Final Drive Ratio - 650 Vs 750

[NM Bonny]

Paul Henshaw put up a video of a test ride on an OIF BSA (apparently with the same frame, forks, and hubs as my '71 OIF Bonny) that had a 750cc conversion on it. He noted it had a 21-tooth front sprocket added, and it's rpms at speed (disregarding the tach which he said reads almost 1K too high) seemed closer to what I'd like for highway cruising. The NM highways I'd like to explore typically have 70 mph limits (or lower).

I realize the tranny sprocket clearance may not be the same on the BSA as with my Unit 650 Triump, but would like to know if whenever I do the 750cc conversion to my bike whether I can fit a 21-tooth front sprocket as well? I'm running what I believe is the standard 46-tooth rear ring.

Thanks as usual.

 

[speedrattle]

sure you can. i run 21s on my stuff sometimes. you ll have to fit the 21 onto the mainshaft, press it up against the inner chaincase wall and mark the positions of the teeth, missing the screw holes. then take it off, cover everything and file/dremel 21 notches so the sprocket will slide in.

 

[NM Bonny]

sure you can. i run 21s on my stuff sometimes. you ll have to fit the 21 onto the mainshaft, press it up against the inner chaincase wall and mark the positions of the teeth, missing the screw holes. then take it off, cover everything and file/dremel 21 notches so the sprocket will slide in.

Thanks for this - and my apologies for posting the question on January 8th--which, as every true American knows, is Elvis's birthday. Shame on me...

Seriously, though, this is good to know. Per the other thread, I'll be buying a set of H-beam con rods to throw into the '750 conversion' box. So, I'll keep this info in the related file.

Oh - well, of course this may be rendered moot by KM Jones's investigation into the possibility of having Rebel Gears make up a run of 46-tooth rear conical hub chain rings. We'll soon see...

 

[speedrattle]

--21/46, you will be turning 3980 at 70mph

--21/47, you will be turning 4065 at 70mph

--20/46, you will be turning 4175 at 70 mph

--a stock 19/47, you will be turning 4490 at 70 mph

20/46 and 21/47 are just about indistinguishable. the difference is real, but i dunno how noticeable it would be on reasonable highway riding. all this assumes an 81.4-inch rear tire, like an avon roadrider.

i loved the 21/47 on my 72 T120. its a morgo 750, with some head work and careful attention on tuning, but what the 21/47 does is make your first gear a real gear that you can use. with the stock 19/47, first is done in after 10 mph. the 21/47 requires you to run the rpms up in nnormal riding, because if you dont you will lug it horribly. but once you adapt to that, you will use the higher rpm to accelerate and then you can loaf a bit at higher speeds.

i found that i usually rode around significantly faster with 21/47, rather than the often expected same speeds/slower rpm.

i havent tried 20/47 because i cant find the box with my five-speed sprockets in it. as soon as i do ill take out the awful 19/47.

gearing can change the personality of your motorcycle radically. if you spend a little time testing and keeping notes, you can make your machine into something that works precisely as it should, for you.

 

[NM Bonny]

--21/46, you will be turning 3980 at 70mph

--21/47, you will be turning 4065 at 70mph

--20/46, you will be turning 4175 at 70 mph

--a stock 19/47, you will be turning 4490 at 70 mph

20/46 and 21/47 are just about indistinguishable. the difference is real, but i dunno how noticeable it would be on reasonable highway riding. all this assumes an 81.4-inch rear tire, like an avon roadrider.

i loved the 21/47 on my 72 T120. its a morgo 750, with some head work and careful attention on tuning, but what the 21/47 does is make your first gear a real gear that you can use. with the stock 19/47, first is done in after 10 mph. the 21/47 requires you to run the rpms up in nnormal riding, because if you dont you will lug it horribly. but once you adapt to that, you will use the higher rpm to accelerate and then you can loaf a bit at higher speeds.

i found that i usually rode around significantly faster with 21/47, rather than the often expected slower speeds/slower rpm.

i havent tried 20/47 because i cant find the box with my five-speed sprockets in it. as soon as i do ill take out the awful 19/47.

gearing can change the personality of your motorcycle radically. if you spend a little time testing and keeping notes, you can make your machine into something that works precisely as it should, for you.

Cool, thanks for all this information. Unless Kyle Jones is successful with the 46-tooth conical hub chain ring, I'll be running 20/47 initially, as those are the parts I've got in-hand. I did some work on the bike today, to be posted on the other thread shortly...

 

[speedrattle]

with 20/47, you will turn 4270 at 70 mph