UPDATE on my 19 Inch BMW Wheel Cracks

[richard in NC]

Here is a question for the engineers. To me, it seems that RFT's would help in this situation. If you hit a pothole, a stiff tire would push the rim up away from the impact whereas a less stiff tire would allow the impact up into the rim, damaging it. Or does the softer tire act like a crumple zone and reduce the G's that the rim has to take? But in a car crash, too much crumple zone, and the impact reaches the passenger compartment, negating the usefulness of it.

[Rafale]

Hmm no the RFTs being too hard will pass all of the vibration and force of the impact onto the wheel. It will not protect them. There is obviously some optimization to be made. There could cases where the tire is so soft that the sidewall acts as if it is non existent in case of shock and would let the rim hit the pavement as you described. It needs to be optimized for the intensity of the shock. The RFTs appear to be way too stiff no matter what the shock is when used on low profile tires. They seem to work just fine on 55-65 profile tires.

[HerrK]

Originally Posted by richard in NC

Here is a question for the engineers. To me, it seems that RFT's would help in this situation. If you hit a pothole, a stiff tire would push the rim up away from the impact whereas a less stiff tire would allow the impact up into the rim, damaging it. Or does the softer tire act like a crumple zone and reduce the G's that the rim has to take? But in a car crash, too much crumple zone, and the impact reaches the passenger compartment, negating the usefulness of it.

I would guess that the tire is likely pushing the rim with the RFT's. After seeing the picture concept design of the Bridgestone RFT's I would find it hard to believe that an impact load would ever completely compress the sidewall and inner membrane with normal air pressure to provide a point contact that would fracture the rim. I think what is happening is that a high relatively even load is being transmitted to the rim. The design of the rim (be it material properties - ductility, ultimate tensile, yield strength or cross section area) may not be up to the task of the load being imposed. It looks like a brittle fracture of the material. Then again it could be a bad "lot" of castings. Numerous possibilities for potential failure.We, the general public, do not have all the tire, rim, impact load information however and all this is simply conjecture. Not enough information to build a stress model.

I imagine/hope that BMW is at least doing some failure analysis on the cracked items they are getting back to determnine the exact problem - if they don't already have a idea. The short sidewall of the 30 & 35 series 19" tires (RFT or Reg) will likely as you state "crumple" with a non RFT and you will likely get the same effect - cracked rim.

[cdeslandes]

The amount of camber and stiffness of the sidewall play little role, if any, on the impact to the rim. The height of the tire does, and a 19" wheel has very low profile tires.

Potholes are irregular, you're unlikely to hit the hole's rim head-on, and coming at your wheel at an angle with such a low profile tire it's no surprise the wheel took a direct hit.

Any wheel hitting concrete inside a pothole at 70 mph is going to suffer damage. The low profile tires just allow it to hit more often and harder.

Also, as noted above: cast=brittle, forged=resilient

Finally, having had flats with low-profile non-run flat tires, I can testify that even the non-RFT have quite rigit sidewalls these days and can be driven flat for a while, and I don't believe that RFT are that much more rigid or contribute to wheel damage.

[Alext]

Are the cracks in these wheels formed more often laterally or radially?

If they are radial in nature it would indicate failure of the rim material due to gradual stress, but if it is a lateral crack then I imagine it would indicate a sharp trauma of some sort.

I had a cracked rim once. It was a 17 inch rim + 40 profile tire that I went over a kerb. Small lateral crack near the edge. It was not able to be repaired.

[Mr. ///M3 RD]

Alext

The crack runs in the transverse direction (in line with the axel) as you can see in this picture here.

Please click here for the photo -----> One of the photos showing the crack on the 19 inch rear wheel on the driver side it is a BMW Type 296

[Vintage]

very disturbing as I tend to drive my wife's 35i with 19s at very high speed when no one is looking - the new non-RFTs come in Friday and I will have a close look at each wheel during the tire installation

[teagueAMX]

Originally Posted by HerrK

I would guess that the tire is likely pushing the rim with the RFT's. After seeing the picture concept design of the Bridgestone RFT's I would find it hard to believe that an impact load would ever completely compress the sidewall and inner membrane with normal air pressure to provide a point contact that would fracture the rim. I think what is happening is that a high relatively even load is being transmitted to the rim. The design of the rim (be it material properties - ductility, ultimate tensile, yield strength or cross section area) may not be up to the task of the load being imposed. It looks like a brittle fracture of the material. Then again it could be a bad "lot" of castings. Numerous possibilities for potential failure.

HerK, I would respectfully disagree. I believe you are underestimating the force of a tire/wheel hitting an object at speed for the following reasons:

1. Maybe if the tire was solid rubber it might rebound 100%, however, we know that the tire profile is taller when inflated with air so it provides al least a minimal cushion effect.

