Heavy flywheel equals more torque? (2014)

So I thought I would expand on what I know about the balance of a Norton.

First off, Knut's math is 100% sound, but what it and most all balancing formulas figure is that the main bearings are static , IE they can not move. This is close enough to be considered true with any engine -except a single or most twins [without balance shafts].

With a single cylinder or 360 degree twin the un-opposed forces are such a large percentage of the engines weight [or with a rigid mount engine the whole motorcycles weight] that the engine and attached parts will attempt to rotate with the main bearings around the crankshafts center of mass. How close the whole engine [or motorcycle] comes to actually rotating around the cranks center of mass depends on the engines weight [including the crankshaft and cradle] and the frequency [RPM].

If the engine is rubber mounted and light weight, then it will rotate in a fairly large oval. Adding weight to the engine [or crank] will make the oval smaller at the same RPM.

A rigid mounted engine will mean the whole motorcycle moves in an oval, but since the ratio of un-opposed weight to motorcycle weight is higher then the motion is less.
Of course you feel it more since your in direct contact with it.

The shape of this oval is determined by the balance factor. A high balance factor means the oval is going to be like an egg that is laying down. A lower balance factor means the oval will be like an egg standing on end.

A Commando will transmit less vibration to the handlebars if the egg is standing on end [lower balance factor] since the front mount is large and soft and lets the engine move freely up and down without taking the chassis with it.

A rigidly mouted engine generally transmits less vibration to the rider if the egg is laying down [a higher balance factor].

comnoz said:

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.

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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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Comnoz and Fast Eddie both addresses a different topic which has no direct bearing to the discussion of crankshaft mass and moment of inertia - namely, how well are the isolastic bushes acting as spring and damper, taylored to varying weights and excitation forces of the powerplant? How much does the powerplant vibrate in terms of amplitude - the frequency is given by Omega and Omega/2 for the primary and secondary force respectively, and how much is transmitted to the frame in terms of amplitude, force and frequency (eigenfrequency of the suspended powerplant comes into play).

This is a big new topic which I am not going to solve here. However, just to give you a clue of how to address the subject: In a simplified mathematical model we have a lumped mass suspended by a spring and a damper towards ground (i.e., the frame) which itself is a mass suspended by a spring and a damper, but let's keep it simple here. The lumped mass of the powerplant weighing in at approx. 130 lbs (59 kg) for a Mk.3 is excitated by a hormonic force (the unbalanced forces and the centripetal forces due to crankshaft bending). I am neglecting a possible coupling with torsion harmonics here. Then we have a system like this (q is a generalized coordinate - in our case the vertical axis coordinate z having origo at center of gravity of the powerplant):

Fig. 1


This system will have a resonant frequency - Omega_0 - not to be confused with the circular frequency of the crankshaft - Omega.
Omega_0 is the square root of the lumped mass divided by the spring stiffness (i.e., that of our isolastic bushes).

Fig. 2


Now, the frequency response of the lumped mass (i.e., the powerplant) in terms of amplitude and phase can be calculated and plotted. It is dependent of the excitation frequency Omega, the resonant frequency Omega_0 (please replace f/f0 by Omega/Omega_0 in fig. 3), and the damping coefficient d, which goes into the dimensionless damping parameter greek Zeta in Fig. 3. It is very low in our case, probably in the range of 0.1 . At idle engine speed we are apparently in the region of frequency ratio 1, as observed on the bikes. As revs pick up, we are moving to the right and the amplitude ratio of the mass lump gets successively smaller as it approaches nil for really high revs.

Fig. 3


The Force transmitted to the foundation (the frame, if we neglect the suspended arrangement actually in place) is the sum of spring and damper action forces (-Fk-Fb) in fig. 1.

This is of course a much simplified model, but it should be usable for assessing some of the basic parameters. I will do the math if someone could tell me the spring and damper characteristics.

A side note, Jim - did N-V change the rubber characteristics for the Mk3? Maybe they did. If so, is shaking of the powerplant better or worse if the Mk3 rubber suspension is put to use on a Mk1 or Mk2 850?

Now, the coupling of linear excitation with torsion reaction is an interesting subject .....


Regards,
Knut

It appears that you are typing in English Knut... and yet I understand naught !

I shall leave this conversation for the grown ups now...!

mdt-son said:

(the unbalanced forces and the centripetal forces due to crankshaft bending)

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Can you clarify this statement of condition. I seem to recall you referring to crankshaft bending elsewhere in this thread and I question the significance; if not the relevance when we are discussing a range of possible rpm.

