The Secondary Drive consists of the Gearbox Chain Sprocket, the Drive Chain and the Rear Wheel Sprocket. The chain that I will use is made by the Japanese firm EK and their part number is 520RX. It has the dimensions 5/8” x ¼” ( pitch x inside width ). The length will have to be decided once everything else is in place. The Gearbox Sprockets are by design limited in the number of teeth one can have and I had ordered 4 sprocket sizes with the gearbox. 18 , 19, 20 & 21 teeth is now what I have got to play with at this end.
So now we are the point of deciding on the size ( number of teeth ) on the Rear Wheel Sprocket. We have a couple of factors to keep in mind. The overall target is to, on a given race track, choose a total gear ratio such that the motorcycle reaches it absolute top speed at the end of the longest straight, just before one has to hit the brakes to reduce the speed enough to be able to tackle the following curve without causing an accident. At the same time you have to make sure the bike can accelerate hard enough over the stretch of the shorter straights, but that is more a matter for selecting the right gearbox gear. Now, we don’t really care what the actual top speed is, measured in for example km/hour. we just have to make sure that we are selecting a target that will at least make us as fast as the competition. Ideally of course faster but since this is not Moto GP or Formula 1 we are not looking at stretching the boundaries. We just want to be able to run with the rest of the crowd. Still, in order to select a suitable rear wheel sprocket we need to have an idea of where we should be in terms of maximum speed. I was searching without results until I found this Austrian guy Herbert Tuscany and his website. Herbert is racing a 750 Norton with some Steve Maney power enhancing hardware and he is winning races in whatever series he is participating. Herbert’s and my bike are there for a similar purpose and therefore I decided to use his top speed claim as a target for me. Herbert says his 750 is good for 221km/h so now I have my first target number. Because my engine by design should give some 10-20% more power than Herbert’s Norton, my bike should be able to do say 5% more top speed than his. That brings us to a second target of 230 km/h or thereabouts. The rest is just mathematics:
I am assuming that my engine will produce peak power at 8000 rpm the calculations are as below. Any halfway educated kid today would have put this into a Microsoft Excel formula but since I don¨t know how to do that I followed the following.
8000 engine revolutions per minute, with a 40 tooth engine pulley and a 68 tooth clutch pulley, and a gearbox internal ratio of 1:1 in sixth gear, result in a gearbox output rpm of 4700. The rear wheel tyre will, according to AVON TYRES, need to complete 507 revolutions to cover 1 kilometre. If the gearbox and rear wheel sprockets have the same size. this will result in a theoretical top speed of 4700/507×60 = 556 km/h. Now we can start to juggle with different sprocket sizes to see how we can come to where we want to be in top speed.
The following final drive sprocket sizes will result in the corresponding theoretical top speeds:
Gearbox Rear Wheel Top Speed
21 43 271
20 43 258
21 46 254
19 43 245
21 48 243
20 46 242
21 50 233
20 48 232
18 43 232
19 46 229
20 50 222
19 48 220
18 46 217
19 50 211
18 48 208
18 50 200
As can be seen, with my 18, 19, 20 & 21 tooth gearbox sprockets, I can adjust the top speed from 208 to 220 to 232 to 243 km/h with a 48 tooth rear sprocket and from 200 to 211 to 222 to 233 with a 50 tooth sprocket. So, I’m getting a 48 and a 50n teeth sprocket to play with. The 50 teeth will still keep my top speed over Herbert’s and will give a superior acceleration compared with the 48 teeth.
Now that the sizes have been settled on I need somebody to make/supply the sprockets to fit the rer wheel. Not an easy task but at last I found a UK company called TALON ENGINEERING who says they can make an aluminium sprocket to my dimensions and hard anodized in aluminium colour to match the rest of the rear end components. It will look something like this, except for the colour of course.
