Commando Crankshaft Porn

Titanium has limitations because it work hardens when it is repeatedly stressed. It might not be so good as a crankshaft. I'd use it in conrods, however the crank needs to have mass unless you have a large external flywheel. The mass of the crank affects the torque characteristic. Would you want a commando that nearly stalls in a stiff breeze - two strokes tend to do that ? I don't know what life of an Arrow titanium conrod is, however I suspect it would be less than steel. The advantage in lighter pistons and conrods comes from the fact that on every single rev of the motor, they reverse direction twice. So a reduction in reciprocating weight helps the motor accelerate better.
I suggest the ultimate in crankshafts for a Commando would be a 75mm stroke forged billet crank made out of 3% Ni, 1% Cr steel which has been selected by doing sulphur prints to ensure there are no inclusions. The main problem is that if you use bar-stock, there are usually sulphide inclusions in the centre of it, which can cause failures .

http://arrowprecision.com/pages/conrods

Dances

PM for you

Snotzo

Gandini = GPM, so are stock pistons and yes they are light, fit and forget, but not much on them to modify.

comnoz said:

Jeandr said:

I don't know if it has been asked before, but why not put the counterweights on each side, very close to the crankcase shell :?:

Jean

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I has definitely been a subject of discussion with many points of view.

My personal view is that the counterweight should be split evenly with the counterbalance for each cylinder split and located on each side of that cylinders rod.
On my crank it would mean splitting the center weight and likely adding some heavy metal on each side weight.

I probably will not add heavy metal to this crank but when I get to removing the excess counterweight that is present I will be splitting the center counterbalance and removing the excess weight from the center to move it closer to what I would consider ideal. Jim

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Jeandr and Jim.

This was a topic at page 29 of this tread.

There are two moment equations affected when moving counterweights from a distributed pattern to an outboard pattern. The dynamic balance (i.e. a possible rocking couple at the bearing locations) is unaffected, i.e., both configurations produce a zero rocking couple. Local bending moment in the center of journals will be twice as large for the configuration with outboard counterbalance (C*d) vs. the configuration with distributred counterbalances (0.5*C*d). Bending moment in the middle of the crankshaft will be zero for the distributed version vs. constant (C*d) for the outboard balance version.
Here, C is the dynamic force of the counterbalance and d is the distance from the throw mass center (crank cheek mass center) to center of the journal (and conrod).
Hence, using an ouboard counterweight pattern is actually increasing the crankshaft loading locally.

Secondary effects like crank flexure (bending deformation) due to insufficient stiffness may be favourable for the outboard counterweight pattern, but they will be overshadowed by the 100% increase in journal loading.

-Knut

Just got news letter from Andover Norton an they show a one piece crank for 750 and 850 listed as 06.3106/1 not as exotic as the ones on here but it does look like an improvement over old factory bolt up type. Not cheap though for the money there may be other options an it will need balancing.

Does anyone know if the woodruff keys are in the same alignment for this pre 72 weslake/NRE low inertia crank and the standard commando crank ?



Thanks in advance