Road Cycling It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle. -- Ernest Hemingway

Rotational mass

Old 06-02-07, 07:37 PM
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roost5o
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Rotational mass

I know that adding weight to automobile wheels by going to a larger (heavier) wheel (but keeping the same overall tire diameter) will slow the vehicle down in timed acceleration runs. I now believe that lowering my rotaional mass is better than removing weight from the vehicle itself (100lbs from the wheels is better than 100lbs from the body)
Now to move on, would I be better off getting lighter wheels, cranks, chain, hubs, pedals, tires, etc as opposed to lighter...say bottle cages, seat post, saddle, bars, forks, etc?
Is money better spent on the lighter drivetrain parts versus the lighter add-ons parts?
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Old 06-02-07, 07:38 PM
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yes



but you'll really want those expensive CF cages that break halfway through a century.
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Old 06-02-07, 07:52 PM
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Dude, you can spend a whole lot of money on all that stuff to save grams or you can ride your bike, a lot, and lose a few pounds for free. I'm not trying to harsh your buzz but...
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Old 06-02-07, 07:57 PM
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mcoomer, I think you missed the point of my thread. I am not talking about reducing weight of me or the bike accessories but specifically of the drivetrain even if the overall weight of the bike is identical
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Old 06-02-07, 08:23 PM
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As i'm sure many will say, overall weight of the bike/you is only factors in on hills/accelerating. Rotational weight only affects your acceleration. An increase in your wheel's rotational inertia (most people say weight/mass, it's really it's inertia) will decrease your acceleration. i.e. it'll take longer to speed up, and longer to slow down.

How is money best spent? that's a tough one, but hopefully that helps you with your cost-benefit analysis
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Old 06-02-07, 08:33 PM
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Rotational weight only affects your acceleration. An increase in your wheels rotational inertia (most people say weight/mass, it's really it's inertia) will decrease your acceleration. i.e. it'll take longer to speed up, and longer to slow down.

Thanks for the INTELLIGENT response. I was just wondering if the overall weight mattered how it was distributed.
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Old 06-02-07, 08:39 PM
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Simply, yes, reducing weight by, say, 1.5 lbs in the wheels will be more beneficial than from a non-rotating part. It's more than just the wheel weight though. Reducing hub weight carries almost no rotational intertia advantage. The rim makes a big difference. The tire and tube makes a big difference.

Still, at the rates we accelerate, even sprinting at 1500w, the rate of RPM change in the wheels is very gradual, so lowering the RI doesn't actually add much value. In seated acceleration, you'd never feel the difference between an average training wheelset and a lightweight carbon tubular. It could probably be measured though.
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Old 06-02-07, 08:40 PM
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Less rotational inertia also helps braking, but I guess no one here cares about that.

"Ain't no stop signs in the fast lane baby!"

Az
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Old 06-02-07, 08:42 PM
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Originally Posted by roost5o
I know that adding weight to automobile wheels by going to a larger (heavier) wheel (but keeping the same overall tire diameter) will slow the vehicle down in timed acceleration runs. I now believe that lowering my rotaional mass is better than removing weight from the vehicle itself (100lbs from the wheels is better than 100lbs from the body)
Now to move on, would I be better off getting lighter wheels, cranks, chain, hubs, pedals, tires, etc as opposed to lighter...say bottle cages, seat post, saddle, bars, forks, etc?
Is money better spent on the lighter drivetrain parts versus the lighter add-ons parts?
All that makes sense after you have trimmed every last excess lb. from your engine.
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Old 06-02-07, 09:05 PM
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Thanks folks, I thought I would just throw that out there is all.
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Old 06-02-07, 09:10 PM
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Originally Posted by 2Tired2Shift
All that makes sense after you have trimmed every last excess lb. from your engine.
It is not the "engine" that I am referring to...I KNOW rotaional mass makes a difference in acceleration in vehicles but was just curious what you guys thought about it as it pertains to bikes
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Old 06-02-07, 09:24 PM
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Well, assuming you personally aren't carrying extra weight, if you're looking to shave weight, the best place to take it from would be the wheels, the further out from the axle the better. This of course is provided you're not compromising strength to do so.
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