Max sustainable RPM?

Does the size of drive sprocket affect this?

I have a 23T. 80mph is about 4100rpm. On the one hand, the engine is rotating more slowly. On the other hand, it's still doing the same amount of work as with a smaller sprocket.

I happily sit at 70mph / 3600rpm. It is smoother than my modern tourer. Not sure I'd be willing to test it to do this all day though, out of respect to the old girl. :)
 
Fast Eddie said:
No it doesn’t.

It does affect the discussion about what max MPH is sustainable, but not what max rpm is sustainable.
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Or put in another way, it affects RPM at a given speed. A higher tooth sprocket will lower the RPM at a given speed.
 
Fast Eddie said:
No it doesn’t.

It does affect the discussion about what max MPH is sustainable, but not what max rpm is sustainable.
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I'm not sure that's right. 6,000 rpm is 120mph for me, vs. 100mph for smaller sprockets. My bike would be working much harder to sustain that 6,000 revs.
 
TomU said:
Or put in another way, it affects RPM at a given speed. A higher tooth sprocket will lower the RPM at a given speed.
Click to expand...
Absolutely agree.

But the question of this thread is ‘max sustainable rpm’ …
 
Mart UK said:
I'm not sure that's right. 6,000 rpm is 120mph for me, vs. 100mph for smaller sprockets. My bike would be working much harder to sustain that 6,000 revs.
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As stated above, the question of this thread is what is the maximum sustainable rpm of the engine.

Whether you have trials gearing that tops out at 60mph… or IOM gearing for 150mph… is not relevant to the actual question.

Gearing dictates, as you rightly point out, how fast you will be going at any given rpm. But has no real bearing on the engines maximum sustainable rpm.
 
Mart UK said:
Does the size of drive sprocket affect this?

I have a 23T. 80mph is about 4100rpm. On the one hand, the engine is rotating more slowly. On the other hand, it's still doing the same amount of work as with a smaller sprocket.

I happily sit at 70mph / 3600rpm. It is smoother than my modern tourer. Not sure I'd be willing to test it to do this all day though, out of respect to the old girl. :)
Click to expand...
Hi (again),,
It is my gut feeling and I will encourage people to correct me if I’m wrong, that horsepower is not so much the issue with our engine components disassociating themselves but rather, the rpm that is required to make the power. In other words, if you foolishly chose to ride your bike all day in second gear at 100km/hr on flat ground, you would still find the bike running into unreliability problems. This is contrary to high powered turbo diesels where high boost and low rpms cause problems such as piston melting and crank failure. High rpm is the enemy of 360’ twin cylinder engines! Numerous slights of hand such as isolastics can be utilised to hide the problem but the mechanical stresses still exist.
We can all speculate on what would have happened if Norton had of taken different paths in the 60s or early seventies but that did not occur. A 90’/270’ twin may have been the solution but would it have been acceptable to the buying public? Their venture into rotaries was a brave but ill fated attempt to move to the other extreme of engine design. Such a shame that a mid path between the two extremes could not have been found. Obviously multi cylinders are the simple practical answer but even that was not truly achieved with the Trident due to inadequate investment and short sightedness. The good news is that the ultimate extension of the pushrod parallel twin, our Commandos, have wonderful riding characteristics and are surprisingly competitive provided we accept their limitations.
There is no real way around the fact that our twins can never primary balance and the faster we rev them the worse the vibration and associated fatigue gets.
alan (again)
 
@Fast Eddie Why? :) I don't think you've understood me. Im asking whether the max sustainable rpm is lowered if the speed it results in is increased. To my mind, it must follow that my bike is working much harder at, say, 6,000rpm and may well not sustain it for as long as a lower geared bike without damage. All else being equal, of course.
 
