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Discussion Starter #1
I know all four are supposed to be the same diameter ... but ...

Wondering if anyone has replaced just two (on the same "axle")? If so, what happened?

My two rear tires are much more worn than the fronts.
 

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What will happen is the vehicle will constantly think that the rear tires are slipping and apply torque to the front, an axle that doesn't need torque. On a dry road, the weak link is the clutch packs in your transfer case, which will then circulate clutch material through the unfiltered fluid. Degree of wear is the important part... I seem to recall a 1-2% difference in overall height the maximum.

Replace all 4.

Or rotate every 5k miles to prevent this.
 

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If I was in your shoes, I'd look at ebay or bestusedtires.com and try to find tires that match the two good ones. I've done this multiple times and been pleased with the results. These guys are your friend when you drive AWD vehicles.

AM.
 

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On Frau Putzer's xDrive, the back tires were 50% faster than the front ones. On my sDrive (RWD), the back tires wear 100% faster than the front ones.

The bigger the difference in tire outside diameters front-to-back, the more you're wearing the xDrive's transfer case clutches on dry pavement while traveling in a straight line.

Tires usually start out with 10/32nds inch of tread. If you've worn down the back tires 6/32nds inch (to 4/32nds inch remaining tread), you've probably worn down the front tires 3/32nds inch (to 7/32nds inch remaining tread). So you now have 3/32nds inch difference in tread depth between the front and back. If you buy new back tires, you'll have 10/32nds inch tread in the back and still have 7/32nds inch of tread in the front. So, you'll still have that 3/32nds inch tread depth difference front to back. That difference will diminish as your new rear tires wear faster, until the new rear tires' tread depth is the same as the front tires' tread depth. By the time the new rears are worn out, the original fronts will also be worn out.

If you have a square set-up, you can rotate your tires and eventually even out the wear front-to-back. Tire rotation is a lot cheaper ($25) than two new tires (~$500) or four new tires (~$1000).

Four new tires now, or two used tires for the rear that match the tread depth of the front, will eliminate xDrive clutch wear on dry pavement now. Rotating or two new tires will allow the xDrive clutch wear on dry pavement to continue, but it will be diminishing.

There are a lot of used tires for BMW's around. That's because some people ditch RFT's soon after getting them. A lot of BMW leasers keep the OE RFT's for re-installation right before lease turn-in. A lot of leasers with worn out RFT's will buy used ones just meet the lease turn-in tread depth requirement (4/32nds inch). That's what's created a market for mostly worn out OE RFT's for BMW's.

What is important to the xDrive clutches is the average diameter of the front tires compared to the average diameter of the rear tires. So, you could rotate one side from to back now and eliminate clutch wear now. But, it's very dangerous having tires on one side performing better than tires on the other side. That's how violent spins start.

I made a point to order Frau Putzer's X3 30 xDrive with a square set-up so that I can rotate the tires. I ordered my 535i with a square set-up for the same reason, so that I can buy tires four at a time and eventually sell the car with four worn out or almost worn out tires on it.
 

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It's possible to change only 1 pair of tires. As mentioned before - check the tire diameter (car on the ground) and adjust tire pressure accordingly. e.g. 2.4bar front 2.3bar rear

it's not a rocket science, it's a simple matter of abs comparing the speed of front vs rear
 

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Discussion Starter #6 (Edited)
Yes, but ... has anyone actually done it? And what was your first-hand experience?

P.S. #1 is an M-Sport with different front/rear sizes, so rotating front/back is not an option.
 

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Ah, if staggered set-up, measure the O.D. of your fronts, and find a tire whose actual measurements match up. The brands of tires will probably be different front-back, but that's fine. All depends on how worn the fronts actually are.
 

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Yes, but ... has anyone actually done it? And what was your first-hand experience?

P.S. #1 is an M-Sport with different front/rear sizes, so rotating front/back is not an option.
The staggered set-up sizes are such that the outside diameters are very close. My E46 M3 had 225/45-18's in the front and 255/50-18's in the back. The theoretical outside diameters were 25.97" in the front and 26.03" in the back.

I went through three sets of rear tires and two sets of front tires. When I had old tires on one end and new tires on another, the car would have either massive understeer or massive oversteer when I was at (beyond) the limit of adhesion.

With a staggered set-up and greater wear in the back, you're always going to have some slight difference in the rolling diameters front to back, except when both pairs of tires are brand new. Even with four new tires, you will continuously be increasing the slip on the xDrive clutches as you wear your back tires faster than the fronts. But, that's just the nature of the beast.

You're only good options are buying two tires at a time, either new or used. Although BestUsedTires.com has a great reputation, there is always risk with buying used tires. With shipping and installation costs, the savings from using used tires is less.

With two new tires in the back, you will have clutch slipping, but you have it now with no immediate adverse affects. With two new tires in the back, you clutch slipping will diminish as the rear tires wear faster.

If you go the used tire route, pick some back tires that have more tread than your front ones. You will have slipping, but it will diminish to zero as the diameters of the front and back come together. Then, slipping will come back as the rear tires continue to wear faster.

