Max sustainable RPM?

[Fast Eddie]

I suppose the gearing might have effect on longevity at a very high RPM.
This is sort of hypothetical as no one operates this way....no wait I know of one fellow who did and destroyed his Vincent.
Anyway, the question is what is the safe maximum sustainable rpm, presumably one that you could drone on for hours and hours at.
If you had a 20 tooth bike and ran it at 5500 rpm for hours, you would be running at about 89 mph or so. This would be too much for my aging body, and I'm not sure the engine would like it either.
If you insisted on running at the same 5500 rpm with 23 teeth that would be about 103 mph. I'm definitely out on this one
The 89 mph @ 5500 is actually going to be a little easier on the engine than flogging it with a 23 tooth at 102 mph, provided the thing had enough power to do the 102. You are taking a lot more horsepower off at 5500 with the 23 than at 5500 with a 20 tooth. The call it "horsepower hours" in tractor world.

But only a few very strange people would ever ride this way.
Most folks cruise at a speed that suits them, not an rpm. They put a big sprocket on to drop the revs for a given speed and that lower rpm should help the motor live a bit longer, as long as it is not being lugged!

Glen

I’m still not convinced Glen. Even though I understand your point about less power being used, I’m not convinced that this is the decisive destructive force.

Power is a significant factor in max possible RPM in some engines, ie drag race engines blow up in seconds when they get it wrong.

But this thread is about max sustainable rpm, I believe (currently at least) that the biggest factor in this question is the destructive forces caused by rpm and lack of balance etc.

There are dozens of factors in play, temperature on the day, amount of wind to provide cooling, type of oil used, age and condition of oil, carb settings and consequent affect on heat, overall engine condition, etc, etc.

But I believe the intent of this thread is to ask, if all else is equal, what is the max sustainable rpm of a Commando engine.

I cannot see that having a 20 or 22 tooth sprocket is going to fundamentally affect that personally.

Its obviously a complicated question though, cos 6 pages in we still haven’t got an answer !

 

[Matchless]

Hi Michael,
I'm not quite sure of the logic you are using when you say it should be able to run at peak torque (5000rpm) all day?
Certainly that should be the goal of an engine designer but I’m not sure if it applies to our Nortons.
Peak torque is a function of geometry, tuning etc where maximum sustainable rpm is a function of materials and construction. The two are totally different things.
I think this interesting discussion can really be defined as determining the limits of a 360’ four stroke engine (without counterbalance shafts) in both size and horsepower output. Simply put, considering the materials and construction techniques available, an 830cc parallel twin engine is difficult to make reliable and have acceptable vibration above a certain rpm. Demands for power output in the Norton’s case has put the maximum torque rpm at the upper limit of it’s continuously reliable rpm range. This would be disastrous for say a truck engine or even an underpowered car engine that in everyday use spends a lot of it’s time approaching maximum engine output in normal operation but could be ‘got away with’ on a high power motorcycle.
I guess Norton could justify this shortcoming at the time by stating that because of the high power output of the engine the average rider would have no need to spend extended times at high rpms, simply using the power for short bursts.
This would probably be the case on the slower British roads in the 60s before freeways were built. Not the case with bikes ridden in Europe or America.
Out of interest, one of my other strange passions in my 1965 Humber Imperial car. It has a beautifully smooth and powerful (for its day) engine and is capable of quite high speed touring but it feels grossly under-geared (19mph/1000 rpm, max rpm 5300). It has oodles of torque and could easily accept higher gearing but I guess in its day there were not that many roads available to make use of a better ratio (Note: overdrive was available on manual versions). It feels a little busy at 110km on Australian freeways although it is capable of over 160km/hr
So, in brief, our motorbikes are (excellent) compromises. I’m sure Norton would have liked to have made the motor more reliable and rideable at higher rpms to increase power and compete with the Japanese onslaught but cost and basic geometry and harmonics were against them.
I should point out that where I live in Australia the roads are fast and mostly remote from police. Potholes and wildlife being our biggest problem. Riding my Commando, especially in the company of friends on modern ultra sports bikes developing multiples of my horsepower the speed I ride at is a constant compromise between what I want to do, what is reasonably safe and what I think is acceptable to the motor. I tends to settle for around 5000rpm or approximately 80mph. I suspect the bike would be happier below 4500, but hell, it’s a pampered bit of Pomie iron, even it occasionally has to work for a living
Alan

Sorry to derail the thread, but my first car was a 1965 Humber Imperial. I owned it for five years and loved it. Maybe Jerry could start a Rootes group section.
Sorry, I'll get me coat.

