What variables are considered when determining wheel size and how do they affect these variables?

Question

What variables are considered when determining wheel size from a theoratical perspective and how do they affect these variables?

I noticed that city bikes tend to have smaller wheel sizes, whereas sometimes I see very large mountainbike wheels.

Answer 1

• The first variable is the size itself. When talking about folding city bikes, they have to be as compact as possible so you can take them as normal luggage when folded.
• There's also the aspect of weight - smaller wheels are lighter but also have lower rotational inertia. This means they are easier to accelerate but once they gained speed, they don't hold it for quite as long as large wheels.
• In mountain bikes the wheel size is a very individual thing. Smaller wheels offer better acceleration an better maneuverability (such bike is generally shorter and of course requires less force to turn a wheel). On the other hand, large wheels give better angle of attack meaning it's easier to roll over any terrain obstacles. Also, on a larger wheel you can have larger contact patch giving better grip.
• There's also a question of stiffness, Small wheels, especially in cheaper ranges are stiffer because of shorter spokes used.
• In most disciplines of cycling the wheel diameter is 550-650mm (not too precisely), so the differences are not enormous. Smaller wheels are used in folding bikes and BMXes - in the latter because of easier handling of a sherter, lower bike both on the ground and in the air.
• I've partly mentioned the changes in bike geometry, but I'd like to make it clear - wheel size can affect your position on a bike. In particular when really large diameter wheels are used, handlebars will be up pretty high and there's little you can do about it with manipulating the frame geometry.

There's no single standard of wheel size for any bike types, which shows that manufacturers change their approach and priorities over time. It's also hard to determine the perfect wheel size for each person (talking about MTBs here), because it depends on people's personal preferences, experience and use of the bike.