Why Rotating Weight Doesn't matter

Just wondered if anyone else had watched this - quite interesting though of course coming from GCN i took it with a pinch of salt…but if the science backs it up…

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Follow the money…

A guy representing a company that sells aero wheels that are heavier than similar aero wheels from other manufacturers tells me that (rotating) weight doesn’t matter… And the proof for that is a simulation (not an actual IRL experiment) which is not available to the public…

Additionally I find the assumptions - at least for my use cases - not fitting. I don’t race / ride TTs on closed roads. I ride often in cities with unfortunately a lot of breaking and accelerating.

I mean, I’m no engineer. He could be right. But if something smells like marketing BS and sounds like marketing BS, I don’t have to stick my finger in it and lick it to figure out if it’s marketing BS.

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Like you i’m very suspicious - just be interesting to hear where the basis for rotating weight being worse came from in the first place and what that was based on.
Aero does seem to be a saving if you’re doing lots of flatish rides anyway.

Some google junk:

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Whats the TL:DR consensus? :relaxed:

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@mcneese.chad thanks for the google junk :slight_smile:

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This has been going on for decades.

If constantly accelerating/decelerating then rotational weight is an issue. If not, then no biggie as it’s overall weight that counts.

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in this case i’ll let it die - sorry wasn’t as aware it had done to death. That’ll teach me for falling for a GCN vid. :scream:

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Sounds like the GCN content is pretty solid, based on @d_diston’s tl;dr and all Chad’s google junk?

Rotating weight only comes into play if you’re accelerating or decelerating…With a lighter wheelset on the Sa Colabra climb, you [Ollie] were potentially 4 seconds faster up that climb with a 400g weight savings on the bike…When we decouple just the rotational mass, because there’s no change in acceleration, zero difference. The 4-second saving comes purely from the 400g you’re not having to carry up the hill.

GCN talked to an engineer about a question a lot of people have and produced an answer that a lot of other sources corroborate. That seems like a win for all the new (and/or clueless like me) cyclists who watch GCN :smiley:

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It’s also an issue when changing directions. The gyroscopic inertia of the wheels resists changes in direction. Probably more of an issue on MTB vs road.

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Not that new so I must be clueless :joy: but it’s one of those myths that I’d heard of and never really thought too much about or ever challenged (I do/did happen to be looking at whether to get some aero wheels or not) and came across this video.

I’ll probably leave it as I have far more to gain from loosing weight off myself first. :+1:

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I’m not sure this is the case.

Rotating weight – or any weight, for that matter – will cost you if you accelerate and then brake because you have to put more energy in to it to get it up to speed.

You have to accelerate the whole system – rider, bike, and wheels – at the same rate, since all of the parts stay together.

The kinetic energy of a bike is:
image
where w is wheel components, b bike, and r rider.

Figure a worst-case where all of the mass of the wheel is on the outer edge, because then the moment of intertia is simpler:
image

Since the wheels are making the bike move, wheel rotation rate and bike speed are strictly related. If you look at the kinetic energy just in the wheels:
image

So the fact that the wheels are rotating seems to add about 2.5% (1/4pi^2) to the “effective mass” of the wheels.

Note that you can only accelerate the whole system, though. If you have a 60 kg rider, 5 kg bike, and 1 kg (each) wheels, then the effective mass due to rotation is less than a tenth of a percent of the mass of the system.

Unless I’ve made an error somewhere?

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I think you have an extra 4pi^2 in there?

Omega = v/r

Effective mass of the wheels I believe doubles.

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That’s it. I’ve been spending too much time with rotation being frequency instead of radians. :stuck_out_tongue:

In that case the actual weight distribution of the wheels is relevant: rim/tire weight is 100% extra, spokes are about 50%, and hub weight is some but not much.

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GCN also did a video where they proved, or tried to prove, that frame stiffness is a load of marketing crap that doesn’t matter. I forget the outcome

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I’m surprised nobody mentions this:

When in doubt that something more expensive is better, stick to the cheaper alternative until proven beyond reasonable doubt that it actually is better.

I’ll stick to my cheap but reliable, heavy non-aero wheels.

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I don’t think you are drawing the correct conclusion here - no side of the argument is challenging the aero effect of wheels so you’d be best off with cheap, slightly heavier but aero wheels instead.

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I like the science in the video but I think the assumption that you are moving at a constant speed on a climb isn’t accurate to the real world. I’m not talking about Contador style attacks or surges but every pedal stroke you have a momentary pause in power output where you are effectively decelerating, particularly on a steep climb. So each pedal stroke is in effect a micro acceleration where the rotating weight will play a part. The effect might not be very much but over a climb like the one in the video this is surely going to add up?

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I suspect the sweet spot between system weight, aero, and price is the new generation of 40-45 mm hookless TL wheels.

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That’s a terrible philosophy bruh!

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