Thinking of Going to a 22T Gearbox Sprocket

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Dan1950

Dan1950

1974 MK II Roadster
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Here's my reasoning:

First of all I live in a rural area about halfway across the Northern tier on New York State. Riding in traffic will be brief, perhaps less than 10% of my riding. I live on the Northern edge of the Adirondack Park.

Second, I want a good highway bike that will also be fun in the twisties. A dual purpose machine so to speak.

Third, I have found that lower gearing can have a "placebo affect'. All the noise and fury gives the elusion of greatly increased acceleration when sometimes, that is not really the case. In fact, too low (numerically high) gearing can actually have a negative effect on acceleration once out of the hole. It all depends on the torque curve.

Last, I have a 110/90 X 18 rear tire that is almost 2% smaller than the stock 19" rubber, so increasing the sprocket tooth count by 5% wont have a significant affect on acceleration but will reduce RPM somewhat..

Here are some screen shots of the various combination. Let's concentrate on the 3500 - 4500 RPM range.

1: Stock 21t with 19" rubber
Thinking of Going to a 22T Gearbox Sprocket


3500 RPM in 4th = about 64 MPH, not bad.


2: Current gearing 21t with 18" rubber
Thinking of Going to a 22T Gearbox Sprocket


3500 RPM in 4th now equals just over 61 MPH. heading in the wrong direction IMO


3: Custom gearing 22t with 18: rubber
Thinking of Going to a 22T Gearbox Sprocket


Now we are doing just over 64 MPH at 3500 RPM, just a bit more than stock. For every 500 RPM the speed in 4th increase by just over 9 MPH so 75 MPH will be at just a little over 4000 RPM and 80 MPH will be attained with less than 4500 RPM. 3rd gear ought to be a hoot for overtaking.

So given the fact that I have 18" rear rubber, my riding style and the environment I will be riding in I think adding a tooth to the gearbox sprocket to be worthwhile, especially since I already have the gearbox exposed.
 
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You may very well be overthinking Commando ownership. Trying to make this bike all things for all purposes will end with mediocrity.
The bike came with a 20 tooth sprocket and is a very nice torque machine. 22 tooth will surely make it a better highway cruiser, but
that is not how this bike shines. I put a 21 tooth sprocket on my '74 and it make a big difference in how long it's legs are. A 22 is
guaranteeing that you will spend lots of time in 3rd gear in the twisties and adding additional stress to a transmission designed for
earlier lower powered Nortons. But you seem to know what you are doing, so have at it.
 
lazyeye6 said:
You may very well be overthinking Commando ownership. Trying to make this bike all things for all purposes will end with mediocrity.
The bike came with a 20 tooth sprocket and is a very nice torque machine. 22 tooth will surely make it a better highway cruiser, but
that is not how this bike shines. I put a 21 tooth sprocket on my '74 and it make a big difference in how long it's legs are. A 22 is
guaranteeing that you will spend lots of time in 3rd gear in the twisties and adding additional stress to a transmission designed for
earlier lower powered Nortons. But you seem to know what you are doing, so have at it.
Click to expand...
The specs I am seeing for Commandos is 58 HP @ 6800 RPM for the 750 and 60 HP @ 6200 RPM for the MK II 850. That's nearly a 10% reduction in redline for the 850 over the 750. Why would one want to gear the 850 the same as a 750?

20t to 22t is also a 10% difference.

As far as powerband usage, wouldn't the 22t on the 850 be similar to a 20t on a 750?
 
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Dan, mine came with a 23T and I absolutely love it. I have no problem in traffic, it pulls strongly and 3500rpm is 70mph in 4th. I typically ride it at 2500 to 4500 rpm and when I'm having fun, only feel the need to use up to 5500rpm. I don't find i need to drop down the gears to overtake.

Even though my touring bike is a Z1000SX, I'm not a fan of revvy 4cyl m/cs, the Norton is my favourite bike.
 
