Strange that it was not picked up during the PDI…
Not that strange when you consider the tyres appeared to be all correct, and there have been many reports of new cars with shipping pressures on the tyres, resulting in a very hard ride, till the pressure was dropped to the correct setting.
That means apparently that Mazda on a PDI don’t check tyres to the 'nth degree. Neither do some dealers either.
Now we know why Mazda don’t supply a space saver spare wheel, in case it’s fitted to the rear axle.
First of all, congrats to OP for a successful outcome! Secondly, well done to all who have contributed their ideas to help sort this issue Thirdly, and for me, more importantly, what an earth is going on? These cars are not some high end Italian ( or other ) exotica, but a relatively low cost, fun, open top car. Only making this comparison as I was told a story about a chap who owns a Porsche Carreras 4 wheel drive thing. He had a puncture and ALL the tyres had to be replaced! Uneven tyre wear being the reason, upsetting the system! This ND issue is not a widely publicised problem, if you’re not reading this on here, how would you know about it? Further down the line, as these cars hit the non franchised, second hand car dealer, who knows what may have happened in the cars life? Do they fit a part worn, non matching tyre purely for the MOT and then condemn the new owner to out of warranty replacement diff costs further down the line? Fair enough, someone quoted the handbook recommendactions/advice but again, this may not be with the car at point of sale, a few years down the line? Why are cars that complicated? I presume it may extend to other " normal " cars from other manufacturers? Not sure this is progress!
Other sad thing is this car, bought from new, was fitted with a non matching tyre on the rear axle, even though the correct make and size, it had a mismatched tread pattern? Beggars belief! Was probably sourced and fitted in good faith but turned out to be a " variant " of a good brand of tyre?
Barrie
My post #8 diagnosed the fault correctly.
Exactly the same as the one new Potenza on the rear of my Mk 3.5 the tyre was also slightly off pattern when studied very closely, causing the wandering problem.
Please excuse a question, which may to those with an engineering background or who are mechanically competent seem stupid. But would the aforementioned situation be more problematic to cars with limited slip differentials than to those without?
Sensible answers please, not snide comments.
Thanks!
More or less odd tyres have the potential to cause a handling problem on any car, but I would not expect an open diff to have been damaged in these circumstances.
We haven’t been told, but it is implied I think that the two rear tyres here ended up with a different rolling radius, which is what caused the LSD damage and would be consistent with the handbook advice for the LSD which concludes -
“If these instructions are not followed,
the rotation of the left and right wheels
will be different and will thus apply
a constant load on the limited-slip
differential.
This will cause a malfunction.”
The tyres were identical with the exception of the tread pattern (and possibly the compound). The rolling radius were the same. The tyre with the different tread pattern was wearing differently to the other 3 - possibly due to compound or the tread itself. Apparently the tread difference would be enough to affect the load on each rear wheel from the LSD. 90% of my driving over the last 4-6 months has been over very rural and windy roads.
Even so, it seems that the operational tolerance of the LSD is so fine that even the slightest asymmetric wear on the rear tyres could potentially cause a very expensive problem. Interestingly, I wonder whether anyone with NBs/NCs with LSDs has ever had a similar problem or is this in any way symptomatic of poor engineering design, quality of manufacture or attempts to reduce weight (and cost) taken too far on the ND…
Crikey! What happens when you have an irreparable puncture on a rear tyre where both rear tyres are part/well worn. Would you have the problem if you replace the damaged tyre with a brand new tyre.
In the manual Mazda also advise a tyre rotation system with diagonal changes. This could also lead to you having a pretty worn tyre and a part worn tyre on the rear of the car as well. I think that I will give this advice a miss.
So does this come down to the fact that you need to not only have exactly the same tyres on the rear but they must have the same amount of wear otherwise the diff will fail? I mean I can’t see two very slightly different tyres of the same size having a difference in circumference greater than the difference in circumference of two tyres of the same type, one worn and one brand new
This is not a new problem.
My old Vectra-B complained about mismatched tyres (ABS light came on after a few minutes at 70) even when there was no measurable difference in circumference, only the specified rolling resistance. So I often needed to replace a puncture with a pair of tyres, or all four. SWMBO tended to clip one particular left turn kerb with the rear wheel, so there were few too many punctures…
I’ve only just picked up on this thread, but find the whole saga incredible.
To read that slightly mis-matched tyres, of the type discussed here, could affect the handling so markedly, and could also result in a limited-slip-differential being wrecked in only a few thousand miles by the same, is almost unbelievable - as is the need to have to replace both rear tyres should a non-repairable puncture on a part-worn tyre, mean that one new one is necessary.
