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      10-18-2011, 06:05 PM   #23
klammer
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Now we're having an intelligent discussion. Thanks for all the info on the subject... Keep discussing
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      10-18-2011, 06:29 PM   #24
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People fail to account for one major factor when comparing 18's to 19's. I think it has been agreed upon and the truth is that the mass furthest from the hub/center is the most detrimental mass on the wheel as it effects moment of inertia and thus acceleration/stopping of the wheel.

18s would clearly be better given the same mass if they ran on no tires, however keeping total diameter the same, 18's will have thicker sidewall tires that have a mass of 2-4 pounds heavier per tire and this mass is at the very furthest point from the center, making the wheel mass itself the much less detrimental aspect of the rotational mass.

All being equal if you found a tire that weighed the same in 18 as 19 than yes lighter 18inch wheels would be better. To make up for the extra pounds in side-wall at the very furthest distance from the center, you have to make-up that weight in the 18's so that they cannot just be lighter but needs to be signifigantly lighter per wheel to make up for the difference and start to show any benefit.
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      10-18-2011, 06:44 PM   #25
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Quote:
Originally Posted by Ateam View Post
All being equal if you found a tire that weighed the same in 18 as 19 than yes lighter 18inch wheels would be better. To make up for the extra pounds in side-wall at the very furthest distance from the center, you have to make-up that weight in the 18's so that they cannot just be lighter but needs to be signifigantly lighter per wheel to make up for the difference and start to show any benefit.
That's far too generalized a statement. You'll end up being correct maybe 30% of the time, if I'm feeling generous and include only race-oriented wheels. On the order of 10% if you include all wheel / tire combinations possible.

You underestimate the impact of the MoI compared to the impact of unsprung weight. The MoI impacts the vehicle in far greater proportion than the actual weight of the wheel / tire combination. Any reduction of wheel weight is comparatively reduced by the action of the spring and damper according to the standard equation of motion (vibrations simplified: F(t) = mass*accel + velocity*damping + susp_travel*springrate). By comparison, the MoI impacts the amount of work required to accelerate as follows: KE_rot = 1/2*MoI*w^2, where w is, essentially, the RPM of the wheel.

All this isn't to say that lighter isn't better than not. But, just because you see 'lighter,' don't automatically assume better.
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Last edited by CT; 10-18-2011 at 06:52 PM.
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      10-18-2011, 07:30 PM   #26
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It's just 19^2 / 18^2 (mean rim diameter into I = 1/4*m*D^2). I can do the long-hand calc if you'd rather
assuming both rims are the same style. which on this car they are not.
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      10-18-2011, 08:23 PM   #27
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Quote:
Originally Posted by captainaudio View Post
......There are many factors at play here, unsprung weight, rotational mass and slip angle among others. These tend to interact with each other and as you change rim size to improve one parameter to gain performance you may actually be downgrading performance by making another area worse......
Perfectly stated !!!
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      10-19-2011, 05:53 AM   #28
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Quote:
Originally Posted by J08M3 View Post
assuming both rims are the same style. which on this car they are not.
This is all a game of assuming things to simplify the equation. Nobody here seems to care about getting precise, fully-derived answers - they just want a Yes or a No or a generalized statement to feel better. So I ballparked it. Of course there are assumptions there that are either wrong or incomplete.
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      10-19-2011, 07:03 AM   #29
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Quote:
Originally Posted by CT View Post
This is all a game of assuming things to simplify the equation. Nobody here seems to care about getting precise, fully-derived answers - they just want a Yes or a No or a generalized statement to feel better. So I ballparked it. Of course there are assumptions there that are either wrong or incomplete.
Well it seemed a pretty safe assumption the OP was talking about the OEM wheels. As far as I know the OEM 19s are lighter??
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      10-19-2011, 07:07 AM   #30
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because there is better selection of tire and size for 18" track wheels.
unsprung weight and all that other crap is minimal at best.
18" for size availability and also the tires are cheaper than 19.
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      10-19-2011, 07:20 AM   #31
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because there is better selection of tire and size for 18" track wheels.
unsprung weight and all that other crap is minimal at best.
18" for size availability and also the tires are cheaper than 19.
That's what I think, it's more about price and selection, the performance criteria is minimal at best, but then again, I'll be trying to shave a few seconds off of my laptimes so this might be something to consider. Once again, the better driver trumps all these "mods"
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      10-19-2011, 07:21 AM   #32
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Quote:
Originally Posted by J08M3 View Post
Well it seemed a pretty safe assumption the OP was talking about the OEM wheels. As far as I know the OEM 19s are lighter??
That may be the case - I don't know for sure. But if we're talking track, like the title implied, I was thinking he'd be going aftermarket to really get into the big weight loss numbers.
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