20 vs 21 sprocket tooth

Road gearboxes are intended to be used for sane riding. My 2.4 litre car has six speeds close ratio - I usually only use every second gear in the box. If I used every gear in the box while driving around town, I'd be booked. So be careful what you wish for.

acotrel said:

Road gearboxes are intended to be used for sane riding. My 2.4 litre car has six speeds close ratio - I usually only use every second gear in the box. If I used every gear in the box while driving around town, I'd be booked. So be careful what you wish for.

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LOL - Try driving one of these bob-tail!! Every 4th gear maybe??

That image alone gave me an immediate headache.... It was like turning on the news.

Glad I'm retired , did not care for that particular trans around town ....

My 850 was born with the 20 tooth sprocket. I found it ok but when I did major surgery a long time ago I replaced the worn 20 tooth with a 21 tooth. That works for me. High enough for open roads in NZ and low enough to take off in all but the steepest hill. The winding roads around where I live do require first gear on occasions when I am having fun. The bigger the g/box sprocket the bigger the load on the transmission [ which was originally designed for 500 cc ] . You are better to fit a bigger sprocket on the engine if it was possible, and that will increase the speed to the gearbox without increasing the load.
There was wisdom [ if you could call it that ] in fitting 19 and 20 tooth sprockets on the 750's and 850's in the first place.

Dereck

I kept with the 19T sprocket on my hotrod Norton it cruises very well at 65 to 90 range and the good thing with the hot motor the stock sprocket works great for quick take offs without putting any strain on my GB, I don't do much highway riding on my Norton and the stock sprocket works so well in the tight twisties, my Norton still goes well over the ton when I need it but with the hot motor my engine will rev well over 8k RPMs if you let it, it revs very freely so go the go easy with my throttle hand lol.

Ashley

When I first started racing my 850, I tried the standard box and immediately thereafter bought the close ratio cluster. First gear in the close box is where second gear is in the standard box. But I found that once my bike was rolling with the close box, I never needed to go back to first. So my motor is pulling at least what would almost be third gear in a standard box, when the bike is going around the slowest part of a race circuit. The main problem with the close box is first gear is too high for a fast start, mainly in part to my own fear. The last time I raced, I revved the tits off the motor and the box did not shit itself, so I got a good start- but I was worried. When I am on the start grid in any race, I always have a look to see what is on the row immediately behind me. If I need to, I go back a row.
There was a period where I had not raced for 20 years. But in about 2001 I started racing the Seeley 850. In my first race there was a guy riding a big bore CB750 Honda. When the flag dropped, his bike spread it's motor. But I was ready for him. I did not back-off, I just jumped sideways and went around him. Most guys in historic racing are not as experienced as I am, and I don't want to get hit from behind. The TTI 6 speed box has thicker shafts but skinnier gears.
In the old days road races were always push-start and some guys still got hit from behind.

If it was not for that very high first gear, an AMC close ratio 4 speed cluster would be perfect even in a road bike - the Commando first gear fits.
If I had a road-going Commando, I would do that with the gearbox, and I would re-balance the crank to 72%, - regardless of the isolastics - (fit the steel plug in the hole).

(Actually, on thinking about it, if I ever get rich again I might buy a Commando and do that. It might be really good fun).

You don’t need to be rich, just persistent ...

I really now have a sudden desire to buy a road going Commando. It could be quite exciting. I have the gearbox, all I would have to do is mod the crank. That isolastic vibration cure will work regardless of whether the vibes are high or low in the rev range.

I really now have a sudden desire to buy a road going Commando. It could be quite exciting. I have the gearbox, all I would have to do is mod the crank. That isolastic vibration cure will work regardless of whether the vibes are high or low in the rev range. I was once on a friend's Commando which was pretty good, but it could be turned into a real boomer.

pierodn said:

Hi All.
Please, what is the difference of RPM from 20 to 21 sprocket tooth?.
Thank you.
Piero

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Easy:
20 rpm vs 21 rpm
2000 - 2100
4000 - 4200
etc..

acotrel said:

I really now have a sudden desire to buy a road going Commando. It could be quite exciting. I have the gearbox, all I would have to do is mod the crank. That isolastic vibration cure will work regardless of whether the vibes are high or low in the rev range. I was once on a friend's Commando which was pretty good, but it could be turned into a real boomer.

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Before you start modifying it to death, just buy one and just ride it for a while. With a bit of fettling, a good working, relatively stock Commando is a joy to ride. IMHO, a road bike must have good smooth manners from 20-80 mph - which a relatively stock Commando has in spades.

There are a very few things that the bikes really need to be a good working road bike, other than making sure everything is in good working order: Good brakes, decent lights, a strong head steady, and a loud horn are amongst my top recommended mod's - mainly for safety on modern roads. Everything else is gravy.

My $0.02 - FWIW

Which gearbox output sprocket do people who have a belt drive fitted prefer ?
Using standard primary of triplex chain with 26T front sprocket and 57T clutch gives 2.19 ratio into gearbox
Belt drive with say 27T front and 54T clutch gives 2.00 ratio into the gearbox speeding it up
Or am I wrong ?
So which gearbox sproket is preferred for normal road riding as described in the posts above?

Depends on the belt drive. My cNw belt is the same as the chain, so no need to change sprocket.

with a 20-1 belt drive there is no direct ratio per tooth. so with a 2.19 and a 21 tooth countershaft sprocket the 2-1 with a 19 is about half tooth lower and a 20 about half tooth higher. 1 tooth on the countershaft is approx 250 RPM. when I have to replace my std ratio norvil I will replace it with an over drive ratio as high as possible and adjust the countershaft to get close to a 21 as that fits my riding style. I still cant understand with matts knowledge why he did not over drive his drive.

bill said:

with a 20-1 belt drive there is no direct ratio per tooth. so with a 2.19 and a 21 tooth countershaft sprocket the 2-1 with a 19 is about half tooth lower and a 20 about half tooth higher. 1 tooth on the countershaft is approx 250 RPM. when I have to replace my std ratio norvil I will replace it with an over drive ratio as high as possible and adjust the countershaft to get close to a 21 as that fits my riding style. I still cant understand with matts knowledge why he did not over drive his drive.

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Because as Derek Wilson has stated above, turning a 4 speed gearbox into a 3 speed with overdrive would feck it up. On winding roads the top gear would become useless.

There is no one size fits all really. So many variables. Where you ride, how you ride, how much you weigh, how hot is your
motor etc.

Bernhard said:

Because as Derek Wilson has stated above, turning a 4 speed gearbox into a 3 speed with overdrive would feck it up. On winding roads the top gear would become useless.

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the over driven primary has NOTHING to do with making it a 3 speed with overdrive but has everything to do with giving the gearbox an easer life. the overdriven primary gives less input torque and better torque multiplication on the output side. it is a win win for the gearbox. as i stated you compensate with the countershaft sprocket to set the overall gear ratio back to what your riding style is with a std primary ratio.

Many people in this string have said they like a 20 or 21 tooth sprocket for normal road use of our elderly Nortons
when using the standard chain or belt drive primary giving a ratio of 2.19:1 for the front:clutch sprockets

Onder says it all depends.....

Bill says he compensates for the change from 2.19 : 1 to 2:1 by carefull choice of gearbox output sprocket

So my dodgy maths would infer to use roughly 16T as a 20T equivalent and 17T as a 21T equivalent if using a 2:1 belt drive
Am I correct ?

Does anybody have experience of using 16/17T sprockets with a 2:1 belt drive ?

Or are people still using 20/21T with a 2:1 belt drive and what's their experience ?