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Technical FAQ: Wheel inertia and bottom bracket types

  • By Lennard Zinn
  • Published Apr. 24, 2012
  • Updated Apr. 25, 2012 at 12:20 PM EDT

Dear Lennard,
I’ve re-read a couple of your old posts on wheel inertia while researching my own homemade torsional pendulum for making wheel comparisons. My question is why people don’t insist on buying wheels based on moment-of-inertia instead of weight.

The concept of rotational inertia is well known, and not complicated… so why don’t the wheel manufacturers tout their low inertias and market their products based on a cycling metric that really matters? Once one of the big guys starts advertising superior low-inertia wouldn’t the buying public start expecting it? From your columns I gather that you, like me, are a rotational-inertia zealot… so I’m sure you must have an answer for why the cycling industry doesn’t promote a wheel inertia metric.

Thanks for your excellent writing on the subject!

P.S. I build a simpler trifilar pendulum which doesn’t require quite as fancy a calibration routine…. Here’s a link to an old paper from 1945 on the setup… see page 7.
David

Dear David,
Actually, I do not have the answer, and I’ve often wondered at rotational inertia’s absence as a measured and published quantity myself and still attribute it to companies assuming that consumers don’t understand the concept. Mavic mentions inertia a fair amount at press product introductions but does not publish moment of inertia specs for its wheels.

I, like you, would think that it would behoove a wheel brand to publish this information, particularly in cases where it doesn’t have the lightest wheel in a given category, but does have the wheel with the lowest rotational inertia. It seems like a no-brainer in that case; if you’re losing sales to a competitor based on weight but actually beat them on a more important weight-related property, what is there to lose in publishing it? I suppose since others don’t publish their rotational inertias, that brand would be in the position of having to list the rotational inertia of its competitors, which could be a slippery slope and perhaps there is a fear of lawsuits. But wheel brands selling aerodynamic wheels often list wind tunnel results of competitors, so it certainly is not without precedent.

Nick Legan and I are actually measuring the rotational inertia of a slew of deep-section aero wheels this coming Thursday.

Your trifilar pendulum idea looks like a good one.
― Lennard

Dear Lennard,
I have been following your recent discussion on the reduction in rolling resistance of larger diameter tires. The insight is interesting, and seems well reasoned. That said, a few comments seem to be misguided, specifically the effect of mass on the rotating wheel vs. non-rotating parts of the bike. I know you have covered this many times in the past, but I think the answer is clear (and does not seem to follow what you have written).

You state, “The rear wheel makes so little difference aerodynamically for most riders in mass-start road races that it makes more sense to have a lower-inertia rear wheel that requires less energy to accelerate or to drag it up a hill.”

We have claimed benefits of lower-inertia wheels. Acceleration and climbing. Let’s do climbing first.

The rotational inertia has no effect on the ability to climb a hill. The most obvious way to show this using basic physics is to use simple conservation of energy principles, taking the system starting at the pedal down to the tire. Increases in the wheel mass have no effect on energy transfer. Any extra energy spent accelerating the wheel at increased periodic pedaling forces will be returned when the forces decrease in a “fly wheel” effect. Even still, this effect is so small that you would never notice (the mass of the total system dominates).

You do have this very small effect of just plain additional mass of the wheel, but this is just mass, so the rider’s bike makes it not make any difference. A pro may care, but the bike mass is fixed by the UCI. A pro should probably get wheels that roll easier or flex less. KOM wheels seem to be a stupid idea in this respect.

As far as acceleration effects, there is a slight difference, but it is so tiny it should probably be ignored. A quick fill in of the [Criterium Corner calculator], where I increase the rim mass by 500g, yields a 5cm gain over 100m. Aerodynamic forces most certainly seem more relevant.

This all leads to further bolstering of the benefits of larger diameter tires (up until the aerodynamics effects start hurting). I wonder how a rim designed for 25mm tires would fare (though the gains seem to be falling off anyway).

One other question: why do people list drag in grams? The unit should be a force. People used pounds, and figured grams would make it sound more scientific? I like the watts representation, as this is what people can relate to.
Jeff

Dear Jeff,
In an ideal system with no energy loss to wind drag, rolling resistance, chain and bearing friction (admittedly small), what you say about climbing is true. But given that those things do exist, inertia does matter when climbing, because NOBODY climbs with a smooth pedal stroke, especially on a steep climb. Everybody has constant variation in the angular velocity of their feet, meaning that the wheel is being accelerated many times per revolution, and if there is continual energy loss, you do not get it all back from the flywheel effect. I admit it is probably a vanishingly small distinction except on a windy day or on a dirt road (lots of rolling resistance).

And when I said, “or to drag it up a hill,” I was making the assumption that the aero wheel was heavier as well as had more of its weight at the rim, and I should have said, “to have a lighter, lower-inertia rear wheel that requires less energy to accelerate or to drag it up a hill.” And of course your comments about the UCI bike-weight mandate apply to pros.

That’s an interesting result on your plug-in into Tom Compton’s Criterium Corner calculator. I wonder if that’s been (or could be) verified experimentally. Two inches (5cm) is not much, but I’m willing to bet that an extra two-inch gap off of the next rider’s wheel ahead does make more aerodynamic difference (resulting in greater energy cost) to the rider than the additional drag of a lighter, lower-inertia, non-aerodynamic rear wheel on the bike of a rider in a pack. And, in contrast to the discussion about climbing, there is no flywheel effect in a criterium corner, since the rider uses the brakes to slow down.

As for your last question, drag results are often measured and listed in grams (of force), as grams are often simply the readout from the balance (i.e., a gram scale) attached to the object in the wind tunnel (the balance is generally underneath the tunnel, attached to the test plate). Yes, grams are normally a measure of mass and not of force, but most people don’t think of the fact that weight is a measure of force and not of mass when they weigh something (particularly if they weigh something in an elevator as it is starting or stopping!).
― Lennard

Dear Lennard,
I’m curious if there is some historical reason behind bottom bracket paranoia. I’ve observed a trend over my eight years of wrenching that anytime someone has a creaky bike it is assumed it must be the bottom bracket. Internet forums also abound in tales of incurable creaky bottom brackets. In my experience, that is the cause of noise less than 10 percent of the time. Was the bottom bracket that troublesome years ago that it has entered popular wisdom as the Achilles heel of bicycles?
Adam

Dear Adam,
I don’t know from whence that came. I can’t remember Ashtabula one-piece cranks with press-in bearing races on my Schwinn Stingrays ever creaking. I guess there was creaking in the days of cottered cranks, if somebody removed and replaced the same cotter bolt. Otherwise, it was no different from now; it creaked if the cups moved in the bottom bracket shell’s threads due to bad threads or under-tightening or lack of grease or threadlock.

The one that cracks me up is when it’s the front hub moving in the fork dropouts and it’s attributed to the bottom bracket.
― Lennard

Dear Lennard,
The inside of my right foot rubs the crankarm on every pedal stroke. This is mostly annoying, but sometimes causes pain on long rides. This happens both on my road and ‘cross bikes, and with both road and mountain type shoes/pedals.

As a result, I’ve had to place the cleat such that it puts my right foot in the most outward position, and actually also so that the heel of my shoe is outward (sort of pigeon-toed on the right side), which doesn’t seem that optimal.

Is there a pedal extender available which can extend the length of the stem a bit so that the pedal is just a little farther outside?
Brent

Dear Brent,
Yes there is.

When it’s time to get a new bike, you might consider one with a BB30 crank, since one of the BB30’s claims to fame is improved ankle clearance.
― Lennard

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Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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