'Wind-up' - changing tires on a ML - do in pairs or risk damaging box? True ?

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fcp
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 My Dad just bought a used 2010 ML from a MB dealer in Guildford (used/ approved) last week, only to have a flat yesterday where one of the evidently previously repaired rears failed with a slow leak on the patch (taking the tire off we found that it had actually been repaired twice before - taking that up with the dealer, along with the oil leak I found on the drive this morning :-( )

When he went to have the rear replaced, he was told that MB mandated that tired must be replaced in pairs, otherwise you risked damage to the transfer box as a result of 'wind up' (different tire sizes by a few mm as a result of wear causing wheels to rotate at different speeds). He questioned this but was told that this is widespread practice with all 4x4s and that it *used* to be that you had to replace all 4 at a time...

He talked to me about it thie evening and I had to say that I can't see the logic behind this.. I mean different rotational speeds - that's kind of what diffs do, huh?   The only situation I can think of where it could be an issue is where a center lock was also engaged (which I don't think the ML even has)

So what's the story, urban myth or is there something to it?

gav.helme
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

My brothers old 1998 V70 AWD Volvo used to state this in the owners manual!!!

That was something to do with the fluid viscous couplings that replaced  / supplemented the diff units to provide 4wd when required.

Uneven tyre wear side to side and front to rear was supposed to screw up these couplings.

I can understand the theory behind this with these types and maybe also Audi's Quattro and their Torsen type diff unit.

But as to the standard diff unit like in the G, i can't see it...

Not sure what type of system is fitted to an ML?

Steve Kirk
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

So presumably this means you can only drive an ML in a straight line....nasty corners might cause your diff or transfer box to explode..........

Steve

peter perfect
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

 the VW towrag had a lot of problems with eating tyres and TC...but are you telling me that 8-10mm difference on a tyre makes or breaks these cars... ?, jeez god forbid those people that do more right hand turns than left and vise versa...but as for ML, wind up would only occur if centre diff or low range was applied surely ?

peter perfect
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

 thought you worked for MB..you should !

Mountain Goat 7
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

I have a 2008 ML and have not had any problems replacing only one tyre following a puncture - my local MB dealer (Inverness) happily replaced a punctured front tyre earlier this year!! Not sure why your dealer is giving the advice he is ..... By the way, my ML has lockable centre and rear diffs plus low ratio and I do occasionally use it off road but more often for towing.

Isle of Man
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

NOT TRUE you can change one tyre, it's only a recommendation to replace both.

phileas
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

i concur with mountain goat: we have a GL (effectively an ML in drag), had a puncture, replaced it with a new tyre and have had no probems....that was about 2 years ago. there is no way a difference of say 10mm should mean the difference between a new t/box or diff......if this were the case then 80% of all the now-off-road-converted classic rangies with BW tboxes would be broken.....but then again....maybe this is why 80% of them ARE broken!    ..... on a more serious note, I think this is a manufacuturer being pedantic and if one should have any problems because of this REALLY MINIMAL difference in the tyre diameter, then there must be a case for taking them to court and making them regret having put such guff into their manuals..........

jeffreyross
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

I think with regards to your vehicle tyres, you should be very careful as this can be a cause of big accident.

johnnytheboy
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

If it was me I would be replacing the pair if the tread was miles away from one side to the other, if one is ok, I would chuck it in the back of the garage or flog it on ebay! why risk bursting up a drivetrain over a tyre!

Its not just MB that recomend this BMW and Toyota do as well.

How does the ML with the centre and rear diff locks go offroad? thats got me interested???

Ford Prefect
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

The reasoning behind the tyre business is that on the whole, a diff is in balance going in a straight line, and this is what it's designed to do most of.   If one wheel is smaller than another then the diff is thinking its going around a corner all the time, and this isn't what it's designed to do.   With 4WD and a centre diff, if the tyres at one end are significantly more worn that the other end, this again gives the same effect as driving around a permanent corner, which again the diff is not designed to do.   The difference on one wheel rotation of a worn tyre and a new one on a wheel the ML size is about  160mm.   So keeping them at more or less the same overall diameter all round is a good thing, and from side to side is an even better thing.   The ML cannot wind up unless it's one with the differential locks.   The traction control works on each individual wheel brake, not the prop shafts or drive shafts.   It's the imbalance in the rotation of these shafts which is called wind up, as one tries to rotate at a different speed than the other, but can't because the locked differential prevents it.
The ML with differential locks and height adjustable suspension is a super vehicle.   Unfortunately not too many people specify it new, so there aren't that many around s/h.

Ljg458
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Re: 'Wind-up' - changing tires on a ML - do in pairs or risk ...

sell the ml buy a G cheeky