2. Please note Bridgestone’s own data showing a deflated tire. The tire will decrease its profile by about 50 percent with the car's static weight alone. We can conservatively assume the static weight on each tire is roughly 700 lbs (3490 / 4 = 698). The dynamic force of a tire hitting an object at 70 mph (or more knowing Rolf ) will be several orders of magnitude greater than 700 lbs. This indicates the tire would collapse on impact, and it can be assumed that it will deflect significantly more than it would with the static load.

As to your point about “high relatively even load is being transmitted to the rim” I’m not sure what you as saying. If the load is not the tire collapse due to an impact, then are you saying that that the entire load bearing foot print of the tire received a load that cracked the rim? That’s sort of a normal load in my book that tires and wheels are designed to deal with. However, again I would disagree based on the fact that you would see many more cracked wheels than is currently the case. I could agree that based on use, it’s a poor design, but not a manufacturing defect.

[Mr. ///M3 RD]

I like to thank you all for your posts and / or comments and positive feedback it is very much appreciated. Do please keep your thoughts coming. As the saying goes “Two heads are better then One”

At this point in time I will reframe from any further comments as to why my wheels have cracked (I have a pretty good idea, however, further checks are required on the one with the open crack).

I will, however, come back and bring you up to date as soon as I hear back from BMW on this matter. At this point in time (as you may have read before) BMW replaced only the passenger side wheel under warranty that has the two (2) hairline cracks. The wheel that has the crack (1/32” open) running transverse to the rear axel of my car as can be seen in the picture below will not be warranted by BMW. The verdict is the wheel is bend. So I am out approximately $1,000- Canadian and I intend to get my money back.

-----Photo ---->Photo showing the crack on Driver Side, BMW verdict Wheel is bend

The $64K question is what came first the crack or the bend? We shall see

This weekend I will personally check the wheel in a vertical boring mill in my friend’s machine shop for TIR (Total Indicator Reading) and document my findings. I will with permission of teagueAMX use one of his fine drawings and record the TIR every 30 degree on both sides of the wheel going with rotation of the wheel. I will then write a report along with supporting documents and a cover letter that will go to the BMW Head-office.

Thank you all again, I will be back you can count on it

[marcw]

So where does all this leave a guy with the Style 296 19" wheels? Frankly I bought this car for the fun factor. Having bent two wheels going only 40 mph I'm now always going to be be worried when I drive the car. Not really my idea of fun. I'm with Rolf that this is really a safety issue. I mean watching the road is one thing, but having to swerve around every tiny little imperfection in the road for fear of damaging wheels is quite another.

So I take it non RFT tires won't help much with these wheels? The profile is just to small to withstand 'normal' American roads? Do I just need to bite the bullet and downsize to an 18" wheel? Or get a forged 19"? I really fell in love with the BBS RE-V, but the price is quite high. Also, there seems to be some debate going on if forged wheels are really THAT much stronger. I do find it interesting that I haven't seen any reports of bent 19" wheels that are not style 296. So what is a guy to do?

-Marc

[HerrK]

Originally Posted by teagueAMX

HerK, I would respectfully disagree.

Hey no problem disagreement is ok with me. I'm a test engineer, sometines I get to try and actually break things. Ceratinly won't claim to be a stress engineer. What I envisioning is the rim being an equally loaded beam (for conservatisim fixed at the ends) because I believe the casting is brittle - not ductile. Now the tire gets a high point load at impact and even though the load is spread locally (say 3 -5") across a circumferential portion of the rim, high localized stress over 1-2" is higher that the tensile strength of the rim. Crack.

Anyway it would be nice to see a cross section of the crack under a few SEM photos. We'd be able to tell if the metal was rapidly torn or stretched. A mass spec of the rim material would help differentiate the alloy and properties. Unfortunately I no longer have easy or "free" access to these methods as we now subcontract out that work.

An old 30 series RFT could be relatively easily sectioned, supported and deflection tested. It would at least give a crude visual load path.

It just seems reminicent of a test when we split a tank of LH2 wide open. Stress engineers said the flight adapter was softer and spread the load into the tank better than our hard stiff test fixture leading to stess fracture of 180ksi CRES.

BMW should be exploring all the potential failure paths or bones of the fish. I love a good fishbone!

I noted what seems to be a month year cast designation on the back of one of the spokes 2 digits for year followed by 5 lines as mine are (10 l : l l l l) I'm guessing that number and location of the dimples in between the lines is relative to yearly quarter and the month. Front rims 1 dot (Jan of 2010) & rear rims have 2 dots (Feb of 2010) It would be a starting point to gather that information for material lot issues.

