Heavy flywheel equals more torque? (2014)

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So if you want to know what the dyno says. I see a little more torque on a steady state pull with a heavier flywheel. But a little less torque when doing a run with inertia only.

Small differences and more difference in the low and midrange than at higher revs.

I theorize that it has to do with the fact that the heavier flywheel has a more consistent speed over the course of the 4 strokes which may help with the breathing or maybe it makes better use of the pressure of the power stroke, but I really don't know.

I do know my track times were better with a heavier flywheel.
 
As comnoz, Canaga and others found out there's hardly a measurable or felt power delivery difference but does affect handling stability as reason most get around better on 'heavy' crank as surely ain't from better acceleration under load. Makes me wonder why Steve Manney and others went to expense/effort to produce a 10 lb lighter crank&wheel. We should contact Manney for reasons and type of use it helps. Two strokes only need flywheel energy return for 180' per rev while 4 stroke needs 270'. More cylinders needs less flywheel to keep spinning between combustions.
 
A dyno shows power and / or torque against rpm.

It doesn’t show time.

So two bikes may show the same bhp / toque curves @ the same rpm plots.

But one could get there quicker than the other, and this would not show.

I suspect the lighter crank might get there quicker.
 
Fast Eddie said:
A dyno shows power and / or torque against rpm.

It doesn’t show time.

So two bikes may show the same bhp / toque curves @ the same rpm plots.

But one could get there quicker than the other, and this would not show.

I suspect the lighter crank might get there quicker.
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An inertia dyno does show time, a steady state pull shows power at a set RPM.
 
Ok, I don’t think I’ve ever seen time on any of mine. The horizontal axis only shows rpm, or sometimes mph.
 
With inertial dyno results, the lighter flywheel will express itself as greater torque. This is torque that would otherwise be used to accelerate a heavier flywheel.

I've found better results with a lighter flywheel on the track as I've been able to capitalize on the greater torque for drive out of the corners and accelerating in the lower gears. Steve Maney could probably weigh in on this also.

I'll speculate that greater BHP at steady state (eddy brake) may have to do with more dwell time near and past TDC that allows combustion chamber pressure to rise more.
 
comnoz said:
An inertia dyno does show time, a steady state pull shows power at a set RPM.
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Since P = T * 2PI * n

in units:

kW = Nm * rpm * PI / 30000
or
PS (DIN) = Nm * rpm * PI / 22080

the steady state pull can always be expressed as torque also. If acceleration (spin-up) is very slow, the two methods of measurement should converge.

Example: The 850 Commando has a quoted power output at the engine shaft (incl. alternator power draw) of 51 PS @ 6250 rpm. The torque calculates to
T = 51*22080/(6250*PI) = 57.4 Nm . This is somewhat lower than the maximum rated torque of 56 lbf*ft = 65.1 Nm @ 5000 rpm which makes sense. At maximum torque the power output is P=65.1*5000*PI/22080 = 46.3 PS.

Regards,
Knut
 
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The steady state power difference, or at least a component of difference, may be attributed to greater drive line hysteresis loss as stated earlier in this thread.

As they say, torque wins races.
 
Hm Dances, something does not compute on claim of better power delivery out of turns as the flywheel mass/energy differences is minuscule compared to road wheels with cycle/pilot inertia. I believe you about better times of course, just not the reasoning. Personally my sense of going a few lb ligther in Ms Peel was less gyroscope resistance to fast lean changes rather than throttle response. I suspect you are a bit braver/handier than others to tolerate/take advantage of the lessor stability/resistance to change.
 
mdt-son said:
Since P = T * 2PI * n

in units:

kW = Nm * rpm * PI / 30000
or
PS (DIN) = Nm * rpm * PI / 22080

the steady state pull can always be expressed as torque also. If acceleration (spin-up) is very slow, the two methods of measurement should converge.

Regards,
Knut
Click to expand...

As weight or gearing got higher and spinup time got longer, the advantage went to the heavier flywheel. Jim
 
hobot said:
Hm Dances, something does not compute on claim of better power delivery ............................
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No claim of better power, better torque.

Analogies:
Power - rate of doing work
Torque - force.....................to get you out of the turn and down the track.

