Originally Posted by oldbmw (Post 169316)

Hey guys, be kind. some of us still have to use dialup.

Margus

If you look at any old Brit bike, or the new Enfield gearbox you will note that the drive train is arranged like this := The engine output sprocket drives the primary chain to a the clutch hub.
There is usually about a 2:1 reduction here. The clutch hub is free to rotate when the clutch is used, but locked to a shaft when the clutch is closed. This shaft goes from the primary chaincase into the gearbox and is the gearbox input shaft. In the space between the back of the primary chaincase and the input side of teh gearbox sits the gear sprocket which drives the rear chain. The gearbox sproket is mounted on splines on a bearing mounted collar that is free to rotate over the input shaft. The collar extends a little way into the gearbox. There is a gear here that takes power from a layshaft driven gear in all the lower gears. In top gear however, instead of driving via a gear onto the layshaft and back out, the output collar is locked to the input shaft via dogs on a gear which is in the input shaft and splined to it. This locks the output gearbox sprocket to the input shaft. It is possible that some gears may be meshed and rotating, but they are idling and not carrying any power. This is one reason these geaboxes whine (straight cut gears) in all gears except top. It may also account for the higher speeds attained with less power than is needed with some newer designs with offset output shafts. After all, a Tiger cub produced only 10hp so energy conservation was important. Or put it another way, a 650 cc triumph Tr6R with 40hp is about 5 mph faster than a BMW 800cc bike producing 50hp. I am not trying to criticise anyones design, They all were (and still are) compromises based on what the conditions were expected at the time of design and marketing fashion. Nowadays bikes have so much power losing the odd 20hp is of no consequence (except to range/mpg) In the case of the BMW, having a centrally mounted input shaft forced them into having an off set output shaft, in order to clear the rear wheel. When they made the K series.. I expect that the reason for lying the engine on its side was to put the centreline of the engine crankshaft inline with the output shaft, thereby making it easier to construct a longer lived gearbox. But I do not know, never having owned a K series bike, but that is what I would have tried to do.