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Rotational Weight

Old 09-06-07, 02:10 PM
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Rotational Weight

Ok, before anyone says anything, I've looked around for a few minutes to see if anyone has posted anything similar to what Im asking for, and I couldn't find anything.


Anyways, Im building up a new bike and i've besided on the Bontrager aeolus 5.0 carbon clencher for wheels. however, I think I found a way to save a few bucks: rather then going with a reynolds ouzo pro fork and the aeolus 5.0s, I could do the Aeolus 5.0 A/Cs and a reynolds UL fork, adding 60 grams to the wheels but loosing 65 with the different fork. Now, it would be cheaper to go with the 5.0 a/cs and lighter, However, since this will be a pure race bike, im somewhat concerned if this will make a difference because of the extra rotational weight.

Pretty much what Im getting at is, a) Rotational Weight, does it exist and B) 60grams, will that make a big difference? Im looking for hard data, not so much opinions.
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Old 09-06-07, 02:28 PM
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Put your current bike in the stand, or flip it over (just watch the computer) and turn the pedals with your hands. Think 30 grams on the rear will make any difference to your hands? Think your legs will notice? Of course not.
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Old 09-06-07, 02:29 PM
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Rotational "weight" or more properly rotational inertia exists. It will decrease your ability to change speed (accelerate or decelerate) more than weight (mass) that does not have to change rotational velocity in addition to changing linear velocity.

The rotational inertia (or moment of intertia) increases with the square of the distance of the mass from the point in rotates around. Thus if all 60 grams of weight difference were at the wheel hubs, the distance of the mass from the hub is small (say only 2 cm) and the rotational issue coudl be ignored. If it were all in the rims, the distance would be roughly 32 cm (I think that is slightly less than the radius of your wheel) so the weight would be about 15 times further out and its effect on the rotational inertia would be 15 x 15 = 225 times more.

With a bit more math, you could calcuate the equivalent of 60 grams in the rim to additional non-rotational mass but I do not have time to do that now.
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Old 09-06-07, 02:36 PM
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A) Yes it exists
B) It's not important

Rotational inertia only exists during acceleration. The *maximum* possible effect is at the very outer edge of the rim, where it takes 2x the energy to accelerate the wheel to speed. Most of the mass of the wheel is somewhat under the 2x ratio.

The amount of energy to accelerate a wheel from 0 to 50kph is not all that much. The difference between those two wheels (60 grams) is tiny. Accelerating 60 grams to 50kph is incredibly easy.

Time for a little math:

Assuming two identical bikes with the same mass, the only difference being distribution with 60 grams more weight on the very outer circumference of the wheels on one bike.

Kinetic energy:

1 Joule = 1 KG * M^2 / S^2

Mass difference = 0.06 KG, velocity is 15 m/s (approximately 50kph)

0.06KG * (15m/s)^2 = 13.5 Joules.

1 Joule = 1 Watt * 1 Second.

If it takes you 15 seconds to accelerate from 0 - 50kph, 60g more of rotational mass will require less than 1 watt extra input power. This is also only a one time cost, once you've accelerated to speed, there's no extra resistance.

The 5 extra grams on the fork will require much more than that over any length of climb.

Different tire weights will have a greater effect than different wheel weights, as all their mass is at the outside of the wheel.
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Old 09-06-07, 03:16 PM
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Originally Posted by Jinker

Different tire weights will have a greater effect than different wheel weights, as all their mass is at the outside of the wheel.
Too true. There are a number of people that spend a fortune on getting the lightest wheel they can afford but run out of money to fit a light tyre and tube. If you are thinking of Rotational weight- then remember your Physics and Cut the weight at the Exremity of the wheel. Light rims and tyres are worth it.
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Old 09-06-07, 03:26 PM
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Originally Posted by Giro
With a bit more math, you could calcuate the equivalent of 60 grams in the rim to additional non-rotational mass but I do not have time to do that now.
Someone's done it for you. https://www.biketechreview.com/archive/wheel_theory.htm

Decreasing rotational inertia by 50%, decreases power required less than ~0.02%. Decreasing wheel mass by 50%, decreases power by ~0.2%. The OP is talking about much less than 50% changes.
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Old 09-06-07, 03:38 PM
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Lift your rear wheel off the ground, and pedal with your hand at 50 rpm. Kick it up to 60 rpm really fast and note how little effort it took. That's a 20% acceleration that would cover most of a sprint, and the sprint would be spread out over 15-25s.

So, yeah, it doesn't matter at all unless you're talking about a REALLY tight finish, which is rare.

