I HATE RFT

[Frank Costanza]

This is sad. I love the look of 296s on the Z4. Hopefully switching to non RFTs will prevent this from happening.

[BOGONY]

I was in for tire number 7 and the dealer saw a bubble on another tire.
Number 8 being installed this week. One of the salesmen said that he went through 12 tires.

I had Porsches before my Z4 and never tires go like this even with NYC potholes.

At least they are covered by insurance.

[cdeslandes]

Quick comparo of tire weight on the Tire Rack's website:

255/35-18 rear tire:

Bridgestone Potenza RE050A RFT (OEM tire): 27 Lbs

Michelin Pilot Sport PS2 ZP (RFT): 29 Lbs

Michelin Pilot Sport PS2 (same as above, not RFT): 24 Lbs

Michelin Pilot Super Sport (non RFT): 25 Lbs


That's anywhere from 2 to 5 more pounds of unsprung weight per wheel.

Also, the RFT are up to $100 more per tire!

[Mr. ///M3 RD]

[QUOTE=cdeslandes;9336970
Here's a teaser, will post pics in the photo sections one mounted on the car with the new Michelin Pilot Super Sport (non-run flat) just ordered from the Tire Rack![/QUOTE]

Good looking wheel, well done

I don't think you have to worry about cracks in the new wheels they look very strong.

What the parts manager told you is very interesting. I am wondering how many people just put the money on the table and believe the BMW story about having driven thru potholes. Then just pay the bill and never tell anyone.

I maintain the 296 wheel is weak and there is some sort of a design flaw.

[richard in NC]

Originally Posted by Rolf-Dieter

I maintain the 296 wheel is weak and there is some sort of a design flaw.

I absolutely agree with you there. The spokes are offset to the far outside edge of the tire. See post #15. There is great potential for the inside edge to deflect, especially given the negative camber. I have had the inside edge of high offset wheels warp, but apparently the 296's crack instead.

I am not sure about RFTs contributing. I've always read that extra tire pressure causes less energy to go into the rim deflection and more into the suspension deflection. The RFTs should maximize the energy transfer into the suspension.

[Mr. ///M3 RD]

Originally Posted by richard in NC

I absolutely agree with you there. The spokes are offset to the far outside edge of the tire. See post #15. There is great potential for the inside edge to deflect, especially given the negative camber. I have had the inside edge of high offset wheels warp, but apparently the 296's crack instead.

I am not sure about RFTs contributing. I've always read that extra tire pressure causes less energy to go into the rim deflection and more into the suspension deflection. The RFTs should maximize the energy transfer into the suspension.

You are properly right on the higher pressure of the RFT, however, during the period of time after my 2 rear wheel cracks happened I checked my tires religesly every week (under the same temperature conditions in my garage). I found that my wheels all 4 of them somehow lost a few pounds over the 7 day period and each week I had to add pressure to each wheel (1 to 2 PSI).

So perhaps the negative side might be lower pressure? Also if a person has a flat driving 80 KM (50 Miles) certainly cannot be very healty for the inner wheel rim the sidewall of the RFT is reinforced with steel shims now inflicting more pressure to the inner outer rim due to the 2.5 degree camber.

[copperplate]

I am neither an engineer nor physicist, so take the following with a grain of salt. It has not been supported by any form of testing or experimentation and therefore has no more basis in fact than unicorns selling bridges for a low low price and extra whitening power for your gear teeth. YMMV.

Seems like pressure could be the exact problem. Think about the air pressure in the tire and the pressure on the rim from hitting a bump separately.

With increased air pressure, you may be transferring X pounds of force, but it's being distributed over a large surface area, the entire inside of the wheel and even the inner sidewalls of the tire...so the pressure, in pounds per square inch on the wheel, of a bump is pretty small when it's distributed by all the air in the tire. But if the force is distributed over a much smaller area, such as would be the case when a bump is transferred by the stiff sidewalls of a runflat to only the area of the rim in contact with the sidewall, the result is much higher pressure on that smaller portion of the rim taking the impact. Even though the pressure from the bump was the same, the ratio of pressure to square inches was higher because there were fewer square inches. Low tire pressures may very well be exacerbating the problem.

