Norton Manx and 88 twins at Daytona.....

Has anyone done or seen a hi-speed video [super slow-motion presentation] of a big Norton &/or Commando crank whirling in anger?

I've seen one set up to do it.
But didn't actually see it.

Apparently they just go round and round.
If there is any flexing, it wasn't apparent.
Not that its easy to simulate the rods pushing on anything....

But, the trouble with cast iron is that the instant there is a crack, it can propagate - CI is brittle, where steel will happily flex, and even if there is a crack it will not generally spread. Chalk and cheese, at this level.

You'll not find too much cast iron in a aeroplane, anywhere.
Possibly with the exception of the engine block, or cylinders, in some early birds.
Later engines almost entirely have steel cylinders - usually turned from the round.

"Simulate" what? I meant using an endoscope-type probe camera in a dyno run scenario, or even via remote telemetry, on the go.
Not much Fe in aircraft Rohan, its too heavy...aircraft mills want less than 1 lb/hp ratio..

Aircraft used a lot of steel, some still do.
My point was that cast iron is brittle (in most grades), and aircraft and brittle don't go together.

If cranks (seriously) flexed, the joints between the central flywheel and the bolt-on journals would show rubbing/wear. Can't say I've seen that - or seen that mentioned.
How they flex may be quite subtle, interested to hear.....

The crank that was famous for flexing was the old Austin 7 bent bit of wire - 4 journals, and only a bearing at each end. They didn't have the reputation for breaking them though ??

Inside the crank while its running is just a huge malestrom of swirling oil, oil mist and gobs of oil - camera obscura is the phrase that comes to mind, not with the original meaning though...

Hobot suggested somewhere maybe some little pegs/sensors set to run real close to the crank, see if flex could trigger them. More likely they'd act as oil scrapers, and at that speed the oil has considerable force to it.... ??

How do you know Rohan? Did you leave a sump plate off & look? This is the 21st century old boy, there are ways & means...

I agree with Rohan on this, it's a maelstorm of oil and air in the crankcase.

Think about it a minute, what do you really anticipate seeing, a few thousandths, several thousandths, several tens of thousandths displacement.

I suppose some sonic or other distance measuring device with high speed data logging would give you more accuracy than a visual. Strain gauges with telemetry might be viable and I recall being used in a similar application.

As I understand it, these cranks do flex, and crank flex led to the development of the hybrids.

Matchless contracted with Reynolds to develop a new frame for their new engine, the 750 version of their 650 found in the G12. AMC developed this engine with 3 main bearings, thinking the additional support would make a more reliable motor. AMC found that, when run hard for an extended period of time, the crank would break. Seems that 3rd bearing in the middle of the crank kept it from flexing, and either the crank or crank case would end up splitting. Not much of an issue with the G11(600cc), but was a bit of a problem with the G12 (650cc) and its longer-stroke crank, and disastrous with the G15/45 (736cc).

Matchless had invested a fair amount of money to develop the G15/45, as this 750 variant was known, and they didn't fair well when desert-raced. Norton's new Atlas had a few teething problems at the start of production, but these were soon sorted and the 750 Norton became a pretty reliable motor. The slimline featherbed frame, however, wasn't as robust as needed for desert racing with that 750.

The story is that the west coast distributor for AMC had a G15/45 with a blown engine and an Atlas with a cracked head stock. Someone thought to stick the Atlas engine in the Matchless frame, and came up with the Atlas Scrambler. The first batch or two had teledraulics, but by 1964, the N15/G15 had Norton wheels and roadholders.

I almost bought a 31CS project last year (the AJS version of the G12). It came with an extra crank, as the one in the motor was broken. Trying to make this engine work was just one of the things that helped drive AMC off its own fiscal cliff.

No doubt they do flex; my point was, what does one think they can see as opposed to measure. Good history by the way.

For well over a century now, scientific advances have shown that the standard human Mk 1 eyeball misses a fair bit of data..
Time/speed factors, non-visible radiation spectrum [seen your own bones lately?] X-ray/C.T. type stuff.
Frank might have something to contribute on this, I`d reckon the likes of Boeing Aerospace have some super-trick lab facilities.

It is well known that Boeing, and most other aircraft manufacturers, let the customers do the flight testing for reliability....

AMC cranks had the crank breakage problems with the cast iron cranks ?
Going to meehanite fixed this.
That mention of cast iron again...

How much the cranks flexed is an interesting question.
It had been suggested the only time they truly flexed was in the act of breaking.....

Could there have been some 'method in the madness' by using a hard/rigid cast flywheel to join the ropy/flexible crank journals?
Encapsulating known movements within an acceptable/practicable range?
That functioned well enough in road/production use , but becoming problematic when raced/stressed beyond metallurgical/elasticity limits?

