- Joined
- Nov 18, 2005
- Messages
- 922
Jeandr said:daveh said:Hobot — you can record your Commandos' top speeds with a Sat Nav or a bicycle computer. Failing those, rev counters are often more accurate than speedos and you can calibrate them from a borrowed electronic tacho (or maybe even from a dyno?). You can then calculate your speed from the workshop manual or (if non-standard) by using a formula, such as:
http://www.rbracing-rsr.com/gearspeed.html
My formula is:
Mph = rpm x .00297 x tire diameter in inches / overall gear ratio
For example, a K81 4.25/85 H18 has a 25.5 inch diameter, a normal Commando has a 4.730769 overall gear ratio in top gear with a 19 tooth countershaft sprocket.
Mph = 7000 x .00297 x 25.5 / 4.730769 = 112 which is about what I saw indicated on the speedo, the real speed may have been a bit lower.
Now, to get a Norton to 130, 150 or 160, the engine must turn a whole lot faster or the gearing must be changed. What is the size of the countershat sprocket needed to get there assuming we keep the same 7000 rpm maximum engine speed?
for 130 the countershaft sprocket will need to be a 23 tooth, for 150, a 26 tooth and for 160, a giant 28. While a 23 tooth sprocket is available, it would limit acceleration in the lower gears to get the high speed as for the others, I don't think they are available and the accelaration would suffer even more if thew were. While it could be said the engine could be reved faster, I don't think it would survive very long at higher than 7000 rpm, especially while trying to produce a lot more horsepower to acheive those speeds.
Jean
I have an excel spreadsheet that you can enter all this into and it spits out the speed.
Post your sprocket sizes and rpm and I'll tell you your speed.