Reverse weighted Norton crankshaft ?

Rohan said:

Should we anticipate any problems picking an infinitely heavy bike up off the sidestand ?
Or any gyro problems in corners ??

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Let's use an imaginary and theoretical black hole in the very center of your crankshaft with no rotational inertia. It would make it impossible to pick the bike off the stand and would make it impossible to accelerate the bike but it would damp out all vibration forward and aft, up and down while the engine was spinning. There would be no additional gyro problems in the corners but the second you attempted to lean the bike to turn it would go splat!





Below is a good example of a bad example. By inspection it looks like the builder was trying to move as much mass to the outside cheeks as possible. Axtell and others made mistakes too you know. It does "look" like it has a lower rotational mass which is desireable in some applications.

rvich said:

beng said:

Tuners have done this. C.R. Axtell used to do it to Triumph dirt track engines he built, and in the photo below a Norton crank built by Heinz Kegler. Axtell said it made the Triumph cranks last longer in his racing engines.

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I don't remember seeing that photo turn up in the "crank porn" thread. It would be nice to include it in that gallery as it certainly fits. Do you have any data on what it weighs or the balance factor?

Russ

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With a Norton 360 crank the goal is to minimize the magnitude of the load reversals due to the dynamic forces of imbalance. This is a trade off between magnitude of bending moments at TDC and BDC (they are similar but not identical) and 90 and 270 degrees. For those really into it you can calculate this for 0, 90 and 270 degrees and graph it. From recollection, for a 750 Commando the target is removing two pounds of weight off of the center bob and adding one pound to each cheek. This is the best compromise for this particular crank from the factory.

As for the comment about wondering if Norton got anything right, I would say their cylinder head was a home run from an intake flow, flow coefficient and configuration standpoint, even if it were arrived at by shear dumb luck (IMHO). I believe (I am speculating here) the drive for the head design was to get the exhaust ports further away from each other to allow more fins and more cooling. This allowed the intake ports to have a straighter approach to the valve head as well as inducing a swirl (turbulence) in the combustion chamber because now the intake ports were pointed more tangential to the cylinders. Best way to visualize this overall enhancement in geometery is to try and visualize what the intake port and valve spatial relationship would look like if the exhaust ports were directly front and the intakes were directly rear (as in a Triumph). Try and visualize rotating the port about the valve stem but out of the rocker arm/valve stem plane and see how that allows a steeper angle of attack to the valve head. This is all achieved while keeping the included angle between the exhuast and intake valves at a relatve minimum (ie not much of a dome necessary on the Norton piston because the valves are relatively flat laying (compare to a Triumph or BSA)


When you look at the discharge coefficients of a stock or slightly modified Norton intake the values are superior to those of IC engines of the day and well into the future.

Tintin said:

Rohan said:

Would this give less potential flywheel flexing ?

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Yes, off course, but also more vibration could be felt by the driver. IMHO the bikes are actually running overcritical - meaning beyond the first resonance frequency of the crank train - at least at peak rpm and thus are actually "self-balancing" themselves to a certain extent. The crank most likely is already on its way back from the first order node by then. Very unusual by todays standards but then there is a reason that a Norton twin engine doesn't look like a BMW P-twin. The AJS/Matchless twins with middle bearing are known to shake substantially more then their Norton and Triumph counterparts.

In a Commando where the Isos take care of the vibes it would make sense to reduce the load on the mains by moving the bobweights as far out as possible. In a Featherbed race bike it would mean the rider would have to tolerate more vibes.


Tim

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Moving mass from the center bob weight to the outside bob weights will do nothing for vibration. It will change the magnitude of the bending moment reversals due to the natural out of balance nature of a 360 crank.

Dont know about the position of the weight, but would have a thought a 1 piece crank as used on the Norton race bikes, would have been a much better idea than messing with the position of the weight?

Dances with Shrapnel said:

Moving mass from the center bob weight to the outside bob weights will do nothing for vibration.