 

[hhh]

If I had a road going Commando, I would spend on gears before I would buy a hot cam or raise comp. ratio. I tried racing with thec standard box, it was absolutely hopeless. The new gearset cot $700.

I disagree. Raising the compression ratio of an 850 is the single best and cheapest improvement you can make for both power throughout the rev range and economy.

 

[worntorn]

I’m still not convinced Glen. Even though I understand your point about less power being used, I’m not convinced that this is the decisive destructive force.

Power is a significant factor in max possible RPM in some engines, ie drag race engines blow up in seconds when they get it wrong.

But this thread is about max sustainable rpm, I believe (currently at least) that the biggest factor in this question is the destructive forces caused by rpm and lack of balance etc.

There are dozens of factors in play, temperature on the day, amount of wind to provide cooling, type of oil used, age and condition of oil, carb settings and consequent affect on heat, overall engine condition, etc, etc.

But I believe the intent of this thread is to ask, if all else is equal, what is the max sustainable rpm of a Commando engine.

I cannot see that having a 20 or 22 tooth sprocket is going to fundamentally affect that personally.

Its obviously a complicated question though, cos 6 pages in we still haven’t got an answer !

For sure the main cause of destruction is high rpm x time. That's what Phil Vincent was shocked to learn about their v twins in1954 during the Picador testing.
Keeping the rpm and time the same but drawing off 50 hp rather than 40 hp will add extra heat to the mix. Heat can be a big problem with these engines, especially when running at maximum sustainable rpm , whatever that number is.

Glen

 

[WZ507]

I’m still not convinced Glen. Even though I understand your point about less power being used, I’m not convinced that this is the decisive destructive force.

Power is a significant factor in max possible RPM in some engines, ie drag race engines blow up in seconds when they get it wrong.

But this thread is about max sustainable rpm, I believe (currently at least) that the biggest factor in this question is the destructive forces caused by rpm and lack of balance etc.

There are dozens of factors in play, temperature on the day, amount of wind to provide cooling, type of oil used, age and condition of oil, carb settings and consequent affect on heat, overall engine condition, etc, etc.

But I believe the intent of this thread is to ask, if all else is equal, what is the max sustainable rpm of a Commando engine.

I cannot see that having a 20 or 22 tooth sprocket is going to fundamentally affect that personally.

Its obviously a complicated question though, cos 6 pages in we still haven’t got an answer !

I believe it would be difficult to overstate the difference in power required to operate a Norton engine running 5500 rpm at 89 mph vs 102 mph. As we’ve discussed many times in the past, the HP required to increase speed scales as the cube of the change in speed. In this case, the HP required to increase speed from 89 mph to 102 mph would be 1.50X, and that is nothing to take lightly. Now add an additional 10, or 20, or 30 mph headwind to the equation and you are now running wide open and I don’t see that ending very well for the user or the equipment. Or restated how long could a Norton run at WOT at 5500 rpm on a brake dyno? I’m feeling fairly optimistic that it would melt down long before it would shake itself apart (would only take a few detonation events before you could kiss it all goodbye).

Make what you will of the above, but it should at least raise an eyebrow regarding gearing, speed, power, load and thermal considerations.

Back to the initial question posed in this thread. As another reference point, Comnoz bike (large displacement, fuel-injection, billet crank, 70+ HP) geared at ~ 3.93:1, ran ~ 80 mph @ 4100+ rpm. I recall him posting that he was uncomfortable running continuously above 4200 rpm. I realize this is not your average Commando, but nevertheless another data point for us.

