80 HP at 8700RPM by Herb Becker

A cast iron flywheel explodes from centrifugal force. If you lighten it by reducing the diameter of the flywheel then you reduce that centrifugal force. But its not enough because Norton cast iron flywheels come from the factory with some material already removed at the bottom counterweight area - they come supplied out of round and so you end up cutting the OD everywhere but at the bottom - and that is the heaviest area and that's where they come apart. Sometimes a chunk of metal will pull out of the counterweight only.

Below is a photo of the lightened crank I raced in the 1980s. The OD is reduced and the crank was rebalanced to a higher factor. The diameter was reduced in a lathe all the way down to the journal flanges - in order to reduce centrifugal forces to help keep it from exploding. After machining on the lathe - the remaining web at the top of the crank became thin so it was removed or narrowed. Shallow dimples were drilled instead of deep holes on the upper 1/2 of the crank (to achieve the higher balance factor). This crank was frequently and relentless revved into valve float range. I started developing the lightweight pistons way back then. I didn't even have a tach - I just revved it as high as it would go. It never blew up. But a chunk still could have flown away from the bottom counterweight area because not much diameter was removed there. Some flywheels blow and some don't - mine didn't.

Most importantly - the PTO shaft was radiused to keep it from breaking as you can see in the photo (as explained in my race manual). This is a must and I do it to all my cranks - including street because this is the 1st place they will break if not radiused.

As said in the threads above - the only way to guarantee against flywheel explosion is to change the cast iron to a steel flywheel. I loved the lighter flywheel. It revved quicker, accelerated harder, It didn't slide the Rear tire as badly going into a turn as a heavy flywheel did because it would spin up faster & quicker when down shifting. Most importantly it was more throttle responsive and that made shifting quicker because you could blip the throttle to the desired RPM quicker between gears. Plus it took several pounds off my race bike - all of it adding up to lower lap times.

jseng1 said:

Some flywheels blow and some don't - mine didn't.

Click to expand...

So, the $64,000 question.
Is this the result of that you machine yours differently,
or just the luck of the draw.....

Rohan said:

jseng1 said:

Some flywheels blow and some don't - mine didn't.

Click to expand...

So, the $64,000 question.
Is this the result of that you machine yours differently,
or just the luck of the draw.....

Click to expand...

Probably both. Nature and Nurture.

Swetune said:

You shouldn´t be racing with a cast iron flywheel. It´s a potential exploding bomb between your legs. The material is brittle and when you machine it for getting it lighter it weakens it a lot. Then also the material of cast iron is not tensile and there are small cracks in the material because of the metalstructure of cole and iron. It is porous and good as liner for example as it may keep oil in the pores. I machined onedown and theres no strenght to it. Do a new flywheel/balance weight in tensile steel.

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Really a good summary. Lightened or unlightened, the cast iron flywheel is a great big notch waiting to form a crack. I blew up a crankshaft/engine some 15 years ago on my race Commando. The carnage reminded me of some photos from WWII showing a direct aerial bomb hit on a steam locomotive - metal boiler tubes and plate splayed out everywhere. That blow up on the track is crystallized in my memory. There was the crankshaft which was surprisingly still bolted together. Most of the crankcase was gone, scattered about the race track. The barrel was broken and shoved up and the cast iron bob weight was sitting there on top of the gear box, hot and dripping with oil.

Needless to say I was glad there was a steel backbone frame between me and the bob weight though the trajectory wasn't threatening in that instance. Anyone running a Featherbed or Seeley (which lacks the center spine) should appreciate this vulnerability, especially with alloy barrels as I believe the cast iron barrel offered some level of protection.

My crankshaft was lightened much in the same fashion as that shown by Jim S in the picture above except I did not drill mine. We chucked it up in a lathe and turned down the circumference to the edge of the crank cheek flange. One should compare that flywheel in the picture above to a stock flywheel where you will see that there is no cast iron flange left near the crank cheek bolt circle on the lightened crank.

In my opinion, anyone considering revving a Norton in anger should get a steel flywheel. A few year later an near identical blow up happened on my team mates Commando where the bob weight left a nifty divot on the track surface.

Rohan - both flywheels were probably lightened is a similar way as he had my race manual. You can see some deep drilling in his flywheel fragment. It could be the drilling or just plain chance -its unpredictable. I was told by my race friend Mic Olfield that the 850s used a better cast iron but can't confirm.

Dances - Yes steel flywheels would be sweet and a good aftermarket item if they were available (I'd buy one). So far I think you have to make them yourself (look at the Maney flywheels). Stock cheeks with a radiused PTO shaft aren't too bad and might hold up fine in racing conditions - they could also be Nitrided which gives a little more strength - a budget racers racing crank and rebuildable. Ron Wood did something similar back in the day - adding mallory metal to the cheeks to focus the weight closer to the main bearings to reduce flex. Then there's the problem of the cases cracking at elevated HP. Mine would crack until I reinforced the PTO side with 3/8" alum plate.