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Yes, it will. It changes the lever the bob weight forces have (meaning it changes the equivalent imbalance due to changing the reference planes of the individual imbalances), it changes the stiffness of the crank, it changes the load on the bolt planes and thus the pre-load and thus the stiffness of the central flanges, it actually changes the whole system.

It does not change anything if you don't accurately model the rotor, off course. The thing is - I'm not talking about a hand calculation by means of pencil and calculator. I'm talking about a proper non-linear dynamic FEA which would easily consume one man-week for a usable result. The bolted flange would already mean quite a hurdle to take.

It will change the magnitude of the bending moment reversals due to the natural out of balance nature of a 360 crank.

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And you don't see a connection of "magnitude of bending moment reversals" and "magnitude of vibration"? Well, I do.


Tim

Carbonfibre said:

Dont know about the position of the weight, but would have a thought a 1 piece crank as used on the Norton race bikes, would have been a much better idea than messing with the position of the weight?

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Understood. The Norton design crankshaft design is not really that bad; it's the factory materials used that has reached their limits. Steve Maney has some excellent three piece cranks which many racers get many grueling race seasons out of. Steve started with steel flywheels and nitrided factory cheeks but has moved to nitirided cheeks made new from CNC billet.

Factory cheeks are cast steel which is a decent material for general crankshaft manufacture. The factory flyhweel is a coarse grained cast iron which is one big notch waiting for a crack to form; really nasty stuff and a must replace (with steel) in racing applications.

Pretty much any serious build I do has a billet crankshafts and I have sorted out a clean way to getting with a 35mm main journal (instead of stock 30mm) while still honoring the engines main bearing envelope of main bearing width and outside diameter. Adding an additional 5mm to the main journal, fabricating from billet and nitirding makes the crankshaft virtually indestructible. Now I break other things on the engine.

Dances with Shrapnel said:

Below is a good example of a bad example.

Click to expand...

I tend to disagree - at least based on the assumption that modifying stock parts was the only option which it probably was.

By inspection it looks like the builder was trying to move as much mass to the outside cheeks as possible.

Click to expand...

Which is a very good idea if you want to give the crank assy a happier life.

With a Norton 360 crank the goal is to minimize the magnitude of the load reversals due to the dynamic forces of imbalance.

Click to expand...

It might be a language problem but in that sentence I'd expect the word "amplitude" instead of "magnitude". For me "minimizing the magnitude" would actually translate into "divide by (multiples of) ten" in this context but that might be German vs. English then.

Nevertheless, how do you minimize the crank bending moment? Either you minimize the force or the lever (or both).



Tim

Strange the singles and V twins , if going for horsepower at the drawing board , have two bearings ea side ,
to force alignment of the shafts .

In the day , Dunstall came out with his 8 Ball ( big balls ) Main bearings , as tuned Commandos raceing
were disentegrateing / overloading the std Ball Races .

Alledgedly The H.D. ones , or roller bearings , held it all in line , so the CRANK broke not flexing .
or I would say incrrectly flexing / stress concentration . Load concentration area . BANG .

So ? how many rpms are the Maney cranks safe to , say drag raceing . :?:

Dances with Shrapnel said:

I don't know what you mean by "flywheel flexing" but the center flywheel (versus two flywheels to either side) will dampen flexing.

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That would be the case if the centre flywheel would be a real flywheel only in the sense of a rotationally symmetrical item (a Laval rotor) but the Norton design ads quite a bit imbalance to the whole assembly and therefore it also adds quite a bit to the flexing IMHO.


Tim

Tintin said:

Dances with Shrapnel said:

I don't know what you mean by "flywheel flexing" but the center flywheel (versus two flywheels to either side) will dampen flexing.

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That would be the case if the centre flywheel would be a real flywheel only in the sense of a rotationally symmetrical item (a Laval rotor) but the Norton design ads quite a bit imbalance to the whole assembly and therefore it also adds quite a bit to the flexing IMHO.


Tim

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All the mass, balanced and out of balance contributes to the rotational momentum (flywheel effect). The imbalance creates moments which induce strain/stress.

Matt Spencer said:

Strange the singles and V twins , if going for horsepower at the drawing board , have two bearings ea side ,
to force alignment of the shafts .

