80 HP at 8700RPM by Herb Becker

Fast Eddie said:

My question to the collective here is, if I ran the same Commando on a dyno with a stock heavy crank, then with a lightweight crank, with everything else being equal, would this show in any different readings on the dyno?

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mdt-son said:

being exceedingly small, I doubt you will spot the difference at the dyno.

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To Fast Eddie - Simply yes.

From first hand experience you can feel a significant difference and anecdotal evidence from others who have lightened their crankshafts has supported this so it is not small.

As an example, see results of dyno testing of a V8 where they reduced the overall flywheel mass from 31 lbs to 24 lbs

http://www.superchevy.com/how-to/en...to-add-hp-with-a-lighter-flywheel-why-weight/

The above is from 7 lbs reduction in overall flywheel mass and we are seeing people reduce a Norton crankshaft from say 24 lbs to around 17 lbs (7 lbs reduction in mass) and, in contrast, much of this reduction is focused on reduction in rotational mass.

It would be great to see a side by side comparison (before and after on a Norton twin) to quantify for that particular Norton twin. These results would vary based on state of tune of the particular bike.

The flywheel is simply an energy storage device so when you accelerate, energy is stored in the flywheel, energy that would have otherwise accelerated the vehicle down the track. This energy is stored in rotation and translation.

Dances with Shrapnel said:

To Fast Eddie - Simply yes.

From first hand experience you can feel a significant difference and anecdotal evidence from others who have lightened their crankshafts has supported this so it is not small.

As an example, see results of dyno testing of a V8 where they reduced the overall flywheel mass from 31 lbs to 24 lbs

<...>

The flywheel is simply an energy storage device so when you accelerate, energy is stored in the flywheel, energy that would have otherwise accelerated the vehicle down the track. This energy is stored in rotation and translation.

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Pardon me but you are comparing apples and oranges, and the the dyno runs are no proof at all. The change in peak power from 535 to 542 hp is attributed to the acceleration phase. If revs were kept at the same figure, the curves would superimpose!
Caution is imperative when viewing "results" obtained for marketing purposes.

The scientific formulas are indisputable. Looking at the flywheel as an energy absorber and dissipator, its properties are governed by

* rotational impuls : L = I * Omega, where I = moment of inertia = Integral (r^2*dm) and Omega = angular velocity = 2*PI*rpm.
* torque : Mt = dL/dt = I * d(Omega)/dt; I is assumed to be constant here.
* power: P = Mt*Omega = I*Omega*d(Omega)/dt.

What do the equations say regarding the flywheel effect?

* There is no change in torque as long as the angular speed is constant.
* At speed-up the flywheel consumes torque; at deceleration, torque is produced.
* The lighter flywheel (lesser moment of inertia I) consumes less torque and also produces less torque at deceleration. This explains the rider impression in another posting above.
* The flywheel consumes power at speed-up proportional to the product of angular speed * acceleration of angular speed. If angular acceleration is nil (constant revs), then no power is consumed. Likewise, power is produced at deceleration proportional to the angular speed deceleration.
* There will be NO change of power output from the engine if revs are kept constant, even if I (moment of inertia) changes from one run to another.

So, what if I (moment of inertia) changes during the run? This could be the case when running the Norton engine (it's not likely to happen to the V8 engine!) .

** torque : Mt = dL/dt = Omega*dI/dt+I * d(Omega)/dt; I is no longer a constant. The rate of change in moment of inertia will affect torque at the PTO, even if change of angular speed in nil.
** power: P = Mt*Omega = Omega^2 *dI/dt + I*Omega*d(Omega)/dt ; Power consumption is significantly affected by a flywheel which changes its moment of inertia, due to being proportional to the square of the angular velocity.

Another reason for fitting a solid crankshaft!

I hope this helps.

-Knut

"I hope this helps"

Not really Knut...

However, you could summarize your impressive posts with a 'So What' summary / conclusion written in a suitably low intellect style, I personally would find that most helpful.

