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Rotational weight of equal weighing wheels but different depths

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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 weight of equal weighing wheels but different depths

Old 07-16-12, 05:42 AM
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JAX_11
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Rotational weight of equal weighing wheels but different depths

A discussion at Superweek yesterday and this question was posed!

Wheel A is 58mm deep and weights 1450 grams

Wheel B is 38mm deep and also weight 1450 grams.

Is rotational weight the same or is it easier to spin up the shallower wheel due to cutting through the wind on acceleration?

Does it make no difference as to depth if their the same weight?

It almost seems like its hard to start a sprint or acceleration on deeper wheels of the same weigh!
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Old 07-16-12, 07:01 AM
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hammy56
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41
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Old 07-16-12, 07:09 AM
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Deeper wheel should spin faster as the mass is closer to the axle. At least that's how I would think it works.

And, does it really matter?
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Old 07-16-12, 07:29 AM
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waterrockets 
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Yes, deeper rim of the same weight will have lower rotational inertia.

Think about a sprint: you start it at maybe 33mph, and end up at maybe 37mph in, say, 5 seconds. Do you really think you could feel what race wheels are doing in a 4mph acceleration spread out over five seconds? That's a 12% acceleration, and feeling a <2 lb. pair of bike parts on a 200 lbs system.
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Old 07-16-12, 07:29 AM
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...and this is 41 talk.
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Old 07-16-12, 07:42 AM
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O snap.

How come we talk about equipment more than form?
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Old 07-16-12, 07:59 AM
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Does the flap of a butterfly’s wings in Brazil.....
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Old 07-16-12, 10:42 PM
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Originally Posted by JAX_11
A discussion at Superweek yesterday and this question was posed!

Wheel A is 58mm deep and weights 1450 grams

Wheel B is 38mm deep and also weight 1450 grams.

Is rotational weight the same or is it easier to spin up the shallower wheel due to cutting through the wind on acceleration?

Does it make no difference as to depth if their the same weight?

It almost seems like its hard to start a sprint or acceleration on deeper wheels of the same weigh!
You're assuming the rims have similar % weight distribution. It may be that the 58mm rim has only a fraction of its mass in the 20mm of rim height (like a fairing for example). You're assuming similar hubs/spokes etc. If you have a 58mm superduper light rim and a clunky 38mm rim it's different than two virtually identical rims on different hubs.

Taller rims tend to weigh more and, generally speaking, across a manufacturer's line, taller rims are more aero (that's the assumption I think that applies to the two rims in question). If the taller wheels don't weigh more they should wind up faster if they're in fact more aero. If they're more aero they'll be much faster at the top end in ideal wind conditions (cross tail I think, and a slight cross head) - the wheel keeps winding up just a bit longer instead of hitting the aero wall.
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