In the day , Dunstall came out with his 8 Ball ( big balls ) Main bearings , as tuned Commandos raceing
were disentegrateing / overloading the std Ball Races .

Alledgedly The H.D. ones , or roller bearings , held it all in line , so the CRANK broke not flexing .
or I would say incrrectly flexing / stress concentration . Load concentration area . BANG .

So ? how many rpms are the Maney cranks safe to , say drag raceing . :?:

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Others have taken the Maney 89mm stroke billet cheek crankshafts to 7,000 rpm regularly in race and 8,000 rpm in anger for seasons. Where problems have arisen are when the crank is ground undersized and 1.) too small of a filet radius is ground in or 2.) the pieces are not nitrided after the regrind or 1 & 2 combined. I know of two instances where there was failure after the above scenarios. No mistake about it, Steve Maney's crankshafts are top notch and many people use them

Tintin said:

Dances with Shrapnel said:

Below is a good example of a bad example.

Click to expand...

I tend to disagree - at least based on the assumption that modifying stock parts was the only option which it probably was.

By inspection it looks like the builder was trying to move as much mass to the outside cheeks as possible.

Click to expand...

Which is a very good idea if you want to give the crank assy a happier life.

With a Norton 360 crank the goal is to minimize the magnitude of the load reversals due to the dynamic forces of imbalance.

Click to expand...

It might be a language problem but in that sentence I'd expect the word "amplitude" instead of "magnitude". For me "minimizing the magnitude" would actually translate into "divide by (multiples of) ten" in this context but that might be German vs. English then.

Nevertheless, how do you minimize the crank bending moment? Either you minimize the force or the lever (or both).



Tim

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You missed my remark on the picture, I too would not have gone that route. Actually, in the day they (Woods) was using billet cranks. But to be honest, I don't know the date of the referenced picture so I don't know the "day". Dave Nourish was also cranking out billet crankshafts for Nortons since before the earth cooled off.

As far as moving as much mass to the outside cheeks, this is a flawed approach and I would tell anybody not to do this to a Norton crankshaft unless you are looking to optimize it for use as a boat anchor.

I don't know if I can explain in detail the concept of bending moment adequately on this forum; this is engineering statics and dynamics. Forces that induce moments are calculated as one goes down one end of the crankshaft ceneterline. Pick a particular RPM (say 7,500) and for TDC, BDC, 90 and 270 degrees one can calculate the forces due to the dynamics of the crankshaft connecting rod(s) and piston(s) system. The crankshaft is only constrained at two points which are the main bearings. As an example, at TDC you have the forces attributed to the pistons and connecting rods deaccelerating partially balanced by the centripetal forces generated from the imbalance (by design) in the bob weights. If all of your out of balance mass were concentrated outboard by the bearings you still have the two con rods (inboard) trying to bend the sh*t out of the crankshaft. By placing some of the bob weight between the two connecting rods you now have a centripetal force reducing the bending moment. So look at this as a snap shot and then do the same analysis for 90/270 degrees and at BDC which has a different rod/piston deacceleration profile. The crankshaft is in different bending conditions at TDC, BDC, and 90/270 degrees. When considering durability of materials, fatigue is a fuction of the the number of changes in stress and the magnitude of the changes in stress. So by minimizing the magnitude of the load reversals (changes) due to the dynamic forces of imbalance you extend the service life of the part.

Tintin said:

Dances with Shrapnel said:

Moving mass from the center bob weight to the outside bob weights will do nothing for vibration.

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Yes, it will. It changes the lever the bob weight forces have (meaning it changes the equivalent imbalance due to changing the reference planes of the individual imbalances), it changes the stiffness of the crank, it changes the load on the bolt planes and thus the pre-load and thus the stiffness of the central flanges, it actually changes the whole system.

It does not change anything if you don't accurately model the rotor, off course. The thing is - I'm not talking about a hand calculation by means of pencil and calculator. I'm talking about a proper non-linear dynamic FEA which would easily consume one man-week for a usable result. The bolted flange would already mean quite a hurdle to take.

