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      02-25-2013, 05:53 PM   #1
netarc
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AWD tire replacement caveat

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So got a screw in my Alpin PA2 runflat tires after a mere 3k miles - had the tire replaced easily enough, but while at the tire shop (BigO tires) I was stunned by the rep's assertion that on an AWD car, when one tire is replaced you must typically replace ALL tires.

Thankfully, as my car had only ~3300 miles on it, the other tires still had full tread depth, and rep advised I was ok purchasing a single tire. But this was a real shocker/eye-opener for me, having never owned an AWD car before.

Some details re: the issue at...
- http://www.tirerack.com/tires/tirete....jsp?techid=18
- http://askville.amazon.com/replace-t...uestId=2651049
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      02-25-2013, 06:18 PM   #2
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Yep, pretty common on awd cars. Having owned a few quattro Audii, I've dealt with this before.
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      02-25-2013, 06:58 PM   #3
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There's lots of posts all about this, basically problems might occur if the tread depth is too different between tires. Everything from abs, traction, excessive diff wear, and so on.

Note though that the summer tire options for the X1 and all sorts of other cars all have slightly different diameter between the front and rear wheels _stocl_ so some difference is likely fine. Reports from 1%-5% variance seems okay for some situations, most try to stick to 2%-3%.
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      02-26-2013, 02:37 PM   #4
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I would think that with BMW's rear bias, the rears will wear faster and you would be ok replacing two rear tires vs all 4 depending on wear...especially on M Sports with 80/20 rear bias.

I learned firsthand that in situations in which you have a single tire failure (under their protection plan) and two or four tires are recommended, America's tire can give you $25-50 credit per good tire towards new tires...better than nothing.
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      02-26-2013, 03:09 PM   #5
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Quote:
Originally Posted by nospam View Post
I would think that with BMW's rear bias, the rears will wear faster and you would be ok replacing two rear tires vs all 4 depending on wear...especially on M Sports with 80/20 rear bias.
This is a good point, especially considering that BMW does not recommend tire rotation at all (staggered tires or not, AWD or not).

Replacing 4 tires because of one failed tire is nonsense, if the tire is almost worn out, then replace the axle at most, or just shave the new tire.
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      02-26-2013, 04:05 PM   #6
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I'd recommend people read up on tire racks info about awd and wheel replacement: http://www.tirerack.com/tires/tirete....jsp?techid=18 Tread difference can cause some serious issues and possibly void a warranty if bad enough. Unfortuntely BMW doesn't really tell us specifics on ranges and how they effect abs and traction control. There is some info around the net but not all models are the same so it's hard to make educated choices if we're risking problems with diffs and safety.

Rears do wear slightly more on rwd and awd but rotating solves this problem. It's also not just acceleration but braking as rear brakes often have more force applied than the front brakes. Fronts get more edge wear from cornering but so can rears depending on driving habbits.

M-sport doesn't have more rwd biast than non M-sport except in very limited cornering conditions under heavy acceleration. All xDrives are the same "80/20" rear biast.

Rarely will manufacturers pro-rate remaining tread life of the other tire (or 3 tires) if you do need to replace 2 or 4 at once, but sometimes you can fight for it or get a discount but stores don't like dealing with these situations. Most tire tread warranties also require all 4 tires to be under 2/32 before a single tire warranty will be considered for cost reduction of new tires, and if you're like most people the tires can wear up to 2mm different which means it's not safe to get all treads down to a warranty amount (but they love this marketing ploy with fine print). You basically have to watch tire pressure weekly, rotate perfectly, not vary driving, and so on to have them all reach 2/32 at the same time which few people ever do.

If you've opted for the performance/summer tire options you're in the worst of everything as the rear tires are wider and can not be rotated. Also, the few manufacturer that do have tread wear warranties are always fully voided if you have wider/staggered wheels on the rear. (and to note, BMW does "recommend" people never rotate tires but most people do for better tire wear) Add this to the fact that some states or companies have rules that new tires must always be installed on the rear if only 2 are being done. I've known two people who went to have rear staggered tires replaced and ended up having to buy 4 new tires when the fronts were only 6 months old and had about 2mm of tread used which I find completely absurd.

I personally was going to do the performance tires until I found out the disadvantages to them and the lifetime cost of tires for this setup was not going to be a good value. I'll be getting a spare for the trunk and switching to non runflat summer and buying additional wheels for a set of winter tires, all with tread warranties 'just in case' and mainly for the ability to rotate all 4 tires. If I have to replace a single tire mid wear, I'll get a new shaved one from tire rack or get it shaved locally.

