Heavy flywheel equals more torque? (2014)

Al-otment said:

Saying torque is generated in the combustion chamber is the same as saying a canon produces torque when it fires a canon ball. .

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Correct.

And the amount of work/torque produced in an internal combustion otto cycle engine
can be calculated from the pressure diagram that happens in the cylinder, acting on that piston.

Its tough to find a good BMEP Otto Cycle diagram, but the work/torque supplied is the area INSIDE the 4 cornered area.


BMEP = Brake Mean Effective Pressure (inside the cylinder).

Flywheels and conrods and stroke and gearboxes and primary drives and clutches and chains and back wheels clearly all transmit or (briefly) store or multiply or deliver torque, but the ultimate source of all this torque is that controlled burn in the cylinder that moves the piston, and ultimately propells the bike along.

P.S. A gearbox is a well known torque multiplier and torque reducer - simply by changing gear.
Clearly, the revs have to inversely decrease or increase to match.
Since gearboxes in themselves cannot manufacture torque...

Torque is the twisting force on the crank. If it is high at lower revs, the bike is usually easier to ride. If you have high torque and a heavy crank, you can store up energy and transfer it to the rear wheel with minimum loss of crankshaft revs. Once you have the crank spinning in first gear of a CR box, you can step up through the gears accelerating more rapidly than with a light crank which has less stored e nergy. My preference has always been for a two stroke type motor where the revs are extremely high thus the energy is stored by the combination of much higher revs combined with a lower crank weight. A two stroke revving at 12,000 RPM with a 6 speed close box is a thing of beauty as long as it doesn't encounter a wind gust. Norton Commandos cannot usually be revved consistently over 7000RPM, so the approach is different.

Rohan, I think you've assumed that work and torque are the same thing. Work is done by moving a force through a distance, torque is the twisting force on a shaft. Are they the same thing in your opinion ? With torque we are talking about a lever arm, with work - moving a mass through a distance. Is the mass the mass of the piston and it's counterbalance?

acotrel said:

A two stroke revving at 12,000 RPM with a 6 speed close box is a thing of beauty as long as it doesn't encounter a wind gust. Norton Commandos cannot usually be revved consistently over 7000RPM, so the approach is different.

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Which do you think revs higher and which one has more torque,a short stroke,light crankshaft Kawasaki H2 or a long stroke,heavy crankshaft Norton 750 ?
The Kawasaki is the higher torque engine and the lower revving (6800 rpm in stock trim) adding a heavy flywheel would simply smooth the power pulses and as before increase kinetic energy.
Examples can be made with anything,put a light flywheel on that Lister engine down in the cow shed and it would be a pig because of lost,well you know.........

All I know is that my 850 actually works acceptably and I never expected that. To my mind it is theoretically wrong for a race motor. I do notice one thing though, it doesn't seem to matter what overall gearing I use, it doesn't seem to change the acceleration rate much, only the top speed.

Time Warp said:

Which do you think revs higher and which one has more torque,a short stroke,light crankshaft Kawasaki H2 or a long stroke,heavy crankshaft Norton 750 ?

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Kawasaki claimed 75hp and a max of 7500 rpm for the H2,
and Nortons claimed 65 hp and a max of 7000 rpm for a Combat.
Given they had not too dissimilar top speeds, on a good day, that is actually not very different ??

Given too that owners mentioned that the centre cylinder was usually kackered by 10,000 miles,
and Combats without upgraded bearings did about the same mileage, they weren't too different.
But it was easier to change bearings in the Combat !! - and better ones were around the corner !

??

Torque is a twisting force.
But applied to anything over time, it produces work, and that can be calculated out, in a vast chain of related equations....

Rohan said:

Time Warp said:

Which do you think revs higher and which one has more torque,a short stroke,light crankshaft Kawasaki H2 or a long stroke,heavy crankshaft Norton 750 ?

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Kawasaki claimed 75hp and a max of 7500 rpm for the H2,
and Nortons claimed 65 hp and a max of 7000 rpm for a Combat.
Given they had not too dissimilar top speeds, on a good day, that is actually not very different ??

Given too that owners mentioned that the centre cylinder was usually kackered by 10,000 miles,
and Combats without upgraded bearings did about the same mileage, they weren't too different.
But it was easier to change bearings in the Combat !! - and better ones were around the corner !