It will change the magnitude of the bending moment reversals due to the natural out of balance nature of a 360 crank.

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And you don't see a connection of "magnitude of bending moment reversals" and "magnitude of vibration"? Well, I do.


Tim

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Excluding any "structural" design changes, moving mass about a crankshaft will not improve the stiffness. Stiffness is a function of material properties (Youngs Modulus and strength), cross section and in the case of columns, slenderness ratio. These are all intertwined.

There may be an ever so slight increase in vibration due to a greater bending moment if it would result in a significant strain (deflection) resulting in further out of balance so yes I do see this but it is a fart in a wind storm. You are making my point that you want to minimize the change in bending moment and to do this you need to have a proper counter balance mass in the center bob weight as well as on each side bob weight.

Draw a free body diagram (FBD) of the Norton crankshaft and calculate the accel/deaccel forces at TDC and BDC using 100% of rod big end mass and say 53% of the reciprocating mass. Do this at say 7,500 rpm. Then have a look at the FBD at 90/270 where you only have 100% of the connecting rod big end. I know this is not a "proper non-linear dynamic FEA" but it is so apparent that it jumps right out at you once you do/see the analysis. There are different forces (reactions) at different crankshaft positions and the goal is to minimize the change in bending moments resulting from these forces from crankshaft position to crankshaft position.

Do the FBD and calculate the forces and it will become apparent.

This was done for me by a fellow vintage road racer and friend who spent his years at Pratt&Whitney and then at Hamilton Sundstran as a mechanical engineer. As an engineer I can understand his analysis but this fellow who did the analysis lives, eats and sleeps engineering and it comes naturally for him. I would be more than happy to scan and post for further discussion but it must wait until I get back to my office as I am in the field and will not be back until before Christmas.

Tintin said:

Dances with Shrapnel said:

Below is a good example of a bad example.

Click to expand...

I tend to disagree - at least based on the assumption that modifying stock parts was the only option which it probably was.

By inspection it looks like the builder was trying to move as much mass to the outside cheeks as possible.

Click to expand...

Which is a very good idea if you want to give the crank assy a happier life.

With a Norton 360 crank the goal is to minimize the magnitude of the load reversals due to the dynamic forces of imbalance.

Click to expand...

It might be a language problem but in that sentence I'd expect the word "amplitude" instead of "magnitude". For me "minimizing the magnitude" would actually translate into "divide by (multiples of) ten" in this context but that might be German vs. English then.

Nevertheless, how do you minimize the crank bending moment? Either you minimize the force or the lever (or both).

Tim

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Yes Tim, the english language is a stinker at times when it comes crashing into engineering.

Amplitude has a hint of vector and infers movement whereas magnitude is a numerical quantitative measure (scaler). My intent was numerical quantitative measure and perhaps this is where you are confusing moment with movement. Yes with every force there is a stress and resulting strain (thus movement) but I was discussing durability through the reduction of the changes in bending moments of the Norton twin crankshaft through different crank angles. The strain (movement) is a fart in a wind storm.

You need to capture what the counterweights are "countering" at different crank angles to understand. To simply say we are going to "minimize the crank bending moment" is... too simple. As the crank rotates at speed the moment changes and it is the change in the moment that we want to minimize.

Dances with Shrapnel said:

As far as moving as much mass to the outside cheeks, this is a flawed approach and I would tell anybody not to do this to a Norton crankshaft unless you are looking to optimize it for use as a boat anchor.

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I'm not sure I'd buy this line of discussion.
Guzzi took world championships with external flywheel engines, and many vehicles over the centuries had external flywheels (Norton had a line of Manxes with them, not fully developed though). Obvously there is more to resolving internal forces and stresses than as above....

Rohan said:

Dances with Shrapnel said:

As far as moving as much mass to the outside cheeks, this is a flawed approach and I would tell anybody not to do this to a Norton crankshaft unless you are looking to optimize it for use as a boat anchor.

Click to expand...

