Asynchronous tyre wear damaging your diff

[ColinsCornflakes]

Several times recently I've heard people saying that if your tyres aren't matched to 1mm or so across an axle with a diff (fwd/ rwd & 4x4) then you're taking a massive risk and your car is probably going to explode or something more serious.

Is this real? Isn't the purpose of a diff to allow one wheel to rotate at a different speed to the other one, is this just a rumour being put about by tyre wholesalers?

(Sorry for the typo in the title)

Edited by ColinsCornflakes on Thursday 15th October 23:00

[irc]

My guess. If you can drive for 50 miles on a spacesaver with a huge difference in diameter then 3mm difference will make no difference.

[5lab]

if you've a limited slip or torque vectoring diff it can wear whatever device is used to apply torque. If its a wide open dif I can't see any way it can apply wear.

[swisstoni]

Sounds like one of the things they say to keep petrolheads awake at night.

[Geertsen]

I may be being simple but wouldn’t mismatched tyre pressures have the same effect? If so, there are so many variables that I don’t believe it. Unless the tyres were wildly different maybe.

[jimmyjimjim]

Depends on the car. Calibra 4x4 was notorious for center diff issues caused by uneven tire wear. Other vehicles, not so much.

[RicksAlfas]

Isn't this a thing which keeps BMW X-drive owners awake at night?

[Chris-ozpmv]

RicksAlfas said:

Isn't this a thing which keeps BMW X-drive owners awake at night?

Certainly is! F25 2014 X3, recommendation is I think front/back need to be within 3-4mm of tread. Had to replace 2 fronts as one had a nail in it and was unrepairable. The rears are perfectly fine but probably about 4mm left on them and the drivetrain clonks at low speeds now... :-( I don't really want to spend another £400 on rear tyres as well just because BMW can't engineer a decent 4x4 system but will probably have to...

[QuartzDad]

Son has a 2013 320d XDrive, he had a very ominous rear axle clunking until he sorted out the tyres.

[RazerSauber]

jimmyjimjim said:

Depends on the car. Calibra 4x4 was notorious for center diff issues caused by uneven tire wear. Other vehicles, not so much.

Cavalier and Calibra 4x4 were plagued by transfer box issues due to uneven tyre wear, and they are to this day. Those boxes are really thin on the ground now too. All 4 tyres had to be the same make and model, with matching tread. Imagine 1 blow out and you're left needing 4 brand new tyres. Not fun.

It is a real thing but a lot of manufacturers will have engineered ways around it by now I would think.

[chris1roll]

Same with the early Volvo AWD systems that used a viscous coupling
All needed to be within ~1mm of wear and of the same make.
People broke them within 50 miles of having just two tyres replaced, or by using the special spare.

[Kawasicki]

Wear has almost no impact on rolling circumference.

[Don Roque]

They used to say this about the Audi A8 I used to own (a D2 model). Torsen AWD, needs all four tyres to be within 1mm of wear or it'll explode...etc. I was always more worried about the gearbox blowing up than the AWD system, since they were notorious for it.

[foccer]

Kawasicki said:

Wear has almost no impact on rolling circumference.

Is that why speedos are bang on accurate throughout a tyres life?

FYI - your speedo will vary by about 3% from a brand new tyre to a fully worn one due to the 12mm (leaving 2mm depth) difference in diameter of a new vs worn tyre.

[Ritchie335is]

I recently had to deal with this on a 2019 Porsche Cayenne.
Porsches recommendation, is that there is no issues with different tyres front / rear (it must have a usually open / switchable centre diff) but side to side on the same axle needs to be up to 15% wear IIRC.
Frustrating when the wife scuds a rock at the side of the road and tears a sidewall on a tyre that is about 20/25% worn.

[Kawasicki]

foccer said:

Kawasicki said:

Wear has almost no impact on rolling circumference.

Is that why speedos are bang on accurate throughout a tyres life?

