will my MK3 pull a 24t front sprocket?

concours said:

True story.
Over gearing will lug the engine, destructive to an engine, just in different ways than over revving.

Click to expand...

Depends how you ride it. Use fourth as an overdrive and third for speed.

cash said:

Depends how you ride it. Use fourth as an overdrive and third for speed.

Click to expand...

In the hands of an engine aware rider, very true.

cash said:

Depends how you ride it. Use fourth as an overdrive and third for speed.

Click to expand...

Yes but you need to be careful and really understand your engines characteristics and your intended use. Depending on the road and type of riding, that could easily mean spending more time in lower gears at higher revs. Resulting in increased average revs over a journey. Thereby defeating the objective.

acotrel said:

I suggest Commandos are simply detuned bullshit. Only about 2 Atlas 750s ever came to Victoria. The first broke its barrel flange off, so very few Atlases were sold. Bob Rosenthal bought one from Athol Paterson. He fitted Map rods and pistons to it, rebalanced the crank. I don't think he even uprated the gearbox. He was successful in A grade road races against the bikes of the early 70s. He became sponsored to ride TZ750 Yanahas.
Nortons built motorcycles with a lot of racing experience in their past.
All I have done with my 850 motor is undo the things which were holding it back.
I would never take the same approach with a modern motorcycle.
Some of our kids are racing Suzuki XR69 replicas. The frames are lookalike and the motors are GSX1100 Katana. The factory XR89 which Crosby raced in Europe was a GS1000 motor with a close ratio 6 speed box , two valves per cylinder with proper race cams.
If somebody tried to race an XR69 with a Katrana engine, close box, full race cams and separate pipes - they would probably almost immediately demolish themself.
As far as I have been able to determine, the only road-going Suzuki which has a close box is the GSXR750L The factories do not usually sell stuff which makes their competitors faster than the factory riders.
I suggest a Commando has more potential than many people might expect. An Atlas gives the clue to the Commando motors' potential. But a race bike usually makes a bad road bike. And converted road bikes do not make good race bikes.

Click to expand...

How many times Al ?
The commando is not detuned FFS

I've always considered 4th to be "direct drive" (my term) meaning you were coupled to the main shaft when MS 3rd slide into MS 4th ( sleeve gear). It is really safe as you are no longer straining the other gears. ( like the 1st or second gears) that can be broken when you ride like I once did. On my stock bikes I'm counting on 4th to take me from about 100 to 120.( when geared appropriately) at sea-level.

I would think there is less friction too as it is turning 1 to 1. ( isn't it?)

If the bike is stock it might only do a little over 100 at altitude. ( there are many variables in tuning and riding ability). A quick highway run would solve the speed issue.

Carl H is right.
The AMC Norton gearbox is a 3-shaft gearbox.
I believe most motorcycle gearboxes are 2-shaft.
( clutch and sprocket on opposite sides)
With a 3-shaft gb, there are 2 pairs of gears involved in the lower gears, eating up quite a few HP.
(2-shaft gb only one pair, but then in every gear)
The top gear in a 3-shaft is direct drive, so no gb power loss.
Best ride in top gear.
I doubt an MK3 can pull 24T.
How heavy are you? ..

ludwig said:

The top gear in a 3-shaft is direct drive, so no gb power loss.

Click to expand...

Not strictly "no" power loss as the layshaft is still driven by the sleeve gear when in 4th.

No one seems to have mentioned the clutch slipping. Fitting a 24 t sprocket will make clutch slip worse. Then again if your going to all the knob ache of fitting a 24 t sprocket, fitting a super duper clutch on the rebuild is possible...

ludwig said:

Carl H is right.
The AMC Norton gearbox is a 3-shaft gearbox.
I believe most motorcycle gearboxes are 2-shaft.
( clutch and sprocket on opposite sides)
With a 3-shaft gb, there are 2 pairs of gears involved in the lower gears, eating up quite a few HP.
(2-shaft gb only one pair, but then in every gear)
The top gear in a 3-shaft is direct drive, so no gb power loss.
Best ride in top gear.
I doubt an MK3 can pull 24T.
How heavy are you? ..

Click to expand...

Good explanation of a complex idea. When I broke layshafts , direct drive got me home as it would ease the pressure and the milling of the Quaife case below the LS 1st, Luckily the casing had lots of meat there as I did in several in the good old days on my stock MK2a , Known as Big Red. (A very famous Commando in my neck of the woods) 100 mph in the 1/4 and 120 @ 7K on route 140 . The speedo was quite accurate as it read 100 as I tripped the lights at New England Dragway.

Carl H said:

I've always considered 4th to be "direct drive" (my term) meaning you were coupled to the main shaft when MS 3rd slide into MS 4th ( sleeve gear). It is really safe as you are no longer straining the other gears. ( like the 1st or second gears) that can be broken when you ride like I once did. On my stock bikes I'm counting on 4th to take me from about 100 to 120.( when geared appropriately) at sea-level.

I would think there is less friction too as it is turning 1 to 1. ( isn't it?)

Click to expand...

The mainshaft turns the 3rd gear main and the 3rd gear main turns the 4th gear main so in that regard, it is direct drive and therefore 1 to 1 in the gearbox. The layshaft is still turning but engaging nothing so it will only add a tiny bit of friction. Edit: Should have said: The layshaft is still turning but engaging nothing that is doing anything other than spinning so it will only add a tiny bit of friction and thereby power loss.

If I have this correct then when in top gear, the sleeve/mainshaft drives the layshaft at x1.64285 (23 x 14) the sleeve/mainshaft RPM so some small power loss there perhaps.
Also, in top gear, the other three gear sets continue to rotate. Splined mainshaft 1st and 3rd gears driving freewheeling layshaft 1st and 3rd.
Splined layshaft 2nd drives free mainshaft 2nd, the free gears rotating at different RPMs to their respective shafts so perhaps some small frictional losses there too.