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kommando said:
Peak power is at a lower rpm for the 850 but the redline is still 7000 and mine regularly gets taken to the red line in the lower gears as it still makes good power at 7000.
Click to expand...
I found that when I increased the stroke on my Panhead it accelerated much better with taller gearing. Also, it would pull to 6000 RPM but I got better 1/4 mile times shifting at 5000 which brought the RPM into the fat part of the power band.

My '06 Gen III Hemi was just the opposite, I would rev to 6600 even though peak HP was at 6250. that also brought the RPM into the fat part of the power band on the upshift. Even stock the peak HP was at 5400 but I ran better times shifting at 6000.

Perhaps the fact that one engine was undersquare and the other was oversquare had something to do with that phenomenon.
 
EstuaryBoy said:
Classic Bike magazine put two 'well maintained' Commando's on the dyno (750 and 850). Neither made anywhere near those outputs!
Bottom line: Ignore 'printed' numbers and go for what works best for you......
Click to expand...
Indeed.

When I built my first hopped up 850 motor (JS #1 cam, JS pistons @ 10,5:1, FCR carbs, etc) it was like a different bike next to my mates stock (ish) 850, hugely more powerful.

It produced 56rwhp…
 
Fast Eddie said:
Indeed.

When I built my first hopped up 850 motor (JS #1 cam, JS pistons @ 10,5:1, FCR carbs, etc) it was like a different bike next to my mates stock (ish) 850, hugely more powerful.

It produced 56rwhp…
Click to expand...
Trouble is, their article left more questions than it provided answers for, sadly.
They ran 750 and 850 back to back, NO mention whether gearing was stock or not, THEN they put a different 850 on the dyno ?????!!
Both ran up to 110mph, with acceleration graphs very close up to 70, then the larger bike wheezed up to max (but with peashooters on unbalanced system).
ON paper (to an alien), ALL humans seem alike: Two arms/legs/eyes etc. etc..... Same with these bikes: No two identical, whatever the specs may say...
 
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EstuaryBoy said:
Classic Bike magazine put two 'well maintained' Commando's on the dyno (750 and 850). Neither made anywhere near those outputs!
Bottom line: Ignore 'printed' numbers and go for what works best for you......
Click to expand...
Fast Eddie said:
Indeed.

When I built my first hopped up 850 motor (JS #1 cam, JS pistons @ 10,5:1, FCR carbs, etc) it was like a different bike next to my mates stock (ish) 850, hugely more powerful.

It produced 56rwhp…
Click to expand...
EstuaryBoy said:
Trouble is, their article left more questions than it provided answers for, sadly.
They ran 750 and 850 back to back, NO mention whether gearing was stock or not, THEN they put a different 850 on the dyno ?????!!
Both ran up to 110mph, with acceleration graphs very close up to 70, then the larger bike wheezed up to max (but with peashooters on unbalanced system).
ON paper (to an alien), ALL humans seem alike: Two arms/legs/eyes etc. etc..... Same with these bikes: No two identical, whatever the specs may say...
Click to expand...
Unless the engine was strapped directly to the dyno (not in the chassis) HP values from a "chassis dyno" will be less that what the output at the crank will indicate. Parasitic loss thorough friction and such. Tire pressure can have a HUGE affect on a chassis dyno. (remember how much faster your bicycle was once you pumped up the tires?)

In an automotive application the loss is about 20% through the drivetrain. Probably less with a motorcycle but there none the less. If 20% was the factor in the 56 HP cited above, the engine's true output at the crank would be 70 HP!

Even now that auto maker use "net" HP rating (all accessories installed) as opposed to the "gross" ratings used before '73 Bare engine block, no mufflers, air filters, alternators, etc, they still rate the HP on an "engine dyno"
 
Or pointed it downhill!!!
(Apologies for the above, but again, we can get bogged down with numbers that raise more questions than answers....)
Of course, raising the gearing should improve consumption.... But that in turn means each ethanol poisoned gallon will stay longer in the tank, doing it's destructive business!! A consideration? :)
 
22 on my '74. 60mph = 3000 rpm, 70= 3500, 80 = 4000.
Never, ever, ever wish for or consider a shorter gear. Including when enjoying the uphill switchbacks on the Snake, Dragon, Hurricane Mountain Road & many, many others. Makes the best use of the 850's torque. Cruising at THE SAME SPEED as modern traffic on I-95, 495, 90, etc. is a huge safety consideration for those times I have to "slab it" to get to good riding. Dawdling in the right lane at 65 is just asking for a Honda Civic enema, IMWO.
 
