[CinBW]

Hi guys,

I know this must have been discussed a million times, so my apologies in advance for creating yet another thread.

I have an F25 3.0d, slightly modified, 420Hp, 820Nm. Due to the increase in power, I changed tyres from a staggered setup to a square setup running 275x35R20s with a 35mm offset all around. Good grip with no clearance issues.

I had the Transfer Case issues before and had it replaced, with the square setup I had more peace of mind due to the same tyre size on all four wheels.

Unfortunately, I have to replace all my wheels, this may result in running a staggered setup once again. This would be an Insurance Job, so what would be your opinion? Fight like hell to keep a square setup or settle for Staggered and risk the Transfer Case? Not sure if the extra torque would affect the Transfer Case with different rolling circumferences?

[pungo]

A proper staggered configuration has all the same tire circumferences therefor should not have any different effect on the transfer case compared to a square set up.

[383vett]

Originally Posted by pungo

A proper staggered configuration has all the same tire circumferences therefor should not have any different effect on the transfer case compared to a square set up.

Correct. The width of the tires will be different front to back, but the circumference and diameter will be the same if done correctly.

[CinBW]

Thanks guys, I would have agreed and felt more comfortable if the recommended sidewall size was the same, but the recommendations are not the same.

I'm struggling to wrap my head around this but with the following recommended sizes sidewall height is different, so how would the rolling circumference be the same?

245/40ZR20 99Y front axle 8.5Jx20 ET38
275/35ZR20 102Y rear axle 10Jx20 ET51

[Brandoch]

Originally Posted by CinBW

Thanks guys, I would have agreed and felt more comfortable if the recommended sidewall size was the same, but the recommendations are not the same.

I'm struggling to wrap my head around this but with the following recommended sizes sidewall height is different, so how would the rolling circumference be the same?

245/40ZR20 99Y front axle 8.5Jx20 ET38
275/35ZR20 102Y rear axle 10Jx20 ET51

To the googles

[pungo]

Originally Posted by CinBW

Thanks guys, I would have agreed and felt more comfortable if the recommended sidewall size was the same, but the recommendations are not the same.

I'm struggling to wrap my head around this but with the following recommended sizes sidewall height is different, so how would the rolling circumference be the same?

245/40ZR20 99Y front axle 8.5Jx20 ET38
275/35ZR20 102Y rear axle 10Jx20 ET51

Yeah, that's not how it works, as Brandoch mentioned "To the googles", tons of explanations on exactly what those number mean, or use one of the many online tire calculators

[CinBW]

Originally Posted by pungo

Yeah, that's not how it works, as Brandoch mentioned "To the googles", tons of explanations on exactly what those number mean, or use one of the many online tire calculators

Thanks guys,

I have Googled this and it's of no use when doing the math of the sidewall relation to width, it's simply not accurate enough in my opinion.

When using online Tyre Calculators I get the following when comparing the two sizes:
Size 1/ Size 2. Diameter 27.7"/27.6"/-0.4%. Width 9.6"/10.8"/+12.5%. Sidewall 3.9"/3.8"/-2.6%. Circum. 87"/86.6"/-0.4%. Revs/Mile 728/732+4.

The 0.4% or 0.1" sidewall height difference is insignificant, however, there would still be an imbalance in the transfer case. After blowing one transfer case I might just be paranoid about the numbers.

I guess BMW knows best about the imbalances the vehicle can handle. I would still prefer a square setup, but if it comes down to staggered I'll make peace with it.

Appreciate the answers!

[ClutchFC]

Check tire manufacturer for specific tire info, or other distributors such as TireRack where they list the specs. Width can be whatever you want, the main focus is diameter as you want that as close to the same front and back, as possible. I've read no more than a 10% difference, I would shoot for 5% to be safer.

BMW will also put a 40 series sidewall on the front and 35 on the rear on some of their cars from the factory. Even visually you can see the difference, so there's probably some leeway there they allow. But I understand you want to avoid as much difference in tolerance as possible.

[pungo]

Originally Posted by ClutchFC

I've read no more than a 10% difference, I would shoot for 5% to be safer.

Pretty sure the allowable difference is up to 1%, not 10%, that's a pretty large difference and would definitely not be healthy for the transfer box.

[Wgosma]

Originally Posted by pungo

Pretty sure the allowable difference is up to 1%, not 10%, that's a pretty large difference and would definitely not be healthy for the transfer box.

Attached is the BMW tire spec from the TIS

[ClutchFC]

Originally Posted by pungo

Pretty sure the allowable difference is up to 1%, not 10%, that's a pretty large difference and would definitely not be healthy for the transfer box.

Yeah probably so, I'm definitely wrong there

[CinBW]

Thanks for the answers guys, it just makes sense to go with a square setup and the same tyres all around. I'm not sure how flimsy the Transfer Case is, but I'd prefer not to replace it out of my pocket.

I often tend to drive aggressively, losing traction on a wheel here and there happens all the time - track days . I can't see how it would be healthy for the Transfer Case to always shift traction between axles, especially with tyres differing in size (even slightly).

With the tyre setup I'm running I get excellent traction. Putting 800Nm down and the tyres not matching up will result in the Diffs as well as the Transfer Case absorbing additional forces that would cause increased wear. Even with increased Tyre wear the Drivetrain Gears would still take some punishment and who can say how long it'll last?

[HPR]

75k miles with BMW OEM staggered setup and no drive train problems.