Max sustainable RPM?

Fast Eddie said:

Perhaps I misunderstand your content here?

But I just tried to compare your numbers to real world scenarios and I’m stuck with this 8x power increase for 2x speed increase equation.

How much power does it take for a Norton Commando to hit 30mph? Looking at mopeds etc I’m going to take a wild stab at somewhere around 5 BHP. Even if wrong, it’ll do to play with for now.

5 BHP at 30mph would then require 40 BHP at 60 mph. And 320 BHP at 120 mph.

Thats not right is it ?

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I think you understand my content perfectly, but perhaps completely misunderstood my intent. The explanation I provided is not “my” numbers, but rather an explanation, for our reader’s edification, of well accepted mathematical relationships equating power, drag, and velocity.

Yes, your well-intentioned calculation is all correct. If it takes 5 HP to go 30 mph then it takes 320 HP to make your Commando go 120 mph. When you bring in cubic relationships stuff multiplies up in a hurry, as evidenced here. Your issue arises due to a poor assumption right out of the starting blocks with 5 HP required for 30 mph. Change this to about 0.75 HP and everything will work out for you.

Since you indicate an interest in “real world scenarios” perhaps we could leave Lambrettas behind and get back on the Norton track. We’ve got no shortage of contributors here that throw out 120 mph as a top speed for their stock CDO – and although that is debatable, please accept this number for the sake of this discussion. Please also accept that said Norton makes ~ 45 RWHP. Thus, we’ve set a stake in the ground equating 45 RWHP to 120 mph.

Now let’s move to a higher speed as described here for ELDO’s bike, which went 154 mph.

https://www.accessnorton.com/Norton...its-after-andy’s-154mph-bonneville-run.25886/

In this example speed increased by a factor of 1.28 (154/120 =1.28). Applying our cubic factor we come up with a power increase of 2.11X (1.28^3 = 2.11) required to increase speed from 120 mph to 154 mph. Applying this factor to the original 45 HP engine gives us 95 RWHP (45 * 2.11 = 95) to go 154 mph. What a coincidence, 95 RWHP just happens to be the RWHP of the Herb Becker prepared engine that powered the bike to 154 mph! Well, maybe it might be even more than a coincidence.

I can’t give any better or more convincing explanation than offered above. But Google is your friend, look, read, verify, etc.

file:///C:/Users/kurtc/Downloads/HP%20required%20to%20overcome%20Aero%20drag.pdf

Try a calculator.

http://www.wallaceracing.com/Calculate HP For Speed.php

In the end it will all lead to the same endpoint – the cubic factor.

WZ507 said:

I think you understand my content perfectly, but perhaps completely misunderstood my intent. The explanation I provided is not “my” numbers, but rather an explanation, for our reader’s edification, of well accepted mathematical relationships equating power, drag, and velocity.

Yes, your well-intentioned calculation is all correct. If it takes 5 HP to go 30 mph then it takes 320 HP to make your Commando go 120 mph. When you bring in cubic relationships stuff multiplies up in a hurry, as evidenced here. Your issue arises due to a poor assumption right out of the starting blocks with 5 HP required for 30 mph. Change this to about 0.75 HP and everything will work out for you.

Since you indicate an interest in “real world scenarios” perhaps we could leave Lambrettas behind and get back on the Norton track. We’ve got no shortage of contributors here that throw out 120 mph as a top speed for their stock CDO – and although that is debatable, please accept this number for the sake of this discussion. Please also accept that said Norton makes ~ 45 RWHP. Thus, we’ve set a stake in the ground equating 45 RWHP to 120 mph.

Now let’s move to a higher speed as described here for ELDO’s bike, which went 154 mph.

https://www.accessnorton.com/NortonCommando/herb-becker-visits-after-andy’s-154mph-bonneville-run.25886/

In this example speed increased by a factor of 1.28 (154/120 =1.28). Applying our cubic factor we come up with a power increase of 2.11X (1.28^3 = 2.11) required to increase speed from 120 mph to 154 mph. Applying this factor to the original 45 HP engine gives us 95 RWHP (45 * 2.11 = 95) to go 154 mph. What a coincidence, 95 RWHP just happens to be the RWHP of the Herb Becker prepared engine that powered the bike to 154 mph! Well, maybe it might be even more than a coincidence.

