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KEEP M3POST ALIVE BY DOING YOUR TIRERACK SHOPPING FROM THIS BANNER LINK! |
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09-23-2008, 07:19 PM | #23 |
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BMW-M-Mexico reports nearly zero difference at the track with 20s. I run 20s currently. The only reason I want a set of 19s for track is for the better tire choices. There aren't a lot of good tires in the 20" size. 18s have the most options but 18s won't fit my BBK.
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09-23-2008, 08:11 PM | #24 | |
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CCW one-piece wheels will fit the Brembo 15" 6 piston monoblock BBK, hard to believe but true. |
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09-23-2008, 09:38 PM | #25 | |
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See post #15 for my thoughts. And if you think Turner is into just selling big wheels with enormous staggers, then you are wrong. |
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09-23-2008, 10:42 PM | #26 | |
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With the same design, 20s will for sure have higher "I" than 19s. Period. Now, how much impact does that extra "I" have on the performance of the car? I don't know. Probably need to do some math to figure out. |
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09-23-2008, 11:06 PM | #27 |
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I bet it isn't much at all. Hence Nissan put 20's on the GT-R. They must not be big on the whole inertia thing. And if you put 20's on that are 3-4 lbs per/wheel lighter than the stock 19's. It would have to offset some/most of the negative affect.
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09-24-2008, 02:22 AM | #28 | |
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As briefly as possble rotating bodies obey an equation that is a perfect equivalent of Newtons law, force = mass x acceleration. Less mass, same force = larger acceleration. For rotating bodies torque = moment of inertia x angular acceleration. Again low I is absolutely fundamental to making objects easy to "spin up" (i.e. have a high angular acceleration for a give applied torque). All rotating masses have to be accelerated linearly along with the translational movement of the car, this takes force and power. As well rotating objects have to not only be accelerated they have to be spun up, this takes even more torque and power. Surely you need to fight weight as well, both are key. Large I values in your wheels not only affect acceleration but also braking and turning, all negatively. In other places that contain rotating mechanical parts like an engine, axles, driveshafts, etc. it is always a balance to have low weight, low I and acceptable stiffness and strength. For these components low mass and I also produce much less bearing loads. There is a similar reason to be greatly concerned with unsprung mass - that of the suspension and everything that moves along with the suspension. High unsprung mass is even worse than high sprung mass (say like the car body or a passenger). Lastly (and again, and again...) these effects are NOT huge. We are probably talking tenths of seconds in most acceleration metrics and maybe a couple feet in braking metrics will separate 18s, 19s and 20s. Very rough educated guesses here. However, do recall that saving X lb in a rotating component is MUCH more significant than saving the same X lb from a non-rotating component. |
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09-24-2008, 02:53 AM | #29 | |
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09-24-2008, 03:42 AM | #30 | |||
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I will say it again. 20's for the street and a seperate 18 inch track setup. It isn't that difficult. |
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09-24-2008, 08:52 AM | #32 | |
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Turner sells lots of things, some make your car work better, others are to satisfy the driver's sense of aesthetics. |
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09-24-2008, 10:40 AM | #33 |
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Turner actually just released a track setup for the E90/E92 M3 and it is with 18" wheels 18X10 with 275 square setup - they say it is the perfect setup for the track
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09-24-2008, 12:43 PM | #34 | |
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What is "best" for the GT-R is clearly NOT best for the M3, why is that so hard to understand? |
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09-24-2008, 01:13 PM | #35 |
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Actually I run a 285 rear and 245 front. Oh, I see you've confused me with another poster. Nevermind, carry on...
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09-24-2008, 02:59 PM | #36 |
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09-24-2008, 03:11 PM | #37 |
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A little stiff but handling is still great and ride quality is acceptable...
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09-24-2008, 05:51 PM | #38 |
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A couple people on here have mentioned density. Can someone explain to me why density matters and not just mass? It doesn't matter how dense something is. For this example it only matters what it's mass is. Isn't it like that old kids game....Which is heavier a 1lb of bricks or a 1lb of feathers? At the end of the day it's the mass that affects the rotation right? Sorry if that is a stupid question. I'm obviously not an engineer.
Taking the "stick" example someone else mentioned I have another question. If 2 sticks are 13.365 inches long and 1 stick has an extra 2 lbs attached to the last 3 inches wouldn't that stick have greater "I", or inertia and be more difficult to rotate and/or stop rotation? Please don't answer with sidewall stiffness, lap times, predictable behavior of at the limit adhesion, etc. I understand all that, don't necessarily disagree, but quite frankly don't care. This is a street setup on a car that is not my daily driver. I drive my Escalade during the week and race a 125cc Tony Kart on weekends. Other than practicing my heel-toe downshifting I don't race my M3 on the streets. My stock wheels/tires will be used for track duty. I'm merely just trying to satisfy a curiousity. Thanks. |
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09-25-2008, 01:41 AM | #39 | |
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You are right on with the stick example. In your example though one had a greater m and greater I. Here is how to take the understanding one step further. If you placed the extra 2 lb right at the center of the stick, call this condition A, and the extra 2 lb at the end of another identical stick, call this condition B. Both A and B would accelerate equally if you placed the same force on them (in such a way that you did not cause them to spin!). However, one will take more more torque to obtain a given angular acceleration. |
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