In light of the above I found it interesting that the OP, milfordite, offered the following a few pages back regarding operating at 4000-4200 rpm, which sounds quite reasonable (assuming his 22T sprocket provides 4.19 gearing).

As the OP, I'm glad to see all of the opinions! As I said, for a stock bike, I still think 5K is a reasonable limit for holding it there between gas stops. That said, I still run mine at 4 or 4,200 rpm all day with a 22 tooth sprocket. I use Royal Purple HPS 20/50 on trips, especially when it's going to be hot. When I rode to the Oregon rally years ago, it was 105 degrees F coming up the coast, in Oregon , and in Kansas on the way home. Glad to have the extra protection!

 

[ashman]

I don't know when I am cruising on a open road say 60 mph when I want to overtake a slower car or truck I just open up the throttle without dropping it back a gear, I just use the torque of the motor you be way ahead than by the time you drop it down and open it up, my Norton pulls pretty quick in top gear but then I am not running a stock motor or restricted exhaust but I am running stock gearing and even up in the twisties I very rarely drop it down a gear, only if it's a very tight corner but most of the Ranges I ride up in I know where I need to drop in a lower gear and that be as I come into the corner, putting the power on while in the corner is how a Featherbed frame works the best.
But even on the open roads and with the work done to my motor very rarely I need to push it up to Max RPMs so 5000 and 5500 is always plenty and will do it all day, my motor will happily freely run to 8000 RPMs but why would I where most of its torque is between 4000 and 6500 RPMs, but 4000 to 5500 my motor will sit on that all day without any problems but it's great knowing it will pull a lot more and love it.
But as Glen said its the body that can't sustain high RPMs unless your young and silly and want a death wish.

 

[worntorn]

I believe it would be difficult to overstate the difference in power required to operate a Norton engine running 5500 rpm at 89 mph vs 102 mph. As we’ve discussed many times in the past, the HP required to increase speed scales as the cube of the change in speed. In this case, the HP required to increase speed from 89 mph to 102 mph would be 1.50X, and that is nothing to take lightly. Now add an additional 10, or 20, or 30 mph headwind to the equation and you are now running wide open and I don’t see that ending very well for the user or the equipment. Or restated how long could a Norton run at WOT at 5500 rpm on a brake dyno? I’m feeling fairly optimistic that it would melt down long before it would shake itself apart (would only take a few detonation events before you could kiss it all goodbye).

Make what you will of the above, but it should at least raise an eyebrow regarding gearing, speed, power, load and thermal considerations.

Back to the initial question posed in this thread. As another reference point, Comnoz bike (large displacement, fuel-injection, billet crank, 70+ HP) geared at ~ 3.93:1, ran ~ 80 mph @ 4100+ rpm. I recall him posting that he was uncomfortable running continuously above 4200 rpm. I realize this is not your average Commando, but nevertheless another data point for us.

In light of the above I found it interesting that the OP, milfordite, offered the following a few pages back regarding operating at 4000-4200 rpm, which sounds quite reasonable (assuming his 22T sprocket provides 4.19 gearing).

Thanks for reminding me of the relationship between speed and power. I was thinking that horsepower requirements increases at the square of velocity, not cubed.
Yes that would have a great effect on heat.
Another thought on rpm and engine damage- sometimes an engine is better off producing x horsepower at a higher rpm than at a lower rpm. An example of this is climbing a steepish hill two up in top gear with the engine lugging along at a given speed, say 60 mph. Drop down into third and maintain the same speed. Now the rpm is up, but the engine is running much cooler.
We have a clubmate who hates to allow his bikes to rev a bit. He feels he is being gentle by short shifting and getting into top gear asap. He does not believe in downshifting when climbing hills.
He has seized up 3 engines in the last 5 years, always on a hill in hot weather lugging away.
High rpm is not the only way to blow up an engine.