I can’t give any better or more convincing explanation than offered above. But Google is your friend, look, read, verify, etc.

file:///C:/Users/kurtc/Downloads/HP%20required%20to%20overcome%20Aero%20drag.pdf

Try a calculator.

http://www.wallaceracing.com/Calculate HP For Speed.php

In the end it will all lead to the same endpoint – the cubic factor.

Click to expand...

I still don’t get it cos I still cannot get my head around the fact that it only takes 0.75bhp to propel a Norton Commando and rider to 30 mph…

But that’s just me, and it’s a moot point IMHO because we are simply not talking about the kind of speed you mention above, or that kind of differential between the different sprocket sizes, for the issues caused by the required power to be the decisive limiting factor in the topic of this thread.

hi (again, again),
I have a gut feeling that no matter how long this very interesting discussion goes for there is no exact answer, it is a choice we make balancing the joy of riding a fast bike and trying to reduce maintenance issues. However, for a standard Commando the answer is going to be somewhere between 4500 and 5000 rpm. In other words, it is like asking how long is a piece of string but in general, staying at or below these rpms most of the time should give us as good an engine life as is reasonably possible, baring disasters, presumably caused by unidentified faults in manufacture or materials 50 years ago.
Everything is a compromise and certainly with my Trident that has been with me since 78, it is expected to take any amount of abuse I care to dish out to it and seems to accept that in good spirit. I’m a little more restrained with my 850 as she seems to tell me that she is not happy as the rpm creeps above about 5500. Either way my bikes don’t get that many miles put on them and are pampered in every other way, so like a race horse that gets the best of care during the week, it might have to accept the crop on race day.
In short, I’m sure that above about 4500 rpm wear and the risk of catastrophic failure increases exponentially with additional rpm. If you chose to never exceed 4000rpm and only with a small throttle opening you may be rewarded with better reliability but what would be the point. You pays your money, you take you chances.
I battle to maintain much more than 80mph for extended periods on a naked bike, 75 being more comfortable, so that kind of answers the question for me
regards all
and thank you to others for such interesting thoughts
Alan

Fast Eddie said:

I still don’t get it cos I still cannot get my head around the fact that it only takes 0.75bhp to propel a Norton Commando and rider to 30 mph…

But that’s just me, and it’s a moot point IMHO because we are simply not talking about the kind of speed you mention above, or that kind of differential between the different sprocket sizes, for the issues caused by the required power to be the decisive limiting factor in the topic of this thread.

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Hi Eddie,
I think we could get too caught up in the formula for required power. The turbulence etc surrounding a bike and rider would make an incredibly complex equation to take it from zero mph to warp speed. Suffice to say the cube law probably works between 60 and 160mph
just a thought
regards
alan

Fast Eddie said:

I still don’t get it cos I still cannot get my head around the fact that it only takes 0.75bhp to propel a Norton Commando and rider to 30 mph…

But that’s just me, and it’s a moot point IMHO because we are simply not talking about the kind of speed you mention above, or that kind of differential between the different sprocket sizes, for the issues caused by the required power to be the decisive limiting factor in the topic of this thread.

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My sense is that gearing is just a factor in the equation, if the equation is asking " where will it break"
For sure you are asking a lot more of the engine when you ask it to propel you and the bike along at 102 mph (23 tooth) vs 89 mph (20 tooth), even though the rpm is identical in both scenarios.

Commandos run pretty hot and I wonder how often heat is responsible for a blow up.
I recall Jim Comstock posting about a fast 2up run in the mountains when the pyrometer fitted to the head showed a very high temp that was approaching the point where aluminium is taken for annealing! IIRC it was 550f or thereabouts.
He wisely backed off and the engine lived on.

Glen

I couldn't find the post about the hot head ride but I did find this little Comnoz tidbit which hopefully is sort of relevant to the topic


"To completely anneal aluminum would be heating to 500 C /930 F. and then quenching in saline solution. this would render a case useless and change the dimensions.

Softening of previously heat treated A356 alloy starts at around 400 F. / 200 C. Below 500 F / 260 C. the damage is minor.

That is why Norton heads slowly get softer and the distance between the bolts holes increases as the miles increase."

worntorn said:

I couldn't find the post about the hot head ride but I did find this little Comnoz tidbit which hopefully is sort of relevant to the topic


"To completely anneal aluminum would be heating to 500 C /930 F. and then quenching in saline solution. this would render a case useless and change the dimensions.

Softening of previously heat treated A356 alloy starts at around 400 F. / 200 C. Below 500 F / 260 C. the damage is minor.

That is why Norton heads slowly get softer and the distance between the bolts holes increases as the miles increase."