Hoping that these 2 instances are out of family occurances and not tending to be the norm. Its a big puzzle and reverse engineering this thing could be done but it would require some time & $. I hope I don't have to go there with my 296's but I will if it happens to me and I am pushed/ignored by BMW. It's too late tonight and I don't even want to start thinking about F=ma, free body diagrams, and dynamic factors. Its really much easier for someone with FEA experience and it would be a complex model to say the least and that is not my bag.

Thanks for your thoughts.

[teagueAMX]

Originally Posted by HerrK

Hey no problem disagreement is ok with me. I'm a test engineer, sometines I get to try and actually break things. Ceratinly won't claim to be a stress engineer. What I envisioning is the rim being an equally loaded beam (for conservatisim fixed at the ends) because I believe the casting is brittle - not ductile. Now the tire gets a high point load at impact and even though the load is spread locally (say 3 -5") across a circumferential portion of the rim, high localized stress over 1-2" is higher that the tensile strength of the rim. Crack.

Anyway it would be nice to see a cross section of the crack under a few SEM photos. We'd be able to tell if the metal was rapidly torn or stretched. A mass spec of the rim material would help differentiate the alloy and properties. Unfortunately I no longer have easy or "free" access to these methods as we now subcontract out that work.

An old 30 series RFT could be relatively easily sectioned, supported and deflection tested. It would at least give a crude visual load path.

It just seems reminicent of a test when we split a tank of LH2 wide open. Stress engineers said the flight adapter was softer and spread the load into the tank better than our hard stiff test fixture leading to stess fracture of 180ksi CRES.

BMW should be exploring all the potential failure paths or bones of the fish. I love a good fishbone!

I noted what seems to be a month year cast designation on the back of one of the spokes 2 digits for year followed by 5 lines as mine are (10 l : l l l l) I'm guessing that number and location of the dimples in between the lines is relative to yearly quarter and the month. Front rims 1 dot (Jan of 2010) & rear rims have 2 dots (Feb of 2010) It would be a starting point to gather that information for material lot issues.

Hoping that these 2 instances are out of family occurances and not tending to be the norm. Its a big puzzle and reverse engineering this thing could be done but it would require some time & $. I hope I don't have to go there with my 296's but I will if it happens to me and I am pushed/ignored by BMW. It's too late tonight and I don't even want to start thinking about F=ma, free body diagrams, and dynamic factors. Its really much easier for someone with FEA experience and it would be a complex model to say the least and that is not my bag.

Thanks for your thoughts.

Are you a destructive testing engineer?

I see where your going with your analysis and agree the wheel is brittle. We have no empirical data - it's all guesswork - so your theory is just as valid as mine.

It would be nice to do so some real test and analysis of the tire and the wheel, which is the only way to know for sure how/why it happened. I suspect that enough of these turn up cracked you may have your wish.

Cheers

[Vintage]

It would be nice to have a wheel and tire combination that did not destroy wheels and maybe even car components over the long term.

[richard in NC]

Originally Posted by VintageBMW

It would be nice to have a wheel and tire combination that did not destroy wheels and maybe even car components over the long term.

The bad news is that BMW is covered, with all of their statements that "low profile wheels and tires are more susceptible to damage". It would take quite a case to prove a safety issue, and even more to prove liability.

[Silver-Bolt]

Some more food for thought. All tires and wheels have a load rating. However the rating is for static loads not dynamic. As mentioned above the static load at each corner is under 900 lbs which is well within the limits of both the tire and wheel. Throw into the equasion some negative camber, cornering "g's", and even a minor impact from a pot hole and it would be very easy to transmit a load well beyond the wheels rating.

Cast wheels have a porous cross section. Molten aluminum is poured into the molds and allowed to cool. Tiny air bubbles are trapped withing the material as it cools. Forged wheels are made under pressure to insure the bubbles are forced out before the aluminum cools. That is over simplified but you get the basic concept. RFT sidewalls are so stiff they transmit more shock to the wheel rather than absorbing and disbursing the load.

[Mr. ///M3 RD]

More food for thought ...

My car and wheels I think have undergone the ultimate exposure during my 3 week US road trip ... some 13,303 KM in all (8,266 Miles). One of the stretches was Las Vegas, NV. to Bishop, CA some (293 Miles). The route we took was I-95 to Beatty then NV-374, I-199 thru Dead Valley over to CA-136 then US-395 up to Bishop, CA. It was a hot day on July 31, 2010 in fact we stopped several times. At the lowest point in Dead Valley 6 feet above sea level the temperature on my BMW read 49 C (120 F) 15 minutes later the temperature dropped by 20 C as we made our way out off the valley ... it was 29 C (84F). Now we did not rush at all our 293 mile trip took us with short stops along the way some 5 hours.

