MX-5 OC Forum

taff eunos:

Rich M:

Yes, completely agreed. I hope that's also how my description of it came across too, and that which Robbie posted from Mazda also.

TBH I hadnt read yours or Robbie`s description of how it works (not for long though...) I had my own idea about it going back some 9 months ago, once one tab breaks the loading on each other tab on that side is increased, its guaranteed failure

denzibaby sent me the bits from his own failed diff, he would better know how the tabs on the dishes are located in the diff carrier as he has had first hand experience of dismantling one.

On another thread Robbie was advising an affected owner to `carry on driving until it fails altogether` or words to that effect see: http://www.mx5oc.co.uk/forum/forums/p/14186/83333.aspx#83333

this is why I became so very incensed at the situation in the first place. Robbie also works for a Mazda dealership

Hi Taff,

Yes I've read the thread a couple of times and I do agree with you.

It's a hard thing to advise on what folk should do in this situation. It must be even harder if you work for Mazda and is perhaps one of the few times Robbie might have to abstain.

If I found bits in my diff I would view the situation as I would a nail in a tyre; drive carefully and get it fixed. The driver has his own safety and convenience in mind but must also consider others in the car with him or coming the other way. It's not like a thousand other things on the car that could go wrong. It is something that has gone wrong.

I work for a Mazda dealer, not for Mazda. Please let's not get the 2 confused.

I have abstained on this topic in the past and the former Club Chairman, John Cookson did some follow up work on behalf of members. John Cookson is at this time not contributing to this issue as a Club official. I posted the technical description of the operation of the LSD as factual information as I thorght it would be of benefit as you were trying to figure out how the LSD function works.

With my Mazda dealer hat on, if you came into my dealership with an affected vehicle I'd advise you, as a customer to contact Mazda customer services. If the car had a full Mazda service history I'd probably ask the warranty department to put in a good will claim on your behalf. I could also submit a PQI (product quality information) form to Mazda's technical help desk.

sidewayssam:

whether this occurs due to the rbt or not,i cannot prove,but as i cannot think of anything driving a wheel from the wheel side back to the diff,it would seem likely.

Things that might cause a force driving the diff from the wheels:
Lifting off the accelerator.
Changing down a gear without rev matching (nice shock load too).

Things that might cause a force driving the diff from just one wheel:
Doing anything from the above list while driving round a corner.
Doing anything from the above list while driving on a road with bumps on it.
Driving with a seized brake caliper (on or off, doesn't matter).

HTH

 hi there,sorry but all of the above are incorrect in the situation in which the reverse torque is being applied,in none of the above is anything driving the wheel from the wheels side back through the driveshaft,and back through the diff,to the prop.

All of what you have quoted have the force in the opposite direction from the prop,through the diff to the wheel,

lifting of the power can cause driveline shock if dropped down a gear,but both wheels would be rotation at the same speed,and would not break the diff,

driving over bump with the wheel lifting would also not effect in the same way,as the power is coming from the prop side not the wheel,if a breaking force is applied through the wheel hitting the floor ,or by a seized calliper,they are both breaking forces and would apply that force to the bottom of the tab that is breaking in the diff.

with force travelling in this direction,the tabs,and diff appear to be functioning ok,but by driving the wheel from an external power source(ie brake roller)the torque being applied is massively different from any of the above.

hope this clarifies matters, end of the day ,there still failing,we need to find the cause of failure,

regards

Tim

sidewayssam:

 hi there,sorry but all of the above are incorrect in the situation in which the reverse torque is being applied,in none of the above is anything driving the wheel from the wheels side back through the driveshaft,and back through the diff,to the prop.

