Commando Crankshaft Porn

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acotrel, if what you say is so about availability of suitable steel, unless one is hell bent of fabricating a crank in the homeland with the alloy steel that you must ship in, it may be more economical to have it fabricated over seas and then shipped in at say maybe 20% of the original billet mass weight.
 
I've always liked Nourish cranks, however I think by importing we take a lot on blind faith. I've adopted the mindset that 7,000 RPM is the upper limit for my motor. I've long expected it to explode, that is the reason that I delayed racing the bike for years. I think most of the old British twin motors used to have a manufacturers' recommended limit of about 6,300 RPM and I'm not that far above it. In the old days we used to run 650 Triumph motors to 8,000 - most of them blew up. I hate to think about what the 850 pistons are doing to the aluminium conrods. When I get some more money, I will do a Jim Schmidt on it. I think his long rods and short pistons are a superb concept.
 
Norton conrods are tougher than tough. What you need to be concerned with is if you are still running with a cast iron flywheel. I was lucky when mine let loose as it was in a Commando frame and the big round center backbone stopped a ~9 lb. chunk from coming up from between my knees into my chest cavity. Think of what you have between you and the engine in that Seeley, couple of carbs & manifolds and an aluminum gas tank full of race fuel (Methanol) - Yikes!

If you do upgrade internals see to it that you at least convert over to a steel flywheel as that cast iron center flywheel is one great big notch
 
Watch dem flywheels .
Commando Crankshaft Porn


Common across automotive field to uprate for intended purpose .
Commando Crankshaft Porn

Mass x Veloctiy . E=Mc2. :?
Commando Crankshaft Porn

Hence Scatter Shields & driveshaft loops . CHAINGUARDS need consideration as to restraint of flails rather than oil fling . :)

Al , WHAT sort of Tacho's that SEELY run ? typical magnetic crap has 200 rpm lag / swing . A impulse types essential for accurate observation of rpm's .
Id think 7500 would only tear the guts out of the sucker earlier . Young Dunstall found it ordinarrily acceptable , chaining the wrenches to the benches .
 
Ron's Norton's vs Norton's Norton. Rubbing grind compound sticks cheeks together.

Commando Crankshaft Porn
 
Actually the crank is finished except for adding some metal for balancing. Before I could do that I needed the pistons to get the weight. They were a bit slow getting here.

Now that I have the pistons I am going to cut some 1/4 stainless plate into bolt on weights for the outer two counterweights. I got everything weighed and the weights drawn up last night. Jim
 
What lets go first - the cast iron flywheel or the holding bolts ? That crankshaft is the main reason the bike sat unraced for many years. I think the biggest threat is the piston weight in the 850.
 
Jump rope bow of over loaded crank by piston jerk down inertia acceleration resistance puts huge prying apart uneven leverage forces on bolt strecth which cracks cast iron before the bolts let go. It is not the pure centrifugal forces that bend and break our 360' end supported cranks.
 
acotrel said:
What lets go first - the cast iron flywheel or the holding bolts ? That crankshaft is the main reason the bike sat unraced for many years. I think the biggest threat is the piston weight in the 850.
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The flywheels will explode, usually somewhere over 7500 rpm. I have never seen the bolts break. I have used stock cranks to 8500 with a steel flywheel and a little careful prep. Jim
 
I've solved the problem, and cheaper than fitting a steel flywheel - I've fitted a 23 tooth Jawa engine sprocket to replace the 21 tooth item, and also raised the overall gearing at the rear by one tooth on the gearbox sprocket. My biggest worry has been the lack of lag after every gear change with the 4 speed CR box. When the weather cools off and I find some more funds I will try it with the 6 speed TTI box. I think the way the motor pulls it should be able to handle the much higher gearing. I've never liked spinning it up too high - 6,500 RPM is enough for any old British twin. If it spins up slower it should be more manageable.
My tacho is off a Suzuki RG250 - Japanese, NOT British. I've had Smith's tachos which have suddenly read zero when the motor is doing 8000 RPM in my old Triumph.

I really like these, they look so right :

Commando Crankshaft Porn
 
When my flywheel let go at Mid Ohio all the bolts were intact. Bolts and flanges are not an issue. Thinking that overall gearing will keep the flywheel intact is pure folly.
 
Where can I buy a reasonably priced steel flywheel with the exact weight characteristics of the standard cast iron one, and with the holes reamed to fit the standard bolts, to balance a near standard 850 to 72 % ? I don't have much money at present, however I will work towards it.
 
acotrel said:
Does prayer help ?
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I came across of an 'on bike camera' You Tube clip of yours recently. We don't have many (any?) circuits in the UK where you have to hang on to WOT in top for the length of time you were in that clip!
Seems to me that your flywheel is quite tough!!
 
Sounds to me like a billet crank and Jim Schmidt's rods and pistons are the only sensible way to go. I think the 850 pistons might pull the top off a rod before the flywheel lets go. In the meantime I will concentrate on getting the revs down to a safe level. Surely the 850 motor should regularly cop 6,500 RPM without destroying itself ? I'm just amazed at the way that silly motor behaves with minimum modification. I've been pulling very high overall gearing, and it is still difficult to avoid over-revving it.
 
Well after getting my new pistons and figuring the needed weights for the crank it came out I was going to end up with more reciprocating weight than I had with my 880 with Carrillo's.

I decided I just as well go all the way.

A new pair of billet Ti rods will be here in about 6 weeks. The price was pretty reasonable since the engine is built for a common small block Chevy rod.

That will let me get my total reciprocating weight down to less than the original 823 cc motor and still have a 914cc stroker motor. Jim :D
 
Jim, I 'phoned Arrow Precision for a price on a pair of Titanium conrods. I was told they would not be suitable for a road going motor due to their limited life expectancy. What are your thoughts on this?
Martyn.
 
Matchless said:
Jim, I 'phoned Arrow Precision for a price on a pair of Titanium conrods. I was told they would not be suitable for a road going motor due to their limited life expectancy. What are your thoughts on this?
Martyn.
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Ti does have a fatigue life but it is a lot higher higher than aluminum. Swanson, who is supplying my rods, didn't think it would be a concern. I hope he is right. Jim


The SN of forged aluminum is .38
The SN of Ti is .5

The SN number is what fraction of the ultimate tensile strength can be applied to a specimen 50 million times before failure.

Swanson rods have a bushing to hold the big end shell which prevents fretting and a coating on the sides of the big end to prevent thrust damage. They use an Ampco 45 bushing.
 
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