Boxerworks.com

melville wrote: ↑Thu Oct 04, 2018 5:23 pm I think there's a reduction gear on the input that is not part of the gear ratio calculation for the oilhead trans but that is part of the calculation for the airhead trans.

From this:

http://largiader.com/tech/oiltrans/sixspeed.html

See the input shaft at the bottom:



That looks like at least a 2:1 input ratio. So the Airhead ratios measure the difference between crank rpm and output rpm, while the published Oilhead ratios measure the difference between the two shafts with all the gears on them.

Thanks for that info. I was aware that both transmission used 3 shafts, and there is some reduction with the extra shaft.
But was not aware that BMW didn't calculate gear ratios on the Oilhead trans without including that reduction.
Doesn't make sense to me. Since all those gears and shafts are in the trans, they should be providing the ratios as input shaft vs output shaft. On bikes that have primary drives, I understand that being a separate value.

Thanks for the extra info I didn't have.

For an airhead gearbox the reduction ratios are:

5th gear 1.5
4th gear 1.67
3rd gear 2.07
2nd gear 2.86
1st gear 4.4

Airhead gearboxes. I've got no idea how they work, I just know how to rebuild them.

Has anybody produced a better than OEM return spring?

ZM asked:

Has anybody produced a better than OEM return spring?

That stupid little spring has such a simple, yet vital, function. All it has to do is is assure that a tiny arm falls to one side, rather than resting where gravity dictates it should. It does not have to exert much pressure, and if it were physically possible, it could be replaced with a common hooked end spiral spring like commonly found on automotive drum brake adjusters and would probably last forever. However, our gearboxes were not designed around the selector mechanism. I have a feeling that the selector mechanism was designed after the main layout of the gearbox.
I haven't spent a lot of time dwelling on it, but I'm sure the designer/engineer that made the first one did.
It would be easy to hook a small spring to the selector arm to pull it in the right direction, but there is no obviously easy place to hook the other end of the spring. Space is tight. The ingenious thing about our pawl spring positioning is that the whole arm that the selector attaches to also pivots, so tension is the same on the pawl spring regardless of what gear you are in. The 2 other springs in the selector mechanism are huge in comparison, and I've never seen those break.
If there were a bit more room, a slightly stronger (thicker, more durable) spring could be used, but there is no room to spare.
The broken springs I've examined always seem to have broken from wear (by design the two sides of the spring rub against each other). They did not appear broken from being over-tensioned in either direction. There was no sign of distortion.
Maybe all we need is someone with a degree in metallurgy to come up with a spring made of some other material, same dimensions and tension as OE, but more durable.

The funny thing is that if we were riding our bikes upside down, that pawl spring would no longer be necessary.