# Rotating mass



## tduncan (Jan 21, 2006)

I've heard several comments on the importance of reducing rotating mass. Can anyone explain how that relates to the overall weight reduction of the bike ie. why it's more important? Thanks Tim


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## hitek (Mar 13, 2006)

*rotating mass*

take a 3 lb rock on a sting and a 2 lb rock on a string. spin them as fast as can. after a minute that 3 lb rock will fill 10 lb rock


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## colinshark (Jun 18, 2006)

tduncan said:


> I've heard several comments on the importance of reducing rotating mass. Can anyone explain how that relates to the overall weight reduction of the bike ie. why it's more important? Thanks Tim


To put it simply, a moving wheel has to carry forward velocity PLUS rotating velocity. It takes more energy to get a wheel up to speed than something that doesn't spin and is just along for the ride.

The mass of the rotating piece doesn't necessarily matter. The only measurement that tells you the whole story is moment of inertia. Mass concentrated at the outer edge of the object makes for a very high and very poor moment of inertia, so bicycle wheels automatically have a terrible moment of inertia unless they are small.

In conclusion, saving 1 lbs of wheel weight will help you more than losing 1 lbs of frame weight. It won't be any better up hills or cruising, but you'll have an edge in acceleration and probably handling.


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## alienator (Jun 11, 2004)

Well, I don't know what the hell the spinning rocks on a string is all about, but if you do a search here on "moment of inertia", "rotating mass", "acceleration", and etc. you'll find posts wherein calculations show that in terms of performance, rotating mass doesn't really affect performance on a bike as much as people think it does.


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## odeum (May 9, 2005)

it's true, once the bike is rolling, put simply, the wheels roll with the bike, so overall weight has precedence over rotating weight, in light of the fact it does not take an appreciable amount of force to "spin up" wheels of different weight.
www.yarchive.net/bike/rotating_mass.html





alienator said:


> Well, I don't know what the hell the spinning rocks on a string is all about, but if you do a search here on "moment of inertia", "rotating mass", "acceleration", and etc. you'll find posts wherein calculations show that in terms of performance, rotating mass doesn't really affect performance on a bike as much as people think it does.


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