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Rotating weight vs. static weight

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Rotating weight vs. static weight

Old 03-14-10, 12:14 AM
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chinarider
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Rotating weight vs. static weight

Its often said that rotating weight is more significant than static weight. Is there any kind of conversion standard, i.e. saving 1 lb rotating weight = 2 lbs static weight? or 3 lbs or 5 lbs? I realize speed plays into this too. Just wondering.
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Old 03-14-10, 12:44 AM
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I believe that it is 1lb rotating weight saving equals a 1lb reduction in rotational inertia + the 1 lb static saving, so would require a 2 lb reduction in static weight to provide the same reduction in overall reduction in inertia.
I think it only matters when you are accelerating either horizontally or going up a hill (force against gravitational acceleration).
Hmm I once would have been able to explain this properly, but alas no longer, I just realised I am not really a proper engineer any more.

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Old 03-14-10, 12:49 AM
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Originally Posted by davida View Post
I believe that it is 1lb rotating weight saving equals a 1lb reduction in rotational inertia + the 1 lb static saving, so would require a 2 lb reduction in static weight to provide the same reduction in overall reduction in inertia.
I think it only matters when you are accelerating either horizontally or going up a hill (force against gravitational acceleration).
Hmm I once would have been able to explain this properly, but alas no longer, I just realised I am not really a proper engineer any more.

davida
That's really not how it works. It depends not just on mass, but how that mass is distributed around the center of the object. You can't just say 1 pound rotating mass equals two of static.

IIRC this is discussed in Bicycling Science, and the effect of rotating mass on a bicycle is generally far less than you might think. I'll look it up later. The wheels really don't go around that fast.
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Old 03-14-10, 12:49 AM
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Only considering weight that's located on the outer edge of the tire and only for the hypothetical case when almost all of your energy is going into acceleration of the bike (as opposed to overcoming gravity, air resistance, mechanical losses, etc.), then the ratio would be 2:1. But since in the course of a typical ride only a few percent of your energy goes into actual acceleration of the bike the effective factor is much, much closer to 1:1, more like 1.05:1.00. OTOH, in a race where the outcome depends on the final sprint then acceleration plays more of a role, so there the ratio edges a little closer to the hypothetical 2:1.
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Old 03-14-10, 02:20 AM
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Old 03-14-10, 02:35 AM
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Nono, on a bicycle you need to worry about unsprung weight, thats where the real savings are.

But really, from what I've read on here the difference between losing a pound from your wheels and a pound out of your gut are very close to the same.
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Old 03-14-10, 02:45 AM
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Originally Posted by W Cole View Post
Nono, on a bicycle you need to worry about unsprung weight, thats where the real savings are
European or African?
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Old 03-14-10, 03:34 AM
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Old 03-14-10, 07:55 AM
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Originally Posted by prathmann View Post
But since in the course of a typical ride only a few percent of your energy goes into actual acceleration of the bike the effective factor is much, much closer to 1:1, more like 1.05:1.00.
You have the right idea, but need to check your math. The actual ratio works out to 1.001:1 for the case of rider in a Pro,1,2 Crit accelerating out of a corner. This can be taken to be toward the upper end of expected values. For someone just riding around and not sprinting out of corners, the ratio would drop by around another order of magnitude, 1.0001:1.

https://biketechreview.com/reviews/wh...el-performance
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Old 03-14-10, 08:15 AM
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