Miniscule? No. One good data point is that the engineering dynamics and analysis by Knut supports it. Take a look at his analysis and run some numbers. Supported by real world experience.
 
comnoz said:
As weight or gearing got higher and spinup time got longer, the advantage went to the heavier flywheel. Jim
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Sounds like something amenable to a Harley Cruiser.

Not sure what you meant "as weight" got higher? Weight of the bike? Is there some rolling resistance element here we are talking about?
 
Dances with Shrapnel said:
No claim of better power, better torque.

Analogies:
Power - rate of doing work
Torque - force.....................to get you out of the turn and down the track.

Miniscule? No. One good data point is that the engineering dynamics and analysis by Knut supports it. Take a look at his analysis and run some numbers. Supported by real world experience.
Click to expand...

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
Al-otment said:
A spinning wheel with a cast iron rim will produce a twisting force (torque) at the spindle. A wheel with an aluminium rim of similar dimensions as the cast iron rim and spinning at the same speed will produce a lower torque because of it's lower mass at the rim.

Engine should be going back together tomorrow but I've got a last minute question. I now have a Steve Maney lighter flywheel fitted to Mk2a crank shafts/journals with Carrillo rods.
I've still got the 150,000 mile Mk3 crank with standard flywheel and 35,000 mile aluminium alloy billet rods. But I'm wondering how noticeable the difference will be between the two regarding engine 'drive-ability'. Anybody had any experience of light compared to heavy flywheels?

If I'd known the time it was going to take etc etc I may well have done things differently but I am where I am. Hind sight's a wonderful thing.
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I have done lots of experimenting from extremely light to heavier than stock.
The results will be pretty much just as you would expect.
The lighter flywheel will sound hotter when revving in neutral.
It will be a little tougher to get a consistent slow idle.
It will vibrate more with the lighter flywheel and heavier rods.
A little harder to launch but a little quicker to rev in the lower gears with the light wheel. Jim
 
Robert_Norton said:
If two otherwise identical bikes were travelling down the road, except for light and heavy flywheels, and the heavier flywheel is storing more energy, where/how is that energy dissipated, or transferred to the rear wheel?

Once a heavy flywheel is spun up to speed, isn't the force of the spinning mass helping to propel the bike forward in a way that a light flywheel is not? It's part of the drivetrain, right? So once up to speed, with the greater forces involved from the heavier flywheel, isn't it easing the work of the combustion chambers, by spinning around with a greater 'contained' energy than a light flywheel?

I think I've read every post in this entire thread, but I would like to have read more regarding the discernible differences of light vs. heavy flywheels, for road-use.


.
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Re: your 1st paragraph

The heavier flywheel has more stored energy in the form of angular momentum. Once spun up, it is not dissipated or transferred to the rear wheel. The stored energy comes into play when the vehicle decellarates...... in your example of two identical bikes except for rotating mass (The mass could be flywheel and wheel mass) should both riders hit the kill switch, engage neutral, the bike with the greater angular momentum in the wheels will coast further down the road. If the engine remains engaged to the rear wheel, the effect is similar, the bike with the heavier flywheel (wheel mass equal) will tend to move ahead of the other bike.

Re your 2nd p.

The energy is simply stored, same as your surplus cash is stored in a bank at zero interest..... It does nothing until it is called for......that is, when decellerating, or when you have a need for cash.

As FastEddie points out, the land speed racer wants the extra energy in the event a 3 mph wind gust comes up at 130 mph..... The extra stored energy will keep the slow down effect to a minimum.

When a vehicle acellerates, some of the engines horsepower output is used for linear aceleration (getting from point A to point B). Some of the engines horsepower is used to spin up the flywheel and wheel mass. The drag racer wants the horsepower output of his engine to get him from point A to point B in as little time as possible ..... he has no interest in puttng some of the engines output into spinning up a mass, and so wants as light a flywheel and wheels as possible.

The main function of the flywheel is to smooth out the power pulses. A flywheel of adequate mass makes the bike more "ridable". The flywheel mass simply does not enter into the physics of torque or horsepower.

