Staggered tire diameter affecting xDrive AWD?

[Derek Zoolander]

I know the recommended tire sizes for 18x8 & 18x10 would be like 225/40 and 255-265/35.
Would running a 245/35 rear cause problems with the xDrive transfer case or diff? There would be a 0.3" difference in tire diameter and 11 revs/mile compared to the 225/40 front (-1.352%).

[Shomegrown]

So you want more understeer in a car that will already be understeer prone?

[lojasmo]

Don't do it.

[Grinch337]

Quote, originally posted by Shomegrown »

So you want more understeer in a car that will already be understeer prone?

Wouldn't narrower tires in the rear help to neutralize the handling characteristics of the car, all else being equal? The front tires are still going to break away at the same point, while now the rear would break away earlier than stock due to a narrower contact patch, right?
OP: I would think not due to the sophisticated computers controlling the AWD system, but truthfully I dont know. You should probably consult either a tech at your dealer, or maybe try one of the dedicated BMW boards that has knowledgeable people about your AWD system. PM me for another suggestion as well as to how to find out the information.

[Shomegrown]

Quote, originally posted by Grinch337 »

Wouldn't narrower tires in the rear help to neutralize the handling characteristics of the car, all else being equal? The front tires are still going to break away at the same point, while now the rear would break away earlier than stock due to a narrower contact patch, right? OP: I would think not due to the sophisticated computers controlling the AWD system, but truthfully I dont know. You should probably consult either a tech at your dealer, or maybe try one of the dedicated BMW boards that has knowledgeable people about your AWD system. PM me for another suggestion as well as to how to find out the information.

You're correct. [IMG]http://*****************.com/smile/emthup.gif[/IMG]
I didn't know the car came with staggered tires. I thought he was talking about going from same sized tires to a staggered setup.

[strictlyA2]

You can get staggered fitment wheels on new x-drive BMW's.

[Subwoofers]

Do it. Jersey FTW.

[VR6 NRG]

just dont try to lower the car, the whole half-shaft going through the middle of the oil pan thing puts a kabosh on any kind of sport suspension. Thats engineering for ya
quattro FTW

[Corbic]

Quote, originally posted by VR6 NRG »

just dont try to lower the car, the whole half-shaft going through the middle of the oil pan thing puts a kabosh on any kind of sport suspension. Thats engineering for ya quattro FTW

If you are buying an AWD BMW, you don't care about sportyness.

[mrothwell]

you should be extra hella flush and put a 225/40 on the your 18x11s

[Derek Zoolander]

I'm not concerned with handling and performance. It's a 328xi.
I'm already lowered on KW coilovers, as well as the other 1000 people on e90post.com, so this statement is garbage.

Quote, originally posted by VR6 NRG »

just dont try to lower the car, the whole half-shaft going through the middle of the oil pan thing puts a kabosh on any kind of sport suspension. Thats engineering for ya quattro FTW

Quote, originally posted by msrothwell »

you should be extra hella flush and put a 225/40 on the your 18x11s

No. I'm asking about 245s on a 18x10" wheel.

Quote, originally posted by strictlyA2 »

You can get staggered fitment wheels on new x-drive BMW's.

Yes, but my OEM tires are 225/45/17 and 255/40/17 which is a negligible difference in rolling diameter.
I understand TCL doesn't like stretch, but I'm looking for actual, intelligent answers.


Modified by Derek Zoolander at 11:35 AM 1-7-2010

[mrothwell]

yea, I think that the 335xi with the sport package comes with 18x8.5 with 255/35R18 rear and 18x8 with 225/40R18, so I think thats what I'd go for. Last thing you want to do is have your AWD system blow up and then have your warranty voided because you ran the wrong tire size.

[SomeRandomGuy]

You'd be better off balancing the handling with air pressure, than by fooling around with tire widths, imo.
Put more pressure in the rear tires and they'll break free sooner than the fronts.
I learned this trick when I used to auto-x my integra. I would jack all the tire pressures up, but the backs especially. It really enhanced the lift-throttle oversteer I could accomplish.

[Derek Zoolander]

Quote, originally posted by msrothwell »

yea, I think that the 335xi with the sport package comes with 18x8.5 with 255/35R18 rear and 18x8 with 225/40R18, so I think thats what I'd go for. Last thing you want to do is have your AWD system blow up and then have your warranty voided because you ran the wrong tire size.

But I want to run a 245 for fender clearance. I'm not asking for tire size suggestions. I know what I should run. I'm asking about the consequences of variation between front/rear rolling diameters.

[mrothwell]

Quote, originally posted by Derek Zoolander »

But I want to run a 245 for fender clearance. I'm not asking for tire size suggestions. I know what I should run. I'm asking about the consequences of variation between front/rear rolling diameters.

Hate to say this then, I think you bought the wrong size/offset wheels if you can't fit the OEM size tire in the fender.

