I’m currently set up with a hub based power meter. I’ll be buying new wheels soon, and I think I’m also going to switch to garmin vector pedals for my power meter. Comparing pedals though got me thinking: Is it more advantageous to have light pedals and keep the added weight of the power meter on the hub? The vectors are heavier than if I just got speedplays, and we turn those pedals over 200 times per minute. Would I be more efficient keeping my cranks/pedals/clips/shoes as light as possible?
Yes, conceptually and in reality, lighter is better for any moving part, and especially so for rotating mass mentioned here.
The reality is that it is not likely to matter in any measurable way for most individuals. It falls well into the “marginal gains” category unless you happen to be in the top percentage of cyclists in the world.
Theory says that non-rotating weight is better than rotating (you only see the difference when accelerating, though), and if rotating the closest to the hub is the best (lower moment of inertia) - and again you only see the difference when accelerating. You could concentrate the entire weight of the bike and yourself into the pedals, and you would only perceive a difference during acceleration (well, apart from the fact that you personally may somehow suffer from being at zero mass).
Practice says the differences are negligible.
If this matters, you’re at a level where you get your equipment for free. Right?
Pedal based PM’s are about 60g heavier than a standard Ultegra pedal. A G3 hub (which is rotational weight too) is 100g heavier than a DT Swiss 240. That 60g is pretty small when you consider the overall rotational mass of the cranks, pedals, shoes and your feet. I have no idea what a human foot weights, but certainly there would be a rotational mass of well over 3000g.
Because a wheel goes down as well as up, rotational weight only matters when accelerating. And therefore can be largely ignored for anything but a crit. Because of gearing, there is not much variability in cadence/rotational speed of pedals - they are either ~90 rpm or they are stopped. Wheels are rarely stopped, but display a much greater variability in rotational speed.
I would suggest that the 40g overall weight saving is probably more beneficial than any decrease in efficiency due to rotational mass.
That’s what I tell my wife!
Well I am a TR user so…
Thanks Chad, good info.
Great analysis, puts it in good context. Thanks!
Wheels also have a much larger diameter, so weight on the rim has a much larger effect (on moment of inertia) than the same weight on a pedal (and even more than at the hub). Rotational acceleration is torque times moment of inertia, and moment of inertia of a rim is proportional to the square of the radius; so the same mass on double the radius will have 4 times the impact in acceleration at a given torque.
All of which nobody cares too much about in a TT, but really do in a criterium.