Does anyone supply a standard type commando crank in about 60 TSI steel ? My bike stood unraced for 20 years after I built it, because I'd had a good look at the crank - especially the bolts which hold it together. I have only raced it a few times and always expect the bang. I think (hope?) when the crank is balanced at 72% (dry), the risk of explosion is less because what is going up and down is better balanced by what is going around. I wonder what balance factor the cranks which blew up Jim's motors were - standard to suit isolastics ?
I've not had the problem down-changing with the heavy crank since I started using the close box. It's a beautiful motor for racing if you don't think about what is inside it. - 'ORRIBLE ! !

Rohan said:

jseng1 said:

Some flywheels blow and some don't - mine didn't.

Click to expand...

So, the $64,000 question.
Is this the result of that you machine yours differently,
or just the luck of the draw.....

Click to expand...

It´s in the material and the process of casting iron. Not every batch of cast iron has the same percentage of iron and cole component. It´s just like baking when you get air bubbles and parts of the component not having the exact similar blend in the mix. When you heat up the material sometimes there are porosities, not an even homogenous mix of the material. Therfore there can be different quality of cast iron and thus different qualities in balance weights.
Look at google pictures of the metallurgy of cast iron compared to different steel types the you will understand with those big chunk pieces in the material.

jseng1 said:

Rohan - both flywheels were probably lightened is a similar way as he had my race manual. You can see some deep drilling in his flywheel fragment. It could be the drilling or just plain chance -its unpredictable. I was told by my race friend Mic Olfield that the 850s used a better cast iron but can't confirm.

Dances - Yes steel flywheels would be sweet and a good aftermarket item if they were available (I'd buy one). So far I think you have to make them yourself (look at the Maney flywheels). Stock cheeks with a radiused PTO shaft aren't too bad and might hold up fine in racing conditions - they could also be Nitrided which gives a little more strength - a budget racers racing crank and rebuildable. Ron Wood did something similar back in the day - adding mallory metal to the cheeks to focus the weight closer to the main bearings to reduce flex. Then there's the problem of the cases cracking at elevated HP. Mine would crack until I reinforced the PTO side with 3/8" alum plate.

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My gut is telling me that only drilling the perimeter for weight reduction would be less problematic than turning down the circumference as it would preserve some of the flange near the bolt circle but it is neither here nor there as I absolutely will not use a modified cast iron flywheel for racing and would absolutely recommend not to use an unmodified cast iron flywheel for serious racing. This from lessons learned, an understanding of the material and what is at stake with regards to health and safety of the rider and competitors.

Unfortunately I don't think Steve Maney sells his steel flywheels alone so I agree those interested will need to go it alone and make their own unless someone steps in to offer steel flywheels. Again, considering what is at stake, steel is a must in certain applications. With my flywheel failure, it was from an 850 E-Start where the engine was sleeved down to 750.

There are three main types of cast iron - grey cast iron, Meehanite which is created by adding calcium to the melt to precipitate calcium silicide and make the material much stronger. The third type has Nickel added and is strong enough to be used for gears. Which one are Norton flywheels made out of ? A few years ago the father of one of out top Australian riders machined discs for an RG500 Suzuki out of fork lift discs, to solve the problem of the chrome spalling off the usual aluminium discs. The rider experienced the discs exploding off his bike at the end of Conrod Straight at Bathurst. We lost a very good friend that day.

If I was serious about road racing these days, I'd only use a billet crank made out of 3%nickel, 1% chrome steel. The trouble is buying it without the sulphur and phosphorus inclusions which usually occur right in the centre where the mainshafts would be. How many industries make barrels for big guns these days ?

acotrel said:

Does anyone supply a standard type commando crank in about 60 TSI steel ?

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The standard Commando crank is already "about 60 TSI steel." It's made with EN16 steel. 60 TSI is within it's range of tensile strength, depending on the heat treatment. Most of the aftermarket crankshafts are made from higher tensile strength material, EN24, 4340, or similar.

I get the debate about heavy vs light cranks on lap times, and I believe it is basically down to rider style / preference.

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?

I think the bike dictates the rider style. If you can ride the bike extremely aggressively with no anxiety, all is good. If you are tippy-toeing around, there is usually a reason. I sold my 500cc short stroke Triton back to the guy who built it in the 50s. I rode it again after he had fitted sticky tyres and a 5 speed box. It was much better, however I still got it sideways in one corner. With the good tyres, I simply rode through the incident. What I've found with the Seeley 850 is that I do things I feel I should not be doing - riding it on the gas as much as I can all the time, is great fun. And I know from the last time I raced it, the motor is as fast as it needs to be to win races.
You don't learn to ride well by riding bikes that try to kill you.