I understand unbalanced forces or even unbalanced centripetal forces but “unbalanced forces and the centripetal forces due to crankshaft bending” is either a new and unusual concept to me or bad sentence structure.

Just trying to understand what you are seeing.

Dances with Shrapnel said:

I understand unbalanced forces or even unbalanced centripetal forces but “unbalanced forces and the centripetal forces due to crankshaft bending” is either a new and unusual concept to me or bad sentence structure.

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Please read my entry on page 14 of this thread (the one with formulas for unbalanced forces). (Additional) centripetal forces arise because of crankshaft flexibility. Mass elements serving to counteract the rotational and translational mass forces of conrod + piston are no longer in their original position. They are displaced radially by a small amount due to the crankshaft bending. This in turns increases bending which dispaces even more, and so forth. Actually this is kind of an iterative process which obviously has an asymptotic result, unless the crankshaft explodes.

Basically it's the same effect seen in this video:


I hope I made myself clear.

Cheers,
Knut

You clarified things; I assume you are being thorough, which is a good thing.

In my opinion, from what I know of the forces and crank stiffness, the "additional" imbalance due to flexure of the crank is insignificant, especially at lower to mid range rpm of the Norton twins. Even at peak rpm. Case in point is the rather wide range of acceptable balance factors for the Norton twin based on the specific application.

Knut,
I don't know if NV changed the rubber mounts for the MK3 in particular. They did add the spring which changes things. And the compliance of the rubber has changed over the years. The original Silentblock mounts used on the early Commandos were the best and most compliant.

Figuring the reaction to the shaking forces becomes impossible when you add in the variable damping added by the pull of the swingarm on a Commando or the difficulty of accessing the amount of flex and in what direction with a rigid mount. At this point cut and try seems to be the most viable method.

But what I am coming back to is adding mass to the crankshaft is basically the same as adding mass to the crankcases. Adding mass to the crankcase means that the crankcase is going to move less in reaction to the un-opposed weight. If the crankcases move less then there is likely to be less vibration felt in the seat and bars. This is what I mean when I say there is going to be less vibration with a heavier crank.

what was the answer for - Heavy flywheel equals more torque?

84ok said:

what was the anxwer for - Heavy flywheel equals more torque?

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No more torque- in any practical way.

More torque in a significant way. See Knut's calc's for an idea of how much more; something like 4.4% greater available to accelerate and when you factor in that you are not dragging around (translating) that extra mass it equates to a net gain of 5.2%. Thanks Knut.

A few suppliers of Norton cranks have gone lighter for obvious performance reasons. At steady state RPM, the jury is still out if there's a difference and if there is, the phenomena cannot be explained.

Dances with Shrapnel said:

More torque in a significant way. See Knut's calc's for an idea of how much more; something like 4.4% greater available to accelerate and when you factor in that you are not dragging around (translating) that extra mass it equates to a net gain of 5.2%. Thanks Knut.

A few suppliers of Norton cranks have gone lighter for obvious performance reasons. At steady state RPM, the jury is still out if there's a difference and if there is, the phenomena cannot be explained.

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And a few have gone heavier -for obvious performance reasons. It all depends on what your looking for.

And the torque is not lost or gained in any practical amount. It is just moved.

comnoz said:

And a few have gone heavier -for obvious performance reasons. It all depends on what your looking for.

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I seem to recall Ro Dy Machine in Michigan may be a good example and they have catered to dirt track. As stated earlier, heavier may enhance traction control and the metering out of power/torque on loose surfaces. I cannot attest to this bit about heavier crankshafts for dirt application first hand, only through what I have read and heard. I am reasonably sure that Ro Dy and others will cater to one's desires and needs. There's also those who want stock or maybe heavier for less vibration (greater damping of vibration).

Many years ago I had an Ossa Enduro that had been modified for flat track racing at Portland.
Dave Wildman , the Vancouver Ossa dealer and Westwood racer, sponsored the bike and made all the changes.
It was very quick.

It came with three (external) flywheels and had the heavy brass flywheel fitted. I switched to the superlight Al. Flywheel and felt an immediate difference in acceleration.
The front end on that bike had always been prone to lifting but now it was ridiculous.
My times were significantly better with the heavy flywheel in place, so it eventually went back on. I can't recall the outcome with the midweight flywheel, but it must have been slower as well or the heavy flywheel would not have been the final choice.
Weight difference from heavy to light was 8 lbs.