I'm not sure I'd buy this line of discussion.
Guzzi took world championships with external flywheel engines, and many vehicles over the centuries had external flywheels (Norton had a line of Manxes with them, not fully developed though). Obvously there is more to resolving internal forces and stresses than as above....

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You are confusing flywheel mass with my threads above about shifting counterbalance (or throw) weights from between two rod journals to outside of the two rod journals.

Yes, there are a variety of very successful motors with flywheel mass often found on the end of the crankshaft. Have a look at any car and what do you see.

In the case of the Norton twin, having the extra mass between the two rod journals helps damp the bending moments since the out of balance centripetal forces must now translationally accelerate the extra mass of the flywheel in the center. If the Norton had a center main bearing then there would be other possibly more viable places to place or distribute the flyheel weight. This is does not interest me.

By example, take a 1 pound weight rotating about a slender but stiff shaft at 44.5mm radius; the shaft is supported by bearings. Spin it at 7,500 rpm and the centripetal forces (impulse) at the bearings will be significant. Now add a 20 ton balanced flywheel on this shaft in the same plane as the 1 pound mass at 44.5mm radius and spin it. No where near the centripetal forces will be reporting to the supporting bearings as impulses. The centripetal force from this one pound out of balance mass must react against the 40,000 pound (20 ton) mass that is rotating but with virtually zero translational velocity up down, forward or backwards. We have never attempted to quantify the damping the flywheel provides while being located between the two rod journals.

One take away on this discussion of flywheels is that the heavier the Norton flywheel, the less the main bearings will be loaded due to out of balance force (ie less vibration). Think of my infinitely massive flywheel analogy.

Yes, there are lots of other complexities in terms of harmonics but this is fundamental engineering statics and dynamics.

As for "buying it", this reminds me of my youth while sitting in an Abnormal Psychology class I chimed in about something theory I "did not buy"; the Prof was quick to say "you don't need to buy it, you just need to get it right on the test". I'll never forget that lesson.

Your discussion reminds of the ferocious battle about whether v-twins can have side-by-side conrods, or knife-and-fork concentric rods.
Needless to say, both types are perfectly successful.

If Mr Axtell successfully used that variety of crank as the pic shows, then there is more to this than you suggest.

Maybe a central flywheel mass to damp forces are as detrimental as beneficial, if Mr Axtell prefers his type of crank ?
Whether the counterweight is between the journals or outside of the journals, the Norton crank is almost in the realm of a single cylinder flywheel design. They can pretty much have the counterbalancing anywhere convenient. Overhung, in some types.....

Dances with Shrapnel said:

As far as moving as much mass to the outside cheeks, this is a flawed approach and I would tell anybody not to do this to a Norton crankshaft unless you are looking to optimize it for use as a boat anchor.

Click to expand...

I would agree to a certain extent if it wasn't for the bunch of bolts in the middle of that particular crankshaft.

I don't know if I can explain in detail the concept of bending moment adequately on this forum; this is engineering statics and dynamics. Forces that induce moments are calculated as one goes down one end of the crankshaft ceneterline. Pick a particular RPM (say 7,500) and for TDC, BDC, 90 and 270 degrees one can calculate the forces due to the dynamics of the crankshaft connecting rod(s) and piston(s) system. The crankshaft is only constrained at two points which are the main bearings. As an example, at TDC you have the forces attributed to the pistons and connecting rods deaccelerating partially balanced by the centripetal forces generated from the imbalance (by design) in the bob weights. If all of your out of balance mass were concentrated outboard by the bearings you still have the two con rods (inboard) trying to bend the sh*t out of the crankshaft. By placing some of the bob weight between the two connecting rods you now have a centripetal force reducing the bending moment. So look at this as a snap shot and then do the same analysis for 90/270 degrees and at BDC which has a different rod/piston deacceleration profile. The crankshaft is in different bending conditions at TDC, BDC, and 90/270 degrees. When considering durability of materials, fatigue is a fuction of the the number of changes in stress and the magnitude of the changes in stress. So by minimizing the magnitude of the load reversals (changes) due to the dynamic forces of imbalance you extend the service life of the part.