??

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The unlikely two stroke still makes more torque.
You read to many books or internet myths,like the CBX that I maintained for the best part of a decade including revving it to 11000 rpm I am a long time triple owner and tuner (since 1980)
Even the multi bike shoot out L.A.B. posted stated the dynoed hp and revs,no 75 no 7500 rpm but I knew that based on actual experience.
Great thread though,it won't stop until everyone agrees that a heavy flywheel adds torque,even on a RS125. :roll: :lol:

Time Warp said:

The unlikely two stroke still makes more torque.

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It fires EVERY rev !!
Even though the fire inside is not as strong, twice as often soon adds up...

Time Warp said:

You read to many books or internet myths

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I know several folks that owned them, or have them in their collections.
It used to be easy to find them with the centre cylinder dead.

Its a heck of a job if the bearings need redoing, a lot of them on the crank.
And 2 smokes don't supply a lot of oil to the bearings....

Time Warp said:

Great thread though,it won't stop until everyone agrees that a heavy flywheel adds torque,even on a RS125. :

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In that case we are wasting our time here.
A heavy flywheel allows good use of the torque - ask any HD owner.
(Incredibly heavy flywheels, so they can open the throttle HARD from really slow revs.)

But as every steam train driver knows, its how strong the fire burns that produces the torque at the driving wheels.
All the bits-n-bobs in between just get it there...

What a remarkable conversation this has been and still is! When Al-otment drafted his initial innocent inquiry regarding how a slightly lighter or heavier flywheel might affect drivability of a Norton, could he ever have possibly imagined where that inquiry could lead?

To keep everybody happy (of course we know there is no way to keep everybody happy) I'll be TimeWarp's 1st conversion and agree that

"a heavy flywheel adds torque even on a RS125. ".

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There, the 1st (and maybe the last) convert has spoken.

Perhaps revisiting the derivation of torque and HP might cast the subject in a slightly different light. Because Watt's figure of 33,000 foot pounds per minute per horsepower has survived through both the 19th and 20th centuries, we generally measure the torque of an engine, and in turn calculate HP by bringing in the element of time, via engine rpm, i.e., work done per some time interval = power. (Apologies to the people that talk SI units of measure, us Yanks are still stuck to measurement units from past centuries).

In the most general terms, HP = [torque * rpm]/5252, thus at 5252 rpm, HP= torque. If torque remains constant, HP increases linearly with rpm, i.e., if an engine provides constant torque, and if the engine speed doubles the HP likewise doubles.

Some may wonder where the constant 5252 comes from? Because we are dealing with angular (rotational) motion we require a means of equating linear to angular motion, i.e., equating rpm to a distance. Since we are talking in ft-lb units, angular motion is occurring at a distance of 1 ft from a center point, thus we move circumferentially around a circle having a radius of 1 ft. The circumference of said circle is 6.28 ft (2*pi*radius = 6.28). So if we take the 33,000 ft-lb/min and divide by 6.28 ft, we get the constant 5252.

So why am I wasting your time talking about derivation of HP? To point out that power is all based on torque, and torque relates to the efficiency of the combustion event in an Otto cycle engine (as Rohan points out above). All other things being equal, anything we can do to improve the efficiency of the combustion event boosts the engine's torque, e.g., improving cam timing/duration, optimizing IN/EX flow and phasing, increasing compression ratio, providing optimum shaped combustion chamber and optimum ignition timing, etc. All these things affect combustion efficiency and in turn torque. A heavy or light flywheel does not appreciably affect the combustion event, and therefore can not affect the torque the engine produces.

For me, sometimes thinking about extreme examples of a system help me better grasp concepts. To that end let's consider a pair of well and identically prepared 50 HP engines that are fit with different flywheels. Engine A has a 20 lb flywheel (a Norton?) and engine B has a 2000 lb flywheel (like a large hit & miss stationary engine). Both engines were dyno tested between 3000 and 4000 rpm, and both engines produced 35 ft-lb of torque. How can that be? Maybe the real question is "how could it not be?" The engines are identical, the only difference between them being the load they were accelerating, therefore they have to produce the same torque. The Engine A dyno run occurred almost instantaneously, because the load was light, whereas the Engine B dyno run required a dramatically extended time period to complete, due to the load being 100 fold higher. When the engines were shut off, Engine A came to a stop almost instantaneously, whereas Engine B required a dramatically extended time period to come to a full stop. Did Engine B, fit with the heavy flywheel, produce more torque in the dyno test? NO. Did the heavy flywheel of Engine B store a lot more energy than the light flywheel of Engine A, and dissipate the energy upon deceleration? YES.

