Broken crank and high compression

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This crank was subjected to 13.5 C.R. on gas (too much). Then it went 30 more races with 10.5 C.R. pistons on gas. It was a stock crank and finally gave in to stress.

Broken crank and high compression


Fortunately the rods and the pistons survived and will be raced again.

Broken crank and high compression
 
Are you blaming high compression or revs ?
I had the Seeley sitting around unraced for about 20 years, because I believed the Commando crank would not cop racing and I didn't want the expense of buying a billet crank. The billet crank in my old 500cc Triton was the only reason I ever raced it for so long. It was almost impossible to blow up, if you didn't tangle the valves.
 
That is a familiar sight to me. I've broken two Norton cranks over the years, one on my 650ss many moons ago & the other on my 850 mk2a. On the 650 I was lying flat on the tank doing an indicated 110 mph, but the conrod held the crank together long enough for me to pull the clutch in. On the 850 the drive side mainshaft sheared as I shut off, after deciding not to do a risky over take. This is why I now use Mr Maneys finest in my bikes, as the consequences of a crank breaking could be fatal.

Martyn.
 
It's hard to tell on the screen but it looks like a fatigue crack - sort of a smooth progression until the final catastrophic break.
 
Surely the most important comment here is '30 MORE' races on a '40 year old' stock crank.

It was going to go sometime!

Or was it 'new stock'? at which point one questions the logic of a stock crank when alternatives are available.
 
Matchless said:
...... On the 850 the drive side mainshaft sheared as I shut off, after deciding not to do a risky over take. This is why I now use Mr Maneys finest in my bikes, as the consequences of a crank breaking could be fatal.......Martyn.
As I was reading this thread and tabbing down, I was thinking to myself "Well I should be okay with a stock crank on my old MKIII street bike" then I got down to your entry Martyn and you have a very good point. Cj
 
You just don't know for sure when it comes to stock cranks. I lightened them and gave the PTO shaft a radius and they never broke on me when racing - but I may just have been lucky. The piston lightening probably helped.
 
SteveA said:
Surely the most important comment here is '30 MORE' races on a '40 year old' stock crank.

It was going to go sometime!

Or was it 'new stock'? at which point one questions the logic of a stock crank when alternatives are available.

It is one of the main reasons that I stick to racing on short tight circuits where the top speeds are lower. I never worry about a get-off except where the speeds are extreme. I previously crashed onto the non-skid surface at Phillip Island at about 90 MPH - down the front straight it would be 130 MPH or more, even on a slow bike. Phillip Island is lovely - the kids love it, but every crash is a biggie. A crank coming out of the motor there, is unthinkable.

https://www.youtube.com/watch?v=IhLEJxpWwTE
 
Generally, there are three classes of Norton big twin crank failures:

1.) Fracture at filet on drive side main bearing
2.) Catastrophic failure of cast iron flywheel; usually associated with unjudicious lightening of the flywheel and abuse or racing
3.) Failure of the rod journal (only seen drive side failures) which invariably go between outer filet and oil feed hole.

Note that I "go between" as I am note sure where the failure starts, but my hunch is the oil hole.

Anyone want to chime in about knowledge is timing side rod journal failures or the start of fractures on the drive side journal?

I have personally done all three and witnessed a few others.
 
Perhaps a close ratio gearbox might reduce the risk of a crank failure ? I tend to keep the revs well up, with minimum drop on the up-changes and always well match the revs on the down-changes. A wider ratio box might cause the crank to be subjected to more stress ? One thing I've noticed with the kids these days, is that many of them will change down two gears when approaching corners and sometimes even lock the rear wheel using the crank inertia. A lot of them seem to be more tyre dependent that riders who were brought up racing when we didn't have sticky tyres. - Bad habits ?
 
I'm not sure a close ratio box makes much difference. I broke a fair number of stock crankshafts racing, and I only used Quaife 5-speeds, which have a close spread between gears. I think it's more a case of fatigue from the crank flexing that finally catches up with the race engines. I've not had one break in a street bike yet, but I've seen some that have. I do think that high rpm is the big culprit here. Race Commandos spend a lot of time at 7,000 + rpm, because that's where they make power, and more power means more crank flex. In any case, with the aftermarket cranks available now, that shouldn't be the problem it's been in the past.

Ken
 
Most of the kids I see doing that are going nowhere. It often happens when they are trying to go faster, but end up going slower. In the old days, with the tyres we had, it meant an early crash. As soon as the rear end moved, you were down. With modern tyres you get away with a lot more stupidity.
 
Spot on Ken. The Commando crank cheeks are cast steel which is a decent material for road service. From a durability standpoint, the metallurgy of the newer (billet) cranks is far superior to the cast steel, add to that the nitirding for extra toughness and wear resistance makes the modern billets pretty much indestructible, but......never say never.
 
I think the combination of a billet crank and Jim Schmidt's long rods and light pistons might be good. As far as I am concerned they might only provide reliability and I cannot really afford to race much these days. So I live with the standard crank and the risk. I just don't go near big race circuits and avoid over-revving the motor beyond 7000 RPM and make every gear-change SMOOTH. I know that one day the motor has to explode, I just hope the crash is small.
 
My racing cranks cracked at the PTO shaft - so I started putting a radius there. I have seen a couple cafe street bikes (Combat) with cracks at the PTO shaft base. It takes a massively strong pair of magnets and magniflux powder to detect a crack. My oversized magnets came from a military radar station. Place the magnets on the shaft and the cheek, blow off the powder and you can see what remains along a crack. I was able to catch a couple cranks that were cracked before they failed.

Broken crank and high compression
 
acotrel said:
Most of the kids I see doing that are going nowhere. It often happens when they are trying to go faster, but end up going slower. In the old days, with the tyres we had, it meant an early crash. As soon as the rear end moved, you were down. With modern tyres you get away with a lot more stupidity.

what you call stupidity others might call advancing the sport by taking advantage of newer technologies... just like in every other powersport that's been around since the turn of the century...
 
At Beezagent blogspot/com see 'The Roland Pike Story', and in Chapter 22 of the Roland Pike Autobiography there is a description of how at BSA they overcame a problem with breaking of twin cylinder engine crankshafts by burnishing the crank radii.

Has such burnishing ever been applied to a Norton crank ? I know not, but perhaps someone who does can tell us .
 
Snotzo said:
At Beezagent blogspot/com see 'The Roland Pike Story', and in Chapter 22 of the Roland Pike Autobiography there is a description of how at BSA they overcame a problem with breaking of twin cylinder engine crankshafts by burnishing the crank radii.

Has such burnishing ever been applied to a Norton crank ? I know not, but perhaps someone who does can tell us .

A similar process is used on Heavy duty diesel truck cranks - I have seen the radius of the journals shot peened to increase their strength. I have never heard of anyone burnishing or shot peening the radius of Norton cranks. Someone needs to get on this.

Nitriding is something else to consider.

see this article:

http://www.nitriding.co.uk/crankshaft-case-study

and

http://www.shaftech.com/nitride.html

Many crank manufactures say that nitriding improves fatigue strength. I know of a machinist who had the journals of a nitrided Norton crank ground and he didn't bother to re-nitride it - the crank broke.
 
That is anecdotal evidence - what do the statistics say ? A failure at the PTO is understandable, that is the most stressed part of the crank. However sharp corners are also stress-raisers.
 
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