Bike design starts by looking at things that cannot be easily changed. Certain components are somewhat standardized. Tires and wheels are two such items. Unless you want to make your own rims and tires, you need to design your bike around what is available.

There are three common wheel sizes for modern bikes.

• 622 mm rim diameter - commonly know as 700C
• 571 mm rim diameter - 650C
• 559 mm rim diameter - MTB, 26 inch

Thanks to sheldonbrown.com for this info.

You could use a different size wheels on the front and rear, but there are some good reasons why you might not. For one, UCI racing rules specify that both be the same size. A more practical reason for using a common size is the need to only carry one spare tube in case of a flat tire. So we will start with two circles, each 622 mm in diameter. This is actually the diameter of the bead seat; the outer diameter of the rim is usually about 642 mm for a clincher. Of course we have to mount tires as well. For a cyclocross bike, this is going to add 70-80 mm to the overall OD. So if we are designing a 700c bike, this means we will have a wheel/tire combo with an OD of at least 712 mm. If we were going with nominal 26 inch wheels, we would have someting like 642 mm. More on this later.

Assuming that we are designing a standard bike and want the back wheel and the front wheel in the same plane, this pretty much sets the lower limit of the wheelbase for us. In practice, having the wheels this close might not work out too well. Wheelbase doesn't get all that much attention in most bike brochures, but it has a lot to do with how the bike will ride. Since the wheelbase is generally determined by a number of other more critical measurements, designers and marketing types don't pay much attention to it. Another reason is that on most design projects, the wheel size decision is made ahead of time, and this takes away a lot of freedom with respect to wheelbase, chainstay length, and most important, front-center distance. The wheel diameter also determines how long the fork blades need to be, and so this also limits how low the front of the bike can be.

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