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  #1  
Old 07-24-2011, 07:24 PM
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Runflats

These tires really suck. Compared to the regular tires in the same sizes that I had on my 4.8, the runflats are harsher but less responsive, drift wide in turns, and are slower in transition. Worst of all, when lifting off throttle and hitting the brakes they get very squirrelly. I never once had that happen on the 4.8. Granted I only have 300 miles on the car so far, and I understand why BMW uses them for safety reasons, but I'm going to swap them out this week and see the difference when going back to regular tires.
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  #2  
Old 07-24-2011, 09:39 PM
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BMW engineers the car's performance based on the RFT's weight, stiffness and ride characteristics. I will not replace mine for a non-RFT as I don't have any of the issues you mentioned or do I want to put a tire on it that wasn't spec'd out when it was engineered.
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  #3  
Old 07-25-2011, 06:54 AM
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There's a big difference between the 4.8 and the M. Runflats are not performance tires, and I guarantee you that the engineers at the M performance division would not have used runflats unless they had to. They made the car perform as well as possible given the tires they had to work with, but that doesn't mean that it wouldn't perform better with better tires.

The fact that you haven't experienced any of these qualities is probably that you don't have anything for comparison. On my previous 4.8, I swapped the stock runflats for regular tires and drove it for 3 yrs. I had an e53 4.4i for three years before that on regular tires. The only negative to switching to non-runflat tires is that you run the risk of getting a flat tire...which happened to me exactly 0 times in the last 6 yrs with an X5. Losing 34 lbs of unsprung, rotating mass will only improve this car's already amazing performance.

I'll report back with my results after I make the switch.
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Old 07-25-2011, 09:53 AM
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I have the Dunlop Sport Maxx those are summer performance tires. I think people people tend to complain because of a stiffer ride. I had an e53 for 9.5 years on regular tires. There have some that have made the switch to non-RFTS and have had issues it's been documented (and some of those tires are heavier in weight than the RFTs). The ride will no doubt be smoother with a non-RFT but I don't see how performance will be better unless you go to a non-RFT summer performance for that size like Yokohama (there are only so many tire manufacturers for the 20" size). I don't see how the Michelin Lattitude Tour or Conti Contact are going to be a better performance tire--they are designed for durability not performance.
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Last edited by BGM; 07-25-2011 at 11:17 AM.
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  #5  
Old 08-04-2011, 01:22 PM
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Switched to non-run flats. To me, overall the turn-in, stability, and handling are improved, and it feels lighter on its feet and marginally quicker (although this could be in my head). I tried to quantify using the same road where I did my previous performance runs but it was raining. Despite the wet road, I posted identical matching times using the same technique, power braking and no LC (0-60 in 4.5, 0-100 in 10.9, 1/4 in [email protected]). Also, on a separate run I got 4.7 to 60 without power braking, just mashing the throttle.

Drove it a bit more today and did a few runs. As these tires are M+S rated, they probably give up a bit in off the line vs. the stock summer tires, but make up for it in lighter weight...not that I can notice but I see it in the numbers. I did a LC run for the first time (my X5 only has a bit over 1300 miles on it), and posted a 0-60 of 4.29, 0-100 of 10.63, and 1/4 mile of [email protected] Definitely some room for improvement there, and hopefully the engine will loosen up a bit more as time goes on. Even if it doesn't, with a set of stickier non-run flats I should be able to improve on those numbers a bit, although options for this tire size are pretty limited.

One other interesting point is that it logged a peak acceleration of 0.967g using launch control...that's impressive.
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Old 08-04-2011, 02:45 PM
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what tires did you get?
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  #7  
Old 08-04-2011, 04:12 PM
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I used the Toyo Proxes STs that I had from my previous X5 4.8i. They are not new but have good tread still. That said, since they are a M+S tire, they are not as sticky as they could be. Despite that, the handling feels improved over the runflats.
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Old 08-04-2011, 05:07 PM
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Wow! .96g on the LC?????
That a hard leaving truck. I posted similar performance time. But only saw .8** Gs.
Is it possible that it leaves that hard due to the regular tires? The runflats are a lot heavier and take more to get them going.
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Old 08-04-2011, 05:14 PM
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I hate my run flats

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  #10  
Old 08-04-2011, 07:40 PM
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Quote:
Originally Posted by donho View Post
Wow! .96g on the LC?????
That a hard leaving truck. I posted similar performance time. But only saw .8** Gs.
Is it possible that it leaves that hard due to the regular tires? The runflats are a lot heavier and take more to get them going.
Yeah I was surprised. Looking closely at the graph, it spikes at 0.969 and then drops off to 0.65 for a full 1/2 second before recovering at 0.87...all during first gear. This dip corresponds to a slight flattening of the acceleration curve, so I suspect the tires lost traction and then recovered. This no doubt cost me a few tenths.

Also keep in mind that all of these runs were done mid-day in South Florida, with temps around 90 degrees with high humidity. On a cooler, dryer day (which I'll have to wait for a few months to see), it will definitely be faster...as you know these turbo motors like the cold air. I wouldn't be surprised to see a 1/2 second improvement with the temps in the low 50s with low humidity.
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