Click to expand...

I would perfectly agree with that statement if there was a middle bearing and no bunch of bolts.

The reason why I would like to do said non-linear dynamic FEA is that I have done my share of hand calcs on this kind of stuff - and based on my experience from working with some really good FEA and MBS specialist on cranktrains all I can say is that a simple FBD is flawed to some extent that the necessary abstraction level is quite high in comparison. We have e.g. run cranktrains covered with strain gauges everywhere and the reality behind the models is so complex that IMHO too many details are dropped on the path to your FBD. For BDC I agree with you that the mass distribution with similar portions of the bob weight either side would help the crank - for TDC it heavily depends how the roughly 50kN of gas forces are fed into the system - and that occurs either side. For 90/270 degree 100% of the bob weights outboard and a Laval rotor in the middle would be the best solution in terms of bending moment.

In terms of just the bending moment the whole system is flawed due to the absence of the middle bearing and either way you shoot yourself in the foot IMHO - you just have to decide which foot. In terms of vibration the weak rotor might actually help but without a proper analysis this is just a gut feeling as I said.


Tim

Dances with Shrapnel said:

Excluding any "structural" design changes, moving mass about a crankshaft will not improve the stiffness.

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Well - by moving the imbalance from one side of the flanges you are changing the structural design quite significantly IMHO. That is the point you are missing.

Do the FBD and calculate the forces and it will become apparent.

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Honestly - it doesn't really and it will become apparent when you throw in the gas forces as well. That particular installation is far to complex IMHO. There are good reasons that nobody would design anything like this today - the missing symmetry under firing in the first place.


Tim

Rohan said:

Your discussion reminds of the ferocious battle about whether v-twins can have side-by-side conrods, or knife-and-fork concentric rods.
Needles to say, both types are perfectly successful.

If Mr Axtell successfully used that variety of crank as the pic shows, then there is more to this than you suggest.

Maybe a central flywheel mass to damp forces are as detrimental as beneficial, if Mr Axtell prefers his type of crank ?
Whether the counterweight is between the journals or outside of the journals, the Norton crank is almost in the realm of a single cylinder flywheel design. They can pretty much have the counterbalancing anywhere convenient. Overhung, in some types.....

Click to expand...

Do you have any idea of what that Axtell crankshaft has been through? Whatever it did or did not survive through, it is only a point estimate; hardly enough to model the universe from. If Axtell used that variety then why is there more than one? Give me that crankshaft and the appropriate balance factor and I can break it; guaranteed. As stated earlier, for true bullet proof crankshafts for a Norton twin one needs to go with billet construction and larger main journals, and yes the extreme would be a center bearing but this is not necessary since there are other engine constraints one runs up against to limit the power.

You can certainly argue away against the laws of physics as to what makes sense to you about where a flywheel mass would be most beneficial for a Norton twin.

rvich said:

beng said:

Tuners have done this. C.R. Axtell used to do it to Triumph dirt track engines he built, and in the photo below a Norton crank built by Heinz Kegler. Axtell said it made the Triumph cranks last longer in his racing engines.

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I don't remember seeing that photo turn up in the "crank porn" thread. It would be nice to include it in that gallery as it certainly fits. Do you have any data on what it weighs or the balance factor?

Russ

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This crank weighed around 19 pounds and it's balance factor was set at 70%. I have a lot of Heinz' information from when he made the crank, including over two pages of him calculating balance factor and the inside volume of each part so he could figure the weight of the oil in them. It showed up in the INOA newsletter about a decade ago. Last I heard the guy that Heinz made this crank for has not to date run it. The idea behind the crank was not to make it lighter, but to move the weight out of the central flywheel and close to the bearings so that it would flex less in use and maybe be more durable at high rpm.

Heinz started to develop cranks with the weight moved out to the main bearings because he was having problems breaking crank-pins in his 72.6mm stroke 500cc Norton twin road racer, which he had tested to 9000 rpm and had run to 8k in races.

Go ahead and add this photo and info to the Crank Porn thead, I don't know where that thread is.