Also if you spin the wheels of one axle, the traction control will sense them turning with the other axles not, and apply the brakes to the axle being tested.

Traction control is overridden when the brakes are applied.

VOSA would not be amused if they knew that a Tapley meter was being used when the vehicle could be tested properly.

It has passed the MOT OK. The brake test was done on the road using a decelerometer. Thanks for the help.

ref. Alan C
Drum brakes used for the handbrake must always have one leading and one trailing shoe per drum. This provides equal braking effort whether the vehicle is facing up or down a hill when parked. (It is also easier to engineer).

Series aint a problem, cos they can be tested in 2WD.
And wot Vic says bout rear drums.

I'm not a brake expert, and it's actually been a few years since I've changed or worked on drum brakes, but wouldn't the leading edge of a shoe become the trailing edge when the drum is rotating backwards? Also aren't the shoes designed to "bite" into the drum as they're bought into surface contact and this action wouldn't work in reverse? I could be talking utter rubbish of course........

most drums have 2 shoes, one will always be using a leading edge, and vice versa no matter which is the direction of rotation!

Members who know about drum brakes please look away now.

A shoe is described as either leading or trailing by its position relative to its pivot point in the direction of rotation. With a leading shoe the effect of friction between the drum and the shoe acts so as to increase the operating pressure, whereas in a trailing shoe it reduces it. A leading shoe is therefore very much more effective, up to three times more powerful than its trailing shoe comrade. (In the days before disc brakes it was usual to have two leading shoes in the front drum providing powerful braking in the normal direction of rotation, and a leading/trailing arrangement at the rear). A twin leading shoe configuration requires two pivots and two operating system, thus is more complex than the leading/trailing system.

As Alan Cstated when the direction of travel reverses a leading shoe becomes a trailing shoe. As I stated earlier it is necessary to have a leading/trailing arrangement to provide adequate power for parking on hills no matter which way the vehicle is facing.

Brakes are designed so that shoes contact the drum over the whole of their surface, not just the leading edge and take-up should be smooth and not grab. Wear ensures that the distributed contact is maintained.