The summary in short is as stated in the second and third sentence. The change of flywheel mass will not affect power readings if revs are kept constant. There will be a small deviation in the power curves while acceleration and deceleration takes place; if the revs were kept at a certain figure, the "better" power and torque curves would move down onto the "heavy flywheel" curve.

As for the Norton crankshaft, bending of the shaft increases 'r' (the distance of a point mass from the shaft axis). The effect on I (moment of inertia) could be quite strong even for modest shaft deflections, as moment of inertia is proportional to the square of 'r', see formula in my previous posting.
I am not even counting the possible movement of mass point 'dm' due to material stressing.
Thus, having a large flywheel coupled to a flexible crankshaft is very disadvantageous to torque and power output of the engine.

-Knut

Sorry about using the old tons per square inch system of units. For much of my working life as an industrial chemist I used SI units in my profession and British units when talking to tradesmen. I spent a lot of time assisting in developing steel for use in making gun barrels. In later life we tried using Comsteel 4130 which was the commercial equivalent to one of our specifications. I cannot remember the strength values in megapascals, however our steel was usually about 70 TSI and when hardened and tempered came up to about 90 TSI. The commercial stuff was always too dirty

mdt-son said:

The summary in short is as stated in the second and third sentence. The change of flywheel mass will not affect power readings if revs are kept constant. There will be a small deviation in the power curves while acceleration and deceleration takes place; if the revs were kept at a certain figure, the "better" power and torque curves would move down onto the "heavy flywheel" curve.

As for the Norton crankshaft, bending of the shaft increases 'r' (the distance of a point mass from the shaft axis). The effect on I (moment of inertia) could be quite strong even for modest shaft deflections, as moment of inertia is proportional to the square of 'r', see formula in my previous posting.
I am not even counting the possible movement of mass point 'dm' due to material stressing.
Thus, having a large flywheel coupled to a flexible crankshaft is very disadvantageous to torque and power output of the engine.

-Knut

Click to expand...

I am confused by your caveat 'if revs are kept constant' ... I would have thought that the primary effect of changing flywheel weight would be a change in the way the engine revs. My question is therefore what effect does this change in the way the engine revs, have on the dyno readings?

As far as the power and torque curves are concerned, - as with all things motorcycle, there is probably an optimum crankshaft weight and stroke to suit the gearbox and chassis. I found I could not race with the wide ratio gearbox combined with the heavy crank. I could never do anything smoothly and acceleration was too slow. The close 4 speed gearbox changed that for the better, however first gear was too high for the clutch start in races. I find the heavy crank suits the way I like to race. I can have the bike cranked over in the middle of a hairpin bend and pour the power on and let the bike steer it's own way out of the corner. It means I often have the run on bikes which should be faster down the straights. The heavy crank encourages aggressive riding because there is no jerkiness in the power delivery which would cause the back end to step out under power.

As far as Eddie's short stroke 750cc motor is concerned - it is probably an exercise worth doing. My feeling is that a 75mm stroke light crank would be very good as long as the rider is 110 % on top of the consequences. I found my short stroked 500cc Triton both mentally and physically exhausting. It was extremely fast for what it was, however it was always trying to kill me. My 850 commando engine in the Seeley frame is a world apart from that. However, perhaps I'm getting too old ?

acotrel said:

As far as Eddie's short stroke 750cc motor is concerned - it is probably an exercise worth doing. My feeling is that a 75mm stroke light crank would be very good as long as the rider is 110 % on top of the consequences. I found my short stroked 500cc Triton both mentally and physically exhausting. It was extremely fast for what it was, however it was always trying to kill me. My 850 commando engine in the Seeley frame is a world apart from that. However, perhaps I'm getting too old ?

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Mine is a stock stroke 850.

mdt-son said:

Dances with Shrapnel said:

To Fast Eddie - Simply yes.

