4-Matic and tyres.

[milleplod]

Somewhere, a while ago, I was reading about fitting new tyres on 4-Matic vehicles, and I can't for the life of me find it now. I'm sure I read that tyres should only be replaced with ones of an equivalent rolling radius - ie if, for example, my fronts are down to, say, 5mm of tread....one gets a nail through the sidewall and is written off....I shouldn't replace the ruined one with a brand new tyre unless I also replace the other one with a brand new one.

Can the 4-Matic system not cope with the difference in size? I can't see anything in the handbook that mentions this, although I've only had a quick look.

Pete

 

[st13phil]

Can the 4-Matic system not cope with the difference in size?

Sounds very unlikely to me. I haven't done the maths, but I would have thought that the effective rolling radius of a wheel/tyre could potentially be affected as much by the difference twixt minimum and maximum loading as the (potential) 6.4mm of tread wear twixt a new and a barely legal example.

 

[David404]

Am sure MB would recommend throwing £££ at the job and in an ideal world it would be the absolute ideal solution BUT I cant see the difference in rolling circumference between a part worn and new doing any damage. You'd get the same effect if one tyre were low on pressure.

I would however replace with the same make/model of tyre.

 

[rf065]

I've read this for various 4x4 vehicles over the last 20 years or so, and no doubt in extreme circumstances it is possible. But, I've also owned a couple of 4X4's over an 8 year period and no matter what tyre or tyres have been replaced, I've never had any problems.

Russ

 

[whitenemesis]

Thought this was only an issue on those AWD systems that a fixed transfer box, or could run with all diffs locked?

 

[David404]

Thought this was only an issue on those AWD systems that a fixed transfer box, or could run with all diffs locked?

On these old systems with a fixed centre diff and or diff locks you should only drive in 4x4 mode or with the diffs locked on loose/slippy surfaces this allows the wheels to 'slip and skip' avoiding 'wind up' in extreme cases the wind up can sheer drive shafts.

If you think about what happens when you go around a corner each wheel follows a slightly different path, each wheel covers a slightly different distance. If the transmission is locked up it forces each wheel to travel the same distance, if the wheels cant slip and skip a bit as they do on loose surfaces the result is transmission wind up.

 

[syncropaddy]

I have both a w124 4matic and a VW Syncro - both from the Styer Puch empire - and both of these cars engage their 4WD automatically by sensing the difference in speeds between the front and rear axles (amongst other ways). The difference in wheel diameter with new or old tyres could be enough to trigger the engagement of the 4WD if the difference is between the front and rear axles. If the difference is from side to side then it usually wont trigger the 4WD

 

[milleplod]

Thanks for the replies folks, and the info therein. I did think it was odd that, if there was a 'problem' whereby only matched tyres should be fitted, there was no mention of it in the handbook. I've had another look, but there's zilch.

Pete