Heavy flywheel equals more torque? (2014)

Hi Knut,

What I have in mind here is that with greater impulses emanating from an engine with a lighter crankshaft there is greater elastic strain of all components down stream including the primary and secondary drive chains and belts, frame, wheel and tires. I am not focusing on torsional displacement but all strain. With each impulse or cycle there's a loss which results in heat. Taking it to a limit, think of a perfect and precise torque delivery system without impulse; it would induce one strain event while accelerating and one strain event while decelerating. As you point out, for our les than perfect Nortons, this is occurring around 100 times per second. Furthermore, as comnoz points out, the engine will vibrate more with a lighter flywheel and this must be constrained by the frame and suspension which "strains" and absorbs the motion and energy.

I hope I have responded to your question.

Dances with Shrapnel said:

My hunch is the slight increased net in indicated steady state power of a heavier flywheel is what they are after with land speed attempts.
Still curious as to why the net difference in indicated power.

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Who says there is a repeatable difference in indicated power as measured on a dynamometer and under constant environmental conditions?
If the crankshaft stiffness (both in bending and torsional stiffness) remains constant, there should be none.

-Knut

So what does a big flywheel do in a drag race?

If a lighter flywheel and a heavier flywheel are both balanced optimally, does a light flywheel really vibrate more? Or does it just vibrate differently?

Fast Eddie said:

If a lighter flywheel and a heavier flywheel are both balanced optimally, does a light flywheel really vibrate more? Or does it just vibrate differently?

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Well, Your not likely to feel it with a rubber mount engine but yes, a lighter crank will transfer more vibration to the engine cases.
The amount of the vibrational force is the comparison of the balanced weight to the unbalanced weight [force]. A heavier crank will not move as much in reaction to the unbalanced reciprocating weight.

comnoz said:

Not do discount Knut's research and math but it does not take into account the differences in engine output due to changes in the rotational


It will vibrate more with the lighter flywheel and heavier rods.

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Jim,
why is that, if the crankshafts are equally well balanced and of identical stiffness?

The bending of an end-supported crankshaft like we have on the Norton will be lesser on a lighter crankshaft of equal stiffness to the heavier crankshaft. This follows from elementary physics. As mass elements (dm) strive away from the rotational axis, the centripetal force is dm*r*Omega**2, where r is the radial displacement due to bending and Omega is the angular speed (radians/sec). As mass elements are reduced, so will the force, so will the bending and so will the vibration. At least according to the book.

When balancing, the normal rule is to balance 100% of the rotating mass and 50% of the translational mass (i.e., piston and small end of the conrod).

Reaction forces from unbalanced forces which acts on the support bearings are, for the primary and secondary unbalaced forces, for vertical and horizontal forces respectively,

Fuv = mub * r * Omega**2 * ( cos(phi) + cos(2*phi) / Beta)
Fuh = mub * r * Omega**2 * ( sin(phi) + sin(2*phi) / Beta)

mub: Unbalanced mass (i.e., 50% of the translational mass)
r : Crankpin radius (=stroke/2)
Beta: Length of conrod L to r (=L/r)
Omega: Angular speed (=2*PI/rpm)
phi : Crankshaft angle measured clockwise from piston position at TDC.

Thus, forces transmitted to the bearings and the cases are solely dependent of the unbalanced mass (lower piston mass is better) in addition to the bending of the crank. The latter depends of weight distribution (and thus moment of inertia) and stiffness of the crankshaft, in addition to Fuv and Fuh which also affects the bending.

Going by these formulas, a lighter crankshaft should vibrate less and this is further amplified if a piston of lower weight is fitted.

-Knut

This reminds me of sitting at the back of the class at school trying to blank out the conversations of the clever kids whilst I struggled to catch up with last weeks lesson...!

comnoz said:

So what does a big flywheel do in a drag race?

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Hardly a proof. The Harley chopper has a fat rear tire, almost all weight at the rear and thus excellent traction. It also has a much larger engine (apparently) and a completely different torque vs. rpm curve, providing strong acceleration in the lower rpm band where most sport bikes are notoriously weak.

The acceleration is impressive nonetheless

mdt-son said:

Jim,
why is that, if the crankshafts are equally well balanced?

