MOT brake test on rolling road ?

[Saigon]

My 4200 is approaching MOT time and I intend using my local garage, but I recall reading somewhere that the brakes should not be tested using a rolling road. Did I dream this up, or is that true ?. If so, what is the reason. If this is true then most likely my local garage knows this to be a fact, but I would like to know in advance the possible implications. Cheers.

 

[Gp79]

Rear diff has a limited slip diff so I don’t get mine tested on rolling road and wouldn’t, not worth the risk.

The mot station I use have a gauge inside the car and drive it at a few mph to test brakes and handbrake.

 

[Zep]

As Greg said, ask your tester to use their Tapley meter for the brake test instead of the rollers, it's a short road test with a calibrated meter in the footwell. All MOT testers should have one.

 

[allandwf]

I think normal use will stress the diff much more than the rollers. Were they not recommended for so equipped cars as a LSD car if locks up may jump off the rollers? Can an MOT tester possibly confirm?
It would be nice if someone could explain how and why the diff may be damaged. I also have always been told they shouldn't go on the rollers but this was way back in the 70s 80s, surely still not the case?

 

[alfatwo]

Also tell them not to yank the handbrake on when the cars moving or on the rollers as it can rip out the rear shoe retaining clips, as its just a parking brake for when the cars stationary!


Dave

 

[Swedish Paul]

Here in Sweden I asked the question, and maserati answered all is fine. And they’re honest over here

 

[Zep]

I think normal use will stress the diff much more than the rollers. Were they not recommended for so equipped cars as a LSD car if locks up may jump off the rollers? Can an MOT tester possibly confirm?
It would be nice if someone could explain how and why the diff may be damaged. I also have always been told they shouldn't go on the rollers but this was way back in the 70s 80s, surely still not the case?

The issue is that a mechanical LSD uses a clutch pack to limit the speed difference between the rear wheels. The rollers force one wheel to turn and holds the other still, wearing the friction material on the plates. This is much worse than any on road operation as even if one wheel has no grip this will not force the diff to slip, the friction material just locks them together.

Other types of LSD are fine with the rollers, torsen and helical and viscous types don't rely on friction plates to match the wheel speeds so there is no harm on the rollers.

 

[allandwf]

The issue is that a mechanical LSD uses a clutch pack to limit the speed difference between the rear wheels. The rollers force one wheel to turn and holds the other still, wearing the friction material on the plates. This is much worse than any on road operation as even if one wheel has no grip this will not force the diff to slip, the friction material just locks them together.

Other types of LSD are fine with the rollers, torsen and helical and viscous types don’t rely on friction plates to match the wheel speeds so there is no harm on the rollers.

Thank you. So "ours" use mechanical LSDs? So it is untrue to say that any car equipped with a LSD cannot be tested on rollers?

 

[Saigon]

Thank you. So "ours" use mechanical LSDs? So it is untrue to say that any car equipped with a LSD cannot be tested on rollers?

Based on what I have read so far, due to the fact that ours are are mechanical LSD’s they should Not be tested on rollers ! But those which are equipped otherwise can be tested on rollers. I wonder how aware of this fact are the general MOT service stations, I would assume a lot if not most are aware, but there are bound to be some where this possibly needs to be tactfully highlighted.

 

[Zep]

Yes and Yes

It think it is a popular misconception, probably borne from the desire not to make a mistake. The Q2 diff in Alfas for example is a torsen (torque sensing) so it uses a gear arrangement to direct torque to the wheel with the most grip up to a predetermined ratio, so it isn't a LSD in the conventional sense. This type of diff works on the principle that the wheel which is turning slowest has the most grip and so the slower turning wheel always receives the most torque. If one wheel has no traction at all it acts like an open diff, meaning you are still going not going to move. Thats why a lot of soft roaders are rubbish off-road.

A Plate type LSD on the other hand doesn't care which wheel has the most traction, it just wants to turn them both at the same speed and the traction part sorts itself out. The allowable difference in wheel speeds is just the difference in arc between the inside and outside wheels going round a corner, in practice much less than that applied by the rollers.

Thank you. So "ours" use mechanical LSDs? So it is untrue to say that any car equipped with a LSD cannot be tested on rollers?

 

[CatmanV2]

I know when I take my GT to Auto Alfa, they know not to test it on a roller. Of course, that may not count on a GT

C

 

[mjheathcote]

Certainly when the details of mine are entered into the MOT test system, it flags up to the tester that the car has a LSD.
Whether the tester bothers to take notice however...

 

[alfatwo]

You just tell them if you break the diff however you carry out the test, it'll cost you a new one..

This usually wakes them up!

My local MOT tester, who's a mate, did the very same thing by mistake, It cost his company 5K!

Dave

 

[ANY]

I don’t know how the rolling bed looks like in UK. Are they only testing one wheel at a time? Here both are rolling and tested at the same time. I suppose that makes a difference. However they are not able to test 4wd cars.

 

[Zep]

I don't know how the rolling bed looks like in UK. Are they only testing one wheel at a time? Here both are rolling and tested at the same time. I suppose that makes a difference. However they are not able to test 4wd cars.

They have two separate rollers and they can test one wheel at a time to see if there is an imbalance across the axle. This is what can do the damage.

 

[Swedish Paul]

I think that here is Sweden, you simply get two measures on a 1m dial display, one for each wheel indicating the brake measure, and if the arrows don't point to the same place, you have an imbalance. I don't think they try to apply different pressures to each wheel. The 4WD comment, they test our 4WD on the same rollers.

 

[Zep]

I think that here is Sweden, you simply get two measures on a 1m dial display, one for each wheel indicating the brake measure, and if the arrows don't point to the same place, you have an imbalance. I don't think they try to apply different pressures to each wheel. The 4WD comment, they test our 4WD on the same rollers.

It's the same here, there are two dials, however the manual indicates that where possible they should test wheels individually for safety.

 

[Swedish Paul]

We don't do safety in Sweden, we do snow scooters....

 

[zagatoes30]

They don't test my Q4 4WD on rollers, no idea what they do with the mud plugger, never been present when its had an MOT, but they are Land Rover specialists so I would expect them to know what they are doing.

 

[Zep]

They don't test my Q4 4WD on rollers, no idea what they do with the mud plugger, never been present when its had an MOT, but they are Land Rover specialists so I would expect them to know what they are doing.

This is probably for the best, the Q4 has a locking centre diff, so if you do one axle at a time it will try to lock the front and rear wheels together