Edit: Oops, went away and came back to finish this post, didn't mean to duplicate what Bill said...
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      02-26-2013, 04:44 PM   #7
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Quote:
Originally Posted by edx1 View Post
M-sport doesn't have more rwd biast than non M-sport except in very limited cornering conditions under heavy acceleration. All xDrives are the same "80/20" rear biast.
Incorrect, standard xDrives have 60/40 rear bias and M Sports have 80/20.
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      02-26-2013, 05:16 PM   #8
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Quote:
Originally Posted by nospam View Post
Incorrect, standard xDrives have 60/40 rear bias and M Sports have 80/20.
Some how I've missed any information on this, is there something that states this? I know the M Sports have Performance Control, but I haven't found any literature around the different power application.
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      02-26-2013, 06:03 PM   #9
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Some how I've missed any information on this, is there something that states this? I know the M Sports have Performance Control, but I haven't found any literature around the different power application.
It used to show up on the build-your-own site but doesn't seem to show the M Sport performance control details any more.

It has been discussed in the past: http://e84.xbimmers.com/forums/showthread.php?t=775070
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      02-26-2013, 06:11 PM   #10
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Quote:
Originally Posted by nospam View Post
It used to show up on the build-your-own site but doesn't seem to show the M Sport performance control details any more.

It has been discussed in the past: http://e84.xbimmers.com/forums/showthread.php?t=775070
I just wish it was stated a little clearer, as it might have swayed us towards M Sport.
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      02-26-2013, 06:12 PM   #11
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Quote:
Originally Posted by nospam View Post
Incorrect, standard xDrives have 60/40 rear bias and M Sports have 80/20.
The 80/20 is referred to as "Dynamic Performance Control (DPC)" which if I recall only is active during cornering between 29mph and 57mph, or near those numbers. I'll have to dig later for the docs on this though to back up my statement.

Edit:
Found different documents, not the ones I remember. I seem to think it might have been in a 3 series or X1 manual I read about the speeds.

In this document it states it's a mechanical clutch for each rear wheel:
https://docs.google.com/viewer?a=v&q...MoGF2fwmcwQ5jA

In other forums for bmw models people state that it's purely software controlled and costed either $100 or $300 depending on the model for this option to be selected (or done via coding for free)

You can also google and find references on bmw pages that state that some models are even brake based. No coding docs are out for the X1 so I don't know if the X1 uses braking or a clutch and if the clutch is in all X1's and just enabled via software or if it's a unique clutch to the MSport but my assumption is it's there on all xDrive for traction control and there's a software tweak to give extra biast to the back.

Online there are also references that the 80/20 biast is on all the time and other pages that state it's only under hard acceleration in cornering.

If someone has specific info for the X1 let us know.

Last edited by edx1; 02-26-2013 at 06:30 PM.
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      02-26-2013, 06:22 PM   #12
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Quote:
Originally Posted by edx1 View Post
The 80/20 is referred to as "performance control" which if I recall only is active during cornering between 29mph and 57mph, or near those numbers. I'll have to dig later for the docs on this though to back up my statement.
I think you may be confusing the 80/20 rear bias with the DTC torque vectoring feature of PC.
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      02-26-2013, 06:29 PM   #13
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http://www.edmunds.com/bmw/x1/2013/road-test.html
Quote:
Opt for the M Sport package and you also get what BMW calls Performance Control. This delivers a 20-to-80 front-to-rear torque split in standard conditions in an attempt to replicate the feel of rear-wheel drive.
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      02-26-2013, 06:49 PM   #14
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Quote:
Originally Posted by nospam View Post
I think you may be confusing the 80/20 rear bias with the DTC torque vectoring feature of PC.
Too many acronyms...

I'm pretty sure I'm not mixing DTC and DPC. DTC if I recall brakes a single wheen below 25mph, wheels in pairs to 50mph, and above 50mph it's engine control only. When I was reading on the mph numbers for DPC I recall an image similar to this: http://www.bmw.com/com/en/newvehicle...e_control.html

Also if I'm reading everything correctly, Dynamic Performance Control has nothing to do with front and rear power biast but only and specfically rear right vs rear left biast.
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      02-26-2013, 06:57 PM   #15
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Sorry for all of the acronyms. The X1 and X3 do not feature Dynamic Performance Control which is true torque vectoring featured on the X6 and X5/6 ///M. The xDrive M Sports use Dynamic Traction Control to independently brake wheels to simulate DPC.

My apologies for taking this thread WAY off topic.
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