The road surfaces all the aforementioned US highways were perfect, in fact thru Dead Valley the road was superb nice curves, like class not a road defect in site (no ripple, or pothole of any kind). In fact my daughter mentioned in one of her video commentaries "what a lovely road almost like it was made for the Z4. The team of Top Gear should come here for one of there road trips and check it out" The tires and wheels had further workout later after I drove alone again (left my daughter in LA to conduct her business) from LA to Portland, OR. Nice roads as well and very nice curves just right for a Grand Touring Car like the Z4 BMW.

The point I am making and yes there is a point ... I wonder if Wheel or Tire Manufacturers expose there product(s) to tests of this magnitude.

Anyone knows what tests wheels and tires are exposed to before release for use on high end Grand Touring Cars like the Z4?

[HerrK]

Originally Posted by teagueAMX

Are you a destructive testing engineer?

I see where your going with your analysis and agree the wheel is brittle. We have no empirical data - it's all guesswork - so your theory is just as valid as mine.

It would be nice to do so some real test and analysis of the tire and the wheel, which is the only way to know for sure how/why it happened. I suspect that enough of these turn up cracked you may have your wish.

Cheers

In 25 yrs of aerospace testing, I've done my share of R&D, IRAD, qualification & production testing. Did a lot of "to failure" testing during the R&D days. Currently I'm doing production acceptance testing.

Dang it - I spent good money on this car, for aesthetics & proper engineering. Wheel failure IMO are not exclusive. If my 296's fail, I will not be happy and I will do my best to spread the pain to the hardware supplier.

Looks like its time to start thinking about a good failure analysis test plan.

[Mr. ///M3 RD]

Originally Posted by HerrK

In 25 yrs of aerospace testing, I've done my share of R&D, IRAD, qualification & production testing. Did a lot of "to failure" testing during the R&D days. Currently I'm doing production acceptance testing.

Dang it - I spent good money on this car, for aesthetics & proper engineering. Wheel failure IMO are not exclusive. If my 296's fail, I will not be happy and I will do my best to spread the pain to the hardware supplier.

Looks like its time to start thinking about a good failure analysis test plan.

I have a nice wheel for you all cleaned up and shinny

When all this is over I properly convert it to a nice table with a chess glass top for our basement I know it will be a nice conversation piece.

[Vintage]

New tires are now on as of today - what a world of difference. We will NEVER go back to the RFTs! An already great car is now amazing: much quieter, no vibration tranlating into cabin, smooth but planted to the road, drive in Sport all day long and it feels better than Normal with the RFTs. I could go on but once the new tires are worn in a little I will push it and give an update. I went with the affordable but awesome Continental Extreme Contact DW (I think that is the name - WOW).

[Mr. ///M3 RD]

Originally Posted by VintageBMW

New tires are now on as of today - what a world of difference. We will NEVER go back to the RFTs! An already great car is now amazing: much quieter, no vibration tranlating into cabin, smooth but planted to the road, drive in Sport all day long and it feels better than Normal with the RFTs. I could go on but once the new tires are worn in a little I will push it and give an update. I went with the affordable but awesome Continental Extreme Contact DW (I think that is the name - WOW).

Thanks for your post, my BMW service manager also recommended the continentals can you (when you have a chance) let me know the sizes and types you got?

Thanks

[Vintage]

Originally Posted by Rolf-Dieter

Thanks for your post, my BMW service manager also recommended the continentals can you (when you have a chance) let me know the sizes and types you got?

Thanks

Yes indeed, Continental Extreme Contact DW in 235x35x19 and 265x30x19 (I think) and they are great. My local Discount Tire store did a Tirerack price match for me; so, total installed for all 4 was $969.30. That is not bad and the RFTs already had a lot of wear even though they only had about 10k on them (2k in Europe and about 8k at home).

I will never go back to RFTs; the Michelin PS2 was a strong contender but the Tirerack ratings pu the Continental just ahead of it for less dough.

[Mr. ///M3 RD]

Originally Posted by VintageBMW

Yes indeed, Continental Extreme Contact DW in 235x35x19 and 265x30x19 (I think) and they are great. My local Discount Tire store did a Tirerack price match for me; so, total installed for all 4 was $969.30. That is not bad and the RFTs already had a lot of wear even though they only had about 10k on them (2k in Europe and about 8k at home).

I will never go back to RFTs; the Michelin PS2 was a strong contender but the Tirerack ratings pu the Continental just ahead of it for less dough.

I will order them as well. Thank you very much for the information

Keep the forum boys updated say after 5K miles and so on it will be good information.

BTW, the service manager of course recommended continental RFT, little does he know I too will never go back to RFT once I change. I've been reading horror stories in the Roundel Magazine about RFT and how people even gone to Audi and Mercedes ... left BMW after 20 .. 30 years.

Thanks again ... and happy travels.

PS. Someone told me (I don't believe it) that if you change from RFT to NRFT you need new wheels ... you certainly proved them wrong Good job