fixed hub you mean? There also called fixies…there typically made from old frames and then fitted with new very classy parts…basically for most its a fashion statement. Usually there used by couriers, in London they all use them…problem is though because its a fixed hub you can’t freewheel so you have to keep pedaling, so careful in the corners! If you have seen velodrome racing those are fixed hub bikes, there more efficient at transferring power and very light. But of course you have one gear more than likely for meating out a nice 15–20mph so hit a hill and unless your a good powerful cyclist your in for a bit of trouble…even if you are you might have to get off lol…for city use I have a Carrera Subway which has a sealed hub gear system, so one cog at either end, nice cruiser and no maintenance but still skinny frame and race tyres :)