I apologize for preaching to the choir here, but it is obvious from the varied responses above that we are of many minds on many of these subjects. Bringing it back around to how drivability is affected by flywheel weight - it all depends. If you want to drop the clutch at an idle and expect the bike to launch, go heavier. If you want it to feel really snappy and responsive around the torque peak I think you would be pleased with the lighter flywheel option. If you're going to be racing the XRs on a flat track, you'd have to be lucky like HD and serendipitously stumble into that magic combination of power and flywheel mass that allows you to put the power to the ground better than competitors having a 15 HP advantage but lighter flywheels.

My $0.02. Excuse me while I slip into my helmet and flack jacket as I have it on good authority that incoming shrapnel will be arriving here shortly.

" For me, sometimes thinking about extreme examples of a system help me better grasp concepts. To that end let's consider a pair of well and identically prepared 50 HP engines that are fit with different flywheels. Engine A has a 20 lb flywheel (a Norton?) and engine B has a 2000 lb flywheel (like a large hit & miss stationary engine). Both engines were dyno tested between 3000 and 4000 rpm, and both engines produced 35 ft-lb of torque. How can that be? Maybe the real question is "how could it not be?" The engines are identical, the only difference between them being the load they were accelerating, therefore they have to produce the same torque. The Engine A dyno run occurred almost instantaneously, because the load was light, whereas the Engine B dyno run required a dramatically extended time period to complete, due to the load being 100 fold higher. When the engines were shut off, Engine A came to a stop almost instantaneously, whereas Engine B required a dramatically extended time period to come to a full stop. Did Engine B, fit with the heavy flywheel, produce more torque in the dyno test? NO. Did the heavy flywheel of Engine B store a lot more energy than the light flywheel of Engine A, and dissipate the energy upon deceleration? YES. "

I think that man got a good mark in High School physics (or an entry level university physics paper) (and a teacher ??)

Nice explaination.

A very nicely explained 2 cents worth, with a true value of $$$$. Thankyou.

I studied Physics at 3rd year university level, however I now find my memory failing me. I seem to think that power is the 'time rate of doing work'. Rohan's comment which seems to equate work with torque interests me. I cannot quite reconcile it. I do know this - the experience of riding my old 500cc short stroke Triton with the light billet crank compare with my 850 commando engine Seeley are poles apart. I loved my Triton, however after riding the Seeley I've learnt that the theory which sends you towards extreme revs, big overlap cams etc. can be a real bastard, and I've got the injuries to prove it.
I think the proof of the pudding is when you come around a tight hairpin bend leaning well over and going quickly and accelerate into the next straightaway. With my old Triton, it was so savage that I tended to always run wide and get it upright before giving it heaps. With the Seeley, It tightens it's line however it is extremely smooth in it's power delivery, so while it is cranked right over I can safely pour on a heap of coals. In that situation the Seeley must beat the Triton - the Seeley always has the run. (Of course there is also a capacity difference in this comparison, however my Triton was bloody fast if geared right ).

With that H2 Kawasaki that was mentioned - always a hi- side waiting to happen. Been there, done that. You won't beat him in a straight line with a commando, so a tight twisty circuit is essential.
That photo on the right was taken at Mount Gambier in the early 70s - what is bike number 9 ?

Two engines .one 20lb flywheel and one 2000 lb flywheel at 3000 rpm will not have the same torque
what you must understand is the net amount of torque comes from the amount of btu's consumed to drive the crank, simple = 2 oz of fuel would generate heat to drive the lighter crank to say 5000 rpm..but the heaver crank to 50 rpm...so the torque is the same,,there cannot be a free lunch !

johnm said:

