Bike Sizes: What does it mean?

Factual Questions
1

If someone says their daughter has a 20" sidewalk bike, what is 20"? IIRC, it is the length of the post from the seat to the pedals. Is this correct?

2

If it’s a childs bike it refers to the diameter of the wheels.

3

Other than different-sized riders, what benefits are gained from different sized wheels?

4

Bosda,

Larger wheels will maintain angular momentum, allowing for better maintenance of speed, along with better stability (due to larger gyroscopic effects of the spinning wheel). Smaller wheels will provide better manuverability and accelleration.

Since kids bikes tend to be unisize, it’s often how you size the bike to the kid. For “adult” mountain bike styles, you buy a frame that matches your size.

5

The straightdope on the lack of gyroscopic effect on bike wheels.

6

  1. Bike size can refer to the wheel size (27 inches for road bikes, on down to clown-bike sized), or, usually only for road-type bikes, the frame size, defined (as the OP thought) by the length of the seat tube (usually in the 20-something inches range).

  2. Larger wheels do make for a smoother and easier ride on moderate surfaces. But they’re weaker (for the same level of construction). Which is why road bikes have larger wheels than BMX bikes. Although the wheels size of mountain-bikes (just slightly smaller than standard road bikes) is as much a historical accident as anything.

7

Tapioca Dextrin, the article you linked to doesn’t say that bicycle wheels don’t have a gyroscopic effect, just that it isn’t necessary or important for staying upright.

Bike wheels do have a gyroscopic effect, as any rotating mass does. That’s why it says that Jones used gyroscopes (or extra wheels) to counteract the gyroscopic force of the wheels. You can’t counteract what ain’t there.

8

The way this term is used, it’s mostly a way of categorizing the bike. 20 inch is kid size BMX bikes or the kind of thing you’d buy at Toys R Us with a banana seat, but not the super small kids’ bikes, which probably have something in the neighborhood of 14 inch wheels. Adult sized mountain bikes have 26 inch wheels.

Remember that quote in “Mr. Mom” where Michael Keaton is asked if he’s putting in a 220 volt circuit and he says, “yeah, 220, 230 … whatever it takes”? It’s kind of like that. You wouldn’t say, “my 20 inch bike is getting a tad small, I think I’ll move up to a 21 inch.”

On the other hand, adult sized bike frames are sized in small increments, so you might go up an inch in frame size. But you’d still have the same 26 (or whatever) inch wheels, so your bike would be a 26 incher with a (say) 19 inch frame.

9

Surprisingly, the man at Toys R Us was knowledgeable here. Because a store like Toys R Us will carry so many styles and brands, the 20" wheel diameter is the only quantity that’s consistent. (Smaller bikes use wheel diameter, too). However, in a bike shop that deals with a few brands, the measurement does refer to the post of the frame (from seat to pedals).

Take it for what you will.