Won't running on El mirage surface consume a fair bit of hp , that will lower the bikes ability to rev out with tall gearing , do some testing somewhere.

I said "less friction". Like in 1st gear the main shaft is spinning in the sleeve gear bushes at a high speed and your going about 40 mph (the sleeve gear is spinning slower..) In 4th they are spinning as one ( together). And a lot less wear on the parts. A good reason to be in 4th. Now we await the results at El Mirage. But A Commando is only as good as the rider/ mechanic and thought put into it. A stock Commando is a formidable weapon in the right hands.

baz said:

How many times Al ?
The commando is not detuned FFS

Click to expand...

Ever had the feeling you are wasting your time?

If I'd spotted that post sooner I would have deleted it as it's another acotrel off-topic racing ramble.

ludwig said:

Carl H is right.
The AMC Norton gearbox is a 3-shaft gearbox.
I believe most motorcycle gearboxes are 2-shaft.
( clutch and sprocket on opposite sides)
With a 3-shaft gb, there are 2 pairs of gears involved in the lower gears, eating up quite a few HP.
(2-shaft gb only one pair, but then in every gear)
The top gear in a 3-shaft is direct drive, so no gb power loss.
Best ride in top gear.
I doubt an MK3 can pull 24T.
How heavy are you? ..

Click to expand...

185

Since the OP’s question relates to two variables (gearing and altitude) he might be interested in roughly calculating jetting changes (which roughly translate into power losses) by doing a desk exercise on this site intended for airplane guys. In other words, he is thinking about gearing way up to a 24 tooth c/s sprocket while losing horsepower to increased altitude.



I used this site for years roadracing with AHRMA, where we raced at elevations from sea level and 60 degrees F in Daytona to Miller, at 4500 feet and 85 degrees F, and most relevant for the OP, we raced at Willow Springs, in the Antelope Valley at say, 80 degrees. Needless to say, getting jetting even in the ballpark in that range of temps and altitudes was always a challenge and this site really helped get it in the ballpark. I used Mikuni carburetors, which number their jets in very rough proportion to their flow, so a percentage change in air very roughly translated to a change in main jet which made jetting off this calculator conceptually easy.

You can get real numbers from the weather service. But you can calculate percentage changes for comparison purposes using plug numbers—100% air is about sea level, 30.0 barometer, and 60 degrees F, and the calculation is not very sensitive to humidity, dew point or barometer, so for a rough desktop calculation of how much power you’ll lose to elevation you can use 30.0 for barometer in both parts of the comparison, sea level and 2500 feet for elevation, and 60 degrees and expected Antelope Valley temperatures (80?) for when you will run, and knock off say 30% of the temperatures for dew point just to plug something in.

So that’s a lot of words but cutting to the chase what you’ll find (if my recollection of my calculations for Willow Springs is right ) is that you’ll be running through the traps with 92% air— so you might do something like jetting down that much to be in the ballpark, and expect something akin to that much power loss, for comparison purposes only. With, let’s say, something like 92% of the power as you would have at sea level, or something like just a bit less than 10% power loss you’d lose something like 4 hp on a 50 horse engine — certainly nothing precise here but it gives you an idea of power loss to altitude—and you are proposing gearing the thing up 20%, with less power. I’ve never raced my street 850 Commando, but my impression is it’s gearing is maybe a bit short with a 20 tooth c/s and it might be able to pull maybe one more tooth on the c/s at sea level but possibly not with 8% less power. Anyway, this is just another thing to consider to rough in an answer to your gearing question for running at altitude.

By the way, this site now contains a link to a similar calculator for engine tuners

L.A.B. said:

Ever had the feeling you are wasting your time?

If I'd spotted that post sooner I would have deleted it as it's another acotrel off-topic racing ramble.

Click to expand...

Witha 54% balance factor a Commando is definirely detuned. The Norton twin probably performed better when it was an Atlas. But the Atlas did not sell. An Atlas is a development of the 650SS. A 650SS was probably better.
Back in those days, the ultimate road-racing motorcycles had four cylinders - that made the CB750 very attractive. I rode one of the first CB750 Hondas which came to Melbourne - I did not like it - it was smooth but did not have anything else. When I raced my 500cc Triton aganst them, they did not go until they were fitted with CB450- pistons - made them become 830cc
In push-start races the Honda guys used to push the button, that was the only reason they were ever in front.
I only ever saw one Commando 850 win a race - it was in the hands of a very experienced A-grade rider - Jeff Curley who was sponsered by Stanco.
Don't get me wrong, for what they are Commandos are excellent. They are not designed to race. Even the frame puts them behind the 8-ball, and the gearbox is hopeless.

acotrel said:

Witha 54% balance factor a Commando is definirely detuned.

Click to expand...

What has the static balance % got to do with HP, its different because of the isolastics FFS.

Where'd the OP go?

With the heavy Commando crank, if you improve the engine's torque, you usually do not detect it until you raise the overall gearing and the bike is faster. If you raise the overall gearing without tuning differently, the bike will usually not change it's performance. When you use slower taper needles and feed the throttle on slightly slower, you stay closer to the optimum mixture, you get better performance. The taper on the needles is there to compensate for loss of vacuum when the motor is pulling hard and the revs are not high. Anyone who has ever raced a two stroke has usually found they need to feed the throttle on. A Commando is no different except that it is less likely to seize. With a two stroke, siezures often occur at the end of a straight when you close the throttle - the pilot jets feed fuel when the throttle is closed. With a wide ratio gearbox, the problems are more pronounced, because of the drop in revs on every up-change - the mixture tends to become richer too quickly