I've borrowed this from Helmut Clasen's Facebook page. Just a simple gear ratio calculator when changing sprockets on the motorcycle. Serves as a reminder what changes causes what result.
One practical example of it's use as described by Helmut:
"On my Zuendapp the standard GS ratio is 14-46 = 3.29 Ratio. But the chain was cutting through my front swingarm. In order to get more clearance I chose a 16 tooth front sprocket. To go back to the proper ratio of 3.29 ( or close to it) I had to increase the back sprocket to 53 = 3.31 which is almost identical."
Helmut is the most knowlegeable Hercules rider/owner/mechanic.