MOT Brake Testing

[sparkythecat]

I've read in the past that MOT brake test rollers can damage the limited slip differential fitted to TVR's.
Forgive my ignorance but can anyone explain why and what the risks are? Can I insist that my local MOT station use a decelerometer to test my car? Are they likely to have one or know what one is or do I have to find some specialist to do it? If they haven't got one whatever one is, will a tray of marbles do? If anyone has any handy comparison tables they could post,I could could suggest that they get a standard canteen or motorway service station tray and add two dozen marbles. They could then put it on the passenger seat, hit the anchors at 30mph and count the spilled marbles to work out braking efficiency. Somehow I suspect that if I suggest this they'll probably just tell me to f off. Any advice would be appreciated.

[dannyboyo]

I think it's because an LSD limits the amount that the two rear wheels can move independantly, it makes it so that both wheels are always reasonably matched, so if one wheel turns so will the other, allowing for only slight changes in wheel speed to compensate for turning, but not allowing one wheel to spin and not the other, to decrease wheelspin.

An MOT brake tester will try to force one wheel to turn while keeping the other still to test the braking force required for each wheel.

In effect it's forcing an LSD to do what it is designed to stop!!

I think I'm right in this anyway, I'm sure someone will correct me if not!



>> Edited by dannyboyo on Sunday 25th January 13:07

[the dodger]

I've been told both. At the same MOT station. Last time the boss of the garage did the MOT with me as his assistant (I wanted to know and see what was tested too). He said the loading of the brake dyno wouldn't affect the diff at all. They are both being driven and it measures the difference in braking force only so the differece is minimal.

Anybody got any other stories?