phil yates said:
I was kind of hoping that with the entire world's Norton experts at the tip of my fingers, one, just one of them would come up with a more scientific approach than that!! In 1973, I knew the figures i.e. mph per 1000rpm for 19T and 21T sprockets and I seem to recall there was no 20T option in that year. C/S sprocket was not even listed in the parts book. Not that it would help if it was.
In the factory service manual there is a chart for the speed of the motorcycle at 3 different RPM's using various countershaft sprockets. With that chart I was able to come up with a constant number for each sprocket size.Unfortunately the chart that Norton supplies is not 100% accurate, they rounded either up or down on their miles per hour. But here is what I came up with:
For a 19 tooth sprocket multiply the rpm by .0153 and that will give you the miles per hour. (Example 4000 rpm x .0153 = 61.2 mph)
For a 20 tooth sprocket multiply the rpm by .0161 to get the miles per hour
For a 21 tooth sprocket multiply the rpm by .0170 to get the miles per hour
For a 22 tooth sprocket multiply the rpm by .0177 to get the miles per hour
For a 23 tooth sprocket multiply the rpm by .0186 to get the miles per hour
These would be approximate miles per hour because I am working off Norton's chart. This is also assuming you have the original primary gearing and are in high gear.
A 4.10 x 18 rear tire seems a bit small in diameter for a Norton Commando and your speedometer reading would no doubt be incorrect. When it comes to tire replacement time, I would recommend finding an 18 inch tire that has an outside diameter closer to the original 19 inch tire.
