Gobsmacked! Weight and rotational inertia!

I’m “gobsmacked”, as the Brits would say. A few weeks ago, I took a week off to ride my bike. In place of my Coto Doñana Tour, I rode a vintage lightweight steel bike with 650c wheels. For this trip, I wanted a bike that was a little lighter and livelier. And indeed it was livelier — with every punch of the pedals, the bike leapt forward. Amazing what a light ride can do for you.

Or can it? Upon my return home, I weighed both bikes. Gulp…my prized 650c bike is a full pound heavier than my Coto Doñana Tour . Okay. How about the wheels? They’re the same weight even though the 650c bike uses Schwalbe Ultremo 23mm tires and the Tour uses Panaracer T-Serve 32-559 tires. This doesn’t mean the wheels have the same rotational inertia, though. Intuitively, my guess is the 650c wheels have less inertia. But is it enough to account for the “suppleness” of the 650c bike? Or is this a case of mind over matter?

About the same time I was pondering this, Velo magazine’s tech editor, Lennard Zinn, ran a column answering a reader’s question about the importance of bike weight. That has since blossomed into quite a discussion on the topic, taking many interesting twists and turns. How important is frame weight? How important is wheel weight? What about the weight of the rider??? I’ve referenced links to these articles at the end of this blog.

It’s hard to really summarize the discussion since so much information was shared. But the gist of it went like this: weight is important, but maybe not as important as we’ve been led to believe. If you generate x watts of power going up a hill, those x watts will get you up the hill faster on a lighter bike. How much faster? It depends on the weight of the two bikes. And let’s not forget the weight of the rider — the amount of power a light rider can generate is typically less than that generated by a heavier rider. So what’s the tradeoff in weight and power generated? That’s where it starts to get a little murky. And…the biggest factor: how well does the bike fit? Do you think that has a role to play in your power output? You bet!

Wheel weight falls into its own little piece of the puzzle. We all know a wheel is harder to accelerate when its mass is further outboard ( think “flywheel” for the ultimate hard to get started wheel). In physics terminology, a wheel with more “rotational inertia” takes more oomph to get started than a wheel with less rotational inertia. But how much? Marketers like to think it’s a lot, but physicists aren’t so sure. At least not in the general scheme of things.

No one does a better job of not mincing words than Jobst Brandt when it comes these matters, so I’ll let him have the last enticing thought: “Although it seems daunting, when another rider pulls away on a hill…, these accelerations, except in standing starts, are so small as to make the rotating mass story a hoax.”

References:
Does Bike Weight Matter?
Does Wheel Weight Matter?
Wheel Inertia
Jobst Brandt: The Weight Question