Are BMW's recommended tire pressures for the 296 wheel any different than the other wheels? It may actually require more pressure.

[zEdP]

Originally Posted by copperplate

With increased air pressure, you may be transferring X pounds of force, but it's being distributed over a large surface area, the entire inside of the wheel and even the inner sidewalls of the tire...so the pressure, in pounds per square inch on the wheel, of a bump is pretty small when it's distributed by all the air in the tire. But if the force is distributed over a much smaller area, such as would be the case when a bump is transferred by the stiff sidewalls of a runflat to only the area of the rim in contact with the sidewall, the result is much higher pressure on that smaller portion of the rim taking the impact. Even though the pressure from the bump was the same, the ratio of pressure to square inches was higher because there were fewer square inches. Low tire pressures may very well be exacerbating the problem.

I'm not sure a lower pressure will hurt the wheel more. The steal side wall of the tire transfers the force. I guess if you want to take advantage of the air pressure, distributing the force over a larger area and bulge the tire, the air pressure should be much higher and dangerous to use.

There are more factors that can amplify the 296 wheel problem, so what about using the sport+ mode with the adaptive suspension?

[Mr. ///M3 RD]

Here is my theory Gentleman ....

-RFT ... detrimental to the soft alloy wheels
-Low Air Pressure ... will transfer more stress to the inner outer rim.
-The 2.5 degree camber contributes to the overall mess.
-Air Pressure control is very important
-Watch what you put in the trunk when the roof is down you need more air pressure to take care of the extra weight

Feel free to add to the list
Forged wheels, different tires, change the camber on the rear wheels to 1 degree negative. A new car (makes sense when you add the cost for new wheels, tires and changing the camber). As I said it depends on the pocket book and how secure you want to be when you go on a trip. Try finding a wheel or a replacement RFT when you are in small town USA. I've been there done that and have no plans to do it again.

Solutions ... well it all depends on your pocket book

My cracked wheels gave me nightmares and I kicked myself for ever changing from my original M3 to a 2008 Z4 30is then to the 2009 Z4 35i (perhaps if I would have stuck with the 2008 I would not be typing this right now).

[Mr. ///M3 RD]

Originally Posted by zEdP

I'm not sure a lower pressure will hurt the wheel more. The steal side wall of the tire transfers the force. I guess if you want to take advantage of the air pressure, distributing the force over a larger area and bulge the tire, the air pressure should be much higher and dangerous to use.

There are more factors that can amplify the 296 wheel problem, so what about using the sport+ mode with the adaptive suspension?

Ed, we all purchased a high end car this mess should not be happening.

Did you ever Google "Cracked wheels BMW" you will come across a link to "UK Watchdog" there are a lot of people in England with cracked wheels. BMW simply tells the customer "Sorry you went thru a pothole"

You are right when you say "There are more factors that can amplify the 296 wheel problem"

I bet BMW knows all about it only they are not telling the customer.

[cdeslandes]

My 294's with non run flats Michelin Pilot Super Sport are on and they look and feel GREAT!

It's amazing how sharp the handling is, and how smooth the ride is now!

Will takes pics when the rain stops and will post feedback in a new thread.



^ I'm happy!

[zEdP]

If I get a crack in one of my 296's, replacing the damaged one will not solve the problem. The problem will come back.

Switching over a non RFT could reduce the change, but I guess that you have to buy two new rear wheels and not use wheels that ran many miles/kms with RFT. Even then, we have no prove the 296 with a non RFT is a solid combination.

Switching over to the 294 will be very expensive. Trading in the E89 MY09 for a new one, 35i N55, 294 wheels, somewhere next year could be an option.

[habib86]

we really need a sticky thread for the cracked 296 wheels

[Mr. ///M3 RD]

Thanks Site Moderator

For putting up the Sticky on the BMW 296 Wheels