Did your stock Commando crank blow at 8,000 RPM?

Would all those who have seen 8000 rpm on the tacho, and 2 seconds later your crank disintegrated, please respond here.

Might be a mighty short response list here too ??

Guess its all in how the question is framed, isn't it....

You might want to broaden your inquiry to also cover bent cranks too.

It might not break immediately if you only did it once.
It could be terminally damaged.From there,the fatigue cracks will spread and grow.It may finally break at a much lower rpm.

For situations of constant cyclic loading,if a part breaks after 10 hrs use the cracks are normally detectable after 1 hour.
If it takes one month of use to develop detectable cracks,expect it to finally break at 10 months.

Where does this 1:10 ratio come from.
Anything scientific about it, from an engine development program for example.

Or just an estimate plucked from, um, mid-air. ??

hobot said:

You might want to broaden your inquiry to also cover bent cranks too.

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Yes, we haven't forgotten your engine's big rev, Steve.

But did you note, as it was revving its heart out, did the tacho get to the magic 8000 rpm ?!

Only times that has happened to me, in lesser shapes and forms, been too busy steering things and switching things off and hauling on the brakes to be faintly interested what the tacho was doing... !

BTW, did we get to see precisely what was bent, and where, and did anything show signs of cracking anywhere ?

Rohan said:

Where does this 1:10 ratio come from.
Anything scientific about it, from an engine development program for example.

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No engine program.I got that one from a metallurgy lecturer,when I studied metal fatigue.

Metal fatigue in what metals precisely, and on what form of stress and heat cycles.

Different alloys and different shapes greatly influence fatigue life.
And I haven't even studied this.... ??

Yes, we haven't forgotten your engine's big rev, Steve.

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Realize how servere that event was on both me and Peel as it stole most a decade from my living it up on her. Not ignoring your poke at me, I got to examine the long term abuse of a sales demo Combat in home of hillbilly tune ups and testing against 750 2 toke triples. Its crank was found bent but flywheel and studs were ok as were the original flat roller bearings, about pristine and sent off to NY expert to verify they still were many decades and 30-40,000 miles on, over 30K on oddometer but we know there's intervals of speedo down times.

To give idea how hard this thing was run, the cases were locked together severely, requiring stacks of razor blades driven in till dowels cleared, over days to get crank out. Then would not hardly mate again empty Ugh, but i eventually did for most a day of same horror to cleave apart again w/o destroying the sealing surfaces. Eventually found the cases had such torque hits they had twisted DS cw to TS and required .003" removed from same side of each locating dowel to make it reasonable to handle again. Over rev affects basic structures than just crank. I over red lined it on 7th day, till it blew the mufflers off, then many times harder opened up on drag strip when God had the day off... I'd only known my P!! prior so thought I was being easy on it.
I only took it apart d/t the oil leaks and beging to smoke. Little did I know what that meant.

DynoDavid had taken me under his wing over the years of pre-Peel and discovered the cases cracked, by Unbelievable Close Up Vision Resolution. David wears coke bottom goggles to see stuff with his nose touching it ok. Amazing! Crack was so fine it was merely the metal grain interface showing maybe 75 microns wide if that. It was found at the machined sharp edge of the engine clamp bosses. I ground them all out rather painfully deeply then took to welder who even more painfully melted deeper and deeper looking for lurking cracks not yet surfaced and showed me some, at limits of human resolution fine dark very torturous faceted line in gleaming melt the elctron flow was blowing clear, then he'd melt beyond it then back fill for wrapage that still hurts me today to recover. This was before Ms Peel whispered name my ear... Cases @ 7-800'F is just getting ready to weld oil free,
Its the welding and locations that does the seam distorting.

Valve float and points float should limit rpm before most cast iron wheels let go but the miss fires and valve/piston clash may be a show stopper the crank survives. Ms Peel rebuild was not stock so not subject of this thread nor any of that damage mentioned by me here at'tall. I think Norton was like Mr. Ford if a part was not failing then he'd weaken it to match the others but did try to fixe those with too short a life, so Crank should begin plastic deformation about the time valve train and cam whip is loosing control and points and carbs hindering further power. Only fools rev stockers into red line and criminals beyond that.

I once had to be stopped from, while in a fit of rage, taking a crescent wrench to the motor a zero RPM. Does that count?

ROTFL. !

Probably more relevant for most of us too.

I think I hit 9000 very briefly the other day.