Depending on how old your tires are, you can probably find the exact brand and model in your exact sizes on TireRack.com.
 

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You're only good options are buying two tires at a time, either new or used. Although BestUsedTires.com has a great reputation, there is always risk with buying used tires. With shipping and installation costs, the savings from using used tires is less.
I've probably bought 20 tires from bestusedtires.com over the years. I only had a problem one time - on an offbrand tire that developed a sidewall bulge within about 3 months of installation. I don't blame the seller for that one - just unfortunate. Still, they offered me a discount on a replacement.

Shipping is free and fast and all tires, new or used, get charged for installation, so the savings are very real.

IMHO, this problem is a no-brainer. Look on bestusedtires.com to see if they have the exact tires you want. If not, check ebay - there are other decent sellers on there.

If you can't find what you need used, then the last alternative is to call tirerack and see if they can shave two new ones down for you.

If you can't get used/shaved, it's new tires all around time.

AM.
 

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Yes, but ... has anyone actually done it?
BMW dealers have, they do/did it on their leases. Sometimes, they even only do 1 tire without caring for tread differences as stated in the service manual....after-all, it's just a lease.

And what was your first-hand experience?
Initially, you'll most likely notice no difference, after ~70K, the t-case will be less responsive and there will be more transfer case malfunction codes (wear on clutch pack)..BUT unless you use a proper scan tool, you will most likely on a daily driving basis feel nothing and the dashboard will give no indications...only the scan tool will give the code(s), and eventually, you'll have to replace the t-case or rebuild it.

Michelin Defender is a good long lasting tire. In general, for tires, you get what you pay for, so don't cheap out here :)
 

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Autoputzer brings up another concern, that the staggered cars should have their transfer case fluid changed perhaps more frequently due to increased clutch wear. 20k miles or less, I'd go.
 

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I've probably bought 20 tires from bestusedtires.com over the years. I only had a problem one time - on an offbrand tire that developed a sidewall bulge within about 3 months of installation. I don't blame the seller for that one - just unfortunate. Still, they offered me a discount on a replacement.

Shipping is free and fast and all tires, new or used, get charged for installation, so the savings are very real.

IMHO, this problem is a no-brainer. Look on bestusedtires.com to see if they have the exact tires you want. If not, check ebay - there are other decent sellers on there.

If you can't find what you need used, then the last alternative is to call tirerack and see if they can shave two new ones down for you.

If you can't get used/shaved, it's new tires all around time.

AM.
Shaving new tires for the back would stop the clutch slipping now. But, it would gradually come back. With unshaved new tires on the back, the clutch slipping would be there now, but would gradually go away, and only come back when the rear tires start getting smaller than the front ones.

Anybody with xDrive and a staggered set-up is gong to have some clutch slipping. The morals of this story are: don't get xDrive unless you need it, and think read hard before getting a staggered set-up with xDrive.

You can diminish the staggered set-up clutch slipping thing by replacing your tires earlier than you would otherwise. But, you're gong to pay either way eventually, either more tires or eventually with a new/remanufactured xDrive transfer case.

I've only bought one set of used tires. I had a Nissan with 120k miles that needed new tires. I'd ordered a BMW to replace the Nissan, but it was delayed. The Nissan had an oddball tire size. I worked on a military base with maybe 2000 other people. The base had a newspaper which I rarely read, and classified ads which I'd never read. But, for some reason I picked up newspaper and looked through the classified ads. There they were, four tires in my size with only 7k miles on them, for $75. They were some oddball Bridgestone's, all-season tires but with a very soft tread rubber compound (and a low DOT treadwear rating). These tires didn't have a treadwear warranty. I put 7k miles on them before my BMW arrived. Later, I ended up buying a new set of those tires for Frau Putzer's Civic. She's a horrible driver, and I figured all-season and soft tread rubber were exactly what she needed in a tire.
 

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Here's another thought.... buy a set of used "square" wheels and sell the staggered set. It's one thing to seek maximum performance at a price from a genuine sports car, but the X3 is hardly that. Maybe put two used tires on for now to buy yourself some time to shop for a deal on some used wheels. Once the tires are wornout, then do the swap and be done with the staggered headache for good.

AM.
 

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I still call BS on the tire specs. My sport model could technically be out of spec right off the showroom floor. The sport runs staggered tires front/back. I've got 138k on it right now and just starting to possibly need a rebuild.
 

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I still call BS on the tire specs. My sport model could technically be out of spec right off the showroom floor. The sport runs staggered tires front/back. I've got 138k on it right now and just starting to possibly need a rebuild.
Out of the showroom, they're likely the same rolling diameter within a few hundredths of an inch (like my M3 was).

The BMW owners manual for our X3 doesn't mention anything about mismatched tread depths. But, there are a lot of things that the BMW owners manual leaves out.

Tire Rack's website (https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18) cites the Porsche, VW, Audi, Nissan, and Subaru requirements:


Audi
As published in their vehicle owner's manual, "rolling radius of all 4 tires must remain the same" or within 4/32-inch of each other in remaining tread depth.
Porsche
Cayenne within 30% of the other tire on the same axle's remaining treadwear.
Nissan
GT-R when replacing less than four (4) tires, each tire continuing in service must have at least 6/32 inch (5 mm) of remaining tread depth.
Subaru
Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.