Glen

 

[Matt Spencer]

INDEED ,

The Exclusive Breathern used to overhaul the local ohc Ciotina engines ( Pinto )

The 1600s got better milage / life , as they had to rev to move in the hill country ,
whereas the old farts'd leave the two litres in top , and slog along . Basically .

So ' Free running ' em , like whilst running in , as in Minimum throttle rpms , Changeing Gear , theres less ' shock load ' .
Intresting with a Vacum Seconday throat acceleration jet , Weber 36 Fiat Carb , on a incline as the load comes on , the vacum increases
and the automatic vacum enrichment ( at speed ) gives a power increase . If your over 80 to hold the speed .

further in and so far ,

A steel L R tank , your less aware of the racket , so revem unworriedly .
hereas , with a little glass Hi rider Tank , in comparison , you think it must be falling apart ,
But if you rev it , the vroom covers the rattle . Or maybe it dosnt rattle when you Go Vroom ! ? .

 

[WZ507]

Thanks for reminding me of the relationship between speed and power. I was thinking that horsepower requirements increases at the square of velocity, not cubed.
Yes that would have a great effect on heat.

Glen

You had the correct answer when you were thinking about the squared relationship, just not the whole answer. The wind resistance does in fact go up as the square of the change in speed. But there is slightly more to it than that. Let’s say we are doubling speed so an increase of 2X. To account for wind resistance, we need to apply the squared relationship, so 2^2 = 4. So, power must increase by 4X to accommodate the wind resistance due to doubling speed. Then there remains one final detail, i.e., if speed doubles we are traveling a given distance in ½ the time. To halve the time requires 2X the HP (power = work/time). In addition to the 4X power increase required to account for wind resistance, we have now added an additional factor of 2X to account for halving the time (doubling speed), for a total power increase of 8X required for doubling speed.

Hopefully the foregoing provides sufficient background to understand why the cubed relationship exists between changes in speed and the power required to achieve said change in speed. If speed changes by a factor of 2X, then power must increase as the cube of the change, i.e., 2^3 = 8, so an 8X HP increase to increase speed by 2X.

The above should also reinforce why LSR guys are generally more concerned with reducing Cd than gaining HP, since significant HP increases alone, on well-prepared engines, are so challenging to make and pay such small dividends relative to reductions in air drag.

 

[worntorn]

Tom Mellor agrees!

 

[KiwiShane]

NAKED....

 

[Fast Eddie]

You had the correct answer when you were thinking about the squared relationship, just not the whole answer. The wind resistance does in fact go up as the square of the change in speed. But there is slightly more to it than that. Let’s say we are doubling speed so an increase of 2X. To account for wind resistance, we need to apply the squared relationship, so 2^2 = 4. So, power must increase by 4X to accommodate the wind resistance due to doubling speed. Then there remains one final detail, i.e., if speed doubles we are traveling a given distance in ½ the time. To halve the time requires 2X the HP (power = work/time). In addition to the 4X power increase required to account for wind resistance, we have now added an additional factor of 2X to account for halving the time (doubling speed), for a total power increase of 8X required for doubling speed.

Hopefully the foregoing provides sufficient background to understand why the cubed relationship exists between changes in speed and the power required to achieve said change in speed. If speed changes by a factor of 2X, then power must increase as the cube of the change, i.e., 2^3 = 8, so an 8X HP increase to increase speed by 2X.

The above should also reinforce why LSR guys are generally more concerned with reducing Cd than gaining HP, since significant HP increases alone, on well-prepared engines, are so challenging to make and pay such small dividends relative to reductions in air drag.

Reading that does make me think that I had under estimated the effects of outright speed vs rpm somewhat, I concede.

Even so, with respect guys, we’re not talking LSR and we’re not talking 200mph.

We are talking Norton Commandos, that will do a little over 100mph, and we are talking about the different gearing that might be used, and actually there is only a few mph difference between each sprocket size, even at max rpm, so much less at sustained cruising speed.

So I still argue that this is ‘just’ another factor, along with the many others already mentioned, that can affect an engines ability to blow up, and is therefore not the decisive destructive force which gives the answer to the question posed in this thread.

