The ton plus 10, what could be the problem?

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Test of a home market Combat Interestate by Motorcyclist Magazine of the UK in September 1972 lists overall gear ratio in top at 4.84 to one.
I believe this correlates to a 19 tooth cs.

Cycle Guide in the US lists the same 4.84 ratio in their US Combat test. Modern Cycle of the US lists the same 4 84 ratio and gives us the info that this bike has a 19 tooth cs.

A test of another UK home market Combat Interstate by Motorcycle Mechanics, also done in September 1972 states " even with the 19 tooth gearbox sprocket the Interstate does not like going over 6500 rpm"

All of the Combats tested in the Norton Commando Ultimate Portfolio , whether UK market or US, list 19 tooth as the as supplied gearbox sprocket.


Glen
 
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Mine had a 19 tooth when I bought it new. Am sure they all did. Changed to a 21 after a year or so. Acceleration was good but it got a bit buzzy on a 200 mile blast up the M6 to Scotland from the Midlands.
 
worntorn said:
Test of a home market Combat Interestate by Motorcyclist Magazine of the UK in September 1972 lists overall gear ratio in top at 4.84 to one.
I believe this correlates to a 19 tooth cs.

Cycle Guide in the US lists the same 4.84 ratio in their US Combat test.

A test of another home market Combat Interstate by Motorcycle Mechanics, also done in September 1972 states " even with the 19 tooth gearbox sprocket the Interstate does not like going over 6500 rpm"

Glen
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Glen yes it does indeed correrlate to a 19th sprocket.. I have always thought the Interstate had a 21tooth . Was told this at the 1972 Jan show.. Possibly it was changed during the year? Wish I could find my old motorbike mags so easily.
I thought Motocyclist Magaa zine was American but nomatter . Certainly Motorcycle Mechanics was a UK publication
 
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Hi Mike

Looks like they went back to the 21 tooth on the 850 models and stayed with that.
The later non Combat 750 might also have been fitted with a 21 tooth
I guess someone realized that the 19 tooth sprocket, while it would help with acceleration, was a negative for keeping the engine together.

My mistake, as you say, Motorcyclist is a US publication.


Glen
 
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worntorn said:
"I think it is interesting that the final engine speed in 4th gear (5300 rpm) occurred very near the torque/HP crossover point for a stock 850 (and I know your 312A cam puts you in a much better place than stock)"

Isn't this the same for all engines, all vehicles ?( capable of at least 5252 rpm)

Torque and horsepower must crossover, in other words, be equal at 5252 rpm.
That's my ( limited) understanding of the simple equations used to compute HP from tq or tq from HP.

Glen
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Yes of course, 5252 rpm is the universal x-over for all torque/HP derivations, and I hope I didn’t suggest it being anything more or less than that.

I was simply calling attention to the fact that from the plots and gear ratios provided, the shift recovery point put the 4th gear engine rpm at ~ 6000, where the engine could not provide the power to sustain the speed of 110 mph. It was reported that it could however sustain a speed of 105 mph, where the engine could support the selected gearing. Serendipitously or otherwise, this happened at ~ 5300 rpm, where with a stock 850 the torque starts to rapidly go south and the HP remains essentially constant with increasing rpm, thus I was hoping to convey that this same scenario might play out here, especially with the advanced cam timing, where torque would be expected to peak earlier and then drop off.
 
Sorry, my misread.

Does seem that the 22 tooth is a bit tall for this bike, if max top end is the goal.
If relaxed cruising is the goal, the 22 tooth is the way to go, except it will raise the "gone smooth" speed point a bit.

Which brings up another point often discussed here. There have been claims that original Commandos were smooth right down to 2,000 rpm.
Again and again in these period tests it is mentioned that the bikes shake considerably in the 2 to 3 thousand Rev range then go magically smooth at 3 k.
Test dates right back to 68.

Glen
 
I use a near-standard 850 cam which is advanced 12 degrees ahead of standard, the motor will go straight through the top of the rev-range if I let it, regardless of the gearing. The main thing which will stop a motor from revving out, is the exhaust system. Any restriction usually takes the top off. I think the Exup system used on modern bikes might be there to keep the noise down while more drastic cam timings are used.
 
If you are talking about the inlet cam, the opening point is 65 degrees BTDC instead of 53 degrees, the closing point is too early. But the exhaust opening is about 94 degrees BBDC. The exhaust noise is very loud but with the 2 into 1 pipe, it seems to work very well. The big thing about the 2 into 1 pipe, is the tailpipe must have the same cross-sectional area as the sum of the two header pipes or you will go backwards. - You cannot do this with a road bike , you will break the noise laws. With any cam you probably need to try advancing or retarding it, if you change your exhaust system.
 
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My 74 850 motor has alway had a 19 tooth sprocket since new and have always kept with the 19 tooth front sprocket, but with the work done to my motor it revs a lot higher than a stock motor, from new the stock motor would valve bounce at 6,500 rpm but with the cam and all work it revs freely to 8,000 and more it you let it but we all know what happens if you do.
One of the weakest points on a Norton motor is the main oil seal area, well thats where mine failed from over reving it, but that was a long time ago when young and no brains with doing burn outs, so much wear and tear from my early days of stupidity, look after these motors and they will give good reliable service, mine was a everyday ride for 38 years with a lot of miles on it, its now semi retired like me but I still love taking it out when I am not mucking around with it.

