I was passing by a row of motorbikes and mopeds yesterday, and noticed they all have really small radius wheels compared to bicycles. Not all motorbikes are equal (some motorbikes are literally bicycles with engines), but generally it seems that motorbikes have smaller wheels.

I assume both vehicle types have been optimised and it's not just a fashion choice on the part of the manufacturers. What are they optimising for which makes them different? And why do mopeds/scooters have the broadest range of wheel radiuses when they're the lowest power type of motorbike?

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    $\begingroup$ The size of a bicycle depends on rider's height, how taller the rider how bigger the bicycle because rider's feet moves in two dimensional space up and down so you need a bigger wheels for taller persons to accommodate enough space which is compatibel with human anatomy and assures his comfort. However, motor riders doesn't need to pedal anymore, smaller wheels can afford higher rpm relative to bigger wheels, and how smaller the dimensions are, how fewer the fuel consumption. Maybe there are other reasons, i leave it to experts. $\endgroup$ – Sam Farjamirad May 27 '19 at 11:47

Kick scooters' small wheels have set the inspiration for the battery powered scooters.

Small wheels make it easy to mount and dismount the scooter. They have little angular momentum, so speed up faster and slow down to a quick stop faster, which is ideal for urban busy traffic.

They turn direction and weave through the traffic or sometimes sidewalks easier.

But the trade off is, they hardly can take the road bumps or puddles. They are wobbly and unstable.

New designs are trying to remedy theses defects by using a larger wheel, suspension, setting the caster angle more leaned back to add stability.

In scooters design, fashion plays a big roll in among other things, sizing the wheels, mostly because of the young consumer in this industry.

In bicycles the engineering of efficiency, low drag, balance call for large, narrow size wheels up to a 29-inch diameter. The frame should be designed to be comfortable, give the rider flexibility of a more vertical gait for power pedaling, at the same time let them lean into the wind when cruising. They have wind tunnels and test drag in different configurations. Another advantage of a large wheel aside from control and a smooth ride is large angular momentum gives the wheel stability when rolling fast if they hit an obstacle.

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It's all to do with the torque output and rpm of the "motor". A fit cyclist puts out about the same torque as a schoolbus diesel engine. But they do so at much lower rpm and therefor much less power. So a bicycle needs to gear the 80 rpm source up by a factor of 5 or so to hit 40 mph. Five is about as high as you want to go with sprocket ratios. The rest has to be done with wheel diameter.

My old Katana spun 9,600 rpm, and needed to convert that to about 160mph. That would be 2400 rpm for 40 mph. That's 30 times the gearing, so instead of gearing up by a factor of 5 as on the bicycle, it would have to gear down by a factor of 6 if it had the same sized tires as a bicycle. It's better to make the wheel diameter smaller and go with less gearing.

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