Currently running a 22T Gearbox Sprocket. Maybe want to change to either a 20T or 21T?

Hi:

1974 850 Norton Commando MKII. I recently removed the inner primary cover on my Commando and noted that my gearbox sprocket is a 22T. I think I would like to run a smaller one, probably either a 20T or a 21T. Right now, at 50 mph in top, I'm running a little bit below 3000 rpm and constantly running into fairly mild, but still intrusive vibes noted between 2500-3000 rpm. At 60 mph and above she is super smooth. I would really like it to be super smooth at 50 mph. So my question is do I go with a 20T or a 21T? I do not spend much time on the highway, but I still do not want to be lower geared than I have to. I want 50 mph and above in top gear to be super smooth, not the current 60 mph and above.

Thank you!

Maybe give your isos a clean & adjust first?
I have a 22 front wth no issues.
2850 at 50mph and 3500 at 62mph (100km/h).Cheers

I am running the stock 19T sprocket that my Norton came out with, and with the Featherbed frame and lightness of the whole bike and work done to my motor I love the quick get up and go, pick up is so good when opening the throttle, second and third gear is my favorite and 4th to me is just right, great up in the tight twisties, on the open highway it still cruises good at 60 MPH + is just under 4K rpm all depend on which way the wind is blowing lol and 70 is just a little over 4k rpm, it loves sitting on 65 to 90 and will sit there all day if you want but with the road speed limits you are open to tickets.
You got to set up your bike for what ever riding you do, if highway use a 21T be good but around town or up in the twisties not so good, as I say my Norton is set up for my riding style and where I ride it the most, suburban and up in the ranges in the tight twisties with a few long straight thrown in and the best thing is my Norton gets up to the ton very quickly, I have no reason to go any faster but it will do it easy if I need too, but I can just putter around in 4th at 40 MPH (its just above laboring the motor) and any slower and it be 3rd gear, but these day a lot of our back streets are 50 km limits so I try to avoid those streets and speeds, I am lucky I don't have to go far to get out and up into the twisties from my home.
For long distant travel I have my 1200 Thruxton for that with stock gearing and 6 speed box, its also good around town as well.
Every bike is different and setting it up to suite your own riding style and where you ride it the most, I am very happy with the stock gearing.

Ashley

Britstuff said:

Hi:

1974 850 Norton Commando MKII. I recently removed the inner primary cover on my Commando and noted that my gearbox sprocket is a 22T. I think I would like to run a smaller one, probably either a 20T or a 21T. Right now, at 50 mph in top, I'm running a little bit below 3000 rpm and constantly running into fairly mild, but still intrusive vibes noted between 2500-3000 rpm. At 60 mph and above she is super smooth. I would really like it to be super smooth at 50 mph. So my question is do I go with a 20T or a 21T? I do not spend much time on the highway, but I still do not want to be lower geared than I have to. I want 50 mph and above in top gear to be super smooth, not the current 60 mph and above.

Thank you!

Click to expand...

Assuming stock otherwise, at 3350 RPM:
19T 52
20T 54
21T 57
22T 60

So, based on what you said, it looks like you want 19T or to adjust your isos.

...or ride around in 3rd gear a lot more?

Thank you everyone for your very helpful comments. ISO's are getting pretty close to optimum - still a bit more experimentation warranted. Just can't help thinking that if I was back in the U.K. (before the age of speed cameras, with the 60/70 mph speed limits, etc) the bikes gearing would be perfect as is with the 22T. On main roads here in the U.S. it just feels too high to me. When cruising it seems like I spend way too much time switching between third and forth. Based on your replies I think I am going to give a 20T a go.

Britstuff said:

Hi:

1974 850 Norton Commando MKII. I recently removed the inner primary cover on my Commando and noted that my gearbox sprocket is a 22T. I think I would like to run a smaller one, probably either a 20T or a 21T. Right now, at 50 mph in top, I'm running a little bit below 3000 rpm and constantly running into fairly mild, but still intrusive vibes noted between 2500-3000 rpm. At 60 mph and above she is super smooth. I would really like it to be super smooth at 50 mph. So my question is do I go with a 20T or a 21T? I do not spend much time on the highway, but I still do not want to be lower geared than I have to. I want 50 mph and above in top gear to be super smooth, not the current 60 mph and above.

Thank you!

Click to expand...

It's personal preference.
The torque of the 850 works perfectly with the 22 on my '74.
At home ANYWHERE, from the switchbacks in the Smokey Mountains, to 400 year old New England cow paths, to 85mph interstate highways.
45,000 miles geared that way.

JMWO

Edit: standard triplex chain primary drive ratio

I only used the standard Norton gearbox once on a race track. The ratios are too wide. I bought a Manx gear cluster for $730 Australian. First gear would be too high for normal road use, but the Commando first gear will fit. You would need to rev the motor a bit higher in 1st to pick-up 2nd smoothly, but the bike would be much better everywhere else.
It is a very silly thing but until you have used close ratios, you do not understand the difference. One of my mates said 'if you have a torquey motor, you do not need a close ratio gearbox'. It is a situation where the theory is different to what actually happens. If you are flying down a road and want to accelerate past a truck, changing down half a gear gives much better acceleration, and with a close box, you have 3 gears close, so from whoa to go, the bike is faster.
I am as mean as cat-shit, - I do not spend $730 for no reason. Anyone who races a Commando with the standard gearbox is an idiot - it is all too difficult. You would not think more speed comes from the gearbox.
One of my mates races a 500cc Manx Norton - he has 3 first gears which he uses for different circuits. I am an idiot - I got frustrated and bought the $5000 6-speed TTI box. I was not thinking clearly. I have never used it in a race, but I think the bike would come-off the start line in a real rush. Nobody needs a million horsepower, a Commando engine is fast enough for anybody without much modification. My 850 motor is nothing special, but the bike is competitive against 1100cc CB750 Hondas.