Glen

Flat tracker essentially try to stay at WOT/rpm all the way around using forks/body to tip rear over enough just befor the turns to use rear spin cross up to slide around trying not to lose wheel rpm. Its more related to drag racer land speed event not road race which must change wheel rpm - so heavier crank and rear help to keep going at max rpm between tire spin and hook ups. For some reason FT rules limit mass of rear but I don't know their reason for this rule.

BF= unbalanced flywheel is separate issue after basic mass of fw is set. Basic mass determines the slight difference in power delivery rates-in-out, spin up-slow down while BF determines how much and what vectors the oscillation is. Some yrs ago I posted BSA reserch article that showed the least spikes of stress on the bearings was 52%.

As accurate as Knuts equations and principles, spring coupled dynamic ares, it don't apply to our engines as Dances states there's just not enough flexure/deformation happening until a dramatic parts failiure, which has to do how the crank is supported and its materials, not harmonic or power delivery smoothing.

As wortorn found out ligher fw delivers power faster but then may become uncontrolable from spin or wheelie, which has to do with CoG of cycle/pilot. So if faster with more fw inertia on events that require up/dn throttle, it means the cycyle configuration and engine power control is beyond pilot ablity to hook up or not wheelie. My front low P!! would not wheelie on throttle snaps or standing launches if over done, just spin in place or turn into pavement flat tracker so easy if took skill not to in public. It wieghed under 300 lb and was my 1st engine w/o much precievable rev time, bBLATT 9000. Second one with distinctly less power was Ms Peel bBLATTT-7500. Both hit redline before could complete WOT snap! I did a lot of off road on both these -BUT found must be in top gear from 20 mph up or would too easy just spin. IF I encountered deep sugar beach like sand or wet grass or mud strecthes I'd down shit on similar throttle power to get fw gryo help trying to stay as steady speed till past them then up shift for hook up out of there.

Face it heavier fw means tamer/easier control, ligher is more manic jerky excitting effective if can control it. My meager experience on these lightened fw Nortons in road race situations wanting turn sharper w/o slowing/braking, instead of backing off I'd be able to spin tire an instant to mostly pivot around CoG, w/o *speeding up* but getting aligned more up right to traction hook out of there. Doing this on normal fw cycles would speed up beyond staying on road and if rear began a spin took too long for it to slow-regrip before sliding crash off path. Kenny Robert was most famous handler of on/off power delivery by riding every which way but civilized smooth. For gosh sakes he'd bounce off barriers on purpose to get knocked into next line of explosive power release. So lower skill/bravery &or poor bike configuration for conditions benefits by more fw taming.

Yeah, I am sure it was my low skill/bravery that made me go faster on a bike with a stock weight crank than with a lighter crank. After all, I only roadraced from 1977 to 2006.

Fast Eddie said:

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

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he did say

Al-otment said:

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

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pretty much covers it, or at least op & the thread subject question - Heavy flywheel equals more torque?

worntorn said:

Many years ago I had an Ossa Enduro that had been modified for flat track racing at Portland.
Dave Wildman , the Vancouver Ossa dealer and Westwood racer, sponsored the bike and made all the changes.
It was very quick.

Glen

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Nice, I also flat tracked an Ossa in the early 70's. Mostly on an indoor coke syrup track. What a gas...

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

Yep, pretty much covers it.

Yes, that was is initial feedback, you’re correct.

He hadn’t done any real mileage though, it was his initial impressions, and how much was placebo and how much was correct would be impossible to tell. He was gonna take on a track day (Cadwell Park I think, a good test) and feedback afterwards. But he left us in suspense !!

With speedway type bikes, the type of tyre and gearing are changed to adjust the slide and drive to suit the power characteristics of the motor and the geometry of the bike. It is a totally different situation to riding on bitumen, even though what happens at relatively low speed on a flat track happens under extreme conditions in road racing. In road racing, we don't usually corner with the rear end of the bike hanging out, because it leads to the high-side. In road racing an old bike, you need two main things - the front brake and the gear-box. If you have got those, the motor is usually fast enough. You can only go as fast as you can stop and corner. The right gear at the right time is very important. Some guys don't even count their gears while racing. A heavy crank or a light crank does not make much difference, the problems are the same. I use the heavy crank because that is what I have got.