Hamster wheels

A hamster needs their back to be straight whilst running on their wheel, to avoid painful back injuries. The minimum sized wheel for robos and dwarfs is 21cm (8 inches) and for Syrians and Chinese hamsters, the minimum is 28cm (11 inches). The bigger the wheel, the better (as long as they can push it) as they have more control.

These wheels are extremely dangerous and painful for a hamster

The wheel also needs to have a solid running surface to avoid your hamster getting bumblefoot. This is when hamsters get extremely painful sores on their feet due to the running surface. Mesh and wire wheels should be avoided.

Wheels with protruding rungs on the running surface should also be avoided.

This is an example of a roborovski hamster on a 26cm (10.2 inch) wheel. As you can see, her back is straight.

Flying saucer wheels are not recommended because hamsters, especially roborovskis can lose control and fall off their wheel, injuring themselves. They also take up a lot of room in an enclosure, leaving less space for enrichment. 

Wheels should always be placed on a platform, so they don't sink into the bedding or (if it's a heavy wheel) crush your hamster if they burrow underneath.