My friend bought one of these. CLICKY LINK
He is quite tickled with it. He has had a problem with breaking spokes. Also the sprocket slides around some. The sprocket clamps to the spokes. He found some heavy duty spokes and is going to respoke the rear wheel (a glutton for punishment). The bike will go 30-35 MPH so you better have good front brakes and paid up insurance. There is another kit that sits behind your seat and has a roller that drives the tire. I would guess these don't work so well wet. I will probably get the one in the link. I have a great old 27" alloy beach cruiser with alloy wheels. Its got a place for front brakes.
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A little more info might be helpfull.
