just because their tractor is fitted with air brakes, they are able to travel at 40kph safely. This is not the case. If not correctly maintained, air brakes can perform worse than their hydraulic equivalent.

Trailers fitted with air brakes are not immune to failing, either. It’s important to check all air pressures are correct, warns Mr Wyatt.

“We’ve found a few tractors where the air pressure has been as low as 5bar coming out of the back of the trailer, meaning there’s only low pressure to activate the brakes.”

What do I need if I want to travel faster than 20mph?

To be legal on the road, a high-speed trailer has to have air brakes, suspended axles, a dual-line braking system and ABS. The handbrake must hold the laden trailer on a 16% slope too.

A high-speed trailer also has to have high-speed tyres – using incorrectly rated tyres automatically de-rates a trailer to 20mph.

I’ve got 10-stud wheels, does that mean I’ve got a high-speed axle?

Not necessarily. You’ve got to look underneath to see what the braking equipment actually is. While ag-spec trailers can have 8 or 10-stud hubs, they are usually fitted with 6 or 8-stud hubs with a typical brake drum diameter and shoe width of 300 x 60mm.

If they have 10-stud hubs they will have 400 x 80mm shoes and use a flat-type cam. High-speed/commercial axles usually have 406 x 120mm or 420 x 180mm drum and shoe measurements and use an S-type cam for progressive braking.

What difference does speed make to braking?

When a driver brakes, the kinetic energy involved is converted to heat in the tractor and trailer braking systems. Indeed, both systems are designed to cope with this at their rated operating weights and top speed.

But if you travel faster or carry more weight, braking requirements rise disproportionately. So if speed is increased by 50%, for example from 20mph to 30mph, the load on the tractor and trailer’s braking systems increases by almost 150%.

Who polices trailer braking?

Agricultural vehicles such as tractors and trailers must comply with the Road Vehicles (Construction and Use) Regulations Act 1986. Regulated by VOSA and the police, these regulations require tractors and trailers to be properly maintained and be safe for use.

This also includes trailer brakes, warns the HSE’s Tony Mitchell. VOSA inspections have increased five-fold in the last five years.

Why carry out a BAGMA brake performance test?

As well as finding out the true braking efficiency of your trailer and knowing that you’re not wearing out your tractor brakes, having a hologrammed certificate and sticker on your trailer means that, if you get stopped by VOSA, you have documentation to prove you’re legal.

What does the test involve?

BAGMA has developed an electronic brake test meter designed for use on tractors and trailers with air and hydraulic braking systems.

The first part of the test involves testing the brakes of the tractor – a modern tractor should give a reading of anywhere between 50% and 70%.

The next thing is to check the hydraulic trailer-brake valve to make sure that is functioning properly. “Once we’ve established there’s nothing wrong with the tractor, we then move on to the trailer,” says Adam Wyatt.

First, the trailer is loaded to its capacity. For a hydraulic system, a pressure regulator is fitted into a spool valve which reduces the pressure to the BS standard for testing of 100bar, operated by the spool lever inside the cab.

The tractor is then driven up to the given speed (20mph) and once that’s met, the driver declutches, pulls the spool lever and waits for the vehicle to stop. The BrakeSafe unit will then give a print out of the efficiency of the trailer itself. Four tests are carried out to establish an average.

Tractors fitted with air brakes are again fitted with a system independent of the tractor’s air brake system and the operator drives it up to speed, declutches the tractor, pushes the button which activates the air brakes on the tractor. When the unit comes to a stop, a reading will again be printed.

A pedal force check is also carried out to see if the ratio between the pedal and the resulting force coming out of the trailer brake valve is correct.

What are the main reasons for poor brake performance?

Lack of maintenance and adjustment are the usual suspects, says Mr Wyatt. “A typical opinion is that because a trailer is only one or two seasons old, it doesn’t need adjusting. On many trailers the paint is still fresh on the adjusters.” Even on older trailers lack of adjustment is the main reason for failure.

Another problem is standard manufacturer’s spec. “Most manufacturers use typical 20-25mm rams and 1/4 inch hosing to keep down the cost for both them and the customer. Even on trailers driven at 20mph equipped with a 25mm actuating ram which is fully adjusted, we’re lucky to get 25% braking efficiency. And this is the minimum – at 25% we should be starting to panic,” explains Mr Wyatt.

Even when the trailer has been re-fitted with new shoes and is fully adjusted, some still only get 18%, he adds. “On some older trailers, we’re seeing a 25mm ram doing both left and right sides of the axle, which is pretty dire.”

Once adjusted, some drivers complain of tyre wear and trailer lock-up when empty, and often slacken off brakes to avoid this, meaning that when fully-laden, there are hardly any brakes at all.

What can be done to bring trailers up to spec?

A simple upgrade involves fitting a larger ram and increasing hose size, typically 35mm ram and 1/2 inch hosing. “The same trailer that gave 18% efficiency when fitted with larger rams and hosing then recorded 32%, just by upgrading the rams resulting in more clamping force.”

A load-sensing unit can also be retro-fitted on both hydraulic and air-brake systems. This unit gives more pressure to the braking rams as the trailer increases in weight and therefore gives better braking at higher loads. “It’s a simple piece of kit and costs between £300-£400,” says Mr Wyatt.

How can I make adjustment easier?

With cheaper-specced trailers, the adjuster extending from the ram to the brake drums is on a splined shaft and the only way to adjust it is to remove the circlip, which often involves getting the gas torch out.

By fitting a manual or auto slack adjuster, which costs about £200, it becomes a five-minute socket/spanner job.

A lot of trailers also have an adjuster on the ram itself, so you can slide it across to give limited movement.

“The only problem with this is that you can alter the geometry to the rod coming down and decrease the efficiency of brakes overall,” he says.

Where can I get a test carried out?

All Case IH, New Holland, MF, Valtra, Fendt, Challenger and McCormick dealers now have a trained technician to carry out the test and are licensed to issue a certificate.

John Deere and Claas are operating their own scheme. For a full list, visit the BAGMA website (

What’s the cost of a test?

A test will cost in the region of £60-£100, but if there is a group of trailers it would be cheaper per unit.

HSE industry advice


• Check brakes regularly and keep them adjusted
• Buy trailers with high enough spec that are easy to maintain and have load sensing and failsafe systems
• Ask suppliers to show evidence of braking efficiency


• Build trailers that have adequate braking and test for actual performance
• Ensure easy maintenance and make sure load sensing systems are correctly set
• Give information for adjustment and service


• Provide trailer brake maintenance and testing service
• See trailers with better brakes
• Make sure customers are provided with infromation on trailer braking and adjustment

How much are repairs?

• Strip and repair tractor brakes: £1750-£2500

Upgrade trailer

• Brake adjusment/new shoes: £0-50
• Upgrade existing trailer: £750-2500
• Buy high spec brakes on new trailer: £900-£2000

Further information

NFU Business Guide on agricultural trailer brakes. Useful guidance and interpretation of the legislation (available to NFU members only)

Tractor & Trailer Brake Testing. Information on training courses available and agricultural machinery dealers trained and equipped to deliver BAGMA tractor and trailer brake testing.