As Testers know, Special Notice 3 0 2013 took effect on 26th June this year. It mandated a new way of measuring and recording brake imbalance by using either ‘lock-up’ or ‘slippage’ readings.
There is, however, a problem. In practice the new method of inspection means that what’s measured is as much the level of traction between the tyre and the rollers as it is braking effect. It is also going to be affected by the weight of the Tester, especially on smaller vehicles. A big, heavy Tester will mean that that the driver’s offside wheel will be more heavily loaded than the nearside, so inevitably the nearside wheel will ‘lock-up’ first, potentially indicating a brake imbalance where none exists.
Other non-braking factors will also have an affect on the two nearside and offside ‘lock-up’ readings, for example
Differential tyre pressures wheel to wheel.
Differential tyre wear wheel to wheel.
If one tyre is wet, and the other is dry (driving through a puddle on one side prior to entering the brake roller, for example)
Any differential weight distribution from one side to the other (a heavy load in the offside of a light van for example).
With the ‘method of inspection’ currently have in place it is almost inevitable that some Testers will apply those rules and fail cars on brake imbalance where such is not the case. Yet if there’s no defect present – what is the owner expected to do to effect a repair?
It’s likely that with medium sized and larger vehicles and Testers of an average weight, any imbalance caused by tyre/roller adhesion differences, or assymetrical weight distribution (that heavy Tester), measured on the RBT would generally fall outside the failure paramaters – but that if, say, a small car like a Fiat 500, with perfect brakes were being checked by a 20 stone Tester, it would probably fail the current imbalance test if the braking forces at ‘lock out’ were compared.
Whilst Testers have the option of either conducting a Tapley check, or a check on individual wheels, what would prompt them to do that if the wheels have locked and an imbalance is shown using the ‘method of inspection’ in the Manual? And in any case with cars fitted with ABS, or modern compensating electric power steering, the imbalance may not show up as either the ABS ‘kicks-in’ on all wheels, or the power steering tries to compensate.
The best way of checking imbalance is to record the braking force just prior to lock-up, and preferably with both wheels being simultaneously driven by the rollers. Currently however, that could get you into trouble as you would be Testing the vehicle in contradiction of the instructions in the Testers Manual.
There has been much talk in our MOT Forum on this, and Trade Bodies have reported an increase in failures due to brake imbalance at their Testing Station members’ businesses.
VOSA need to look into this quickly to ensure they are not inundated with angry motorists with brake imbalance failures that they shouldn’t have