The question about whether the crank mass affects the horsepower. I think it affects the torque and the way the motor spins up. If you use a wide ratio box with the heavy crank and wait for the revs to build up as you accelerate, you will wait forever. With the close box, when the heavier crank is revving at about 6,500 RPM and you race change upwards the higher gear cannot cause the revs to drop much , so you accelerate faster. You use the crank inertia to your advantage. Torque is crankshaft twisting power, not the power output at max revs. A while back I did a silly thing with the 6 speed box - I didn't realise the gear-change was upside down and I gave the motor a heap of revs in about 6th gear and quickly rode the clutch out - and almost stuck the bike through a fence 60 metres away. By all rights it should have stalled. Once that crank is wound up, nothing is going to stop it.

One thing I'd like to point out - the late 60s CR750 Honda had four megaphones. Most modern four cylinder bikes have siamesed exhausts. Top end was never a problem with those motors. Also the progression of Yamaha racing two stroke motors was always about getting decent torque

lcrken said:

acotrel said:

Does anyone supply a standard type commando crank in about 60 TSI steel ?

Click to expand...

The standard Commando crank is already "about 60 TSI steel." It's made with EN16 steel. 60 TSI is within it's range of tensile strength, depending on the heat treatment. Most of the aftermarket crankshafts are made from higher tensile strength material, EN24, 4340, or similar.

Click to expand...

Ken & acotrel,

Excuse my ignorance, but what is "TSI" in connection with tensile strength? Did you mean KSI?
The standard heat treated grade EN16-R has a minimum tensile strength of 700 MPa / 101.5 KSI;
EN16-S yields 775 MPa / 112.4 KSI and EN16-T peaks at 850 MPa / 123 KSI.
These values are of no significance though, as the governing parameter is fatigue strength of cyclic tensile stress. Crankshaft cyclic stress amplitudes above 120 MPa / 17 KSI will not meet the requirements of a utility vehicle (life endurance > 10^7 cycles).
Surprisingly, fatigue strength is much less material dependent than UTS (ultimate tensile strength). Hence, casting the crankshaft in nodular iron may be a viable alternative to EN16-R.
Thus, in crankshaft and engine design it's imperative to design such that cyclic stress amplitudes are kept low. The Norton crankshaft is probably not a good starting point in this respect.

-Knut

acotrel said:

There are three main types of cast iron - grey cast iron, Meehanite which is created by adding calcium to the melt to precipitate calcium silicide and make the material much stronger. The third type has Nickel added and is strong enough to be used for gears. Which one are Norton flywheels made out of ? <...>

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acotrel,

The "third type" is nodular cast iron. This is a high-performance cast iron, often used as a resort when crankshafts made of mehanite cast iron fail. This was for instance the case at AMC with their G12 crankshafts - and surprise surprise - also the case with the Norton Navigator, which had a mehanite unit crank (non-bolted) which failed big time. As Bracebridge street didn't have a metallurgist, the experts at Woolwich stepped in and specified a nodular crank. This solved the problem.
The example cited above with the 850 Mk3 flywheel suggests they were made of Mehanite as well. However, in a race application, remaining fatigue life is consumed rapidly. Increasing the revs from 7000 to 8000 rpm increases crack growth proportional to stress cycles by a whopping 30% !

-Knut

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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Internal work of deformation energy has the units of energy (Joule) and if cycled in a certain time, internal work will consume a differential power (Watt = Joule / sec). This differential power is deducted from the crankshaft output - so yes, there should be a difference but with many parameters at play and this deduction being exceedingly small, I doubt you will spot the difference at the dyno. The displayed resulation is poor also. A run at the dyno is ussually non-repetitive, making the comparison void at the outset. A static crankshaft bending test of a locked crank will not work, as part of the work is due to dynamics (torsion and the afore mentioned centrifugal force). A mathematical simulation will probably show a difference - in the second decimal

-Knut

mdt-son said:

Excuse my ignorance, but what is "TSI" in connection with tensile strength?

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TSI is the old tons per square inch yardstick.

45 ton steel is pretty low, 80 or 90 ton is really high grade steel, better than most high strength bolts, conrods, etc.

Can also be quoted in pounds per sq inch, so you'd multiply by 2240.
e.g. 180,000 psi steel is about the 80 ton mark, and about as good as most common stuff would ever see.

I looked at those metric numbers you quoted, and mentally noted how unfamiliar they looked.

Rohan,

Thanks for the definition. Are you quoting TSI as in the UK or TSI as in the USA?
Was the "60 TSI" figure quoted by Norton in a shop manual or similar?

Notwithstanding, I think the figures match fairly well. 60 TSI (US) = 60*13.7895 MPa = 827 MPa / 120 KSI.
So, if the information holds, UTS would be between that of EN16-T and that of EN16-S.

The information isn't really useful unless it's official.

-Knut