The bending of an end-supported crankshaft like we have on the Norton will be lesser on a lighter crankshaft of equal stiffness to the heavier crankshaft. This follows from elementary physics. As mass elements (dm) strive away from the rotational axis, the centripetal force is dm*r*Omega**2, where r is the radial displacement due to bending and Omega is the angular speed (radians/sec). As mass elements are reduced, so will the force, so will the bending and so will the vibration. At least according to the book.

-Knut

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If your engine balance factor is 50% then you have 50% of the reciprocating weight unbalanced [or un-opposed].
This unbalanced [or un-opposed] weight will remain the same no matter what the weight of the crankshaft is.
This un-opposed weight is going to get more reaction from a lightweight shaft than it would from a heavy weight shaft.
Like a 400 lb sumo wrestler is going to get more reaction shaking a flagpole than a toddler trying to shake that same flagpole.[a quote from balancing school] Or better yet I can shake a 10 lb weight [the crankshaft] a lot farther that I can shake a 50 lb. weight at the same frequency.

mdt-son said:

Hardly a proof. The Harley chopper has a fat rear tire, almost all weight at the rear and thus excellent traction. It also has a much larger engine (apparently) and a completely different torque vs. rpm curve, providing strong acceleration in the lower rpm band where most sport bikes are notoriously weak.

The acceleration is impressive nonetheless

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No question, no proof, too many variables to consider. But amusing just the same.

The crank balance factor is usually adjusted to suit the usable rev range. A top end motor revs higher to get more horsepower rather than torque. With a light crank, the vibration frequency is usually higher and cracking the frame becomes more likely. With isolastics you are probably in a better position than with a rigidly mounted motor. But even with a rigidly mounted motor, when it is revving at a level where the crank balance is right for it, there is often very little vibration. With the heavy Commando 850 crank, a balance factor of 72% causes the motor to be very smooth at 6,000 to 7,000 RPM. But at 3,000 RPM, the crank causes the whole bike to rock backwards and forwards. When you fill the hole in the Commando flywheel with a steel plug the balance factor comes out at about 72 % - obviously originally designed to be used in an Atlas ?

comnoz said:

So what does a big flywheel do in a drag race?

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It is alright doing that, but what about the gearbox ? When I raced with the Norton 4 speed close box, I only got one good start. Every other time I was too scared to rev the tits off the motor and dump the clutch.

comnoz said:

If your engine balance factor is 50% then you have 50% of the reciprocating weight unbalanced [or un-opposed]. This unbalanced [or un-opposed] weight will remain the same no matter what the weight of the crankshaft is.
This un-opposed weight is going to get more reaction from a lightweight shaft than it would from a heavy weight shaft.
Like a 400 lb sumo wrestler is going to get more reaction shaking a flagpole than a toddler trying to shake that same flagpole.[a quote from balancing school] Or better yet I can shake a 10 lb weight [the crankshaft] a lot farther that I can shake a 50 lb. weight at the same frequency.

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Jim, I extended my proof above. In your argument, the un-opposed weight is creating a force which needs a force reaction. This force reaction does not need to "shake" the un-opposed mass. The reaction force does not need to lift the crankshaft up and down either (it does try to bend the crankshaft though). The reaction force is transmitted straight into the support bearings. The "shaking" of the reaction force occurs due to harmonic variation as the crank rotates.

There might be an effect of the heavy Norton bolt-up crankshaft I am missing here (a flexible crankshaft does behave strangely in some ways) but I can't imagine what that should be. When discussing this topic, it's important to keep some parameters constant, such as dimensions, stiffness and angular speed.

-Knut

mdt-son said:

Jim, I extended my proof above. In your argument, the un-opposed weight is creating a force which needs a force reaction. This force reaction does not need to "shake" the un-opposed mass. The reaction force does not need to lift the crankshaft up and down either (it does try to bend the crankshaft though). The reaction force is transmitted straight into the support bearings. The "shaking" of the reaction force occurs due to harmonic variation as the crank rotates.

There might be an effect of the heavy crankshaft I am missing here (a flexible crankshaft does behave strangely in some ways) but I can't imagine what that should be. When discussing this topic, it's important to keep some parameters constant, such as dimensions, stiffness and angular speed.

-Knut

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Yes, You are correct in saying the force is [almost] directly transmitted into the bearings. But with a lighter crankshaft, the whole engine is lighter so the effect is the same.

Now if you were to take 5 lbs off the crankshaft and add 5 lbs to the crankcase then things would work out the same. [except for the effect of the flex in the crank.]