From first hand experience you can feel a significant difference and anecdotal evidence from others who have lightened their crankshafts has supported this so it is not small.

As an example, see results of dyno testing of a V8 where they reduced the overall flywheel mass from 31 lbs to 24 lbs

<...>

The flywheel is simply an energy storage device so when you accelerate, energy is stored in the flywheel, energy that would have otherwise accelerated the vehicle down the track. This energy is stored in rotation and translation.

Click to expand...

Pardon me but you are comparing apples and oranges, and the the dyno runs are no proof at all. The change in peak power from 535 to 542 hp is attributed to the acceleration phase. If revs were kept at the same figure, the curves would superimpose!

Click to expand...

Sorry to bust any bubbles here but I said nothing about power. Someone asked about any change in dyno runs between a heavier and lighter flywheel. I stated yes and linked to an illustration by a third party.

mdt-son said:

* The lighter flywheel (lesser moment of inertia I) consumes less torque and also produces less torque at deceleration. This explains the rider impression in another posting above.

Click to expand...

So this consumption of less torque in accelerating a lighter flywheel is now torque available to accelerate other things such as a motor cycle down the track. Pretty straight forward to me. So I would expect that surplus torque to show up on a dyno and manifest itself some way; at least on an inertial dyno. It is not trivial amount and offers performance enhancements.

Regardless of whether we are speaking about rotation or translation mass, a lighter vehicle/flywheel with the same power will get down the track faster than a heavier vehicle/flywheel. It is really that simple. In the case of an inertial dyno, think of the rotating drum mass of the dyno as mechanically linked to all the other rotating masses throughout the motorcycle (including wheels, tires, sprockets, gears, shafts, clutch basket, crankshaft and flywheel). Reduce rotational mass on any one of the components and it will register as a gain on the dyno in terms of acceleration - a good attribute for racers, don't you think?

mdt-son said:

The change in peak power from 535 to 542 hp is attributed to the acceleration phase

Click to expand...

I think the term you might want to check out is horsepower-second. :lol:

Knut and fast Eddie

Knut - you are refering to HP figures when the RPMS are held constant as on a dyno at a constant RPM. And you are right - at a constant RPM the HP would be the same with a light or heavy flywheel.

But in road racing the RPMs are seldom held constant unless banked to the max in a turn or at top speed. You are always accelerating to the next turn, hitting the brakes and then accelerating again. What Dances with shrapnel is saying is that you are getting more acceleration with a lighter flywheel and he is correct about that for sure. If you are taking 7 lbs off your flywheel you are also getting more acceleration simply because your bike is lighter. Maybe not much but if it adds up to a few feet by the end of the race - it could mean the difference between 1st or 2nd place.

Figures may be the same on a dyno with steady RPM. But I think that Fast Eddie is more interested in lower lap times around the track.

Everyone has their preference and opinion. I raced with a lightened flywheel and won a lot of races. There are a lot of lightweight Maney cranks around and racing - many of them winning. The winningest Norton in recent history was the Dave Watson 1007cc bike which won all its races and the championship etc with top rider Gary Thwaites using a lightweight crank and other lightweight parts.

Check out this link:
http://www.jsmotorsport.com/technical_success.asp

A heavy or light flywheel CANNOT affect how much TORQUE an engine actually produces.

Remember, torque is the result of the gas pressure on the piston(s) pushing the crankshaft round and round.
If you open the throttle a bit, a bit of torque is generated on the output shaft.
If you open the throttle a lot, a lot more torque is generated on the output shaft.
If you open the throttle fully, the full torque available AT THOSE REVS is generated on the output shaft.

What the flywheel CAN DO is influence how that torque is stored (briefly) and delivered back to the rider.
It primarily smoothes the torque flow - so flywheel weight has a huge say in this aspect of vehicle performance/dynamics.

So, flywheel affect and torque output are 2 entirely separate subjects - closely interlinked - in the real riding world.