Slick
 
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comnoz said:
The results will be pretty much just as you would expect.
The lighter flywheel will sound hotter when revving in neutral.
It will be a little tougher to get a consistent slow idle.
It will vibrate more with the lighter flywheel and heavier rods.
A little harder to launch but a little quicker to rev in the lower gears with the light wheel. Jim
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Agree on all points. For street use and comfort, heavier is more comfy, manageable and forgiving. Your last line about revving quicker is what it is all about for road racing. Acceleration out of the turns is "held in the bank" with greater average speed down the following straight. Taking a look at Knut's numbers expressed in net torque as a percentage, it is significant. Lighter is backed by many engine builders other than those noted for dirt and lesser traction conditions. Whatever the apparent higher net BHP may be for a heavier crank, the advantage in land speed records is apparently where the vehicle is approaching or hitting "steady state", an analogy of an eddy brake dyno condition.
 
texasSlick said:
As FastEddie points out, the land speed racer wants the extra energy in the event a 3 mph wind gust comes up at 130 mph..... The extra stored energy will keep the slow down effect to a minimum.
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Agree in principle. Like a double edge sword, it cuts both ways. In the event of a 3mph gust from the tail, coming up on 130 mph, the heavier flywheel will not be able to respond (accelerate) as quickly to take advantage of the gust.

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.
 
Ugh I'm rocket science trained from NASA Cape Canaveral Florida days, calculating pay loads vs thrust vs fuel mass used for particular height orbits and how fast to spin a satellite to hold its instruments at predicable aim. Extra credit calculating rotational moment of inertia of spinning paint flecks following/orbiting around a satellite/capsule in relation to Earth CoG, by slide rule. Not confused as you are on physics principles or its terms-grammar.

"Power delivery" don't mean more-less actual hp/energy, it only means how quick power source energy can change up or dn. In practice, if can hook up the power, torque determines how quick/hard ya get up to some speed, while hp determines how fast the top speed is. Flywheel mass is pretty insignificant factor on dyno or drag strip runs, mainly depend on hp/wt ratio of the craft or dyno flywheel mass. If flywheel mass helps or hurts road race times it must have more to due with gyroscopic effects on resisting tip down and fling up rates.

Interestingly IF pretty powerful hp/wt combo ya can hold throttle steady and shift up down gears with lots of engine rpm changes but hardly any difference in road wheel speed. Same steady total power = same road speed. For instance if I see lengths of mud on THE Gravel or pasture grass bogs I down shift to get distincly more flywheel gyro stabilization Being Very Careful Not to change wheel speed much or rear may spin out passing my shoulders on way down or rear slows up dragging/digging in for messy impact. Similar on dry loose pebbles stones lumpy paths, better be going slow enough to keep both feet down, or go fast enough pure ballistics and engine rpm keep ya facing direction of travel.

Above may effect which way a cycle leans easier. I tested Ms Peel harshly to find she's slightly easier to tip L, which I thot might be d/t more DS components mass + engine slightly L of center line, but now see may be direction of crank rotation even more. I'm heard many explanations of reports of cycles slightly easier to tip L, attributed to heart more on left side or being L handed like me to lop sided construction or cargo loads or rear wheel not in centerline.
 
Dances with Shrapnel said:
The steady state power difference, or at least a component of difference, may be attributed to greater drive line hysteresis loss as stated earlier in this thread.
Click to expand...

I have been pondering which hysteresis effect you are referring to. Apart from the flywheel and the rear wheel, there is only one component in the transmission with a noteworthy moment of inertia - the clutch. The only hysteresis effects I can think of is torsional vibration, which is a spring effect of wind-up and release. Torsional vibration frequency is normally in the 30-150 Hz range (relatively to the crankshafts rotational frequency). The impact on torque delivery will therefore diminish as revs increase (6250 rpm = 104 Hz).

The variation of torque with angle of rotation is very large (due to the engine's combustion). The peak torque value may be as much as 8 times the average torque. Torque impuls varies therefore with half the rpm frequency. No energy is stored in the engine directly and I would say this is not a hysteresis phenomenon.

As for clutch and rear wheel, these a attached to the crankshaft by means of flexible chains. They will soften the variation of torque and I doubt there is any effect of crankshaft torsional vibration at the rear chainwheel.

-Knut
 
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