need to know the right size :p
Rims don't have axles. However complete wheels have hubs that do have axles. Axle size depends on if the wheel is on the front or the back and what type of hub they have
Most of the axles are 3/8 or 10mm. The threads vary with the imperial sized axles. If you are making an attempt to install longer axles on a bike to install training wheels or anything else, DON'T. The large majority of 16 wheeled bikes have frames that simply won't take the stress. which is why the axles are short. If the bike already has training wheels then you can be sure the bike was designed with the extra stress in mind.
