I noticed this nomadic knife and scissors sharpening guy set up his contraption in a market near Oaxaca Mexico. He could roll it along on its single wheel. When he got a customer, he tipped it slightly so the seat was up. He put the leather strap in the bike rim and then he could sit and pedal the wheel. It would turn the grinding wheel, and he would merrily sharpen away, sparks flying. I've seen a lot of makeshift sharpeners, but this one was the most elegantly designed ever.