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“An ounce off the wheels is worth a pound off the frame,” goes the old saying, implying that rotating weight, especially on the wheels, is supremely important. The claim is sometimes laid out in less hyperbolic terms that weight on the wheels counts twice because when you accelerate you have to get it both spinning and moving forward.
Problem is, it’s not true. In 2001
bike engineer Kraig Willett analysed the forces on wheels(link is external) and concluded:
“When evaluating wheel performance, wheel aerodynamics are the most important, distantly followed by wheel mass. Wheel inertia effects in all cases are so small that they are arguably insignificant.”
The idea that rotating mass is important comes from the belief that wheel inertia matters, because it’s inertia that has to be overcome to accelerate a wheel. But Willett clearly demonstrates that wheel inertia doesn’t matter, so rotating weight is also relatively unimportant.
Why not? Well, you don't do much accelerating when you ride a bike, and even when you do the acceleration is relatively low, so the power expended accelerating a bike with ‘heavy’ wheels is only fractionally higher than that needed for light wheels. Overall weight matters when you’re climbing, but even that’s not as big a factor as people imagine and it’s a lot cheaper to save weight off your middle than the bike.
In fact you spend most of your
time, and therefore effort, shoving the air out of the way, and that’s a far better basis for choosing wheels. The roughly tenfold difference in the effect of aerodynamics versus total mass means you’re far better off with a pair of good aero wheels than a pair of light ones.
www.bdc-mag.com
