MX-5 OC Forum

I just booked my car in for a service , check over and MOT - I mentioned the points raised in the club magazine and on these forums concerning brake testing and the LSD. My mechanic was fully aware of this and said the trade have known of these issues and should never put the LSD cars on a rolling brake tester.

He said there is another safe device they use instead...I will ask what the device is called and post it later.... He identified the lsd requirement for the car when he enetred the reg no. in his lap top - he is a qualified MOT tester as well .

teaser56:

He said there is another safe device they use instead

Yep, its called a road test!

Or even a  `Tapley Brake Testing Decelerometer`

Dr. Eunos

that could be the one ....

 My Mk1 has a LSD and it has had a roller test for the last five years, but I will question it when I next take it. I have used a tapley meter in the past when I used to repair HGV's as a guide to how the brakes were performing but how would they test a car's handbrake, or would that be a road test as well.

Phil, your yellow car will have a viscous coupling diff and not affected by RBT, the 94 car will have a Torsen and again not a problem (I think and hope as I have two cars equipped with the same) the handbrake can also be tested with the Tapley gear on road-test

 1991 J ltd definately has LSD which is the one I would have been concerned about. Not sure on the 1994 V spec as I thought it had a standard "open" diff as it is totally different to the J ltd to drive, being much more tail happy and tends to spin a wheel rather easy when pulling away on lock from say a junction. Not sure if there are any visual references to identify a LSD but the wheels will certainly turn in opposite directions if rotated when jacked up so assume it is open (no idea how a torsen diff works). I guess a LSD was an option when new.

Phil H:

 1991 J ltd definately has LSD which is the one I would have been concerned about. Not sure on the 1994 V spec as I thought it had a standard "open" diff as it is totally different to the J ltd to drive, being much more tail happy and tends to spin a wheel rather easy when pulling away on lock from say a junction. Not sure if there are any visual references to identify a LSD but the wheels will certainly turn in opposite directions if rotated when jacked up so assume it is open (no idea how a torsen diff works). I guess a LSD was an option when new.

Hi Phil, on your J Limited its pretty certain that the LSD element to the diff which is a viscous coupling, is no longer functioning as it would have when new. There is a tendency for the silicone fluid to deteriorate via shear forces and the accepted life of one of these types of LSD diff in a Eunos Roadster is around 60,000 miles, after which they will have reverted gradually to being an open diff although I feel they will always have a very partial LSD function at higher milages. The diff itself will subject to wear limits, continue working as an open diff for as long as you own the car, and needs no special LSD oil, just ordinary EP multigrade, as the Silicone fluid is housed in a self contained non-serviceable* cannister within the diff.

The V-SpecII wich you have was originally fitted with a TorSen (torque sensing) LSD which employs inter-meshing worm gears in operation as an LSD, this type does not really suffer with milage (other than normal wear and tear) and will still operate as if it were new effectively for as long as you keep the car, this is why in nearly all respects the TorSen is the better unit. Again, ordinary multigrade EP is used.

The fact that the V-Spec wants to kick its tail out as you pull out of a junction indicates it is doing what it was designed to do, in so much as the mechanism assumes the faster rotating wheel (longer arc) is spinning and power is being transfered to the slower rotating (or inside) wheel causing it to catch up rotational speed with the other and chirping the tyre in the process. A car with an LSD will wear its rear tyres out faster than a car with an open diff because of this. When the J Limited was younger and fresher it would have done the same but to a lesser extent than the TorSen.

For out and out competition (motorsport) neither type is ideal, nearly all RWD Rallycars will employ a diff which is based on the principle of a GKN PowerLock or Salisbury LSD which relies on multiple friction plates and stator plates being clamped together when a wheel spins via the ramp angle used in the set-up, this being adjustable on modern variants to allow more or less level of lock on both acceleration and deceleration independantly, very clever. Require special LSD oil because of the friction packs.

* I think there is a firm in Birmingham offering to replace old silicone from VC diff couplings and re-fill, they defo do VW 4wd `Syncro` units.

Dr. Eunos

 "Taff" thanks for the comprehensive reply. I assumed that the Vspec diff was open due to the way it drives but maybe it just different then. Out of the two cars I prefer to drive the J Ltd thoughas the 1600 engine seems to rev more sweetly than the 1800, and it just seems to corner better and be less loose on the back end.

Have a good day at Autolink if you are going. Can't make it myself unfortunately sounds like an interesting day.

Hi

My '91 I think has an LSD. When the car is jacked up, if you turn one of the back wheels, the other one turns also. Is this not a quick way of telling? Advice needed please as I think that the MOT station puts the car on the rollers.

Cheers

 Not a 100% sure but I thought that a basic way to tell if you have a LSD is when jacked up and one of the rear wheels is rotated, if the other wheel rotates the same way it is a LSD if it rotates opposite to to the direction you are turning it is a standardor "open" diff. However I am sure that there is some one more knowledgable that me that can give you a definate answer.

This may help you identify.

http://www.miata.net/garage/vlsd.html

http://www.miata.net/garage/diffguide/index.html

Gary Beale:

Hi

My '91 I think has an LSD. When the car is jacked up, if you turn one of the back wheels, the other one turns also. Is this not a quick way of telling? Advice needed please as I think that the MOT station puts the car on the rollers.

Cheers

Hi Gary, unless it has been replaced your `91 Eunos Roadster has a viscous-coupling diff, same as for PhilH if it has covered more than 60,000miles it will be less effective than a new one and un-affected by roller brake testing. The problem is really with the `Fujitsu Super-Diff` and seemingly restricted to a few early years of production from 2000-2004 or thereabouts, where through a combination of poor design, rubbish materials and most disturbingly a lack of communication between Mazda Japan, Mazda UK and VOSA as to whether cars had a diff that shouldnt be RBT`d.

If the people at Mazda UK are anything like the people at my local Mazda dealers then their lack of understanding is sadly very easily understood

Dr. Eunos

Thanks Taff

I mentioned this topic to 2 MOT testers at a MOT testing garage a while ago, they did'nt reply , just looked non-plussed. Maybe I was insulting their intelligence, there again, they were unaware of the coming changes to the MOT at that time.

marpar:

I mentioned this topic to 2 MOT testers at a MOT testing garage a while ago, they did'nt reply , just looked non-plussed. Maybe I was insulting their intelligence, there again, they were unaware of the coming changes to the MOT at that time.

To be honest, most MOT testers are very good at checking visible things, what is going on inside transmissions is usually beyond their comfort zone, roller brake testing is `easier` for them to do than a decellerometer and indeed quicker.

Believe me, the very last thing an MOT tester wants to see during a working day is a `smart Alec` who knows more than they do, if you really dont want an LSD equipped car to go on the rollers then tell them at the time of booking, not when they are just about to do it so they can allow a little more time for the test.

They will tend to only want to follow and test `what comes up on the sheet for the car` and automatically assume you are trying to pull a fast one by insisting the car does not go near the RBT machine.

I was discussing the problem with Andrew at Autolink today, they have replaced numerous damaged LSDs for customers where the partial (or indeed total) diff failure has occurred and he is in agreement about the most probable cause being roller brake testing. (at 3 years into a vehicles life, and at the end of warranty)