X-Drive Staggered Question (any new info?)

[dawg1]

Researching the internet, I've seen multiple posts stating x-drive tire sizes must be within 1% of each other. I can't really find the source of this and wondered if any new information has come to light.

I also found this by another poster:

"The xDrive system has a calibration and correct function at startup that looks at the wheel speeds and determines the correct rotational allowance for the tire setup. As of 2015, I've seen some technical documentation that indicates it will consider up a 2% spread as valid."

and "The BMW awd system is not always locked like some other manufactures"...

On the newer models (say '14 and up), is there really a calibration system or is the 1% rule still the guideline? I have access to some good tires that are 2.3% different and am trying to be certain they cannot be used on x-drive.

[DuSh]

Originally Posted by dawg1

Researching the internet, I've seen multiple posts stating x-drive tire sizes must be within 1% of each other. I can't really find the source of this and wondered if any new information has come to light.

I also found this by another poster:

"The xDrive system has a calibration and correct function at startup that looks at the wheel speeds and determines the correct rotational allowance for the tire setup. As of 2015, I've seen some technical documentation that indicates it will consider up a 2% spread as valid."

and "The BMW awd system is not always locked like some other manufactures"...

On the newer models (say '14 and up), is there really a calibration system or is the 1% rule still the guideline? I have access to some good tires that are 2.3% different and am trying to be certain they cannot be used on x-drive.

You will be pushing it.

I calculated earlier in different thread that a usual owner can have as much as 3% spread at some point of time due to uneven wear (after all x-drive is RWD biased system) and other factors. Not to mention that even just during simple turns tires will rotate differently - can't avoid that - it's just physics. What I'm trying to say is that you will always have a spread more than 2-3% occasionally, that's for sure, even with all 4 exactly similar tires there's going to be a spread in everyday life usage scenario. That is exactly why x-drive system has a spread tolerance - otherwise every x-drive car would go in lump mode with broken transfer case after first couple turns
The main difference is that if you constantly have such big spread - then it's going to affect transfer case and transmission causing wear and tear and eventually failure
2.3% is doable, but you will be pushing it - it won't brake right away, but given enough time and mileage it will. If it were me - I wouldn't do it - cost of set of tires is not comparable to expenses to fix x-drive system

[bmwx5er]

Just curious on how you folks are computing % differences? The % difference is rotational distance AFAIK, not just difference in tread depths. The difference between a 20” tire with 10/32 depth and 4/32 depth is only 1.3%. How do you get to 2 or 3%?

[Roundown]

Originally Posted by DuSh

You will be pushing it.

I calculated earlier in different thread that a usual owner can have as much as 3% spread at some point of time due to uneven wear (after all x-drive is RWD biased system) and other factors. Not to mention that even just during simple turns tires will rotate differently - can't avoid that - it's just physics. What I'm trying to say is that you will always have a spread more than 2-3% occasionally, that's for sure, even with all 4 exactly similar tires there's going to be a spread in everyday life usage scenario. That is exactly why x-drive system has a spread tolerance - otherwise every x-drive car would go in lump mode with broken transfer case after first couple turns
The main difference is that if you constantly have such big spread - then it's going to affect transfer case and transmission causing wear and tear and eventually failure
2.3% is doable, but you will be pushing it - it won't brake right away, but given enough time and mileage it will. If it were me - I wouldn't do it - cost of set of tires is not comparable to expenses to fix x-drive system

What are you basing this on? I'd have thought 3% would eat the transfer case?

[DuSh]

I'll have to find my notes and I post then, but there's more factors than just thread depth. One example is tire pressure difference: here in Arizona, when my x5 is parked under shade cover my rear wheels are hit by sun almost all day - difference is usually easily 5-7 psi when I start rolling. That psi difference alone can add to a rotational difference about 1% - don't remember correlation right now, but I can find out. Bunch of other factors like when I'm exiting a 180 degree circle ramp from highway - inner side of tires will get squished by a lot more than 1-2% rotational difference and it's a good 20 sec until you exit ramp - if x-drive was without spread tolerance it would have bailed out on us on the first day. During my spirited driving I'm tossing a car like a Mario Go cart, imagine what forces tires experience and how much rotational circumference changes during that - and all this during harsh acceleration and load on transfer case and yet it's still working fine so far
All I'm saying is that tolerance has to be at least 3% if not higher for short period of times and best when kept under 1% for permanent tire set. As OP was asking 2.3% will be too much for everyday usage IMHO

In my e90 days people used to drive with 2% difference for years and not get into any issues, others would do 3% spread and occasionally get into shudder when going uphill under load. Empirically it looks like 3% is not going to blow x-drive right away, but will probably do so if used for longer periods