Sorry Rohan but what you are stating is, at best, totally misleading. Where do you bound the engine, at the conrod? Maybe call torque lost to accelerating a heavier flywheel parasitic loss but that would be confusing a simple matter. I would expect a brake dyno to show little to no difference between a heavier and lighter flywheel at a fixed rpm. This is great for big diesel generators or land speed records but for road racing or even spirited road riding, in my opinion, acceleration is pretty important.

Energy used to spin up a 17 lb flywheel to say 7000 rpm remains in the flywheel
Energy used to spin up a 24 lb flywheel to say 7000 rpm remains in the flywheel
It takes more energy (sustained torque) to spin up to speed a 24 lb flywheel than a 17 lb flyhweel.

So where do you think this difference in surplus sustained torque goes? A lighter flywheel will accelerate more rapidly than a heavy one with all other things held constant, plain and simple as a lighter bike will accelerate more rapidly with all other things held constant.

Rohan said:

A heavy or light flywheel CANNOT affect how much TORQUE an engine actually produces.

Remember, torque is the result of the gas pressure on the piston(s) pushing the crankshaft round and round.
If you open the throttle a bit, a bit of torque is generated on the output shaft.
If you open the throttle a lot, a lot more torque is generated on the output shaft.
If you open the throttle fully, the full torque available AT THOSE REVS is generated on the output shaft.

What the flywheel CAN DO is influence how that torque is stored (briefly) and delivered back to the rider.
It primarily smoothes the torque flow - so flywheel weight has a huge say in this aspect of vehicle performance/dynamics.

So, flywheel affect and torque output are 2 entirely separate subjects - closely interlinked - in the real riding world.

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The question was not asking what effect crank weight has on the torque and engine produces. The question was (and still is) what will the effect be that is shown on a dyno (as in rear wheel driven dyno) ?

Well, if the flywheel CANNOT alter the torque produced, how can a dynamometer show anything different ??

Depending on the dyno it can hold a steady RPM and take readings or it can be the type that spins up a heavy wheel (spun up with the rear tire) and makes measurements that way. The latter would seem to relate to acceleration (somewhat). I don't know shit about dynos but I think I have this right as it seems obvious from the different videos.

Rohan said:

Well, if the flywheel CANNOT alter the torque produced, how can a dynamometer show anything different ??

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Let's try this as a crude oversimplification....the flywheel consumes torque under acceleration. The greater the flywheel mass, the greater the torque consumed during acceleration.

As I type this, ........ these lyrics are running through my head from Another brick in the wall by Pink Floyd

"If you don't eat yer meat, you can't have any pudding. How can you have any pudding if you don't eat yer meat?"

Rohan said:

Well, if the flywheel CANNOT alter the torque produced, how can a dynamometer show anything different ??

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Because of its effect on transmitting the torque to the wheel?

But I can't answer that... you are asking my very same question Rohan! Will a light crank show a difference in dyno readings?!

Despite some deep and insightful conversation thus far, this still apears rather inconclusive.

Fast Eddie said:

I am confused by your caveat 'if revs are kept constant' ... I would have thought that the primary effect of changing flywheel weight would be a change in the way the engine revs. My question is therefore what effect does this change in the way the engine revs, have on the dyno readings?

Click to expand...

In comparison with the heavy flywheel engine, the light flywheel engine will

1. speed up and decelerate the crankshaft faster (angular velocity),
2. less engine torque is used on speeding up the flywheel,
3. more torque will be available at the PTO as long as the engine speeds up and less as it decelerates, and
4. Provided the resolution and accuracy is good, the Dyno will show a higher torque and power during acceleration and less under deceleration of the crankshaft, if all other parameteres remain constant.

I hope this answers your question.

-Knut

Fast Eddie said:

Will a light crank show a difference in dyno readings?!

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On a brake type dyno, it will not.

HEAVY flywheels allow the throttle to be opened earlier and harder.
Ask any Harley rider....