FYI - your speedo will vary by about 3% from a brand new tyre to a fully worn one due to the 12mm (leaving 2mm depth) difference in diameter of a new vs worn tyre.

I carried a measurement on the rolling circumference of three different tyre sizes, new vs legal limit tread depth. All varied by significantly less than 1%. 3% is not likely, published studies agree. https://saemobilus.sae.org/papers/tire-rolling-rad...

“there was an average change in the rolling radius of 0.34 mm for each 1 mm change in tread depth”…

Your calls are off by a factor of about 3, and the Michelin results are the same as mine.

[Pica-Pica]

Kawasicki said:

foccer said:

Kawasicki said:

Wear has almost no impact on rolling circumference.

Is that why speedos are bang on accurate throughout a tyres life?

FYI - your speedo will vary by about 3% from a brand new tyre to a fully worn one due to the 12mm (leaving 2mm depth) difference in diameter of a new vs worn tyre.

I carried a measurement on the rolling circumference of three different tyre sizes, new vs legal limit tread depth. All varied by significantly less than 1%. 3% is not likely, published studies agree. https://saemobilus.sae.org/papers/tire-rolling-rad...

there was an average change in the rolling radius of 0.34 mm for each 1 mm change in tread depth

Your calls are off by a factor of about 3, and the Michelin results are the same as mine.

I think the static, unloaded tyre will show a change in circumference between new and worn. I suspect the rolling circumferences will be closer. I imagine calculated versus empirical measurements will differ here.
Thanks for the link. An interesting read.
Keeping to the same tyre make and tread pattern, and maintaing correct tyre pressures appear to be the main factors affecting rolling circumference.

Edited by Pica-Pica on Saturday 17th October 12:55

[omniflow]

Kawasicki said:

foccer said:

Kawasicki said:

Wear has almost no impact on rolling circumference.

Is that why speedos are bang on accurate throughout a tyres life?

FYI - your speedo will vary by about 3% from a brand new tyre to a fully worn one due to the 12mm (leaving 2mm depth) difference in diameter of a new vs worn tyre.

I carried a measurement on the rolling circumference of three different tyre sizes, new vs legal limit tread depth. All varied by significantly less than 1%. 3% is not likely, published studies agree. https://saemobilus.sae.org/papers/tire-rolling-rad...

there was an average change in the rolling radius of 0.34 mm for each 1 mm change in tread depth

Your calls are off by a factor of about 3, and the Michelin results are the same as mine.

I'm fairly sure it's down to (fairly) simple mathematics.

Taking a fairly average tyre - 225/45 ZR17

The overall diameter of a new tyre is:

17 * 25.4 + 2 * (45 * 225 / 100) = 634.3mm

The circumference of this tyre is 1993mm

When worn from 8mm tread to 2mm tread the diameter is now 622.3mm

The circumference is now 1955.27, a difference of 37.7mm or approx 1.9%

Obviously, for smaller tyres the % would be higher and for larger tyres the % would be smaller.

[jimmyjimjim]

Ritchie335is said:

I recently had to deal with this on a 2019 Porsche Cayenne.
Porsches recommendation, is that there is no issues with different tyres front / rear (it must have a usually open / switchable centre diff) but side to side on the same axle needs to be up to 15% wear IIRC.
Frustrating when the wife scuds a rock at the side of the road and tears a sidewall on a tyre that is about 20/25% worn.

You can get tires shaved.

[Panamax]

Car gurus on PH say, "it's nothing, don't worry about it".

Toyota/Lexus says rotate your 4x4 tyres every 3,000 to 5,000 miles.

BF Goodrich says rotating your tires is not a gimmick, rotate immediately at the first sign of irregular wear, even before that mileage.

In many cases the diff(s) have either mechanical limited slip or electronically controlled limited slip. Where that mechanism involves friction materials then if it's constantly "working" there will be more heat and more wear.