The road to better braking
Roller brake test results can provide some valuable diagnostic clues on the state of a vehicle’s braking components, explains Chris Manby, UK sales manager at VL Test Systems
Regular brake testing is a very important tool to demonstrate an operator is doing his best to comply with DVSA requirements in maintaining the roadworthiness of the vehicles in the fleet. However, once you have invested in a roller brake tester, making greater use of it in the workshop can also highlight some potential problems and allow you to rectify them.
It makes sense to invest in a brake tester with the DVSA-approved software, so the test is the same as if the vehicle was undergoing its annual test. VL Test Systems’ VLT16733 HGV brake tester, for example, evaluates the vehicle brake performance and calculates the pass/fail criteria according to the latest DVSA regulations.
Bind
Bind is a measurement of rolling resistance or drag. On the test, the permitted amount of drag is four per cent of the axle weight per wheel when the rollers are started. However, if tested vehicles regularly fail the bind test by exceeding this figure, do investigate further.
Too high a reading can mean faulty wheel bearings, too little tyre pressure and/or incorrectly adjusted brakes, causing excessive wear of brake components. And no matter what the reason, a high bind reading will in any case cause excessive fuel consumption to overcome this artificial braking whilst driving, and additionally have an impact on the handling/ steering of the vehicle.
Ovality
On the annual test, brake ovality is measured mainly on the front steered axles and indicates that although the brake pedal force is steady, the brake force at the wheel is not.
If you measure it on all the axles as part of a routine inspection, it can help with preventative maintenance, because a fail on the ovality test can be an early warning sign that a component is failing.
A crack within the drum, for example, could show up as a poor result. The technician then has the opportunity to carry out further checks on the drum or disc. If it is indeed cracked, it can then be replaced before it starts to split and cause brake failure or damage to other components.
Imbalance
Braking imbalance is a test on the service brake, and to pass, the brake effort on each axle wheel must be no greater than 30 per cent difference between opposite wheels. The percentage imbalance is not considered provided both wheels achieve locking on the same axle. Imbalance can be a particular problem on 13- and 15-seater minibuses.
Although imbalance is a routine part of the annual brake test, it is usually measured on one side of the axle. Checking the imbalance by running both wheels together will give a more realistic indication of the brakes’ on-road performance. You will get a visual indication of which wheel is the greatest amount out of balance on the display of all VLT16733 brake testers.
Load simulation
If you test a vehicle unladen, and then again laden or partly laden, it will produce a different result. That, of course, is because the amount of braking effort required is greater the heavier the axle load.
VL Test Systems recommends performing any brake test with a laden vehicle of minimum 65 per cent design axle load for a meaningful test result. If that is not practicable, then our advice is to invest in a load simulator.
Our load simulator is electronically controlled by the brake tester program and counteracts modulation and any kickback from the suspension system. Furthermore, with a constant 65 per cent DAW (design axle weight) from the VLT chassis load simulator, you can rest assured of a meaningful brake test result.
The VLT chassis load simulator is particularly efficient for testing semi-trailers to a laden condition instead of ULTAST (unladen tri-axle semi-trailer), as the system is able to add sufficient load to the vehicle according to the DVSA requirements without damaging the axles or chassis.