Of course, UT_Dude beat me by something like 0.01s in a TT recently After that happened, I said I'd never say "it doesn't matter" any more, but that was a bump from 7th place - I'm back to my cynical self again
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Old 09-06-07, 04:09 PM
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Nah. Doesn't matter. I race my heavy Cosmic Carbones and beat people with their SL's all the time because aero is 10x more important than weight. Get the cheaper combo.
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Old 09-06-07, 04:35 PM
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Thanks every, looks like im going with the aeolus A/c's then.
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Old 09-06-07, 07:22 PM
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Rotational acceleration MAY exist when the bike is on a stand, with the wheels free to rotate, but not when the tyres are stuck to the road with 170+ pounds of rider holding them down

Last edited by 531Aussie; 09-06-07 at 07:58 PM.
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Old 09-07-07, 12:30 AM
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No, it certainly does still exist, but it's relegated to the realm of minutia.
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Old 09-07-07, 12:41 AM
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Originally Posted by Mr.Mavic
on the Bontrager aeolus 5.0 carbon clencher for wheels. however, I think I found a way to save a few bucks...
If you could just get your wheels to relax more, I'd bet they'd be a lot lighter...
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Old 09-07-07, 01:28 AM
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Originally Posted by UT_Dude
Nah. Doesn't matter. I race my heavy Cosmic Carbones and beat people with their SL's all the time because aero is 10x more important than weight. Get the cheaper combo.
+1

unless you're in a horrible crosswind
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Old 09-17-07, 05:05 PM
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Originally Posted by asgelle
Someone's done it for you. https://www.biketechreview.com/archive/wheel_theory.htm

Decreasing rotational inertia by 50%, decreases power required less than ~0.02%. Decreasing wheel mass by 50%, decreases power by ~0.2%. The OP is talking about much less than 50% changes.
I recently had a chance to try some Bontrager Race XXX Lite Carbon Clincher wheels (last year's model). I had a short discussion with the guys at a LBS about this topic, and they said, 'Here, try these; you'll see the difference.' Unfortunately, I didn't. They were very kind to let me try these $1500 wheels, but I couldn't tell much difference from my Richie wheels. I think mine were about 50 g heavier, and the XXX would probably be better for that reason. But after I did some google research, I'm convinced that the advantage is not in the lower moment of inertia.

Here are some of the analyses I found:

This one re-states and references the BikeTech analysis: the effect of rotational inertia (moment of inertia) is negligible compared to wind resistance:
https://www.charles.whitaker.name/wheels.html

Jobst Brandt's take on rotational inertia:
https://yarchive.net/bike/rotating_mass.html

and for the physics students out there:
https://www.physics.mcgill.ca/~royp/P...ProjectLab.doc
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Old 09-17-07, 05:14 PM
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Originally Posted by UT_Dude
Nah. Doesn't matter. I race my heavy Cosmic Carbones and beat people with their SL's all the time because aero is 10x more important than weight. Get the cheaper combo.
Dangit, now you've got me thinking about swapping back to my Cosmic Elites for the last TT tomorrow. Not that I'm fast enough to make a meaningful difference...
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Old 09-17-07, 05:37 PM
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Originally Posted by stapfam
Too true. There are a number of people that spend a fortune on getting the lightest wheel they can afford but run out of money to fit a light tyre and tube. If you are thinking of Rotational weight- then remember your Physics and Cut the weight at the Exremity of the wheel. Light rims and tyres are worth it.
: )

I just bought some used 1220 gram wheels that I've yet to use, because I can't afford $100 a tire race tubulars and am unwilling to run new old stock training vittorias I have in possession.
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Old 09-17-07, 05:44 PM
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Originally Posted by UT_Dude
Nah. Doesn't matter. I race my heavy Cosmic Carbones and beat people with their SL's all the time because aero is 10x more important than weight. Get the cheaper combo.
People, please stop generalizing.

It depends on the circumstances.

In a crit, when you're racing for the line or jumping away lighter wheels are going to spin up faster. Once you're solo off the front, the aero wheels are going to be better suited. However, factor in 8 corners per lap that cause you to have to decelerate/accelerate continuously, and the lighter wheels are better suited. Also consider crosswinds in the equation.

Also consider climbing. If you're on a 7 or 10% grade climbing, suffering out your eyes, the lighter wheel at your 9mph is better as there's virtually no aero-dynamic advantage.

You're in a solo or two or three man break for 20 miles, give me the aero wheels.

So really, it depends on the application. No one wheel is better (unless they're lieghtweights!).

