LSD & MOT

[on2wheels]

Hi,
It has been mentioned before that the e-type with LSD should not have the rolling road test carried out during the MOT.
Put me under the category "worrier" then please answer,
If your car has had the rolling road brake test,
1) What damage could occur?
2) Could a mechanic, who knows what he is doing, put the e on a rolling road and test in a way that would not cause damage?
3) In normal driving how would you detect any damage?
4) Any long term implications?

Thanks.

[1954Etype]

Judging by your post you have already had this done? I have had a number of MOTs with the rolling road and did not suffer any consequences. I think if any damage was caused, it would be noticeable very quickly.

Angus

[PeterCrespin]

1) What damage could occur?

2) Could a mechanic, who knows what he is doing, put the e on a rolling road and test in a way that would not cause damage?

3) In normal driving how would you detect any damage?

1) The clutch packs could wear, either in terms of the friction plates themselves or the Belleville washer (large concave plate in each clutch pack) losing tension, or both.

2) The rolling road is not the issue per se, it's spinning/braking the wheels one at a time that's the issue - i.e. differential rotation between the two sides. This involves overriding the LSD clamping action of the clutches which tries ot keep the wheels rotating the same. The whole purpose of the rolling road test is that it allows Mr MOT to test your brakes individually, where in the old days the inertia brake meters just checked retardation of the car as a whole, and the retardation provided by the handbrake operating on both wheels at the back. It was not able to differentiate between the braking force at each corner hence the test was/is easier to pass.

Consequently, there is no point using a rolling road unless he overrides the clutches and tests the wheels one at a time. He could technically just check the back wheels together AFAIK, same as using a Tapley meter out on the road. you do have the right to aks for a Tapley meter to be used and all stationsm ust have one I think for LSD cars.

However, in practice, provided you use correct LSD fluid (and frankly even if you don't) a short rolling road test with individual braking is not going to trash your diff instantly. On the road the major clamping force comes when the clamping pressure on the clutches is increased, due to the spider gear pins ramping up the slopes of the crownwheel carrier slots. I.e., the harder the torque applied via the input shaft, the stronger the clamping that occurs at the two output shafts - hence the name 'Powr-Lok' to describe this type of diff. This action continues regardless of whether the static clutch clamping loads were compromised by using non LSD fluid or too many rolling road tests.

3) You wouldn't detect any damage in normal driving. If your diff creaks going round tight slow corners (like in a car park) then you might notice it no longer creaking as loudly. The only way to formally assess any change is to take before and after measurements of the torque required at the hub to overcome the clutch packs. IIRC the spec is somethingl ike 50 ft/lbs, so if it were way lower you'd know things weren't great.

Basically, jack up one back wheel with the trans in neutral and see how hard it is to turn the wheel. It should be stiff but not impossible to budge. If it just spins more or less freely, your clutches are dud but even that doesn't mean you'd spin just one wheel on dry tarmac. You would get less LSD benefit on slippery surfaces, but that's about all you'd notice.

4) Any long term implications?
The diff will keep working and would be no worse than an open diff even in the worst case scenario. If the plates were totally knackered you'd want to change/flush the oil and remove the metal particles to preserve gear and bearing life, but that's about it.

My biggest issue is to ask for a Tapley just to make sure you don't get caught out with handbrake imbalance. The Jag handbrake is fine when set up right, but if it's out of whack and you don't fancy grovelling under there, then it's nice to avoid the need for that by getting a Tapley pass based on overall retardation with both wheels. It's a nuisance getting a rolling road fail on a car where the handbrake holds you on a fair hill, just because one side's handbrake is doing significantly more work than the other...

[christopher storey]

I always ask them to use the Tapley meter and have never had a problem getting them to do this

[Mark Gordon]

I know what a "rolling road" is, but what is a Tapley meter? I don't think that we have them on this side of the pond or maybe it's just another of the myriad things that I don't know about.
Mark

[Tony]

Mark,

It is a box that was used in the old days before we had rolling roads.The MOT man would put in on the floor of the car, drive down the road and brake hard.

He would get a reading from the box on the forces generated. If they did not match a set reading the brakes would be considered inadequate.

So the answer to your question is " It's a box with some bits in it"

[Mark Gordon]

Thanks, Tony.
Mark

[christopher storey]

It is a G meter . 1G was considered 100% efficient, but for testing purposes I think anything better than 60% is considered a pass on the foorbrake, and 25% on the handbrake

[PeterCrespin]

...what is a Tapley meter? I don't think that we have them on this side of the pond or maybe it's just another of the myriad things that I don't know about.

See eBay item 110675879139

Basically it's an accelerometer, or in this case decelerometer, which is set up horizontal in the footwell on a heavy base and then gives a numerical readout of braking force. Probably not that sophisticated for the MOT, since it's not use to check ultimate braking, just that it meets an MOT minimum standard. Apart from not being mega-accurate, the ministry moved towards rolling roads for safety and insurance reasons I think. It wasn't good news to have MOT testers tearing round the countryside in other people's cars slamming the brakes on in all weathers....

You sometimes see another type of brake tester by Churchill that sits in the footwell and is checked for horizontal the same and uses a pendulum with a marker in the end that swings fore and aft. When you apply the brakes it swings over a printed graduated card clipped to the curved outer frame so that the pen leaves a trace of the braking 'swing' directly on the printed grid. That is read off as a force equivalent and the car passes/fails.

I don't think those are allowed any longer for the MOT, but being cheap enough I've always fancied getting one just to see how brakes change with tyres, pads and caliper changes etc. The problem is that although the machine is cheap enough it costs a lot to post because it's so heavy so that inertia is all concentrated in the pendulum effect, not the machine moving around .

Pete

[on2wheels]

Hi,
Thanks for all the replies.
Am I correct in assuming if the MOT tester applies the brakes on the rolling road , ie tests the rear brakes as a pair, no damage can occur?

If so that may be safer than "foot to the floor down the road, and then apply the brakes". NO thanks in my old girl!

Nick.

[PeterCrespin]

Correct. He can do anything he wants so long as the rear wheels are allowed to rotate at the same speed.

As for not wanting him to go down the road and test your brakes for real, what's the problem? I'm not saying I'd pay someone to have my accident if the brakes were dud, but if they're happy to be my crash-test dummy for free???