CVW

T he BPW Limited CustomerServices team are frequentlycalled upon to help diagnose

problems with trailer braking systems.Nowadays trailer braking systems arenearly all controlled by EBS (ElectronicBraking System) and this can be an aid tohelp with the diagnosis of system faults.

Problem causes can be largely splitinto three areas: The foundation brake(disc or drum), the trailer brake controlsystem (ABS or EBS) and the towingvehicle. The aim of this and a subsequentarticle is to provide a guide to faultdiagnosis and recti�cation.

Very often the clues to the cause ofthe problem will lie in the condition ofthe friction material (brake linings orpads) or on the friction surfaces of thedrums or discs. Sometimes thesymptoms for both drum and disc brakescan be similar but it must be borne inmind that as disc brakes run hotter thandrum brakes these symptoms will varyacross the 2 systems. What follows is aresumé of some of the most commonproblems that we encounter:

CASE 1A trailer is presented for the annual MOTtest and fails on brake performance; veryoften this is its �rst test. The driver has

not complained about poor brakes, so the�rst thing that we would do is to look atthe condition of the friction material andfriction surfaces. In most cases it will beseen that the friction surfaces are shinyand that the friction material exhibitsvery little wear for the mileage that thevehicle has covered and also has a sheenon the surface. This is a classic case ofbrake under-utilisation, which issometimes referred to as glazing.

With endurance braking (retarders andengine braking) included as standard onmost modern towing vehicles this is now avery common problem. With EBS systemsthe towing vehicle has control of thebraking balance between tractor andtrailer. Sometimes there are a couple oftweaks that can be done on the trailer EBSsystem but very often the remedy lies inliaison with the towing vehiclemanufacturer in order to get them to makean adjustment to the tractor EBS system.

MOT timePeople ask why this occurs at MOT timewhen the driver has previously notcomplained about poor braking. Theanswer to this question lies in theunderstanding of the braking pressuresinvolved. During normal driving the brakepressures delivered to the brake

chambers are below 2 Bar for 95% of thebrake applications. Even during anemergency stop the pressures are nomore than 4 Bar. During the brake rollertest, the pressures are taken right up to6.5 Bar. Air pressure is directlyproportional to friction surface pressureand it is at these higher pressures thatthe under-utilisation e�ect takes place.

Occasionally we still come acrosscases where the vent plug bungs have notbeen removed when the brake chambershave been �tted to the foundation brake.This is especially troublesome with discbraked axles but also causes problemswith drum braked axles. Depending onwhich plugs have been left in place therecan be either a pressure retained in thelow pressure housing, causing a bindingbrake, or there can be a residual vacuumin the housing which has to be overcomeby air pressure before the brake can beapplied. It is this latter case which willmanifest itself as under-utilisation.

NOTE: Sometimes the bungs may be left inon only one or two wheel stations causingthe problem on those corresponding brakes.

CASE 2A telephone call may be received byCustomer Services and the customer

BRAKES Fault Diagnosis

In the �rst of a two part series, BPWtakes a look at fault diagnosis ofTrailer Braking Systems.

TRAIL OF TWOHALV E S

BRAKES Fault Diagnosis

34 CVW OCTOBER 2012

complains of braking issues, normallypoor brake performance. The trailer is 2or 3 years old and in these cases theproblem has usually been reported by thedriver. The clue here is that the brakeshave performed well until now and theowner is asked what has changed. Is therea new driver or a new towing vehicle or isthe trailer being used on di�erent types ofwork? The normal response is thatnothing has changed but we have re-linedthe brakes a little while ago.

Further investigation discovers thatdi�erent or non original linings have been�tted and this has upset the brake balance.During design, the coe�cient of friction ofthe friction materials is balanced to themetallurgy of the friction surfaces and anydeviation in these two components cancause problems. Don’t forget that duringthe manufacture of the trailer a set ofbrake calculations is performed using theknown parameters of the originalequipment speci�cations of the frictionmaterials. If this is changed then the brakeperformance will also change.

CASE 3This is really a combination of cases 1 and2. A customer may have su�ered withunder-utilisation and has overcome thee�ect of this by �tting a di�erent type ofbrake lining which has a higher coe�cientof friction. All may have been well for the�rst few weeks or months but thenproblems arise. The heat generated by thehigher coe�cient of friction may cause thesurface temperatures to rise on thefriction material and friction surfaces,giving rise to surface cracking.

Some surface cracking is normal and itis removed continually by the naturalaction of wear in the friction

components, indeed this is evidence of ahealthy braking system, where thefriction surfaces are a dull pewter colourand the friction material is a matt greycolour. If cracks are generated too rapidlythey become larger and join up,eventually forming long cracks whichpropagate through the brake disc ordrum. In these cases the friction materialwill start to disintegrate because theresin that bonds the material togetherhas been drawn out and burned o�. Theremedy here is to replace the frictionmaterial and the drums or discs if theyare too badly damaged and then toinvestigate why the under-utilisationoccurred in the �rst instance.

CASE 4This case is the opposite really of casenumber 1. The customer will call to saythat they are unhappy with the life theyare getting from the friction material or,worse still, the brake linings or pads areburning out with excessive heat. Normallythese cases demand a visit from one of ourengineers, ideally to see the vehicle beforematerial has been replaced. The engineerwill check the condition of friction surfacesand material on all wheel stations.

If the problem is only down one side ofthe trailer it may suggest a problem withone of the modulator valves in the brakingsystem. If the problem is only on onewheel station then this may point to aproblem with the brake chamber(remember the bungs) or the foundationbrake itself. If the problem is uniform thenthey will look at the data in the EBS system.

Brake calculationsThe brake pressure control settings in theECU will be checked and if BPW Limited

has carried out the brake calculations, theparameters will be checked against thoseon the calculation sheet. The onboarddata will be downloaded and checked forevidence of overloading, speeding, ABScycling, roll stability activations and primemover ESC (Electronic stability control).

ABS cycling where the ABS systemcontinually sends test pulses down to thebrakes because of high cornering speedscan be a problem. This can be resolvedeither by driver training or liaison with themanufacturer of the system to eliminateany programming issues. In cases wherenone of the above checks reveal a reasonfor the high component wear then you mayhave to resort to more high tech testing todetermine tractor/trailer braking balance.

Testing developmentsWe used to do these tests statically byinstalling pressure gauges and regulatorvalves onto the tractor/trailercombination and monitoring pressures inthe complete combination. With thewidespread use of EBS the brakingpressures balance out dynamically whenthe vehicle is in motion and we havedeveloped our own testing equipment tomeasure this.

ECE regulations require that the EBSprime movers must allow static thresholdtesting, but BPW have found thisunreliable. The towing vehicle and trailerare installed with pressure gauges,temperature gauges and accelerometersand data is fed back to a laptop computerin realtime as the vehicle is driven on atypical route. Later, the data is analysedand imbalances in braking identi�ed. Whenthe cause of the problem is identi�ed wecan then work with the tractormanufacturer and resolve the issue.

The four cases described here in thisarticle relate to the most commonproblems that the company see. BPWLimited have available two publicationswhich you may �nd interesting and usefulin identifying problems on frictionmaterial and friction surfaces:1. ‘Wear and damage characteristics onfriction brakes – Disc Brakes’2. ‘Wear and damage characteristics onfriction brakes – Drum Brakes’