Just like any time I have installed an aluminum barrel in place of an iron barrel. There is a definite increase in engine motion on the mounts. This is very noticeable when I have an engine mounted in the cradle on my dyno.

Thanx mid-son for explaining the proper physics acting here and awful confusingly clear equations over my head. Nay saysers should relax and learn, to help all us comprehend how to decide what to spend on.

Reviewing objective measures of dynos and drag times, of which many to search up, there is insignificant power curves measurable, even comnoz states this. Logic, not hobot, dictates, it can not be better acceleration of cycle being affected by various flywheel mass. Illogical to ignore these facts, so to claim otherwise, ie: heavy or light flywheel pulls better out of turns or down a paved strip etc, absolutely not. What's left is gyroscopic forces which are significantly felt by pilot and tire patches, just not yet recognized.

Ugh, torsional stress is same thing as elastic stress, ie: twists bends vibes always returning to original rest shape, which is different from plastic 'stress' deformation, that leaves permanent distortion in structure, after or in between stress removals or reversals.

Almost all vibration generators/agitators use imbalanced wheel, often with changeable weigths/bolts, heavier gives more stronger motion vibes and visa versa - at same motor speed. Nortons are no different. No opinion just a fact.

There's another factor that over laps with what mdt-son expressed but did not continue, that at some threshold structures like flywheels or bridges suddenly over distort>fail w/o warning, similar to an early physics lesion watching a friend grab a dump truck bed to pull him up an over pass instead of peddling, still holding a mostly closed umbrella from a trash pile to suddenly flap open about 45 mph > long enough to lift him a few feet up off bike >then collapse dumping him tumbling down grass embankment yet his bike kept on going straight a good ways before falling over. If ever played with 'buzz saw' button and thread toys, picture a slight imbalance, more or less mass, such as Norton cranks spun while only supported at ends. Resists jump rope shape til suddenly can't then bends/bows/candy canes dramatically levering cases apart horizontally by bearing width high advantage lever action. Why do some like it lighter and others heavier who really knows yet.

glanced over the thread & wondered which way op went after the first post

Al-otment said:

Finally the bikes back on the road! Halle-fuckin'-lujah. Going well, just done 60 miles revving to 5,000, first kick starting, good tick-over and no oil leaks. I'm pretty sure acceleration from a standing start and also roll-on is quicker (lighter Steve Maney flywheel fitted). Only down side is driving through congested town traffic when she seems happier one gear lower than before but this is not a problem and I'll quickly adjust. Leaving for the Britti Ralli week on Friday so I'd be happy if I can get 700 - 800 miles on the clock before then.

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first

Al-otment said:

I'm wondering how noticeable the difference will be between the two regarding engine 'drive-ability'

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woulda thought heavy is ideal for street use driveability

He went with a lighter Maney crank. But he left the forum before giving us his opinion on the before / after comparison unfortunately !

mdt-son said:

Who says there is a repeatable difference in indicated power as measured on a dynamometer and under constant environmental conditions?
If the crankshaft stiffness (both in bending and torsional stiffness) remains constant, there should be none.

-Knut

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You are correct. Premature to jump on that band wagon and accept. Proper and repeatable before and after tests would have to be conducted under controlled conditions. It’s not a given.

Fast Eddie said:

If a lighter flywheel and a heavier flywheel are both balanced optimally, does a light flywheel really vibrate more? Or does it just vibrate differently?

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All things equal, a heavier flywheel will dampen the reactive forces due to imbalance at the main bearings.

BTW, on the obnoxious nasty looking chopper vs sleek wholesome sports bike, the chopper main advantage is extended front resists wheelies, which the spork bike does at end of parked cars to cut power fall back and give up. Nothing at all to do with this subject on flywheel mass. HD flywheels are pretty light for displacement compared to Norton twins but not as light as the sports bike. Again more evidence Flywheel mass is insignificant on pavement as other factors over whlem its significance for get up and go. So back on point its how flywheel mass matter on power pulsed tire hysteria hook up &or its gyro lean ease instability or lean stabilization security that can be stronger than both wheels put together. Ya know best turning boats and aircraft are set up to be unstable if that's a clue to judge preferences.

There's more fast furious fun features of flywheel mass and BF ratios to cover but if not getting over simplest flywheel prinicples vs gyros basics wouldn't beleive either.