Most mountain bikes were made with the same slotted dropout width. The front fork dropouts were routinely spaced 100mm apart with a rear frame dropout spaced at 135mm. This generally lead to enough room for the 7, 8, 9 and 10 speed gear clusters we commonly see. The width of this dropout slot was ~9.7-10mm, so that the standard 3/8″ bicycle axle would slide in with ease.
These types of bikes have become the standard most ebike hub motors are designed to fit. If you have a classic bike or mountain bike you can feel confident most conversion kits will fit your needs. In some minor cases you may need to adjust or file open the dropout slot in order for the axle to slide in, and in other cases you may need so spread the rear frame a little to span to 138mm. This is generally within the structural integrity for most dropouts and you’ll have very few fitment issues with hub motors on the market..