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      03-06-2011, 05:30 PM   #14
Lotus7
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Quote:
Originally Posted by X3buyerScotland View Post
Lotus you are amazing. So let me ask a simple question in hope of a technical answer. Convenience an blow out safety aside, what does the above mean for run flats and would you advocate them for all X3 rims? Issues for me are mainly ride, handling & mpg. Thanks.
35 years of trying to find optimum tire pressures on racecars has taken its toll. Sometimes the setting that makes the car handle the way you want or the way it handled last week is not possible to find. Sometimes it depends on the phase of the moon. Hence my cop-out: "It Depends".

Am a novice re: Runflats. This is the first car that I've had with them. All I know is that the first and second generation runflats had sidewalls that transmitted a lot of road shock. The 2010-2011 versions (Gen. 3) are supposed to be more "shock-absorbing". Personally, I don't really like the idea of driving on a uninflated tire, so I've already equipped our 35i with a "on-the -rim" tire repair kit. A factory BMW toolkit (BMW Europe calls it "The European Driver's Utility Set" that fits perfectly in the left rear cubby hole under the rear side panel), and a small, but reliable 12volt air compressor. If I get a flat while driving, the plan is to pull off the road as soon as possible, find the nail or other puncturing object, remove it, install a fiber/rubber plug, and inflate the tire and drive off.

I find the X3 ride (with P7 tires) to be excellent, in fact it rides better than our Subaru LGT with Goodyear GSD3's or our old Audi A4 with Dunlop summer tires, so I have no complaints regarding the X3 ride, even in the VDC Sport Mode. BMW also claims that all of the run-flats fitted to X3's are designed for extra low rolling resistance (for better fuel efficiency). I suspect that is a function of the sidewall hysteresis coefficient (which should be low) and the tire rubber compound Shore Durometer hardness which is higher for a low-resistance tire. The “Summer P7” has a DOT wear rating of around 200 and "All-Seasons P7" has a rating of 500 implying harder, slower wearing rubber, lower rolling resistance, with less ultimate grip. High performance summer tires will undoubtedly give slightly more cornering grip, but on a high center of gravity SUV like a X3, that's not always something you really want or need.

Sometimes it's better to let the back end drift out or the front end slide a little wide (and let the stability control take care of it) than to have enough cornering grip that the inside tires lift and the car gets tippy. The X3 is not a M3 and it's a mistake to try to make it into one. I have a 5-point harness in the Lotus and a pair of rollover hoops. No such protection in the X3. ..........................................yet
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