[Corbic]

Quote, originally posted by Derek Zoolander »

But I want to run a 245 for fender clearance. I'm not asking for tire size suggestions. I know what I should run. I'm asking about the consequences of variation between front/rear rolling diameters.

Pull your fenders.
What you want us to say is your car will be fine, but no one has the experience to be sure, so bottom line you run the risk of damaging your drive line.

[Derek Zoolander]

Quote, originally posted by msrothwell »

Hate to say this then, I think you bought the wrong size/offset wheels if you can't fit the OEM size tire in the fender.

Wow. I didn't buy the wheels. There is no right or wrong offset, just recommended. My stock rear tires are 255s. I can fit anything up to 285s. Does that mean I bought the wrong wheels?
Don't you realize that people run aftermarket wheel/tire setups larger than stock?
E90 rear fenders are super thick. Pulling would require extra bodywork.


Modified by Derek Zoolander at 2:20 PM 1-7-2010

[Gitcha Sum]

Quote, originally posted by Derek Zoolander »

I'm asking about the consequences of variation between front/rear rolling diameters.

It sounds like you understand that your transfer case will be working constantly to overcome that "-1.352%" from your first post.
I doubt it would explode the first day. I also doubt it would last as long as one riding in a car with balanced corners. There's so many other variables, a definite answer really isn't possible.

[strictlyA2]

Quote, originally posted by Derek Zoolander »

I understand TCL doesn't like stretch, but I'm looking for actual, intelligent answers.

Generally speaking, staggering wheels on an awd drive vehicle isn't recommended, from my experience (Volvo's especially) we would sell a set of four tires if there was a variance in tread depth alone. BMW's come from the factory with very specific tires, not just specific sizes but literally specific tires. If you are staggering a set of tires that BMW does not normally fit (even as a factory option) you are pretty much on your own if you are wondering what will roll equally front to back etc.
The Car Lounge is the last place I would seek advice for something like this, its a silly place unless you want to be insulted, applauded, berated, praised, personally attacked and left with a feeling of regret after only a few thread replies. If I were a woman I would equate the feeling of posting technical questions in TCL the same way I would feel entering/ exiting an abortion clinic.
Give the Tire Rack a call, they are quite helpful when it comes to stuff like this.

[Derek Zoolander]

I've got a thread over on e90post. I was just curious because there are some knowledgeable people here.

[justanotherusername]

By doing this, the ECU is going to sense the differences in rolling diameters as wheel slip. That part is a given.
The question is, is it enough slip for it to try and do something about it? If this passes the thresholds where the xDrive system is going to try and shift power from one end to the other, it's going to cause damage for sure.
If it's not past that threshold, it's going to lower the amount of real world slip it takes to reach the threshold, and it could possibly cause damage, especially when it happens on dry roads.
Personally I wouldn't attempt to run tires with different rolling diameters on an AWD car under any circumstances. The systems needs everything spinning at the same speed to work properly.

[bootymac]

Quote, originally posted by Derek Zoolander »

knowledgeable people here.

[Murderface]

Quote, originally posted by Derek Zoolander »

I've got a thread over on e90post. I was just curious because there are some knowledgeable people here.

Here's what I think...I know AWD has to have some amount of rolling diameter difference tolerance built in to account for tire wear, so you ought to be able to get away with a little difference. You said the difference in rolling diameter would be .3" which = 9.6/32". Most tires come with 7/32 - 9/32" tread depth. Therefore what you're doing would be roughly equal to running identical tires all around but with two brand new tires and two tires with a bit less than half tread. Hard to say whether that'd be liable to damage something or not.
BMW should tell you if you ask them. Someone in the TT forum emailed Haldex asking about quattro's tolerance for the same thing and they responded that it's around 4%...which is something like 3/4" if I remember right.
I wouldn't risk breaking expensive things like AWD systems just to have my car slamd on its nutz, but I guess some Mk4 habits don't leave even after you sell the car and replace it do they?


Modified by Murderface at 6:11 PM 1-7-2010

[drionbh3]

Don't have to take my thought seriously since I'm a complete AWD noob, but here it is:
Having skinnier tires than stock at the rear will induce more oversteer since there's less contact patch, causing the rear tires to break away sooner than stock. Setting yourself in a situation of hard fast cornering, the ECU will sense the slippage and will adjusts the power applied on the wheels of whichever axle it is designed to adjust. To neutralize the car, the ECU would have to make action of understeer.
If the power is adjustable at the rear wheels, the ECU would lower the power at the rear; if it's adjustable at the front, it would increase that; and if both, then somehow it'll have to find balance but that's too complicated of a system. Rear adjustable power would be safer and more practical since the car will slown down, it is easier to "breathe" off power than to apply more, and it is less stressful on the drivetrain. This is completely the opposite if it was front adjustable, but it's more fun since we're going at higher speed!
Figure out how your system works and what your tolerances like others have mentioned. Whatever I said is completely theory and somewhat pulled out my ass...

[2000JettaGLXVR6]

LOL
Double Fail today
https://forums.vwvortex.com/zerothread?id=4712767