RPMs required for various speeds in friction drive

Hi folks – I have put together a table on what RPMs are required in a friction drive system to reach various speeds. Friction drive doesn’t care what size your tire is; the only thing that matters is what size the drive wheel is. As long as there is no slippage, the distance the surface of the drive wheel covers (usually on the top of the tire) is the same distance the bottom of the tire moves on the road. However the diameter of the drive wheel determines how many revolutions it must turn in order to cover a certain distance. A smaller diameter drive wheel will turn more times to cover the same distance, therefore it will need to turn at a higher RPM to maintain a certain speed. As with other mechanical situations, lower speeds means higher torque or power and higher speeds equals lower torque. Highest efficiency is achieved by balancing torque and speed and depends on the motor, etc. Hopefully the following chart will help people figure their needs. I am working on another series of tables to show motor rpms for various speeds for chain drive with different wheel sizes and sprocket sizes, but that’s a little more complicated. In chain drive, wheel size as well as sprocket ratios affect power and speed - jd

Hi Dan - I've actually finished a bunch of tables. It just took a little while with Excel. I am trying to figure a way to present them in some way that won't take up a lot of space and won't be too hard to find your way through. Maybe I'll put them on a web page and link to it here. In the meantime, if you want to figure a specific bike/sprocket combo you could go to Bicycle Analyzer and put in the numbers to get a top speed for your bike. Just set the wheel size then put the output rpms of your motor in pedaling cadence (usually max rpms divided by 5 for a typical China Girl). Enter the drive sprocket teeth in Front Gear 1 and the rear sprocket teeth in Rear Gear 1 and push Analyze My Bike. If you want to compare a few sizes just enter their values in different Front/Rear boxes. It's probably easier to use the Bicycle Analyzer than anything I could put together anyway - jd