All of what you have quoted have the force in the opposite direction from the prop,through the diff to the wheel,

lifting of the power can cause driveline shock if dropped down a gear,but both wheels would be rotation at the same speed,and would not break the diff,

driving over bump with the wheel lifting would also not effect in the same way,as the power is coming from the prop side not the wheel,if a breaking force is applied through the wheel hitting the floor ,or by a seized calliper,they are both breaking forces and would apply that force to the bottom of the tab that is breaking in the diff.

with force travelling in this direction,the tabs,and diff appear to be functioning ok,but by driving the wheel from an external power source(ie brake roller)the torque being applied is massively different from any of the above.

hope this clarifies matters, end of the day ,there still failing,we need to find the cause of failure,

regards

Tim

Hi Tim,

I can see your train of thought but there really does not seem to be enough pre-load to cause a problem on the rollers. I think that to get the unit to operate beyond a little drag would need torque input from the prop.

Please see my check on our Mk3 page 10 and the one Tone did on his Mk2 page 11 where we both found little resistance to one raised wheel rotating (prop free) whilst the other was stationary on the ground. Considering the forces these diffs handle driving on the road under power I think it would be very obvious (and scary) to the operator if they were really locking up so much as to destroy them on the rollers. There would be a big lurch and bang I think. That would be supposing the non-driven roller is locked; I'm not familiar but think some may just be free to roll if driven by a wheel. Perhaps others know more about roller types/makes?

To me the reason looks more like poor quality control during those 2 or 3 years early production of this diff type, or hammering from weeks/months of bad clutch judder that seems to have affected some of those same model years.

Captain Muppet:

Things that might cause a force driving the diff from the wheels:
Lifting off the accelerator.
Changing down a gear without rev matching (nice shock load too).

sidewayssam:

 hi there,sorry but all of the above are incorrect in the situation in which the reverse torque is being applied,in none of the above is anything driving the wheel from the wheels side back through the driveshaft,and back through the diff,to the prop.

All of what you have quoted have the force in the opposite direction from the prop,through the diff to the wheel,

When you lifting off the throttle when driving a car what force do you think it is that keeps the engine rotating above idle speed with the throttle closed? I thinks it a force transmitted from the wheels, through the diff to the engine - the reverse of what happens when it's driving. Moving on and off the throttle reverses the torque through the diff - speeds up and slows down the car (coupled with a little lurch from all the slack in the driveline/mounts being taken up).

What do you think is happening to drive the engine at a speed higher than idle with the throttle shut and where do you think this force comes from if not the wheels?

Rich M:

Please see my check on our Mk3 page 10 and the one Tone did on his Mk2 page 11 where we both found little resistance to one raised wheel rotating (prop free) whilst the other was stationary on the ground.

Rich,

I found something very different on my MK2.5 Sport LSD, which for info I have already extracted broken metal from. With my car completely raised, wheels off and in neutral, turning one rear wheel also turns the other rear wheel at exactly the same speed and direction - as if they were connected by a solid axle.

Thus in my case, I can understand how the RBT (putting signicant torque into one wheel whilst the other is stationary) could stress the diff internals.

Martin 

 Glad I have got a standard dif !!!

MJJ_ZX6RR:

 

Rich,

I found something very different on my MK2.5 Sport LSD, which for info I have already extracted broken metal from. With my car completely raised, wheels off and in neutral, turning one rear wheel also turns the other rear wheel at exactly the same speed and direction - as if they were connected by a solid axle.

Thus in my case, I can understand how the RBT (putting signicant torque into one wheel whilst the other is stationary) could stress the diff internals.

Martin 

Hi Martin,

I didn't check our car with both wheels clear of the ground (just one as that was the supposed problem scenario) but you'll see Tone did and found the same as you. That is to be expected as the wheels are coupled by a little pre-load which slips if one wheel is held. It's also not clear if the stationary roller is locked in these tests, it may run free.

Tone:

With both rear wheels clear of the ground if one is turned the other goes round the same way,  with one on the ground [in neutral, h/brake off] the other turns freely, also tried it [whilst being exstremely careful] in 1st with the engine at idle and no sign of the LSD trying to lock, the wheel still turned freely!!!.  If my findings are typical, my conclusions from this are that whilst the RBT should not be used with an LSD, the speed the rollers go at should not have caused my diff failure.