You are still going to have the vibrations. I would find the cause of those before I change gearing.

In my 850 the biggest causes of vibration was the worn out Amal carbs that did not hold a proper mixture, the next biggest change was replace the rods and pistons with longer rods and lighter pistons.

I run a 23 tooth sprocket and which I had a 24.

@acotel This thread has nothing to do with racing or gearboxes

Plus Al (acotel) you have never ridden a Norton on the road so your input on a stock gearbox for road use is not reverent to the OP question, I find passing cars or trucks on open roads the stock gearbox works great and most of the time no need to drop back a gear to do so, using the torque of the motor works fine for me with the stock front sprocket.

Question....... been doing a bit of reading..... Am I right in thinking that, (from the factory) standard sprocket size for the U.S. market was 19T or 20T? For the U.K. market the standard sprocket sizes were 21T or 22T?

If the above is true, why? Was is it just for the perceived need for increased acceleration for the U.S. market, (my apologies if this suggestion sounds stereotypical). Or was it to avoid vibes at U.S. and U.S. road speeds?

Thank you!

Britstuff said:

Question....... been doing a bit of reading..... Am I right in thinking that, (from the factory) standard sprocket size for the U.S. market was 19T or 20T? For the U.K. market the standard sprocket sizes were 21T or 22T?

If the above is true, why? Was is it just for the perceived need for increased acceleration for the U.S. market, (my apologies if this suggestion sounds stereotypical). Or was it to avoid vibes at U.S. and U.S. road speeds?

Thank you!

Click to expand...

America was OBSESSED with drag racing in that time period and mimicked it everywhere.
My 20" pedal bike had a square profile "Cheater slick" tire in 1967.

Britstuff said:

Question....... been doing a bit of reading..... Am I right in thinking that, (from the factory) standard sprocket size for the U.S. market was 19T or 20T? For the U.K. market the standard sprocket sizes were 21T or 22T?

If the above is true, why? Was is it just for the perceived need for increased acceleration for the U.S. market, (my apologies if this suggestion sounds stereotypical). Or was it to avoid vibes at U.S. and U.S. road speeds?

Thank you!

Click to expand...

Don't take this to the bank! AFAIK, 850s came with 21T and Combats with 19T. I've never heard what non-Combat 750s came with. I also don't know if it mattered where sold. I know my 74 MK2A came with 21T but it was sold in Scotland and imported to the US in 1974.

IMHO: At 55 mph it makes little difference. A 19T is a little more exciting when getting going. A 21T at 70mph would be quite comfortable (4100 RPM). At 55, everything sucks

My 74 850 came stock with a 19T that I have owned since new and have kept it that way, so whether it was different for the US market, but 2 other Norton gear boxes I have rebuilt for mates also had 19T sprockets fitted, one was a 750 and the other was a 850.

marshg246 said:

Don't take this to the bank! AFAIK, 850s came with 21T and Combats with 19T. I've never heard what non-Combat 750s came with. I also don't know if it mattered where sold. I know my 74 MK2A came with 21T but it was sold in Scotland and imported to the US in 1974.

IMHO: At 55 mph it makes little difference. A 19T is a little more exciting when getting going. A 21T at 70mph would be quite comfortable (4100 RPM). At 55, everything sucks

Click to expand...

I believe this correct. My original Mk1 850 came with a 21.
I think the Mk 3 came with a 20.

Not sure about the Combat Interstates. They may have come with 21 vs. 19.

Currently running a 20 on my Mk 850.
Didn't care for the 22t, maybe when I grow up I will...
Your mileage may vary...

That fact that my 850 Interstate will go halfway into redline (7500) in top tells me my bike's gearing with a 22 is not too tall.
Cheers

robs ss said:

That fact that my 850 Interstate will go halfway into redline (7500) in top tells me my bike's gearing with a 22 is not too tall.
Cheers

Click to expand...

I probably misunderstood what you meant, but an otherwise stock bike with a 22T front sprocket at 7500 RPM is going 134 mph. Even if you're tiny and laying on the tank, that seems a bit fast for a Commando. If on the other hand you mean 3750 rpm (1/2 of 7500) , then that's only 67 mph and clearly doable.

marshg246 said:

I probably misunderstood what you meant, but an otherwise stock bike with a 22T front sprocket at 7500 RPM is going 134 mph. Even if you're tiny and laying on the tank, that seems a bit fast for a Commando. If on the other hand you mean 3750 rpm (1/2 of 7500) , then that's only 67 mph and clearly doable.

Click to expand...

Greg - At 7500rpm my speedo is showing a bit above 130mph - hard to precisely tell at that speed.
Great engine - Fullauto head, dynamically balanced crank, light pistons with gapless rings, FCR35s - cNw build #134.
Cheers

Oh ...and I am 80kg and (of course!) crouched down.

OP, remember that if a bike is equipped with a belt drive primary (less reduction ratio) , the OVERAL RATIO is taller.
My post above is with a stock triplex chain.