" For me, sometimes thinking about extreme examples of a system help me better grasp concepts. To that end let's consider a pair of well and identically prepared 50 HP engines that are fit with different flywheels. Engine A has a 20 lb flywheel (a Norton?) and engine B has a 2000 lb flywheel (like a large hit & miss stationary engine). Both engines were dyno tested between 3000 and 4000 rpm, and both engines produced 35 ft-lb of torque. How can that be? Maybe the real question is "how could it not be?" The engines are identical, the only difference between them being the load they were accelerating, therefore they have to produce the same torque. The Engine A dyno run occurred almost instantaneously, because the load was light, whereas the Engine B dyno run required a dramatically extended time period to complete, due to the load being 100 fold higher. When the engines were shut off, Engine A came to a stop almost instantaneously, whereas Engine B required a dramatically extended time period to come to a full stop. Did Engine B, fit with the heavy flywheel, produce more torque in the dyno test? NO. Did the heavy flywheel of Engine B store a lot more energy than the light flywheel of Engine A, and dissipate the energy upon deceleration? YES. "

I think that man got a good mark in High School physics (or an entry level university physics paper) (and a teacher ??)

Nice explaination.

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Ugh Archimedes & Newton should be turning in their graves d/ the mixing up of averaged specific brake torque a dyno and time trials are needed to measure vs instantaneous torque that takes calculus to put a number too. Shaft toque is developed by amount of fuel heat pressure on crank in each single combustion stroke. Fly wheel only needed to store enough inertia to get next piston though compression stroke and then rider/frame luxury of helping to smooth out the piston pulses. Offset cranks in multi cylinders can get by with less or no flywheel if piston strokes arranged so some are at speed while others at reversal points. Beyond this flywheel optimal mass becomes complicated by the intended use of the engine. Flywheels are inertia storage devices so a heavy one can store up and release more energy than a lighter wheel but not as quick to store up same level of spin energy as lighter wheel so for a short time a heavy wheel can get the jump on lighter wheel but very soon after the lighter one allows faster delivery of fuel torque to forward motion. If poor throttle control and/or poor frame suspension set up for tire adhesion then heavier flywheel will be safer and easier to go faster around on, but if able to control throttle well and suspension and tire able to plant power better then lighter wheel will accelerate faster to some speed till the fuel burn just runs out. Heavy flywheels are famous benefit in slow going slow firing uses since forever in case that clarifies my opinion and choice. I like the big ole cast iron one in my stocker Combat wimp to ease its operation but not for the deeds powerful light Peel is purposed for.

john robert bould said:

Two engines .one 20lb flywheel and one 2000 lb flywheel at 3000 rpm will not have the same torque

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They will. !!! = if the engine driving both plots are IDENTICAL.

Physics say they must - allowing for parasitic losses....
A flywheel can only provide out what is fed into it (rpms).
It can only provide out what has been fed into it.
It can provide a small amount of torque evening/damping/call it what you will, however.
That is ENTIRELY why it is there...
It is only the torque evening/damping that a rider will feel any differently.
Note that a 2000 lb flywheel will be EXTREMELY slow to accelerate, and extremely slow to slow down.
As a motorcycle, it would make a good wheat harvester - see next paragraph.
That is not to say that a flywheel cannot be too light for the job.

As a very young lad, I can recall going out to farms for my father to service/repair farm machinery.
The old style wheat harvesters had a VERY heavy flywheel incorporated (500 lbs ?) , in horse/tractor drawn type harvesters.
(No engine of their own, all the power to drive it came from the ground wheel turning and driving the mechanism.)
It provided a stable rpm platform for the harvester to operate, when drawn across sometimes uneven ground.
(Don't ask me to explain how the flywheel was activated - some form of slipper clutch ?).
(And once the harvesting mechanism was out of gear, the flywheel continued to spin).

When the machine stopped, the flywheel continued to spin.
My father commented that you knew the bearings were still good if you could finish your lunch before it stopped spinning.
Its whole function was to smooth out the drive to the mechanism.
Damping the variable rpm drive, in fact.

Not motorcycle related, but flywheel related...

P.S.
I think I may have taken JRB slightly out of context here.
My parasitic losses and his explanation are preaching from the same song book,
but we arrived there via different paths...
Given long enough though, both flywheels are capable of the same rpms - if the bearings are up to it, as noted....

john robert bould said:

Two engines .one 20lb flywheel and one 2000 lb flywheel at 3000 rpm will not have the same torque
what you must understand is the net amount of torque comes from the amount of btu's consumed to drive the crank, simple = 2 oz of fuel would generate heat to drive the lighter crank to say 5000 rpm..but the heaver crank to 50 rpm...so the torque is the same,,there cannot be a free lunch !

