I looked and didn't find any discussion regarding MB's all wheel drive tolerances. Does anyone have any experience with issues, no issues, etc.?

This question has come up recently when I tried to have a garage change only my front tires (rear still has tread left). I'm wanting to switch over from Conti's ProContact to the ExtremeContact and the garage said they wouldn't do it as it would ruin my transmission. So I changed them anyway using another garage with the plan to change the rears at a later date...

So I did some simple calculations...

An 18" wheel with a brand new tire having 10/32's of tread on it has a circumference of 60" and a radius of 9.55"
Calculation of 1/32 worth of tread = .031" difference in radius

With that in mind I now have tires on my car with a 4/32's difference in treadwear. That equates to .124" difference in radius equaling a circumference of 59.25".

That's a difference of 3/4" in circumference between tires, and yes i did measure them 3/4" was spot on. Is that a lot? Surely people have driven with worse, like when a tire gets a flat and a new one is put on. I don't trust tire companies which will make you think you need a whole set. How much of difference would actually ruin transfer cases or differentials? I did look in my manuals but didn't find anything related to this.

Please share thoughts.

 

But have you considered air pressure?
As theoretical tire OD is not actual Rolling radius due to tire deflection based on air pressure and load on tire.

canbe more then most think.

 

I've got the same air pressure in all tires and I measured circumference using using a string around the tire and with the car on the ground so load was taken into account.

Another simpler way to put this: A garage might tell you all tires should be the same on an AWD car. But what does that mean? They should wall be within 1/32s of each other? 2/32's? 4/32's? etc? Granted at some point if they're real different I would expect the traction stability control to start lighting up. Has anyone ever seen this happen?

 

That is a really interesting point! I am surprised no one else has chimed in here, regarding this. I would *think* the OEM & manufacturers who design cars with AWD systems, took into consideration of slight differences in tire diameter, air pressure, wear, etc. But who really knows, until a transfer case, center diff, haldex, or what have you, fails.

Is there a way to reliably design it to include a bit of slip, into the the differences in front & rear axle, or side to side tire diameter differences?

 

What in the world make/type/size of car did you measure this on?? There isn't an automotive tire on earth for an 18" wheel that has a circumference or radius anywhere near those dimensions. It's basically impossible.

Taking one of the smaller MB sizes, the front tires on the A-Class and C-Class are 225/45 R18. A check of any standard tire shows it will have a diameter of around 26", and therefore a circumference of over 81". You didn't really use a string to measure this, did you??

Tires are specified as Width/Aspect RDia, where Width is the approximate, specified tire width in mm, Aspect is the aspect ratio in %, and Dia is the wheel diameter in inches. So the diameter of any tire can be calculated easily, and fairly accurately, by the formula:

Outside Tire Diameter = (Width x Aspect/100 x 2 / 25.4 + Dia. For the above tire, this works out to 225 x 45/100 x 2 / 25.4 + 18 = 25.97". Per Tiresize.com, the diameter of this tire is 26.0", but that will vary slightly with manufacturer.

Anyhow, your question is still a valid one: how closely should tire diameters be to not affect operation or life of the vehicle. We all have heard that we should replace tires in pairs, at least, on the same axle, so they're close to the same diameter. Obviously, the differential will accept any difference, just as it does when we drive on curves or circles. Since MB has a limited slip differential, slippage due to different diameter tires or curves causes some wear, but as it doesn't happen all the time, at least not much, that's rarely a problem. The Front-to-rear differential works the same way, and will accommodate slight variations in front and rear tires.

So how much difference is allowed? One way to get an idea of this is to look at the C-Series sedan tires and their theoretical diameters. There are 3 wheel sizes available with the C300, and each has its own tire size. For 18" and 19" wheels, the front and rear tires are different. These are:

225/50 R17: 25.86" diameter (same front and rear)

Front: 225/45 R18: 25.97" diameter
Rear: 245/40 R18: 25.72" diameter (0.25" smaller)

Front: 225/40 R19: 26.09" diameter
Rear: 255/35 R19: 26.03" diameter (0.06" smaller)

Notice they're all close to 26" diameter (maximum variation 1/4"), or 81.7" circumference, but no where near identical. Expressing that diameter difference in tire wear (1/32"), which is measured on radius rather than diameter, the 18" tires vary in radius, or tire wear, by 1/8" or 4/32" BY DESIGN. Or they can be the same diameter with the 17" wheels. So obviously the 4MATIC system can accept over 4/32 difference (1/4" on diameter), more due to anticipated tire wear.

Yes, that treadwear of 4/32" does result in a circumference difference of slightly over 3/4". But as shown above, with the 18" front and rear tires on the C-Series sedan, they come with that large a difference from the factory. So there should be no problem whatever in that. I would say, by design, we'd want to have tires within that 1/4" diameter or so from each other. If you had new tires on the rear, and bald tires on the front, that would give you 1/2" diameter difference, which IMHO is getting rather large.

The MB manuals simply state to replace tires before they wear below their minimum tread depth point, which varies with type of tire (performance, snow, all-purpose), and as far as I can see, don't even mention replacing two together on the same axle, although we usually do. They have to expect that a destroyed half-worn tire would be replaced, while its mate would be left in place. Now, if both tires were at minimum, then sure, you'd want to replace both.

In your case, with the rear tires still in good shape, but the front needing replaced, while replacing only the front will not give you exactly the same difference in diameter (if any) as original, it will be FAR better than you now have, and have had for probably half the life of your front tires, and no doubt within the manufacturer's expected variation. Unless the new front tires have substantially different diameter, I would think you'll be just fine. Just check the published diameter of the replacement with the original, as tire diameters do vary by manufacturer.

 

While the rolling radius certainly does change with tire pressure, with radial tires, the radial band keeps it a really constant length, and so the OD does not really change much at all with tire pressure.

 

That is what differentials (3 of them on a 4MATIC - Front, Rear and Center) are for. Even limited slip ones. The 4MATIC actually allows wheel speeds to be different (a slipping tire speeds up, so 4MATIC brakes it, providing more traction to the other side).

 

Oops. Your right. I added tread depth to the 18" diameter of the rim and completely forgot to add in the depth of the tire itself. Point is still valid but your number work out more correctly. Funny because I just ran into this again this week. I blew out a tire on a pothole... Went to townfair tire and they wanted me to buy the road hazard warranty. Then I asked if I buy it and someday need a new tire will they put one on for free without forcing me buy all the other tires. The bumblling began.