Commando Gearing Options

Dan1950 · Mar 16, 2023

I have used the online gearing commander to calculate relative speeds through the gears with 20, 21 and 22t gearbox sprockets. Since they had no input for the Avon RR II 400-18 tire I am running on the back, I interpolated the circumference for a 26.4 diameter rear tire.

1st, a 20t gearbox sprocket.

Next, my current gearing with the 21t gearbox sprocket

Last the 22t gearbox sprocket I plan to run after the engine build.

gortnipper · Mar 16, 2023

I had a 20T for most of the time I have had my 74, and all before I rebuilt with my motor similar to what you are building. I now have a 21T.

20T was great when I lived in Seattle with a stock old motor. I am glad I changed to a 21T for living in NZ with a hotter performance motor. It does great for the windy country roads we have with 80-100 kph (50-65mph) limits. A 22T would be too big and I would never get out of second gear.

I think a 22T would only be good with this motor for a lot more freeway riding.

Dan1950 · Mar 16, 2023

gortnipper said:
I had a 20T for most of the time I have had my 74, and all before I rebuilt with my motor similar to what you are building. I now have a 21T.

20T was great when I lived in Seattle with a stock old motor. I am glad I changed to a 21T for living in NZ with a hotter performance motor. It does great for the windy country roads we have with 80-100 kph (50-65mph) limits. A 22T would be too big and I would never get out of second gear.

I think a 22T would only be good with this motor for a lot more freeway riding.

3500 RPM in 3rd gear with my rubber would be 51.5 MPH with the 21t sprocket.

3500 RPM in 3rd gear with the 22t sprocket would be 54.0 MPH.

The same RPM in 4th ear would be 62.7 and 65.7 MPH respectively.

concours · Mar 16, 2023

I run 22T.
Never liked tractor gears.
L O N G legs are my preference.
Never had a single instance where I wished for any shorter gearing. From TOTD to Hurricane Mountain Road.

gortnipper · Mar 16, 2023

Dan1950 said:
3500 RPM in 3rd gear with my rubber would be 51.5 MPH with the 21t sprocket.

3500 RPM in 3rd gear with the 22t sprocket would be 54.0 MPH.

The same RPM in 4th ear would be 62.7 and 65.7 MPH respectively.

What I find matters a bit more to me are shifting points in twisty roads as it matches up to the powerband on my motor with the 312a. For that, I like the 21t better than I would a 22t I think. I like to keep that motor higher than 3500 coming out of corners.

TomU · Mar 16, 2023

I really like my 20T. But i have not gotten over 50 (yet)

Ron Hulton · Mar 16, 2023

Dan ... For the roads you will be mostly riding , the speed limit is 55 mph . I would think that third gear would be the choice so pick a RPM that suits your desire and match it with the appropriate sprocket

Jdub · Mar 16, 2023

When I bought my first Commando it came with a 19 tooth. The look of surprise on Vette owner's faces at the next stoplight was priceless.

Atlas Commando · Mar 16, 2023

Wasn't a 19 tooth part of the Combat's allure?

Torontonian · Mar 16, 2023

Atlas Commando said:
Wasn't a 19 tooth part of the Combat's allure?

My first Norty the 70 came with the 19 T . Loved the pickup and quickness until I tried the hwy .
50 years later it's a 21 T . On a Mk 111 for touring with the wife and camping gear on it .

acotrel · Mar 17, 2023

The heavy crank in the Commando always tends to spin up at the same rate, so lowering the gearing sometimes does not give you quicker acceleration. A lighter crank is more flexible, but I love thev heaby crank. Once it is spinning high, nothing stops it. Every upwards gear change is a leap forward.

milfordite · Mar 17, 2023

I have the 22 on my MKIII, mainly for cruising and touring. I would have a 23, but hard to find in 1/4" width. On my 74 Roadster with a warmed up motor, the 21 works well for me around here, hilly, twisty roads. Love them both!

baz · Mar 17, 2023

milfordite said:
I have the 22 on my MKIII, mainly for cruising and touring. I would have a 23, but hard to find in 1/4" width. On my 74 Roadster with a warmed up motor, the 21 works well for me around here, hilly, twisty roads. Love them both!

Sounds like you have the best of both worlds

olympus · Mar 17, 2023

it'd love to try them all..... but with the shear amount of work needed to change them ill be sticking for the time being with a 21 tooth

Deckard · Mar 17, 2023

I like my 750 with a 19 tooth just fine. Revs quickly and I don't need to go beyond 100mph. Even better when I raised compression to 10:1 and ported head to add 32mm carbs.
The 850 with a 20 tooth sprocket, heavier crank and lower compression is nowhere near as responsive.
That engine will get an injection of testosterone with a Web 312, higher compression pistons and Comstock flowed head in the future.
Similar to what you're doing Dan. Looking forward to how your build comes out.

Deckard · Mar 17, 2023

Jdub said:
When I bought my first Commando it came with a 19 tooth. The look of surprise on Vette owner's faces at the next stoplight was priceless.

Maybe just that short a distance. I had a '71 427 L88 and a friend with another '71 Commando. In the 1/4 mile, I always caught him by 3rd gear and had more top end.

Dan1950 · Mar 17, 2023

Atlas Commando said:
Wasn't a 19 tooth part of the Combat's allure?

I think it was also part of the reliability problem. Too many ham fisted riders over-revving the engine.

hhh · Mar 17, 2023

Deckard said:
I like my 750 with a 19 tooth just fine. Revs quickly and I don't need to go beyond 100mph. Even better when I raised compression to 10:1 and ported head to add 32mm carbs.
The 850 with a 20 tooth sprocket, heavier crank and lower compression is nowhere near as responsive.
That engine will get an injection of testosterone with a Web 312, higher compression pistons and Comstock flowed head in the future.
Similar to what you're doing Dan. Looking forward to how your build comes out.

I thought the consensus was that 750s especially are better of with the smallest port heads. Raising 850 compression is a must though, assuming suitable fuel is available.

Deckard · Mar 17, 2023

hhh said:
I thought the consensus was that 750s especially are better of with the smallest port heads. Raising 850 compression is a must though, assuming suitable fuel is available.

Well, that was 1974 and I wanted to bring my '71 up to a Combat without swapping the cam. Now we know that 30mm ports did flow better. Better if you can find RH10 intake manifolds to mount 32mm carbs on a 30mm intake.
I went beyond, had Jim Comstock flow head and bought Keihin FCR's to modernize for the next build of it.

bluto · Mar 19, 2023

Another consideration is repeat contact numbers...they are surprisingly good for a 21/42 combo despite that being an integer ratio, the 100 link chain works well with it. Here are numbers for 19, 20, and 21.

Commando Gearing Options

Dan1950

1974 MK II Roadster

gortnipper

Dan1950

1974 MK II Roadster

concours

gortnipper

TomU

1972 Roadster

Ron Hulton

Jdub

Atlas Commando

Torontonian

acotrel

milfordite

baz

olympus

Deckard

Deckard

Dan1950

1974 MK II Roadster

hhh

Deckard

bluto

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