I'm working on a project for model airplane propulsion. I'm trying to design a friction drive gear train which will provide power to coaxial propellers. The chosen solution is to use a drive pinion parallel to the propeller drive shafts. This means one driven gear(wheel) will be driven on its OD and the other on the ID. My problem is since the driven gears(wheels) are coaxial and I need different ratios I need to figure the diameter of one driven wheel given the diameter of the other wheel and pinion step. Pinion may need to be stepped so that it has diffenent diameters to provide the desired ratios. So the formula will need to also calculate the pinion size for the unspecified driven wheel. My math skills are not up to translating the variables into a formula to solve for them. While I would prefer "hints" as to how to develop such a formula, perhaps its too simple for a "real" engineer to help with beyond just giving me a formula. I suspect, however, that it involves solving simultaneous equations as the results will be a single solution for any given ratio and size of the gear that is initially specified. The picture shows the two driven wheels and the black pinion, you can see I have not properly sized the pinion so it does not "fit". Of course the smallest part of the pinion represents the motor shaft the pinion would be on.