I am into bicycling and I notice when a magazine reviews a light weight wheel they talk about how it dramatically improves acceleration. I have just been doing a little research on performance rims for cars and I hear the same thing, heavier rims will hurt your acceleration.
Everyone says a heavier wheel is harder to spin up to speed or something to that effect. I don’t get it, assuming the tire has good traction with the ground, you have to accelerate the entire mass of the vehicle to spin the wheel faster.
The only thing I can think of is that a wheel must be accelerated twice, once rotationally and once forward, but when the weight of the entire vehicle is considered I don’t see how this can have the dramatic effect on acceleration that everyone claims. So what am I missing, how does a slightly heavier wheel have such a dramatic effect on acceleration?
I think that’s it. A spinning wheel has both linear and rotational kinetic energy. Another way to think about it is that the top of the wheel is moving forward at twice the speed of the vehicle. Since kinetic energy is proportional to the square of speed, it’s not cancelled out by the fact that the bottom of the wheel is not moving forward. One way to demonstrate it is to prepare two balls or cylinders of equal size, shape and weight, but one solid and one hollow. (Obviously the hollow one is made of denser material) If you roll them down a slope, the solid one rolls down faster. The hollow one has the weight concentrated on the edge and has higher moment of inertia, therefore it accelerates slower.
I do notice that my folding bike (with 20-inch 90-psi tires) doesn’t ride as smooth as road bikes. It jerks forward on every pedal stroke. So it definitely feels like it accelerates better, though it may be just the bike getting ahead of the rider on each stroke.
I also think you’re on the right track, and have always wondered about this myself. I think the first consideration is that weight on the wheel periphery is twice as important as weight elsewhere in determining acceleration.
But it’s also true that we’re talking about a very small part of the bicycle rider combination. You’re not going to guess your body weight within two pounds based on how this combination feels, so why would a difference of one pound in wheels matter? One reason may be that mystical edge where imagination takes over - like shaving your legs to reduce air resistance, or drilling out even the tiniest parts.
There’s another reason, though. To some degree you can separate the acceleration of the bike from that of the rider. Especially when you stand off the saddle, you can feel the bike pull ahead as you pedal. Now you’re feeling bike and wheel weight more like you’d feel the weight of a tennis racket.
Besides, you only get to sample wheel weight as an influence on acceleration simultaneously with other influences, like the degree of gyroscopic effect in the front wheel and the transmission of vibration from road texture. Given that it’s a package deal, it can seem like alot. It’s amazing what a difference wheels make. A bike I had 25 years ago was equipped with a pair of clincher wheels and another pair of sewups, and I’d trade frequently according to the kind of riding. Man, what a difference!