I need some help from people who know, in addition to what my local service reps say. Situation Part 1: My RFTs on the front are shot and I had to replace them...now...with the pneumatic Contis I wanted. So now I have new tires on the front and the rear tires come in next week. No change in tire size (other than tread wear). Situation Part 2: It's a 2006 330xi...AWD and the normal interplay between the front and rear tires. Situation Part 3: I have to go 400 miles this weekend, almost all of it at highway speeds. Question: Does the tire mismatch pose an issue for the drivetrain? If I drive it for distance, at speed, am I going to create a problem? Or should I just park the car at the airport and take a rental for the weekend?
I was thinking that someone with a more technical answer will come forward over the course of the last 24 hours, but I'd have to say from my own experience (not with your particular X drive setup though) that it's not a good idea to drive with newer/grippier tires on the front vs. the back. I've done this on a FWD car (a MINI) and it lent for some pretty scary handling...even in the dry at highway speeds. Any time I hit a bump such as an expansion joint, etc., it felt like tthe back end of the car wanted to/was shifting out of line. (in fact, it was) That said, I don't know if the awd that your car has will somehow manage to even things out .... and I guess that's really the technical detail of your question. Nevertheless, I can't imagine that it's a good idea for the awd to be trying to fight against consistently lower traction in the rear than the front and the constant natural inclination the car would have under those circumstances to swap back with front. If it's only a few more days to wait, I'd just park it, rent a car if you need to, and resume driving when you have the proper tire setup. Just an opinion.
Well, I'm certainly not technically qualified to answer, but I doubt any auto manufacturer would design a vehicle that required a full tire change if you had a failure of a single tire (which is pretty much what you are suggesting above). And, there are certain variables in actual tire sizes between manufacturers (for the very same tire size specification), as well as simple size differences due to tire inflation pressures. That tells me that there is a certain allowance for differences in tire size and traction differences, or X-drive vehicles wouldn't last very long. My wife also has a Mini, and she has, over the years, replaced individual tires (runflats-front and back) due to unrepairable tires with absolutely no noticable change in driving characteristics. Perhaps its because she is not anywhere close to an "assertive" driver, or ... maybe not. I'd be curious to see what the BMW Owner's manual has to say about it. Of course, BMW recommends no tire rotation due to handling changes (yet all the tire makes DO recommend them, and many void their tire tread warranty if rotations are NOT done). I would think that the average driver gets no where near the traction limits of a tire in relatively good condition on an average, day-to-day basis. But, I'm sure there are a few that push the envelope enough to tell a difference.
Actually, the way I read it, the OP didn't replace just one tire but rather put 2 new tires on the front while leaving 2 worn tires on the rear (pending their replacement). Replacing just one tire, so long as the remainders aren't terribly worn and the tire type/tread design is the same is indeed not a big deal. The problem is having 2 rear tires that are significantly worn compared to the fronts. This can result in giving the car a dangerous amount of oversteer...especially in wet or other reduced traction situations. There are lots of places to read about this. Try this link on TireRack for a start. http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=52 Again, the awd/X drive system would probably try to counter this...but indeed if it finds itself constantly working to correct (as might be the case if the 400 miles of highway driving were in the rain), I'd think it really could have detrimental effects on the drivetrain...which was the OP's original question.
Obviously, I read the situation differently... Situation Part 1: My RFTs on the front are shot and I had to replace them...now...with the pneumatic Contis I wanted. So now I have new tires on the front and the rear tires come in next week. No change in tire size (other than tread wear). No where is the condition of the rear tires mentioned. All bets are off if they have no tread left. X-Drive or not, that would never be safe. Also, I checked the wheel/tire extended warranty brochure I have from my BMW dealer, which I feel sure most (if not all) BMWNA dealers offer. If a tire has more than 2/32 treadlife left and is damaged beyond repair, the tire will be replaced. Not the same axle, nor the complete set...just an individual tire. Common sense tells me from that standpoint BMW doesn't see a new tire and a tire with 3/32's tread left on it as a mis-match, or the warranty would be structured differently. Unless one is a spirited driver, and assuming a decent amount of tread remaining on the rear tires, I sincerely doubt it would cause any problem at all. Maybe so on a track, but that was not the intended use of the car.
Thanks folks. I talked to two different BMW shops. One said it should be OK due to the compensation the system is designed to do. The other shops said I'm fine for a while, but he'd recommend not driving too long like that. So, I went safe and just rented a cheapo car from Hertz for the drive. The upside is that after driving a crappy KIA, I have a freshly polished appreciation for my Bimmer! The car's auto tranny shifted far more jerky than my manual tranny ever does, even on my bad days. Hopefully those new rear tires come in this week. I'm not sure about replacing just one tire ever. The tire sensor will alert if either of a pair of tires (front or rear) has a significantly lower inflation than its partner...not if both are low...just if they differ. The reason, according to the Owner's Manual, is the resulting difference in rolling radius and related drivetrain stresses. Not sure if this translates to front/rear radius mismatches or not. Anyway, thanks all for the help.
Glad to hear that it all worked out for you. I'm guessing that after being used to driving your 330xi, you were forcing that poor Kia into quite a few sudden downshifts. Not a happy driving situation...but one that saved you a fair bit of worry from driving your "real" car on mismatched tires.
You got that right! The worry was the big thing. I didn't want to go through gut-wrenching experience of wondering about the health of my Bimmer. And no, I wasn't driving the KIA hard at all. They just....suck! In addition to the normal KIA suckiness, one tire (or multiple) was either out of round or out of balance, and the TPMS light kept popping on every once in a while.
1. The difference in revolutions per mile between a typical brand-new 225/45R17 performance tire (9/32" tread depth) and an identical tire worn to the wear bars (2/32" tread depth) is a scant 1%. Neither the AWD nor the TPM system will complain about or be damaged by such a tiny difference, even on the same axle. The car does have differentials at each end after all.... 2. NEVER mix different brands/models/construction types of tires on the same axle. If you ever must replace one tire only, the replacement must be a twin of its mate on the other side. Mismatched tires produce unpredictable handling due to the differing grip characteristics of the tires. In emergency maneuvers and slippery conditions, your worst-case result can be enough lateral torque to provoke a spin (absent DSC to save your butt).
1. Thanks... I agree. The first time the car makes a turn the wheel rotational "balance" is off between the left and right sides of the car's tires. No car could operate normally with such sensitivity in a normal environment. 2. Agreed. The very same stated tire size can vary between different manufacturers, in addition to the grip characteristics due to tread design, current tire-wear state, etc. But, in the end, I think that...when in doubt, take the safest route. So, in this case, perhaps the rental gave Michgndr89 the "piece of mind" he needed.
I was OK when BMW Shop #1 said it's OK. I was less comfy when BMW Shop #2 said it wasn't so good. Nevertheless, the tire shop called and said my new rear tires are in, so by dinnertime tonight, I expect I'll be back to 100%. I tell ya...having the BMW on limited duty is worse than having a kid sick with the flu! Ha!
Shop #2 was probably thinking of your safety, not the car's. If you left the tires that way and just forgot about it, someday you'd have one tire on the wear bars and one with substantial tread left. Hit a big puddle at speed and one side might aquaplane while the other stayed planted. The resulting yaw torque could toss you into trouble before DSC could straighten out the mess.
My current ride has dead rear shocks (on the to-do list) and fairly worn rear tires. When it rains I'm getting a lot of much-needed practice correcting oversteer.