Click to expand...

I also recall a posting by Comnoz informing that he had seen the oil temp in the EX valve pockets of his engine in the range of 390F (199C).

worntorn said:

My sense is that gearing is just a factor in the equation, if the equation is asking " where will it break"
For sure you are asking a lot more of the engine when you ask it to propel you and the bike along at 102 mph (23 tooth) vs 89 mph (20 tooth), even though the rpm is identical in both scenarios.

Commandos run pretty hot and I wonder how often heat is responsible for a blow up.
I recall Jim Comstock posting about a fast 2up run in the mountains when the pyrometer fitted to the head showed a very high temp that was approaching the point where aluminium is taken for annealing! IIRC it was 550f or thereabouts.
He wisely backed off and the engine lived on.

Glen

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Yes that’s how I see it, as ‘a factor’ I just don’t think it’s the main one, I still hold on to the notion that the main one, in these engines, is rpm.

Which Commando engine will blow up first (assuming all else is equal) the one doing 90mph in fourth, or the one doing 90mph in third?!

Or, in your example above, which will blow up first at a sustained 102mph, the 20 tooth or the 23 tooth bike?

I know which my money would be on.

I agree, very high rpm even in neutral is one way to quickly end the life of just about any engine.
Rev limiters are a wonderful thing.
Having said that, you might get away with a very long run at 89 mph with a 20 tooth bike at 5500 rpm whereas your 23 tooth bike holding the same rpm would come apart.
This is one experiment that I'm not in on, although I have successfully done the first part on the Salmo Creston. It was more like 5800 rpm for 30 miles and lots of climbing.
She held and beat the Vincent twin that was chasing.

Glen

Fast Eddie said:

I still don’t get it cos I still cannot get my head around the fact that it only takes 0.75bhp to propel a Norton Commando and rider to 30 mph…

But that’s just me, and it’s a moot point IMHO because we are simply not talking about the kind of speed you mention above, or that kind of differential between the different sprocket sizes, for the issues caused by the required power to be the decisive limiting factor in the topic of this thread.

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You can't see your weed whip running your Norton to 30 mph eh?

We are obviously at loggerheads on this one and therefore I’ll politely agree to disagree. The OP question requires a pile of qualifiers to even begin talking about the subject, and perhaps that is where our difference of opinions arises. In considering this question I’m thinking more about death by fire (thermal issues), whereas I believe you are thinking more about an engine running under moderate load and death via metal fatigue (case, crank, rod, bearings, etc). These different viewpoints would certainly have us talking apples and oranges, since I’m envisioning tortuous operating conditions and melt-downs, whereas you’re perhaps just wanting to run an engine comfortably at higher rpm where load is well balanced with rpm.

The reason I’m harping on speed and gearing is because over gearing and the various exacerbating road conditions encountered, e.g., steep grades, strong headwinds, high temperatures could all give rise to challenging thermal operating conditions, and once power demands (heat generation) exceed cooling capacity the end is just around the corner.

Just as you can’t accept the cubic factor relating power requirements to speed changes, it’s difficult for me to accept how you can discount differences in sprocket size when you suggest that “differential between the different sprocket sizes, for the issues caused by the required power” are not “the decisive limiting factor in the topic of this thread”. I guess we can view the same information, yet envision totally different outcomes.

I look at the following plot and I see sprocket size being a huge consideration. For example, at 5000 rpm comparing bike speed of 19T to the 23T is a very significant difference (79 mph vs 95 mph). Hopefully you’ll consider the rpm and speed ranges cited relevant to the conversation. Applying the cubic factor to this speed differential indicates that the higher geared bike has to provide 1.74X the power (and heat) of the lower geared bike to operate at the same engine speed of 5000 rpm. To me that could clearly be a decisive limiting factor, especially in light of the exacerbating factors cited above, as they relate to thermal induced failure.

Back when we were trying to answer the question asked, I wrote post #4. A couple of people "liked" it, no one else disputed or agreed with it. IMHO, the question as asked is not answerable. Given two identical Commandos, one being ridden by someone weighting less than 150lbs and the other weighing more than 300lbs, the engine with the heavier rider is doing much more work and getting hotter at any RPM.

I think it's fair to say that people are still trying to answer the question asked.

Glen

WZ507 said:

You can't see your weed whip running your Norton to 30 mph eh?