Rich M:

My findings are the same as Tony's but I wanted confirmation from someone actually trying it without me saying it first, thank you for that.

- Last Fri I jacked up one wheel and gave it a spin, very little drag in either direction with the opposite wheel hard on the ground.

- This car's had 4 RBT MOTs. On Sat I changed the diff oil at my local friendly garage where he guided me onto the ramp and left me to it. Big relief, no swarf or bits at all, just dark grey/black oil and grey paste on the drain plug magnet. Poked around inside with my little finger and some wire but couldn't find anything.

(sorry, they were both on page 11)

 Ater reading some of the threads many would seem it is driving wrong and not the fault of dif design. I used to do stage rallys and one of the mech bits that gave the least amount of trouble was the LSD (lid slip dif) We rallyed over smooth tarmac and very bumpy tracks some times taking off and landing hatd with the power on and loads of side ways driving under power. In the ten years I was rallying we never had a dif break.

mal jones:

 Glad I have got a standard dif !!!

Yes I was thinking I wish I had too a week or two ago. Looking around the web though this Super LSD has only shown problems with MX5 Mk2.5 of a couple of years production that I can see. I don't know if they were early years for production, certainly it was new to MX5 then. Since then it's used on the RX8 and Mk3/3.5 MX5s, 260 bhp Mazda 3 MPS, 161 bhp Mini Cooper S and probably other stuff too without reported problems. I'd expect all these units to be dimensionally different but don't know. 

 Hi there, as you said"wheels in the plural sense"as both wheels are rotating at the same speed in the same direction,this would not put undue torque on the diff causing failure...think you may have missed the point....

 i'm with you...i agree completely...i have never seen one break before either on a lsd...

 Hi mate, yes you have missed the point completley!you are correct in what you have said,but both wheels are rotating in the same direction at the same speed,this would not cause the diff to fail,the lsd on any car in affect acts as a brake when the opposite wheels is spinning faster than the other.

On the diff's affected,the work of thrust pressure  forces the clutch to grip the hub.If you lift one wheel of the ground and spin it,the other wheel will rotate as well,if you now get someone to grip the other wheel,the wheel you have hold of will spin with little resistance.

If you spin this wheel faster(much faster)the wheel on the opposing side will start to grip ,as the thrust from the spinning wheel drives the clutch on the opposing side into the cage.The more torque put through the spinning wheel will transfer to the grip wheel until the both spin at the same speed.

Some people have missed the above point in the thread,when i said i can think off nothing but the rbt that drives the wheels in the opposite direction,i should have correctly worded it,i can think of nothing that drives "one"wheel.Which has probably led to the confusion.

I have stripped many diffs down,and rebuilt quite a few diffs(not something i enjoy!lol)but have never seen one of these diffs affected,so have had to work of the photos on here.Though i do understand how it works,the only weakness in the argument against rbt's is that of the one's i have seen in mot stations,they rarely brake the opposing wheel,and usually it free wheels.As long as both wheels are rotating in the brake test,it should not affect it what so ever!

hope this clarifies things.

many thanks

Tim

sidewayssam:

 Hi mate, yes you have missed the point completley!you are correct in what you have said,but both wheels are rotating in the same direction at the same speed,this would not cause the diff to fail,the lsd on any car in affect acts as a brake when the opposite wheels is spinning faster than the other.

If you look at the second part of my post I suggested ways of applying that force asymmetrically. Lifting off round a tight corner does produce uneven speeds (and torques) applied to the diff from the wheels. Lifting off in a straight line with a siezed brake will apply different torques to the diff. Lifting off with a seized brake round a corner or braking with a seized brake round a corner will produce asymmetric torques and different speeds from each wheel.

The failures might not be caused by the MOT test. Tight turns with a defective brake could apply loads that the diff wasn't necessarily designed for, for example.

I know very little about diffs, and my MX5 doesn't even have a differental (it's welded up) but I thought some ideas of unusual diff loads might be helpful.