I have a bike that I'd like to throw an electric motor on. It has an 8 cog casset on the back and disc brakes. I'm looking for a rear hub motor that can accommodate this (willing to go down in number of cogs in back) but I basically want the same set up. Range and mph aren't important as I just need a little help with my 5 mile commute. I'm pretty out of shape and the city I live in is incredibly windy. Riding head on into the wind sucks and I could probably walk faster than riding. What are my options? Any low cost conversions? For what I need am I better off with a front wheel hub motor? The only thing that worries me with front wheel is the chance of breaking dropouts, but if I'm only going with a low power motor am I ok? Thanks. Maybe I'll just buy a second bike and make It gas...
See my post #9 under Considering electric bike.. Help! Adding to what I wrote using a SLA battery reduces the cost of a conversion by about half but we wanted the lighter weight of both the geared hub and lithium battery since I have to lift the bike in and out of the pickup when we go camping. There are many conversions available so choose wisely because I think once you get an electric you'll find you will ride it a lot more. I know my wife sure does and she absolutely loves it.
A rear hub motor conversion can get pretty involved. Most require 135mm (or more) dropout spacing. If you're converting a modern MTB or hybrid, you're probably ok, but road bikes or older MTBs can be a problem. Most rear hubs are set up for a freewheel, which will get you 7-speeds at most. I've seen a couple with freehubs that will take an 8-speed cassette, but the choices are much more limited.
A front conversion is usually cheaper and easier. You have a huge range of options and a much simpler installation. Your dropouts will be fine as long as you put a torque arm on both sides (which I recommend for safety even on low power motors).
Note that much of the cost of an ebike conversion is the battery, so "low cost" conversions are going to be in the range of $600 - $1000 for a 250W motor laced into a rim, 24V LiFEPO4 battery, battery charger, motor control electronics, throttle, e-brakes, and torque arms. If you want more power or the ability to cruise at 20+ MPH without pedaling, you're looking at $1000+. If you don't mind hauling around the equivalent of a boat anchor and replacing batteries every year, you could go with sealed lead-acid batteries and save at least $100.