We are obviously at loggerheads on this one and therefore I’ll politely agree to disagree. The OP question requires a pile of qualifiers to even begin talking about the subject, and perhaps that is where our difference of opinions arises. In considering this question I’m thinking more about death by fire (thermal issues), whereas I believe you are thinking more about an engine running under moderate load and death via metal fatigue (case, crank, rod, bearings, etc). These different viewpoints would certainly have us talking apples and oranges, since I’m envisioning tortuous operating conditions and melt-downs, whereas you’re perhaps just wanting to run an engine comfortably at higher rpm where load is well balanced with rpm.

The reason I’m harping on speed and gearing is because over gearing and the various exacerbating road conditions encountered, e.g., steep grades, strong headwinds, high temperatures could all give rise to challenging thermal operating conditions, and once power demands (heat generation) exceed cooling capacity the end is just around the corner.

Just as you can’t accept the cubic factor relating power requirements to speed changes, it’s difficult for me to accept how you can discount differences in sprocket size when you suggest that “differential between the different sprocket sizes, for the issues caused by the required power” are not “the decisive limiting factor in the topic of this thread”. I guess we can view the same information, yet envision totally different outcomes.

I look at the following plot and I see sprocket size being a huge consideration. For example, at 5000 rpm comparing bike speed of 19T to the 23T is a very significant difference (79 mph vs 95 mph). Hopefully you’ll consider the rpm and speed ranges cited relevant to the conversation. Applying the cubic factor to this speed differential indicates that the higher geared bike has to provide 1.74X the power (and heat) of the lower geared bike to operate at the same engine speed of 5000 rpm. To me that could clearly be a decisive limiting factor, especially in light of the exacerbating factors cited above, as they relate to thermal induced failure.

View attachment 105928

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Indeed we are at loggerheads! I don’t think we are actually disagreeing as much as we are looking at something differently.

You are 100%… I basically think a Norton, or indeed any old Brit, is more at risk from mechanical failure induced by excessive rpm than it is from thermal loading, as I tried to explain in my previous response to Glen.

But that’s probably because most, if not all, of my impressive record of blow ups has been caused by that !

In fact, when I was racing I learnt that engines built to produce the same, or more, power at lower rpms always outlasted engines producing the same or less power at higher rpms.

Anyway, I don’t think either of us are answering the OPs question, so I’m also happy to agree to disagree and move on.

Fast Eddie said:

At a given rpm the engine is producing a given BHP.

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Only if the throttle opening ie the air/fuel supply is the same. Hence why the gearing comes into it, 5000 rpm at 80mph will require a lower throttle opening to maintain the 80mph than 5000 at 100mph. So the 5000 rpms are not equal as the fuel/air supply is different at the different speeds. Mainly due to the increase in wind resistance at the higher speed.

For an approximate check of the 8x power for 2x speed ratio, one could look at the cbr 250 and cbr1000rr.
The 250 put about 22 hp to the rear wheel on dyno. The stock CBR 1000rr does about 170 rwhp. The top speed of the CBR1000rr is roughly double that of the 250 and the rwhp is very close to 8x.
The ratio seems to work for that example.

Glen

Fast Eddie said:

Indeed we are at loggerheads! I don’t think we are actually disagreeing as much as we are looking at something differently.

You are 100%… I basically think a Norton, or indeed any old Brit, is more at risk from mechanical failure induced by excessive rpm than it is from thermal loading, as I tried to explain in my previous response to Glen.

But that’s probably because most, if not all, of my impressive record of blow ups has been caused by that !

In fact, when I was racing I learnt that engines built to produce the same, or more, power at lower rpms always outlasted engines producing the same or less power at higher rpms.

Anyway, I don’t think either of us are answering the OPs question, so I’m also happy to agree to disagree and move on.

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I don't think we are off topic at all.
For those who feel that their post on the subject was definitive and all else is belly button lint, - this is a discussion, not a true or false quiz!
I have learned something and also remembered some things that might add to the discussion.
Re blowing things up with methods other than high rpm-
A group of us witnessed this firsthand recently on the ride up to the annual Vintage bike rally held in Winthrop, WA.
This ride involves some fairly steep and short grades plus some gentle but long grades.
On one of the long gentle grades the bike in front of me, a plunger 650 BSA Golden Flash, suddenly puffed out a cloud of blue smoke and stopped. We were doing about 55 mph at the time.
His revs were not high as he was pootling along in top gear. This is one of those grades that looks level, but throttle response tells you that it's not.
Most of the group pulled over to help the fellow with the seized up Golden Flash.
One member of the group rode putted by on his Vincent Comet at maybe 50 mph. He gave us all a quizzical look, as if to say " Why on earth are you stopping here"
About 500 yards up the road his bike puffed out blue smoke just like the Golden Flash had half a minute earlier.
Then it seized hard.
Lots of damage on the Comet, not as much on the BSA.
One of the things that interested me was the amount of smoke coming off the engine castings, cylinder, head everywhere. The temp must spike very high right when the seizure occurs as all of the accumulated leaked oil vapourizes almost instantly. So in each case there were two clouds of smoke, one from the engine exterior and one from inside the engine, exiting the silencer.

