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Comment by jackmott | Hacker News Reader

This is one of the most persistent and silly myths in cycling.

When biking at a steady speed, even when accounting for the fact that steady speed isn't entirely steady because humans pulse their power a bit, wheel weight is not worse than frame weight (the extra effort of each micro-acceleration is offset by the reduce slowdown between micro-accelerations and then some)

When climbing a mountain, wheel weight is not worse than frame weight

Yes, when accelerating, wheel mass is worse than frame weight, but only a tiny amount. source: physics: https://docs.google.com/spreadsheets/d/1N2aS3E0K4wrTa5jKqed7...

That is my spreadsheet, it is very very basic physics and it drives me crazy that people spew pseduo physics and comment on this "without taking the time to comapre the sizes of the effects"

take the time!

TLDNR; don't worry about wheel weight any more than frame weight. Addendum: don't worry about frame weight much either.

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> Assumes wheels are solid cylinders

I'm no expert but I would say that wheels are closer to a ring. You're underestimating the moment of inertia of the wheels by a factor of about 2 by doing this. Might be well enough to throw off your conclusions. In fact if I read your spreadsheet correctly this would bring the rotational energy to about the same as the translation kinetic energy.

BMX racer here.

First three pedals are the only ones that count. All we ever, ever do is accelerate from a dead stop and cut people off at the 30ft line. I'm running front-specific rims front and rear to cut weight, and it's working.

This may not be applicable to cycling at large, but it's extremely important for BMX racing.

> That is my spreadsheet

I think your spreadsheet makes several assumptions that are only true in theory.

As other have said, acceleration costs more energy. You don't get that back when braking.

More importantly, in practice, the road is not flat at all. There are small movements at every brick / stone that you cross, slightly rotating the bike forward and back. The momentum (MOI) multiplied with the weight of the bike for those those small movements is lost in energy. Weight at the ends of the bike (mostly the wheels) increases the momentum of the bike far more than weight in the center of the bike. I don't remember the exact energy formula, but think there is a power of 4 with the distance from the center in it...

I too have done the calculations, and with reasonable weight wheels it is totally trivial. Actually the weight of the bike itself is fairly trivial too unless you are a professional cyclist or regularly ride up mountains. The biggest difference it makes is how hard it is to pick the bike up.

Note that it is important if you have suspension though as you'll have a much smoother ride if the mass on the other side of the suspension from you is minimised. If the wheel is heavy the opposite and equal force will be felt when going over bumps.

The suspension reason isn't relevant to lightweight bicycles but important to keep in mind for other bikes.

I went for a bike fitting today and the guy that did the fitting was incredibly knowledgeable about all things bikes. He told me that due to the "physics" involved that wheels were 2 to 3 times heavier at speed. Turns out he was talking nonsense!

I wonder where this old wives tale about wheel weight comes from?

a 26inch wheel when spinning at 80 roations per minute (20km/h or 12mph) will have a Centripetal Acceleration of about 5g (G-Force) on the outermost spot. That does not make the wheel "heavier", but due to the gyroscopic effect and air resistance loose ever more energy, when spinning faster.

Fun wheel videos: https://www.youtube.com/watch?v=GeyDf4ooPdo https://www.youtube.com/watch?v=ty9QSiVC2g0

The gyroscopic effect makes you lose energy only if you turn. Of course you turn a little all the time while cycling.

Car engineers also say. But that may also be a matter of sprints.

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