Which corner tire wears out faster?

[ard]

Here is a question:

What is the design limit on rotational mismatch, front to rear and side to side?

Lets say your rears are 0.2" larger, so they are turning at (say) 700 revs per mile, and the fronts at 715 revs/mile.

So at 60 mph that is 15x60/60 = 15 revs per minute. So the differential (front to rear) is 'compensating' for a mismatch at 15 revs per minute, or one rev every 4 seconds.

(Anyone want to check, just measure the distance from the ground to the central point of the roundel- difference is the rotational radius (without dynamic effects))

Anyone know? My guess is that BMW doesnt publish this....

[JCL]

I think there is some truth and validity in the recommendation to change all four tires at the same time, but as with many things, it is taken to extremes.

I have seen 2% as a recommended limit for differences in tire rolling diameter. Seems reasonable to me. If I had two worn tires, and two good ones, of the same tire model, mounted on an AWD vehicle, I wouldn't worry about it. If it was to the limit, I would put the worn tires on the rear and put 2psi more in them, problem solved. However, if I was replacing two tires and I had a choice of getting two of the same model, or two from a different manufacturer, I would stick with the same design. That is for two reasons: tires from different manufacturers can vary far more than 2% for the same notional size, and tires have different handling characteristics, so DSC would be more effective with the same traction characteristics front and rear. I think that is where the recommendation comes from for all tires to be similar.

For what it is worth, I don't rotate my tires either. I do believe that rotating cupped or worn tires compromises handling. It helped that I got 70,000 km out of a set of OE Michelin tires on my X5 without rotating them, and they had good life left. Not buying tires that are too wide means I don't ever have to deal with rotating them. And BMW doesn't have anything to gain, they don't sell replacement tires. And who would ever go back to their dealer to buy replacement tires?

[gregorio]

You'd be surprised at how many people buy tires from the dealers. After all, many people do not have time or want to spend more time taking the car to multiple places to get the work done.

Your assertion that NOT rotating tires makes them last longer is generally false for most cars. Cupping and uneven wear can cause parts of a tire to wear out long before the full tread wears off. These unevenly worn tires also have degraded handling characterisitcs. Every tire manufacture and automobile maker recommends tire rotation to extend tire life for vehicles that can support it. Obviously, cars with staggered tire sizes cannot easily take advantage of tire rotation without dismounting the tires and swappign left to right. However, this only applies to directional or symmetrical tread designs and not asymetrical treads. The former have an arrow to designate the rotation of the tire and the latter have the word OUTSIDE stamped on them.

[JCL]

Originally Posted by gregorio

You'd be surprised at how many people buy tires from the dealers. After all, many people do not have time or want to spend more time taking the car to multiple places to get the work done.

Your assertion that NOT rotating tires makes them last longer is generally false for most cars. Cupping and uneven wear can cause parts of a tire to wear out long before the full tread wears off. These unevenly worn tires also have degraded handling characterisitcs. Every tire manufacture and automobile maker recommends tire rotation to extend tire life for vehicles that can support it. Obviously, cars with staggered tire sizes cannot easily take advantage of tire rotation without dismounting the tires and swappign left to right. However, this only applies to directional or symmetrical tread designs and not asymetrical treads. The former have an arrow to designate the rotation of the tire and the latter have the word OUTSIDE stamped on them.

I didn't assert that not rotating tires makes them last longer. I said that it helped that I did get that mileage, not that it helped me to get that mileage. I did assert that rotating them compromises handling. I agree that continuing to run cupped tires compromises handling as well. I just don't think that putting that compromised tire on another corner is a good idea. I noted that since my tires lasted so well, likely because I didn't buy tires that were too wide, I never had to worry about rotating them. A well designed suspension, with a tire that is not too wide, and which is properly aligned, will wear tires evenly.

[gregorio]

Originally Posted by JCL

I didn't assert that not rotating tires makes them last longer. I asserted that rotating them compromises handling. I agree that continuing to run cupped tires compromises handling as well. I just don't think that putting that compromised tire on another corner is a good idea. I noted that since my tires lasted so well, likely because I didn't buy tires that were too wide, I never had to worry about rotating them. A well designed suspension, with a tire that is not too side, and which is properly aligned, will wear tires evenly.

Sorry. I misunderstood you. It is not very often that a car is so balanced that its tires wear evenly.

[Weasel]

Originally Posted by ard

Here is a question:

What is the design limit on rotational mismatch, front to rear and side to side?

Lets say your rears are 0.2" larger, so they are turning at (say) 700 revs per mile, and the fronts at 715 revs/mile.

So at 60 mph that is 15x60/60 = 15 revs per minute. So the differential (front to rear) is 'compensating' for a mismatch at 15 revs per minute, or one rev every 4 seconds.

(Anyone want to check, just measure the distance from the ground to the central point of the roundel- difference is the rotational radius (without dynamic effects))

Anyone know? My guess is that BMW doesnt publish this....

BMW did publish this in one of the the training manuals (likely for the chassis dynamics/undercar technology classes) If I had my books at the house still I'd scan the page and post it, but I loaned all my training manuals to my brother when he started working with me so he could read up. Come to think of it, I may have even posted that info up here sometime in the past, but that would be a hard dig. I wish I could access the online training manuals from home... but that would make life easier.

[JCL]

Some threads where the DSC lights came on due to differences in tire sizes. Two of the threads mention that the BMW spec is 3%.

http://www.xoutpost.com/bmw-sav-foru...xed-tires.html

http://www.xoutpost.com/bmw-sav-foru...-inactive.html

http://www.xoutpost.com/bmw-sav-foru...er-thanks.html

http://www.xoutpost.com/bmw-sav-foru...ize-tires.html

There are quite a few more.

[AzNMpower32]

There have been quite a few cases in the X3 where the transfer case makes unpleasant sounds, or fails altogther, because the tires were mismatched.

I'm not saying rotating them is good or bad. I swap wheels twice a year so they usually end up being rotated anyway when I remount them.

Over the summer, I ran into a situation where I had rear tires at the legal minimum wear bars and fronts that were "so-so". Due to budget constraints, I purchased 2 new tires and put them in the back, and drove with mismatched tires for a month before changing out the fronts. There certain instances where DSC intervened for no reason due to the mismatch.