Glen

worntorn said:

I don't think we are off topic at all.
For those who feel that their post on the subject was definitive and all else is belly button lint, - this is a discussion, not a true or false quiz!
I have learned something and also remembered some things that might add to the discussion.
Re blowing things up with methods other than high rpm-
A group of us witnessed this firsthand recently on the ride up to the annual Vintage bike rally held in Winthrop, WA.
This ride involves some fairly steep and short grades plus some gentle but long grades.
On one of the long gentle grades the bike in front of me, a plunger 650 BSA Golden Flash, suddenly puffed out a cloud of blue smoke and stopped. We were doing about 55 mph at the time.
His revs were not high as he was pootling along in top gear. This is one of those grades that looks level, but throttle response tells you that it's not.
Most of the group pulled over to help the fellow with the seized up Golden Flash.
One member of the group rode putted by on his Vincent Comet at maybe 50 mph. He gave us all a quizzical look, as if to say " Why on earth are you stopping here"
About 500 yards up the road his bike puffed out blue smoke just like the Golden Flash had half a minute earlier.
Then it seized hard.
Lots of damage on the Comet, not as much on the BSA.
One of the things that interested me was the amount of smoke coming off the engine castings, cylinder, head everywhere. The temp must spike very high right when the seizure occurs as all of the accumulated leaked oil vapourizes almost instantly. So in each case there were two clouds of smoke, one from the engine exterior and one from inside the engine, exiting the silencer.

Glen

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How long are these gentle gradients? And was it a roasting hot day?

worntorn said:

I don't think we are off topic at all.
For those who feel that their post on the subject was definitive and all else is belly button lint, - this is a discussion, not a true or false quiz!
I have learned something and also remembered some things that might add to the discussion.
Re blowing things up with methods other than high rpm-
A group of us witnessed this firsthand recently on the ride up to the annual Vintage bike rally held in Winthrop, WA.
This ride involves some fairly steep and short grades plus some gentle but long grades.
On one of the long gentle grades the bike in front of me, a plunger 650 BSA Golden Flash, suddenly puffed out a cloud of blue smoke and stopped. We were doing about 55 mph at the time.
His revs were not high as he was pootling along in top gear. This is one of those grades that looks level, but throttle response tells you that it's not.
Most of the group pulled over to help the fellow with the seized up Golden Flash.
One member of the group rode putted by on his Vincent Comet at maybe 50 mph. He gave us all a quizzical look, as if to say " Why on earth are you stopping here"
About 500 yards up the road his bike puffed out blue smoke just like the Golden Flash had half a minute earlier.
Then it seized hard.
Lots of damage on the Comet, not as much on the BSA.
One of the things that interested me was the amount of smoke coming off the engine castings, cylinder, head everywhere. The temp must spike very high right when the seizure occurs as all of the accumulated leaked oil vapourizes almost instantly. So in each case there were two clouds of smoke, one from the engine exterior and one from inside the engine, exiting the silencer.

Glen

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Glen, how much power were these producing as they ‘putted by’ ?

With respect I’d say these two gentleman’s steeds seized due to heat caused by reasons other than power.

marshg246 said:

Back when we were trying to answer the question asked, I wrote post #4. A couple of people "liked" it, no one else disputed or agreed with it. IMHO, the question as asked is not answerable. Given two identical Commandos, one being ridden by someone weighing less than 150 lbs and the other weighing more than 300 lbs, the engine with the heavier rider is doing much more work and getting hotter at any RPM.

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It would be definitely true if going uphill, but on the perfect level, if the fat guy presents the same frontal area as the thin guy (unlikely I realize, but people come in all shapes and sizes) then the work would be the same given equalization of tire deformation via tire pressure.

baz said:

How long are these gentle gradients? And was it a roasting hot day?

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Yes warm day and long gradients.
In this case some higher rpm (shift down and let them rev!) would likely have avoided both seizures.

Glen