We should still be able to take a good Commando engine and ascertain the point at which its sustained use rpm becomes unsafe, ergo determine its maximum sustainable rpm. And that will only be affected a small degree either way with things like realistic / available sprocket choices.

 

[Fast Eddie]

To try and answer the OPs question, IIRC the old adage from back in the day for a ‘sensible’ cruising speed was 2/3s of peak rpm.

I know this is safe though, a T140 red lines at 7,000 so the cruising speed should be at 4,666 but I’ve habitually used 5,000 on them without blow ups.

I know my Commando feels sweet at around a sustained 4,500 (which on my bike with current gearing is as fast as I’d wanna cruise) and I would have no hesitation using that as a prolonged cruising speed.

Now that my Commando has been demoted (or promoted, depending on your point of view) from regular track use to road use only, I may even get to try some decent sustained cruising on it this year

 

[Mart UK]

Definitely "promoted".

 

[Grandad]

old adage from back in the day for a ‘sensible’ cruising speed was 2/3s of peak rpm.

another adage I've used for bikes and cars ..... the most comfortable speed is when the speedo point to 12 - o - clock

 

[Fast Eddie]

another adage I've used for bikes and cars ..... the most comfortable speed is when the speedo point to 12 - o - clock

That’s interesting, on 3 of my old bikes in regular service, that’s 80mph, which about right for me !

 

[Triton Thrasher]

He has seized up 3 engines in the last 5 years, always on a hill in hot weather lugging away.
High rpm is not the only way to blow up an engine.

A common problem with old men on Triumph twins (not so far off topic as Imperials, even if they’re mint). It is unwise to make them pull for long at under about 3,200 rpm.

Some successful Triumph 650/750 twin racing people said the crankshaft breaks after 7-12 hours at the red line 7,000. By not doing any physics or engineering calculations, I deduce that if kept under 6,000 rpm, the Triumph crank won’t break.

Through experience, I know the pre-1959 bolted-up crankshaft can do 5,000 all day.

 

[Fast Eddie]

You had the correct answer when you were thinking about the squared relationship, just not the whole answer. The wind resistance does in fact go up as the square of the change in speed. But there is slightly more to it than that. Let’s say we are doubling speed so an increase of 2X. To account for wind resistance, we need to apply the squared relationship, so 2^2 = 4. So, power must increase by 4X to accommodate the wind resistance due to doubling speed. Then there remains one final detail, i.e., if speed doubles we are traveling a given distance in ½ the time. To halve the time requires 2X the HP (power = work/time). In addition to the 4X power increase required to account for wind resistance, we have now added an additional factor of 2X to account for halving the time (doubling speed), for a total power increase of 8X required for doubling speed.

Hopefully the foregoing provides sufficient background to understand why the cubed relationship exists between changes in speed and the power required to achieve said change in speed. If speed changes by a factor of 2X, then power must increase as the cube of the change, i.e., 2^3 = 8, so an 8X HP increase to increase speed by 2X.

The above should also reinforce why LSR guys are generally more concerned with reducing Cd than gaining HP, since significant HP increases alone, on well-prepared engines, are so challenging to make and pay such small dividends relative to reductions in air drag.

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 ?

 

[SteveA]

another adage I've used for bikes and cars ..... the most comfortable speed is when the speedo point to 12 - o - clock

That even works with my Ducato van!

 

[Schwany]

I can't see how "Maximum RPM" and "reliable" belong in the same sentence with the word Norton.

Speedo at 12 o'clock in 4th gear is definitely a good answer looking at a Smith's speedo running the right gearing. I don't have a tach so no clue what that would be RPM wise. 75mph is just starting to get into the cam operating range on my Norton in 4th. Not relevant though.

I have to think that a world land speed record holding Norton motor comes apart and is checked a little more frequently than the average Norton owner would like to deal with.

Gearing makes a difference building power. I don't think it changes where the power is in the RPM range though. I did not read all the wisdom spread above. I prefer my own delusions.