I've seen it mentioned that larger wheels are easier to turn - is that actually the case? How would that work?
The way I see it, larger wheels will have more angular momentum, meaning they'll take more energy to start turning, but while spinning, will be easier to keep at the same speed?
Larger wheels have lower effective rolling resistance. That is, they roll over obstacles more easily and smoothly because the larger diameter doesn't allow them to fall into gaps as easily. Which means on actual roads you'll roll more easily with a larger wheel. Everything else is much smaller differences.
However, I think the angular momentum may work out about the same: at the same speed, a bigger wheel will have a lower rpm than a smaller wheel. And the ratio of the difference in rotation speed is the same as the ratio of the the angular momentum. To put it another way: I'm too lazy to work out all the math, but I think the angular momentum cancels out and you get the same linear momentum regardless of wheel size. (and having the same linear momentum is an intuitive result. If you're going 15mph, relative to you the bottom of the wheel is going 15mph back, the top 15mph forward, the front 15mph down, the back 15mph up. Relative to the road it's 0, 15, 15 and 30. All independent of wheel size)
Constructing a larger wheel will generally require slightly more mass and might therefore slow your acceleration slightly.
Also, all else being equal, a larger wheel is geared higher and may give you a higher top speed.
I of a wheel of mass m, radius r is kmr^2, where k is some constant (less than one). At a speed v, the angular velocity of the wheel is v/r, and so the angular momentum is Iw = kmvr. So, for a larger wheel, you do have more angular momentum. But momentum isn't what's important - energy is. The kinetic energy is 1/2 Iw^2 = kmv^2 / 2. This means it takes the same amount of work (energy) to get up to the same speed on the different wheel sizes.
It mostly has to do with gearing. Imagine we have a big wheel of 26 inches and a small wheel of 10 inches.
The 10 inch wheel will have a circumference of 31 inches.
The 26 inch wheel will have a circumference of 163 inches.
This means that to travel 163 inches the big wheel will need to spin once and the smaller wheel will need to spin 5 times.
Another way to put this is that the bearing on the smaller wheel will need to travel a distance 5x further than the bearing on the larger wheel.
My physics is too rusty to calculate the force of friction from the bearing but physics public beta opens in 4 days we should ask them.