Motorcycles tend to go in two different directions - cruisers and racers. It has been claimed that 'racing inproves the breed', but for road use a racer can be pretty horrible. The Gilera 4, MV4 and Honda 4 were a big step forward. But for feel-good there is nothing like an old British bike. I once built a very competitive T250 Suzuki based racer. It was on methanol,was extremely fast and handled extreemely well. I won a race with it, and sold it. At last count, the guy who bought it had won 28 races and 4 historic championships. Whenever he replaced the barrels, he just copied the port timing.
If I race, it is not about winning races. It is about the dog-fight and developing the bike. It is more fun when you are behind the 8-ball. If you ride a bike which does not look much, the other guys have more anxiety if you pass them, so they try harder - (the harder you try, the slower you go' ?
The Norton Commando was designed to compete for sales against the Honda CB750 - a decision was made and rthe Commando is what it is. - Not a bad compromise. I have ridden a new CB750 Honda - to me it was disgusting - it did not go, stop or handle. I found the same thing when I rode a Heritage Soft-taIL Harley. But If Norton had continiued to make the 650SS and Atlas, they would have gone broke quicker.
I suggest that most beginners who buy motorcycles do not know much about them. Some people only want a fashion statement. When you race, you can look pretty stupid, but some people do not know how to have fun.
 
Alan L said:
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
Click to expand...

Apologies.... Slight detour, Imperial automatic only, Super Snipe could be ordered with manual and o/d..
(and V8s were tried :) )

Now back to our scheduled programme.............
 
Alan L said:
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.
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They are different, but related. Power is made (and limited) by how much fuel charge you can move through an engine. The engine speed is a factor on how quickly you can move a fuel charge through an engine, that is to a point. At some point, no matter how fast the engine is spinning, it can't move any more fuel charge. Seems like a stock Commando, that point is 5k RPM so there's no point exceeding that even if the engine can spin faster with no undue consequences.

Now if you can move more fuel charge (i.e. Full Auto head/cam/carbs/etc), than more RPM will deliver more power. Then the limiting factor may be the cases and other mechanical components that have to support that energy
 
Mart UK said:
@Fast Eddie Why? :) I don't think you've understood me. Im asking whether the max sustainable rpm is lowered if the speed it results in is increased. To my mind, it must follow that my bike is working much harder at, say, 6,000rpm and may well not sustain it for as long as a lower geared bike without damage. All else being equal, of course.
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I’m not sure it is. At a given rpm the engine is producing a given BHP. So I do not think is it is doing more ‘work’.

It is under more load, I agree. But what impact that has on the engines safe max rpm I don’t know.

There are other factors, like the faster you’re going, the more cooling the engine will recieve.

But I think all these things are relatively minor influences and ultimately it’s the physics of all those moving parts inside the engine that determines what the safe max sustainable rpm is.

But I’m only looking at this through a layman's eyes…
 
The higher you rev them the more likely that something will crack or break - starting with the cases, the crank or rods. There are two solutions - either stronger cases & cranks or lighter pistons or all of the above. As a racer in the mid 1980s when stock cases/cranks were cracking and stronger cases weren't available I had no choice but to special order lightweight Wiseco pistons and strengthen the cases by welding on reinforcement and strengthening the crank by radiusing the PTO shaft. After that I had no failures.

Failed case
Max sustainable RPM?


Herb Becker reinforced case
Max sustainable RPM?


Cracked PTO shaft that I cut off and now use as a tool.
Max sustainable RPM?


PTO radius to prevent cracking
Max sustainable RPM?


Early attempts at lightening stock pistons
Max sustainable RPM?


Much lighter pistons I special ordered from Wiseco (my 1st pair as used in my monoshock 1985). These early pistons used the stock rods and the cylinders had to be shortened.
Max sustainable RPM?
 
The 850 produces its highest torque at 5000 but it produces just slightly less than the max all the way from 3000 to around 5500. That's why they have so much grunt.

Glen
 
With a 72% crank balance factor, my engine is dead smooth between 5,500 RPM and 7,500 RPM. It feels as though it could easily rev to 8000 RPM without breaking. However I have not lightened and polished the valve gear, and I do not regularly replace valve springs. I suspect my cam has a mild lift rate, but I do not really know.
In the 1960s there was a 650cc Triumph twin known as The Saint which was designed to be used by traffic officers. and police. It had one-piece light crank, probably with a low balance factor. If you are building a Triumph, those cranks are cheap. But a waste of money. When you ride the bike which has that crank, it feels most uninspiring.
To me, a bike which feels lumpy is always much better than one which is smooth and does not really go. The Commando seems to perform OK, but I don't think you could really use it in anger. But only idiots ride that way, anyway.
 
