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I'm designing a 2-wheeled cart that I plan to rig to a donkey for hauling work around a farm. I'm wondering if there are mechanical advantages to using smaller wheels (like 40 cm diameter) vs. using larger wheels (like 50 cm diameter).

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Bigger wheels will be less of a torture for the poor animal. After a stop, it is easier to start again walking, because the same force will do a higher momentum on the axis, thus making it easier to evince friction. It will be as well easier to walk over stones. – Eduardo Guerras Valera May 30 '13 at 21:12
up vote 7 down vote accepted

Larger are better because they can roll over stuff better (like gravel or sticks), but have more inertia and so require more force to start or stop.

Also the larger wheel requires less friction to roll because the ratio of wheel diameter to axle diameter is larger.

There are some trade-offs as you try to make the wheels as thin as possible to reduce weight, they become unstable and more difficult to keep upright unless you have really tight clearances in the bearings. A wider wheel is somewhat self righting as the wider footprint provides for a restoring moment.

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I agree with @ja72 answer, except for what seems to me a minor slip (the kind I do all too often). I think he meant more energy to start or stop rather than more force, the same force for a longer time, or more force for the same acceleration.

Also the energy increase is only due to the fact that a large wheel is likely to have a greater mass. If the mass of the wheels is the same, distributed homothetically for the two sizes of wheels, then the innertia of the larger wheel does increase but this has no effect since the wheel angular speed will slow proportionally to its size for a given cart speed. The two changes cancel out for kinetic energy.

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