Ashley
 
acotrel said:
If you are talking about the inlet cam, the opening point is 65 degrees BTDC instead of 53 degrees, the closing point is too early. But the exhaust opening is about 94 degrees BBDC. The exhaust noise is very loud but with the 2 into 1 pipe, it seems to work very well. The big thing about the 2 into 1 pipe, is the tailpipe must have the same cross-sectional area as the sum of the two header pipes or you will go backwards. - You cannot do this with a road bike , you will break the noise laws. With any cam you probably need to try advancing or retarding it, if you change your exhaust system.
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The opening numbers you provided sound very much like a bone stock Commando cam, as those numbers don’t suggest at all that the cam is advanced, rather, that it is slightly retarded relative to the standard Commando cam.
The ton plus 10, what could be the problem?
 
WZ507 said:
The opening numbers you provided sound very much like a bone stock Commando cam, as those numbers don’t suggest at all that the cam is advanced, rather, that it is slightly retarded relative to the standard Commando cam.
View attachment 4760
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The exhaust opening in an 850 Commando engine is usually 82 degrees BBDC, NOT 94. The Combat 750 inlet opening is 59 degrees BTDC, not 65. In the 850, it is 53 degrees BTDC.
 
In answer to the original question, there is nothing wrong with your Commando. Your performance sounds typical of the breed, based on my Commando experience over four decades, with an average rider in normal riding gear, rather than skin tight leathers. The problem lies in believing factory and media hype, uncalibrated speedos and over enthusiastic owners.
 
worntorn said:
Sorry, my misread.

Does seem that the 22 tooth is a bit tall for this bike, if max top end is the goal.
If relaxed cruising is the goal, the 22 tooth is the way to go, except it will raise the "gone smooth" speed point a bit.

Which brings up another point often discussed here. There have been claims that original Commandos were smooth right down to 2,000 rpm.
Again and again in these period tests it is mentioned that the bikes shake considerably in the 2 to 3 thousand Rev range then go magically smooth at 3 k.
Test dates right back to 68.

Glen
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My experience with an early 850 is and a 21 is that it would pull 6500 rpm in top around 110mph, but that's a bit over the peak hp at 6200. I haven't done a true top speed run in a few years, but I'm pretty sure you pick up a mph or two with a 22T or maybe a 23T sprocket.

From memory of the chart in the INOA Tech Digest:
with a 21, 119*65/70=110.5
with a 22, 122*64/70=111.5

Compare this to the 850 power curve shown in the Duckworth book.

With a 750 that pulls hard to 7000 this wouldn't be true.
 
Fullauto said:
In answer to the original question, there is nothing wrong with your Commando. Your performance sounds typical of the breed, based on my Commando experience over four decades, with an average rider in normal riding gear, rather than skin tight leathers. The problem lies in believing factory and media hype, uncalibrated speedos and over enthusiastic owners.
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Yep I agree
 
ewgoforth said:
My experience with an early 850 is and a 21 is that it would pull 6500 rpm in top around 110mph, but that's a bit over the peak hp at 6200. I haven't done a true top speed run in a few years, but I'm pretty sure you pick up a mph or two with a 22T or maybe a 23T sprocket.

From memory of the chart in the INOA Tech Digest:
with a 21, 119*65/70=110.5
with a 22, 122*64/70=111.5

Compare this to the 850 power curve shown in the Duckworth book.

With a 750 that pulls hard to 7000 this wouldn't be true.
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I had a 23 gearbox sprocket on an 850 it was a performance killer I soon dropped it back to 21
 
so in real life with much experience here .... roughly how fast should a '72 Commando be going at 5000 RPM ..... standard size tires both ends K81 4:10 X19" , stock rear sprocket , with 21 tooth counter shaft sprocket , 180lb rider ..... Big Thanks !!! for anyone skilled at the math and takes time to answer .... I have downloaded a speedo app on phone and am curious but still a couple wks out from actual test , due to weather and road conditions here ....
 
Why is it so important to know about how fast your Norton will go, each bike will be different, tuning plays a big part, whats been done to the motor or how much weight has been saved how heavy is the rider it all plays a big part in high speed running, in my younger days riding flat out was the normal thing to do but now more older and wiser its not important to me, my 850 Featherbed is so much lighter than my old stock Commando, it has a lot of good things done to the motor its built for reving hard and handling at any speeds, it will happerly cruise in top gear all day from 60 mph to 90 mph all day while traveling with the stock gearing and when I need to take it over the ton it does it easy without any problems.
In the 60s, 70s as well the 80s was all about going flat out to prove your manhood, to prove who had the fastest bike around, but now I have nothing to prove, I know what my bikes can do , how they perform, there are a lot more people on the roads these day who are out to kill you with all the distraction that life brings to the moden world, to me down low torque is more inportant than high speed flat out runs.

Ashley
 
Thanks, surprisingly have heard that all before .... not sure how state of tune affects this question though as 5000RPM can be maintained no problem .... I had my original speedo repaired several years ago and it bounces a bit .... after all the talk ‘bout speeding and the resulting tickets thought I would ask those more knowledgeable how I was doing at 5000RPM .... maybe save myself a few dollars plus just kinda nice to know how true the speedo is .... really didn’t mean to stir anything up ... just an ask , that it ....
 
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