But I cheat : Maney crank, carillo/schmidt set up... :roll:

I will start by saying I personally did not have this happen to me but I saw it first hand.

Down here here in NZ there is a group of guys who for several years raced Commandos with standard cranks and routinely proudly showed the remains of having yet another exploded crank. I never understood the logic myself and eventually even they seemed to think it was a bad idea and moved on to Maney cranks.

So yes I have personnaly seen at least two exploded standard cranks. Abused at racing rpm at at least 8000 rpm. They lasted longer than 2 seconds however. I would say a few hours of abuse before destruction.

But then anoher case. This time personnal experience.

A standard 500 Dommie crank in a race bike. (So 1/8 " smaller diameter big ends than a Commando crank) Not my bike - a friends. This was a really weird machine. It was on methanol and was the smoothest 500 Dommie I have ever ridden. (Even smoother and totally different to a 90 deg stepped crank 600 Dommie I have ridden) The owner swore it had a standard crank but the thing reved like crazy and felt like a turbine. It was really really smooth. and it just wanted to rev. I was kind to it but the normal rider rode it at 8000 rpm no issues. Im still a bit suspiceous about this machine to be honest however. The owner was a machinist and it would not surprise me it had a stepped crank - but on the other hand it didnt sound like it !!! But that 500 crank reved to 8000 plus for several race seasons no issues.

Interesting. You don't often seem to hear of problems with 500 cranks or rods, its only when they went bigger cc that things started to happen. ?
Maybe someone did all the numbers correctly ?
Although there was a stronger rod later on.
And a few versions of heads with more and more finning.

But, don't exaggerate ?
The early 500 dommie bigend journals are a full 1/4" smaller than Commando ones...
1 & 1/2" versus 1 & 3/4"
Did they get bigger somewhere later on ?

Rohan said:

But, don't exaggerate ?
The early 500 dommie bigend journals are a full 1/4" smaller than Commando ones...
1 & 1/2" versus 1 & 3/4"
Did they get bigger somewhere later on ?

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Way too long ago since I looked so prob got it wrong. I know they are smaller . My own 500 has a Nourish crank with 1.75 " big ends

I raced most of a season with a OEM crank and shifted at 8500. It had been shot peened and the flywheel was lightened and reduced in diameter.
The flywheel finally exploded which destroyed most of the engine and transmission and did severe frame damage . The memory tach said 8400 rpm at failure. The ends of the crank survived with no problems and went into the next motor. Jim

Not a stock crank/flywheel there...

grandpaul said:

Not a stock crank/flywheel there...

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True, that's why I called it OEM.

Then again most Norton parts have been fettled with -or they wouldn't fit. Jim

comnoz said:

True, that's why I called it OEM. Then again most Norton parts have been fettled with -or they wouldn't fit. Jim

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TOUCHE!

I believe a standard commando crank would cop 8000 RPM, but not for long . I think (know) that if the crank has the standard balance factor it would destroy the cases with the offloading at high revs. When even a rebalanced crank is running at high revs it probably has a bow in it at revs other than for which it has been rebalanced. The fatigue aspect of the bolts which hold it together then becomes important if the crank is regularly subjected to high revs. The other thing is the fatigue effect on the aluminium rods when the inertia of the pistons causes stretch at high revs - the loads are higher at high revs. My approach is to use the revs for which the bottom end has been designed, and direct all engine mods at increasing torque, rather than top end horsepower. In any bike the gearbox is extremely important. If the torque output is increased, gearing for it can become a problem when first gear in a close ratio box becomes much higher as the overall gearing is increased to use the extra mid-range urge productively.

So lets review the reports so far, we know that TC ran over 120 WOT 150 8000+ rpm shift points though he told me he sometimes over shot that on his cast iron factory flwyheels and factor Al rods w/o an issue, and I've also spoke with the old school desert racers who would hold on too 8500-ish now an then to show them new upstart inline fours a thing or two on eating dust, then Jim's more sustained 8500 rpm season of use till fracture. A nerd with a slide rule might plug in the peak acceleration values at 8000 and 8500 and length of a vintage race and number of races for Time to Failure estimate graph. IRRC a 750 after redline puts like 7 tons of sling force between the bearings.

The trouble with cast iron is that some % of castings will contain significant internal fractures so its luck of the draw if yours in one of these time bombs or not. I ain't at all worried about factory Trixie as just don't wring her neck off but on the one I intend to abuse a steel wheel is best insurance to initially spend on any serious upgrade rebuild. How lucky do ya feel?