Mart UK said:
@Fast Eddie Why? :) I don't think you've understood me. Im asking whether the max sustainable rpm is lowered if the speed it results in is increased. To my mind, it must follow that my bike is working much harder at, say, 6,000rpm and may well not sustain it for as long as a lower geared bike without damage. All else being equal, of course.
Click to expand...
Hi Eddie,
I for one did understand you. I believe that of course wear is dependent on power produced The higher the rpm the lower the torque (loading) on any individual part but also the higher the unbalanced stresses resulting in fatigue.
Ergo, every engine design must have a sweet spot where it is best suited to make max power. Too low and shock and torsional loads on components become excessive, too high and the motor flies apart.
in short, higher power (faster) requires higher rpm, to tune a motor to make high power at low rpm (without supercharging) is difficult and requires very robust internals.
The test for your question would be to hold your bike in neutral (zero power) at a very high rpm and see how long it lasts (sufficiently cooled etc) compared to riding on the road at equivalent rpm (high power). My gut feeling is there would be not that much difference in the hours they would run for. In other words, the stresses and loads that our engine is subject to are as much a function of rotational stresses as it is power produced.
A modern super high powered multi easily produces reliable power with very small engine components because the vibrational fatigue is not present. Back to basics, power is a function of torque times angular velocity.
Im not sure if I’ve explained anything😵‍💫
alan
 
worntorn said:
The 850 produces its highest torque at 5000 but it produces just slightly less than the max all the way from 3000 to around 5500. That's why they have so much grunt.

Glen
Click to expand...
If you are ever in Benalla, I would let you ride my Seeley 850 on Winton Raceway. You would get a real laugh out of it. When you ride it from a standstill, it just lurches away until it gets to 5,500 RPM. With the 4 speed close box, Ive revved it to 6000 RPM to get it going in a clutch start. The 6 speed box should be much better. I still need to try that. Since Covid, everything has gone stupid at Winton.
 
Fast Eddie said:
As stated above, the question of this thread is what is the maximum sustainable rpm of the engine.

Whether you have trials gearing that tops out at 60mph… or IOM gearing for 150mph… is not relevant to the actual question.

Gearing dictates, as you rightly point out, how fast you will be going at any given rpm. But has no real bearing on the engines maximum sustainable rpm.
Click to expand...
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
 
Alan L said:
Hi Eddie,
I for one did understand you. I believe that of course wear is dependent on power produced The higher the rpm the lower the torque (loading) on any individual part but also the higher the unbalanced stresses resulting in fatigue.
Ergo, every engine design must have a sweet spot where it is best suited to make max power. Too low and shock and torsional loads on components become excessive, too high and the motor flies apart.
in short, higher power (faster) requires higher rpm, to tune a motor to make high power at low rpm (without supercharging) is difficult and requires very robust internals.
The test for your question would be to hold your bike in neutral (zero power) at a very high rpm and see how long it lasts (sufficiently cooled etc) compared to riding on the road at equivalent rpm (high power). My gut feeling is there would be not that much difference in the hours they would run for. In other words, the stresses and loads that our engine is subject to are as much a function of rotational stresses as it is power produced.
A modern super high powered multi easily produces reliable power with very small engine components because the vibrational fatigue is not present. Back to basics, power is a function of torque times angular velocity.
Im not sure if I’ve explained anything😵‍💫
alan
Click to expand...
The only thing which stops a Commando from revving higher is the crank balance factor. The cam and the exhaust system determine where maximum torque occurs. If you fit a radical race cam and megaphione exhausts, maximum torque will occur very high in the rev range, but the situation becomes self-defeating. With any 4 stroke motor, top end is never a problem as long as the motor stasys together, but when you have everything too high in the rev-rane, the bike becomes unrideable. It then becomes faster to go slower in corners.
 
worntorn said:
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
Click to expand...
With a road bike, a lot depends on where you use it the most. When I had road bikes, they were always low geared, because I did not do much highway riding, I was just usually around the town. On a highway, you ride at higher speeds for longer periods. But you often need to pass trucks. It is useful, if you can change down half a gear rather than the full step from top back to third, and that is whem maximum revs becomes an issue. With close ratios, it is easier to keep your speeds high, and acceleration is better.
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.
 
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