When you take a vehicle for a roller brake test, the weight on each axle is critical: the load on each tyre limits how much braking force it can apply before it locks, whatever the condition of the friction materials and the brake mechanism. This is why most vehicles need to be taken for the MOT brake test fully laden, or at least some way towards that state. The DVSA’s recommendation for trailers is that “this normally means at least 65% of the vehicle design axle weight (DAW)”, but a representative has said: “Occasionally, due to the vehicle’s design this can be hard to achieve. In these cases we would accept less than 65% but no lower than 50%.”
But there is a significant exemption for tri-axle trailers, which may be MOT tested unladen: the DVSA’s ULTAST (unladen tri-axle semi-trailer) scheme has been in place for more than 30 years, and is likely to remain. Of course, it is still possible to test a tri-axle either part-laden or fully laden, but in each scenario you need to know what the issues are.
Why is unladen testing permitted? For some types of trailer it is difficult to test laden: spirit tankers, for instance, cannot be tested with a full load, so they must either be filled with water (involving waste and cleanup) or tested unladen.
At a recent seminar, Roger Thorpe, special projects manager at axle and suspension manufacturer BPW, gave a presentation on the issues and risks of brake tests with tri-axle trailers.
CRITERIA
Thorpe explained that the criteria for an MOT pass are different depending on whether the trailer is classified as:
■ Laden – that is, if the axle weights sum up to >50% (and preferably >65%) of the design weight
■ Unladen – with no load except operating equipment (see below)
■ Part laden – with any load more than essential operating equipment.
If the trailer is laden, then the service brake force must meet 45% of the total design axle weight; the parking brake force must meet 16% of that weight. And in both cases, there will be a pass if more than half the wheels lock.
“For a tri-axle trailer and only for a tri-axle trailer, we‘re allowed to go unladen,” says Thorpe, “and this is the type of test that the large majority of operators go for.”
But what is the definition of ‘unladen’? “If you take a trailer unladen, you are allowed to have equipment fitted which allows it to operate as a trailer,” says Thorpe. For example, a timber trailer is permitted to keep its timber stakes on for an unladen MOT test. “You‘re also allowed to include an empty container on a skeletal trailer, but it must be empty – no sneaking a tonne in the back, to give you a bit more traction.”
If the trailer is unladen, again the service brake (45%) and parking brake (16%) forces are the basis. But there is a stark difference: “In the unladen state, you‘re really relying on wheels locking,” says Thorpe. “If you‘ve got six wheels locking, you have to have a total retardation force of 3,000kg; five wheels, 3,600kg; four wheels, 4,200kg. If you've got three or fewer wheels locking, then it must meet 45% of the total design weight. And frankly, you haven’t got a hope in hell with an unladen trailer of getting that.
“On the parking brake performance, all the wheels need to lock,” he adds. “And, at the same time, you need a total retardation force of 1,500kg. If a wheel doesn't lock, they‘ll test it to 16% of the design gross vehicle weight. And again, you're not going to pass.”
Thorpe showed an example of an unladen trailer where only one of the wheels locked during the service brake test, though all four of the parking brakes had locked. This meant that the operator had to arrange an additional (and fee-paid) laden re-test of the service brakes.
“Over the years that the unladen brake test has been enforced,” says Thorpe, “there's enough statistical information to show that it gives a reasonably safe prediction as to whether the vehicle is going to work OK under maximum braking. But it‘s not ideal, it‘s far from ideal. The laden state is by far the most satisfying test result, because we can actually see how good the brakes are.”
For a part-laden trailer – with any load more than necessary operating equipment – the 45% and 16% figures still apply, but crucially, there is no alternative pass when more than half the wheels lock. “The most fraught condition really is the part-laden condition,” says Thorpe, “because you're not going to get the benefit of getting wheel locks, and you've not got enough load on the vehicle to give an appreciation of how well the brakes perform.”
Part-laden, the mathematics is against you: with three axles rated at 8 tonnes each, the braking efficiency is calculated using 24 tonnes as a denominator. So, for a 45% pass, the braking force required is 45% x 24 = 10.8 tonnes.
But if you are only laden to 16 tonnes, that braking force represents 67.5% braking efficiency (ie, 10.8/16=0.675). The situation is even worse if, as with many trailers from the continent, your trailer bogie is rated at 27 tonnes: then your 16-tonne trailer needs almost 76% braking efficiency.
And the situation is even worse, again, when you look at individual wheels: with a 24t design bogie load, each wheel needs to supply 1,800kg of braking force (10,800kg/6). If an axle is loaded to less than 3,600kg, the brake efficiency would have to exceed 100%!
ACTIONS
So what can an operator do to improve their chances of passing the MOT braking test, whatever the load condition?
Service brakes may underperform because the friction materials have ‘gone to sleep’ – become glazed or polished. “You need to generate enough heat and create enough friction to burn off material so you've always got a fresh surface material making contact with the brake drum or disc,” says Thorpe. He adds that the braking system’s mechanical adjustments and the brake pressures set in the ECU must be correct, to give conditions under which the friction material can operate properly. “There‘s no real compromise anywhere in the system.”
If you are relying on the brakes locking, there are a couple of other factors: “The condition of the brake rollers is significant – sometimes they've got good friction material on the roller and sometimes it‘s worn quite smooth. And it‘s best to apply the brakes gradually so that there‘s an opportunity to get maximum retardation before the wheels actually lock. If the brakes are applied quickly, the wheels can lock before the machine’s measured the maximum deceleration.”
Another issue is whether the friction material is warm or cold: “If the vehicle‘s had to stand and wait his turn at the testing station for a length of time then he‘s going to have cold friction material which increases his chances of getting a poor result.”
He adds that, on the road, the friction materials warm up in milliseconds when the brakes are applied, but the low speeds of roller testing don’t put much energy into the brakes: “We've had instances where customers went through the testing station for a voluntary, got a good result, drove round the testing station and came back again some time later for the MOT and failed.”