as for one of your questions, if you wrap it around the wheel, you can wrap it around the other one too. Infact in some machines two rollers directly over eachother have a cable figure 8 around them, Its hard to explain without a sketch, but --8-- the rollers turn opposite ways like they were interlocking gears, the string comes into the middle, up around the top one, though the middle, down through the bottom one and through the middle again. They should be as close together as posible. The advantage of this system is that it eliminates a problem where the string tries to progress to one side due to its coiled shape on a turning drum, then hits the side, climbs over itself and gets caught up. With this method you can coil it one way on one wheel and the opposite way on the other, so it wont climb as much, also you only have to loop it once. We use steel drums and steel cables, but I dont see how it wouldent work with string and a plastic wheel with a rubber rim. A broken printer would be a good place to look for rubber rim wheels, gears, spacers and axels that you need.