I'm about to try out a pair of LEW tubulars that are in at 1220 grams for the first time at the state champ hill climb (20 miles, 6500 feet of climbing). I've been running ksyrium SSL's for about a year now, so I'm curious to see how much I notice the wheels.
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Old 09-17-07, 06:14 PM
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Originally Posted by brianallan
In a crit, when you're racing for the line or jumping away lighter wheels are going to spin up faster.
Perhaps, but how much faster? What good is it if you gain 0.01 sec. on the spin up and lose 10 sec. on the straight?
Originally Posted by brianallan
Once you're solo off the front, the aero wheels are going to be better suited. However, factor in 8 corners per lap that cause you to have to decelerate/accelerate continuously, and the lighter wheels are better suited.
You tell a nice story pardner, but I don't see any numbers to back up your fairy tale. Sadly the data says different. The aero wheels save more energy even sitting in a pack than the lighter wheels (especially if you only consider roational inertial effects). Why don't you just read and understand the article? https://www.biketechreview.com/archive/wheel_theory.htm
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Old 09-17-07, 07:19 PM
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Originally Posted by asgelle
Perhaps, but how much faster? What good is it if you gain 0.01 sec. on the spin up and lose 10 sec. on the straight?

You tell a nice story pardner, but I don't see any numbers to back up your fairy tale. Sadly the data says different. The aero wheels save more energy even sitting in a pack than the lighter wheels (especially if you only consider roational inertial effects). Why don't you just read and understand the article? https://www.biketechreview.com/archive/wheel_theory.htm
I described situations in which one wheel or the other would be beneficial, and I made no claims as to which wheel was "better". As I stated, one has to take a variety of circumstances in to consideration. My fairy tail? You've oversimplified things, trying to describe the advantage of one wheel with brevity, but you fail. Why don't YOU read the article. His account of wheel performance is not the end all be all of the aero vs lighter debate. If this was the definitive guide to which wheels were better, you'd see the riders in the TDf all running 60mm deep clinchers, because some data supports that as the most aero, least rolling resistance wheel.

The fact of the matter is, the course and riding conditions found in "real" racing are extremely difficult to account for in a few physics equations. Terrain undulates, accelerations happen over and over and over again in a pack, and crosswinds are almost inevitable. I don't claim to have the answers, I'm just saying that you don't either.
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Old 09-18-07, 08:57 AM
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Originally Posted by brianallan
The fact of the matter is, the course and riding conditions found in "real" racing are extremely difficult to account for in a few physics equations. Terrain undulates, accelerations happen over and over and over again in a pack, and crosswinds are almost inevitable. I don't claim to have the answers, I'm just saying that you don't either.
The course and riding conditions in real conditions may be complex, but that does not mean they are difficult. Grade, wind, road surface pretty much captures everything of importance, but even that is not necessary. Knowing for example, that rotational inertia effects are always 2 orders of magnitude less than aerodynamic drag means I don't have to spend a lot of time defining the precise accelerations in an event. Another way to look at it is if I take the maximum acceleration observed in a P1,2 crit as a reference value, I know actual accelerations will always be less. If it turns out that aero drag is much more important than rotational effects for that case, there's no need to do any further refinement. The conclusion will not change.

A precise answer is not always needed to gain understanding.
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Old 09-18-07, 09:29 AM
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Originally Posted by brianallan
I'm about to try out a pair of LEW tubulars that are in at 1220 grams for the first time at the state champ hill climb (20 miles, 6500 feet of climbing). I've been running ksyrium SSL's for about a year now, so I'm curious to see how much I notice the wheels.

My understanding is that the Lew's are much lighter than that (claimed 880 grams) Zipp 404 tubulars only weigh 1252g
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Old 09-18-07, 12:56 PM
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[QUOTE=Mr.Mavic;5221707]Thanks every, looks like im going with the aeolus A/c's then.[/QUOTE


Niceeeeeeeeeee hoops.
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Old 05-11-08, 03:41 PM
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Generalizing

This guy hit the nail on the head. I agree that it depends on the application. The best wheel? For what? Racing, Centuries, Recreational.
Racing; what kind of racing? Crit or road race? Sprinter, climber or break-a-way rider?
Century / Recreational rider: Heavy or light rider? How important is speed on the flats vs climbing, or both?
For me, I'm a 188 lb sprinter. Needless to say I suffer badly and don't do so well on road races so I don't do them much. So, I race mostly Crits. The training rides are somewhat hilly so I'm in the red zone often. I have several sets of wheels; Zipp 404 (Clincher & Tubies), Mavic Ksyrium SL, Mavic Ksyrium R-Sys, Shimano Dura Ace 7801 SL, Zipp 909 (TT) and recently Bontrager XXX Lite. So, here is my take on the "best" wheels for me and the kind of riding I do.
Bottom line:
The Mavicís SLís are a sturdy all round wheel. Downright dangerous in cross winds. Non-aero.
The Bontrager XXX's are super- lite clinchers great for all around training and racing. Plus, I donít have to switch out the brake pads for racing.
The Zipp 404 clinchers are great for relatively flat continuous speed rides or cases but a little heavy for climbs so I pretty much keep them on my TT bike for training.
The Zipp 404 tub's are just plain awesome. They do everything superior to any wheel I've owned or ridden, however, I use them exclusively for racing since I donít want to deal with flats on the road.
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