20 vs 21 sprocket tooth

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pierodn

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Hi All.
Please, what is the difference of RPM from 20 to 21 sprocket tooth?.
Thank you.
Piero
 
Hi Piero,
For every 1,000 rpm
20t sprocket = 16.16 mph / 26 kmh
21t sprocket = 17.0 mph / 27.3 kmh
This assumes all other parts are standard.

Reggie
 
Which do you prefer between 20T and 21T on a 750 engine?

I am finding that a stock 19T is not really adequate for today's higher road speeds.
21T would be best, but don't think a stock 750 will like it.
 
I have a 20T on mine. Seems to work well for my uses. To me, it is about trying to keep the revs over 3000 RPM as much as possible (keeping vibration to minimum). So, 50 mph in 4th as a minimum, 40-ish mph in 3rd as a minimum - works good for the mix of rural / small town riding I do. And then being able to wick it up to 70-80 mph for highway/motorway stints when necessary. Good compromise when all you have is 4 gears. My $0.02.
 
Same as Derek. I spend most of my riding in the 35 - 50 mph range and the 20 puts me at 2500 rpm at 40 mph with the ISO's working perfectly. Can sustain 70 - 75 if I want, without straining. Dunno if my 850 is better suited there than a 750 would be.
 
Make that 3 of us. I ran the 21 for a long time then tried a 20.
The 20 tooth is better for the riding I do for the same reasons Derek and Maylar listed.
The Commando must be the only old bike that you rev up to get rid of vibration. All of the others go the other way, that's why my Super Rocket is overgeared and wont pull the skin off a rice pudding.
In fairness, if you rev the Super Rocket to about 6k then dump the clutch, it will pull a rice pudding skin about halfway off before it stalls:)

Glen
 
I've always liked the 20 on the 850.
You guys just convinced me to go back, currently have a 21.
The biggest issue is having to pull a perfectly good primary down to do it.
Think I'll do the gear box at the same time.
 
I run a 21 on my 750. The last other norton owner who rode my bike asked if it was a combat engine when he got off of it, so a 21 tooth sprocket doesn't gut a strong 750's power... at least, not mine. I found my original 19 tooth so low geared that I jumped right over the choice of 20 tooth because I wanted more than a 5% change which seemed small given that the highway speeds limits are 70mph in my area and everyone does 80mph...
 
I've a 20T on my 850 and think it to be just about perfect for anything I'd ever in my right mind wish to do. Go take a look at your Commando and ponder for a moment..... If NV had had no pressure, a large bankroll, & new tooling when your bike was made..... Would you hold it in any higher esteem than you do now?
 
I used to have a 20 on my 850, and rode with that for 15 years in Seattle. Loved it.

After rebuilding my bike after a long dry spell, i put on a 21 here in NZ where it is mostly twisty country roads I ride. Love it.

So, i think it depends how/where you ride.
 
I rarely break 50 on my Combat and find the 19 perfect. It really wants to run 4K and up. I would love to try an 850 with a 21 sometime to feel the difference.
 
I was reading yesterday that Commandos which were sent to the US were lower geared to get better acceleration. But I have found that my Commando goes in the opposite direction. You never know what gearing your motor will pull comfortably, until you raise it. I started with low gearing and every time I raised it, the bike accelerated faster. When I fitted the 6 speed, I raised the counter-shaft sprocket another tooth. Close ratios make a difference. With wide ratios, you lose too many revs on the up-changes and with poor throttle response, you will wait forever for them to come back.
 
I run 21T on my 750 Roadster which started life as a Combat but had a standard 750 engine replacement at sometime in its life.
Works okay for me riding mostly on UK A and B roads.
I found the standard 19T resulted in revs being too high - I like to keep in the 3000 - 4000 range largely out of respect to so many 48-year old parts!
I didn't notice much loss of acceleration going from 19T to 21T and the larger sprocket means if I do have to travel on dual carriageways or motorways I can easily keep up with the 'moderns' in our bike club without over-stressing the engine based on my rev criteria.
The only issue I found was having to get a rear chain with a half-link but that's straightforward enough.
Andy
 
With a belt and a TTI 5 speed I thought I was finally on a bike that could do the dual as well as the lanes. Im indicating 3750
at 70. With the 5 speed you can always drop it down one without a big jump. On the bottom side I lowered first gear a bit and
it really helps for green lane riding. But out in the west country I probably should be seeing 4k for 70mph. This could be solved
by going to 920. :-)
It never ends! BTW 850cc.
 
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