That's good news that you have 138k miles on the original transfer case. If ours lasts that long, it will likely only be a problem for the second or third owner. My rule with BMW's is when the "1" first shows up in the sixth digit of the odometer it's time for a new BMW.

Those "smart" AWD systems are similar to BMW's xDrive. The Subaru's requirement is more stringent than the Audi's.

Rolling radius and tread depth are effectively the same thing, if the tire pressures and load are close to being the same, front to back.

If you apply Audi's 4/32nds inch rule to a BMW, you assume that the rear tires will wear 50% faster than the front ones, you assume new tires come with 10/32nds inch of tread, you would wear the rear tires completely bald (0/32nds inch of tread) and still meet the 4/32nds inch maximum difference requirement.

If you apply Subaru's 2/32nds requirement and the same assumptions as above, and do not rotate your tires, you could only wear your front tires down to 6/32nds inch and the rears down to 4/32nds inch and still maintain a maximum of 2/32nds inch difference in tread depth.

Audi's 4/32nd inch tread depth difference requirement doesn't mean that there will not be excessive xDrive clutch wear below 4/32nds inch difference in tread depths. If there is any difference, there will be some clutch wear. You also get some wear when turning, and when one or more wheels slip and xDrive has to reallocate the torques sent to the front and back.

Our X3 has a square set-up and I rotate the tires every 7k miles. That will keep the tread depth differences between the tires at less than 1/32nd inch. It should also allow us to get somewhere around 55k miles out of the OE non-RFT tires.
 

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I still call BS on the tire specs. My sport model could technically be out of spec right off the showroom floor. The sport runs staggered tires front/back. I've got 138k on it right now and just starting to possibly need a rebuild.


That’s why BMW went to great lengths to approve/offer a very minimal set of tires for staggered setups. Different brands of the same tire size have different measured diameters. In the end, though, BMW only really cares if you make it past the warranty; after that, they’re making money on selling a car or selling parts/labor.

If a rebuild is needed at only 138k...there’s a MAJOR problem with how it’s lived its life.


Sent from my iPhone using Bimmerfest mobile app
 

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That’s why BMW went to great lengths to approve/offer a very minimal set of tires for staggered setups. Different brands of the same tire size have different measured diameters. In the end, though, BMW only really cares if you make it past the warranty; after that, they’re making money on selling a car or selling parts/labor.

If a rebuild is needed at only 138k...there’s a MAJOR problem with how it’s lived its life.


Sent from my iPhone using Bimmerfest mobile app
I don't drink the BMW-approved tires Kool-Aid. The OE "BMW approved" tires on my 2014 535i were the worst tires I've ever owned. Most car manufacturers tweak the tire spec's. They can because they buy tires literally by the millions. It's usually minor stuff, but not always.

My car has the same size, brand, and model of tires used on a Ferrari. But, the Ferrari-spec tire has a lower load rating. My Chevy Cobalt SS has the same size tires that were used on the front of a bunch of air-cooled 911's back in the day. Michelin still makes a Porsche "N spec" tire in that size. But, it has a lower load rating, starts out with only 8/32nds inch of tread, and costs twice as much as "normal" tires. I bought some Conti' DW's instead.

Those manufacturer spec' tires often go out of production when the car does.

The Nissan Sentra SE-R (B13 platform) and the original Mazda Miata had the same tire size, and brand (Dunlop). But the Sentra's tire has a lower tread wear rating and actually had better adhesion. Mazda determined that more adhesion caused an abrupt loss in adhesion. They wanted a gradual loss which would improve the driving experience, and spec'd a harder tread rubber compound.

I had a B-13 SE-R. That car was critical success, but a commercial failure. Dunlop had too many of the Nissan spec' tires laying around. By the time I needed tires, they were highly discounted.
 

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That’s all well and good, however the AWD vehicle manufacturer selects the brand or brands whose different sizes have closest overall diameter in the case of a staggered set up. Load rating, coefficients of friction, and compatibility between Porsches and Cavaliers are irrelevant when the subject is overall diameter and what effects the differences therein will have on the transfer case clutches. If you look up the specs for various makes and models of the same size, you’ll find often substantial differences in actual diameters, so finding two different sizes with the same or approvable overall diameter is more difficult than pulling tires off the shelf. If you want the transfer case to last as long or longer than the car, that is. I’ve got Nexens on mine, $275 installed, and I couldn’t give a rat’s a$$ that they come on Hyundais.


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Discussion Starter #19
Busy here, but hopefully I'll do an experiment soon with above advice on changing the air pressures. I'll see if lowering the existing fronts', and raising the rears' pressures makes a difference in straight-line, dry-road characteristics.

On new tires, I don't think I would ever pay to have perfectly good tires shaved ... :)
 

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Discussion Starter #20
Update: Two new rear tires on order (Cont. Extreme Contact DWS 06). Also four new TPMS senders (Mobiletron's). Will be interesting to see how X3#1 drives afterward.
 
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