EstuaryBoy said:
Or pointed it downhill!!!
(Apologies for the above, but again, we can get bogged down with numbers that raise more questions than answers....)
Of course, raising the gearing should improve consumption.... But that means each ethanol poisoned gallon will stay longer in the tank, doing it's destructive business!! A consideration? :)
Click to expand...
What really matters here is the torque band and where in the RPM range it is on the fat part of the curve. The number value is secondary. Torque will diminish as RPM climbs and will always cross the HP line on the graph at 5250 RPM HP and TQ will be equal at 5250 RPM as long as the engine can pull that many RPM. When one installs a "hot cam" it boosts the RPM range of the TQ band albeit at the expense of TQ at lower RPM. A compression ratio increase is the only free lunch so to speak. It will increase TQ and HP throughout the RPM range and can even improve fuel efficiency too as long as it stays within a reasonable range that doesn't cause detonation that can't be mitigated by fuel octane and some mechanical factors such as effective quench (squish).
 
Dan1950 said:
Unless the engine was strapped directly to the dyno (not in the chassis) HP values from a "chassis dyno" will be less that what the output at the crank will indicate. Parasitic loss thorough friction and such. Tire pressure can have a HUGE affect on a chassis dyno. (remember how much faster your bicycle was once you pumped up the tires?)

In an automotive application the loss is about 20% through the drivetrain. Probably less with a motorcycle but there none the less. If 20% was the factor in the 56 HP cited above, the engine's true output at the crank would be 70 HP!

Even now that auto maker use "net" HP rating (all accessories installed) as opposed to the "gross" ratings used before '73 Bare engine block, no mufflers, air filters, alternators, etc, they still rate the HP on an "engine dyno"
Click to expand...
Dave Nourish probably did more engine brake and rear wheel Dyno testing than anyone in the U.K. outside of the factories.

He always reckoned on a 5bhp loss, give or take.
 
concours said:
22 on my '74. 60mph = 3000 rpm, 70= 3500, 80 = 4000.
Never, ever, ever wish for or consider a shorter gear. Including when enjoying the uphill switchbacks on the Snake, Dragon, Hurricane Mountain Road & many, many others. Makes the best use of the 850's torque. Cruising at THE SAME SPEED as modern traffic on I-95, 495, 90, etc. is a huge safety consideration for those times I have to "slab it" to get to good riding. Dawdling in the right lane at 65 is just asking for a Honda Civic enema, IMWO.
Click to expand...
:oops::oops::oops::oops::oops::oops::oops::oops:
 
That is a figure of speech, to mean, rear-end collision by a driver conditioned by "The Fast & The Furious" movie. There are a lot of them.
60,000 miles per year, I've had a front row seat for the last 28 years.
 
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Fast Eddie said:
Dave Nourish probably did more engine brake and rear wheel Dyno testing than anyone in the U.K. outside of the factories.

He always reckoned on a 5bhp loss, give or take.
Click to expand...

Fast Eddie said:
Dave Nourish probably did more engine brake and rear wheel Dyno testing than anyone in the U.K. outside of the factories.

He always reckoned on a 5bhp loss, give or take.
Click to expand...
It's usually thought to be a percentage especially when RPM is a factor. Even so 5HP is almost 10% on the 56 HP cited above.
 
concours said:
That is a figure of speech, to mean, rear-end collision by a driver condition by "The Fast & The Furious" movie.
Click to expand...
Yes I know. It is so colorful and funny.

Reminds me of the days when I hunted deer and small game almost exclusively with tradition muzzle loading rifles and shotguns. I called breech loaders "suppository guns" as you loaded them by sticking something in their backside.
 
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