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A major sign and symptom of faulty central nervous system is obsessive fascination with spinning objects. While in gear all the spinning items add to flywheel effect.

[video]https://www.youtube.com/watch?v=VSfTUMIEEKc[/video]

Carn't quite see this being right, When you raise a hand hammer 2 Lb above your head and strike a nail that nail react's to the force you hit it with, now hit that nail with a hammer 20 lb , differant result! Because your arm in raising the the two hammers is under going differant imputs of energy to just lift them.
The force of gravity will act at differant levels . as each hammer as differant stored energy. Mass :!:

If you dont think the flywheels are differant ,try grabing hold of each...both revolving at the same speed.....i understand your point...that once a brake is applied the driving force [pistons] will be the power imput and the flywheels will be the same torqe..but at differant speeds...agree?

Rohan said:

john robert bould said:

Two engines .one 20lb flywheel and one 2000 lb flywheel at 3000 rpm will not have the same torque

Click to expand...

They will. !!! = if the engine driving both plots are IDENTICAL.

Physics say they must - allowing for parasitic losses....
A flywheel can only provide out what is fed into it (rpms).
It can only provide out what has been fed into it.
It can provide a small amount of torque evening/damping/call it what you will, however.
That is ENTIRELY why it is there...
It is only the torque evening/damping that a rider will feel any differently.
Note that a 2000 lb flywheel will be EXTREMELY slow to accelerate, and extremely slow to slow down.
As a motorcycle, it would make a good wheat harvester - see next paragraph.
That is not to say that a flywheel cannot be too light for the job.

As a very young lad, I can recall going out to farms for my father to service/repair farm machinery.
The old style wheat harvesters had a VERY heavy flywheel incorporated (500 lbs ?) , in horse/tractor drawn type harvesters.
(No engine of their own, all the power to drive it came from the ground wheel turning and driving the mechanism.)
It provided a stable rpm platform for the harvester to operate, when drawn across sometimes uneven ground.
(Don't ask me to explain how the flywheel was activated - some form of slipper clutch ?).
(And once the harvesting mechanism was out of gear, the flywheel continued to spin).

When the machine stopped, the flywheel continued to spin.
My father commented that you knew the bearings were still good if you could finish your lunch before it stopped spinning.
Its whole function was to smooth out the drive to the mechanism.
Damping the variable rpm drive, in fact.

Not motorcycle related, but flywheel related...

P.S.
I think I may have taken JRB slightly out of context here.
My parasitic losses and his explanation are preaching from the same song book,
but we arrived there via different paths...
Given long enough though, both flywheels are capable of the same rpms - if the bearings are up to it, as noted....

john robert bould said:

Two engines .one 20lb flywheel and one 2000 lb flywheel at 3000 rpm will not have the same torque
what you must understand is the net amount of torque comes from the amount of btu's consumed to drive the crank, simple = 2 oz of fuel would generate heat to drive the lighter crank to say 5000 rpm..but the heaver crank to 50 rpm...so the torque is the same,,there cannot be a free lunch !

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john robert bould said:

..agree?

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I don't know !!
Hammers and grabbing hold of flywheels is getting off track a tad.
And a 2000 lb flywheel in a Norton is a bit of a physical impossibility too..

??

Clearly, flywheel weight has some influence on power delivery.
As near everyone has said all along, if in rather different ways.

But flywheels can't make torque on their own.
So the fire in the cylinder(s) is the ONLY provider of energy to give it energy/torque in the first place.
Open the throttle, and more torque flows to the back wheel.
Close the throttle, and less torque flows.
Couldn't be more clearcut.... ?

I have two similar BMW airheads, one has the heavy original flywheel, the other the later 'clutch carrier" one, the heavy one has a clunky gearshift, slow change and jacking effect when throttle released, complains when going over 6000 rpm. The other my race bike has a nice gear shift, no jacking effect and revs to 8000, Torque.....it will pull past the 1000cc Harleys going up the hill at the track.
I'd have one on a sidecar but otherwise I don't like heavy flywheels.
I must say I surprised when I first got a Commando how keen it was to rev, I always got the impressions that long stroke motors did not like it.