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  1. #1
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    Runflat ruminations

    Came across some interesting test data comparing the difference in forces transferred to suspension mounting points on cars running runflat tires vs. non runflats.

    The comparison involved prototypes of the E90 3 series BMW. The engineers involved discovered that cars using runflat tires experienced a 10-15% increase in the forces transmitted to the suspension when encountering a bump in the road. This was attributed to the increased stiffness of the sidewalls and the increased weight of the runflat tires. As a result, BMW had to rethink the design and strength of the car's suspension as well as the body stiffness, especially in localized areas where the suspension meets the body. The production E90 comes with runflats as standard equipment.

    Now, the runflats involved in this study are the latest generation of the technology and feature softer sidewalls than earlier designs (like the RE-040 used on the Z8) and weighed an average of 2.5 lbs. more than comparable non runflat tires. By comparison, the 245/45-18 RE-040 weighs 4 lbs. more than the equivalent sized Bridgestone S0-3 and the 275/40-18 RE-040 weighs 3 lbs. more. Needless to say, the S0-3s also feature softer sidewalls.

    If you change to lighter weight wheels, the weight savings at each corner can be substantially higher. For example, by putting 19" BBS RGRs (9" front - 10" rear) shod with Michelin PS2s on my Z8, I removed 14 lbs. per corner up front and 15 lbs. per corner in the rear!

    I think it worth noting that the early chassis development work on the Z8 was done with Michelin Pilot Sports on the cars, not RE-040 runflats. I have been told that the decision to use runflats on the Z8 was driven by marketing concerns. It would be interesting to know when the switch was made. For what it's worth, the RE-040s weigh 3 lbs. more per tire up front and 5 lbs. more per tire in the rear than equivalent sized Michelin Pilot Sports which also feature a more compliant sidewall.

    While we wait for BMW to finish work on their chassis strengthening upgrade, replacing the stiff sidewalled RE-040s with lighter weight and more compliant non-runflats seems like a prudent move. If you can give up the beautiful OE wheels, you can take an even greater step towards reducing potentially damaging forces from being transmitted to the chassis of your Z8. Plus, you get all of the other benefits of reduced unsprung weight and can take advantage of more sophistcated rubber compounding. At the very least, food for thought.Grease Monkey


  2. #2
    rjay
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    that's a huge amount of weight off the wheels

    Can you feel the weight difference while driving? I currently have the stock wheels with SO3's. If I can find wheels that I really like, hopefully even better than the stock wheels, then I will probably change them out. What about between the stock 18" and the 19" size, is their any strong advantage to either, or is it just preference on the looks? I assume the 18" RGR wheel/tire combo would be even lighter than the 19's?

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    Re: that's a huge amount of weight off the wheels

    The reduced unsprung weight with the RGRs and PS2s is very noticeable while driving, especially on rough or irregular road surfaces where the handling is far superior. The reason I went with 19" wheels was because the tire manufacturers are releasing their highest performance tires in 19" sizes and ignoring the 18" sizes. The PS2s are a perfect example of this. 18" RGRs only weigh one pound less than the 19" RGRs so not a big consideration. I also like the responsiveness of the shorter sidewall on 19" tires. The Alpina V8 Roadsters run 20" tires but the suspension on those cars was softened to compensate for the stiffness of those very short sidewalls. Some on this Board are running 20 wheels and tires on their Z8s and seem happy with the results. I think the 19" combo is a nice compromise and better proportioned on the Z8.Grease Monkey

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