Not all bikes can be safety converted to e-bikes, motors add extra stresses to the frame or forks as does the heavy battery pack. Probably the best type of bike to convert is a simple steel rigid 26" mountain bike. They are very strong and the cheapest kits seem to be based around 26" wheels. More difficult and expensive to convert a road bike to an e-bike as the hub motors kits for 700c wheels are more expensive and the brakes with motor cut-out switches often supplied are not for drop handlebars.
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