My first experience of MX-5s was a company-car that my wife had from 2011 to 2015, and following the odd puncture over the years, which often required a tyre to be replaced because the damage was in a position that was deemed non-repairable (funny how that seems to happen so regularly !), rarely had identical tyres fitted anywhere on it. Being a company-car, all repairs were paid for by the lease-hire firm, who would specify which type / make of tyre they were prepared to pay for, and I would have hated to have had to explain to them, should one tyre be damaged, for whatever reason, and need replacing, that TWO new ones had to be fitted instead !
The car in question at the time, was obviously my wife’s daily driver (going back and forth to work each day), and I had the use of it too. Neither of us could exactly be called ‘dawdlers’ on the road, but we never ever had any handling issues with it, mis-matched tyres or not, in any weather.
I suppose one’s driving style could have a bearing on things here - after all MX-5s are ‘sports’ cars, and it would be expected that they would quite often come in for some ‘spirited’ driving from time to time, and it would be in these instances I suspect, that any handling issues would become apparent.
I think it rather unfair, and perhaps unreasonable, to expect a mechanic, perhaps carrying out a PDI check, or any other inspection for that matter, to notice, or even consider mis-matched tyres on a car. Having said that, I am the first to admit that my experience as a motor mechanic is very ‘old-hat’ (1970s actually), and LSDs were not something I ever encountered, so perhaps my knowledge and experience is not relevant in today’s motoring world.
Still, as the saying goes - “you learn something new every day”, and being a member of this OC and forum for the past 15 months, I have to say that this site is extremely educational, even if some issues take a bit of digesting !
Looking forward to the better weather !
Seems so Ian.
First I was aware was back in Vauxhall Frontera days…when their diffs were failing wholesale due to owners mixing tyres with odd-ball rolling circumferences.
Earlier Landrover TDi’s in the 80’s had the same issue.
Then I guess…it’s a kissing cousin theory to the much reported (in ye olde days) issue with the “soft” Mk2.5 2003 LSDs getting boogered up (tab snapping) at MOT time on rolling roads.
I had a slow rear puncture years back with our 2002 Sport…it was the ***stress-meshing noises *** that caught my attention before the 10-odd PSI.
Suffice to say I stopped quickly and pumped both rears up to 40PSI…and straight into a local KwikFit.
(May I say I took the precaution of swapping out the diff oil…which was original…for yon loverly Castrol liquid gold…and all was well then and still is now.
Thankfully!)
Therefore as previous posters have said not just MX5’s have had issues with very slight changes in tyre rolling radius and characteristics of tyres. Why are Mazda in the case of the ND advising the “rotation” of tyres and wheels between different corners on the ND where there will be different wear patterns.
Thats an interesting point… as Rodders has mentioned he has seen very high diff temps in track use, the ‘limited’ gearbox issues in the main from US reports seem to be around cars equipped with limited slip diffs too suggesting a fairly narrow optimum envelope where in certain conditions a weakness may be exacerbated.
As others have noted historically diffs in other makes have also disintegrated when mismatched tyres have been on the axle.
As a non LSD owner I’ll carry on with the tyre rotation though take care about the amount of wear on the tyres going opposits side.
I’ll be up to get my diff oil changed as soon as weather conditions allow…
The thinking must be that if they are rotated frequently enough then the wear will even out. I have always managed that with my 4WD cars (6,000 mile change arounds)
Of course if you have directional tyres you can only rotate front to rear on the same side. If you live in Milton Keynes, monument to the roundabout, you will then end up with more worn tyres on the left than the right.
I am not sure there was a rolling radius difference. certainly in my case of the replacement Bridgestone Potenza being of the same stock size there was no difference it was purely the tread block pattern that on close scrutiny was different causing the drifting on power on / off.
Not even sure that the diff was damaged in the ND case, possibly the dealership clutching at straws to resolve the problem. Especially as the problem was still evident after swapping the diff.
Hi
I’ve had my 2009 Mk3.5 1.8 SE just over a week. It’s my first MX5 and great car. But this talk about duff diffs has me a bit worried…
Now then, the car drives fine but i can detect a bit of a noise from the back. It’s not the banshee like whine I experienced on a car years ago and it doesn’t sound like a whistling bearing. It’s more of a whirrrr. (!) It doesn’t increase in pitch with speed. As a 1.8 SE it shouldn’t have a LSD.
So is it a normal MX5 noise or should I be looking further into it. If they all do it then I’ll leave it alone…
Thanks,
Steve
Normal noise, they all do it to some extent.
No need to be worried.
I can assure you. Having been around MX-5’s for almost twenty years this is a very very rare post concerning one possibly faulty diff on a nearly new MX-5.
As Ian says MX5 diffs are reliable in normal use and with oil changes at about 60,000 miles.
However a limited slip differential can fail through having mismatched tyres. A different rolling radius will cause the limited slip mechanism to continually work, whether it is a clutch type or gear type, something that it is not designed to do. This is the same for any make of car that has a limited slip diff.