Its not unknown for the bolted-up flywheel assembly to give trouble on (hard ridden) road bikes ? The MK3 850 got an upgraded/ stronger set of bolts/studs to bolt up the flywheel...

Bert Hopwood claimed that he designed the original Dommie (Model 7) crank as a steel (one piece ? was it ?) affair. But the machine shop said they couldn't machine it like that.
Hence the bolted up design that eventuated.

J.A.W. said:

Could there have been some 'method in the madness' by using a hard/rigid cast flywheel to join the ropy/flexible crank journals?
Encapsulating known movements within an acceptable/practicable range?
That functioned well enough in road/production use , but becoming problematic when raced/stressed beyond metallurgical/elasticity limits?

Click to expand...

I cannot comment on the decision to go with a three piece crank but I am pretty sure the selection of cast iron for the flywheel and cast steel for the cheeks was purely a matter of economics and manufacturing process. What better material then a cast material to get the mass and the peculiar bob weight shapes with a minimum of machining to final tolerances.

The bolt together crank works rather well when not spun too high an rpm on a regular basis. I have not heard of any bolt failures but have experienced one catastrophic cast iron flywheel failure in a race application; the flywheel had also been lightened considerably and may have been a contributing factor. Cast iron has very poor flexural durabilty as it is a cluster of notches just chomping at the bit to start a crack.

Aside from the cast iron flywheel, (from personal experience and experience of many others) the cranks typically start to fail at the drive side main bearing journal filet, the rod journal filet and/or the rod journal oil feed ports.

My understanding is that the OEM crank cheeks were/are cast steel and not Mehanite (variant of cast iron). Upgrading to billet steel cheeks with proper heat treating and nitriding and a steel flywheel (aka Steve Maney race crankshafts) renders a crankshaft that can handle the more modern race rigors (nearly indestructible).

The weakness in a Norton big twin crankshaft is the materials of construction, not the design.

I rebalanced my 850 crank to 72%. I would have preferred to get it up to 78% but after plugging the hole opposite the journals with steel, could only dare to drill the flywheel very carefully, so as not to weaken it. Getting the balance factor beyond 72% seems dangerous to me. In any case the bike rocks back and forward when idling, and is still very smooth at 7,500 rpm. I don't believe the flywheel is ever very stressed, but grey cast iron typically has a tensile strength of less than 20 TSI. You would never make a complete crank out of it, or meehanite, even the grade which contains nickel and can be used for gears , would be suspect. If we are getting serious, a billet crank is the only way to go. Even then you should be careful to machine to avoid sulphide inclusions if you are using bar stock.

AMC crank breakage problems were solved by going to meehanite, I never mentioned Norton there anywhere. Heat treatment is involved there, somewhere ?

Norton cranks are renowned for breaking if the fillets are not redone CORRECTLY when the journals are ground undersize - the factory put out a bulletin about this ?

Mallory metal inserts are a way of increasing the flywheel weight, rather than drilling near the conrod journals. Mallory is tungsten, far heavier than steel....

According to Charles Fayette Taylor, "fillet radius is the most important factor in crankshaft strength."

In Taylor's book, there is a graph which shows the dramatic influence the ratio of filet radius to shaft diameter has on durability. Taylor also suggested undercutting crank webbing to achieve an adequate filet radius when necessary; we have done this on a few cranks on the power take-off main journal.

Of possible interest, Oct`12 Classic Bike mag has a Mike Nicks article covering P.Dunstall, inc interviews discussing the Domiracer:
" I got 1 near-complete 500cc bike, & enough bits to build 3 or 4 more...engines were completely different to the standard road motors. There wasn`t a single nut or screw that was interchangeable. It was a beautiful little thing. It had a much shorter barrel because they used slipper pistons so that the slipper bits of the pistons were going down between the crank webs...beautiful Mahle pistons. On the 650 & 750cc race bikes we used the factory`s little tiny cam followers with the long blown pushrods, & the needle roller camshafts where we could.
We used to get main bearings spinning in the crankcases sometimes; their mainbearings had a flange around them with 3 Allen screws in, & you`d think, '-thats a good idea'."

We used to get main bearings spinning in the crankcases sometimes; their mainbearings had a flange around them with 3 Allen screws in, & you`d think, '-thats a good idea'."

Click to expand...

Manx and Inter engines have been doing that with their bearings for generations.
Before someone invented allen screws, in fact ??

Rohan said:

Before someone invented allen screws, in fact ??

Click to expand...

In 1909–1910, William G. Allen patented a method of cold-forming screw heads around a hexagonal die (U.S. Patent 960,244). Published advertisements for the "Allen safety set screw" by the Allen Manufacturing Company of Hartford, Connecticut, exist from 1910. - courtesy of Wikipedia :wink: