American Eagle 750 GT - Crank Modification & Diet

chrisk said:
First one with sprocket protecting, furthest from camera. It?s only got a bit of a twist and hasn?t been on the same diet as the others
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Twisted in the opposite direction...

piet
 
Quentin's crank is for Australia ... the southern hemisphere.

The rest must be for the northern latitudes allowing for the coriolis effect I reckon.

Or maybe Red hit the turps early that day.
 
Thank you all - especially Red Cawte for the technical feedback.
I think I have as much information as I can gather at this stage to make an informed decision on how to proceed. Purely technical information cannot fully replace experience and I appreciate everyone on the Forum sharing their opinions and expertise with me on this issue.
As a new Laverdian, I am slowly catching up on your collective arcane Italian magics - and look forward to future discussions on the forum.
 
Yep, furthest from the camera indeed - a genuine factory SFC and the first twisted 270 Laverda on the planet I do believe (1991 or thereabouts. Happy to be brought down in flames!).

And believe me, there was more than a little trepidation in going down that path on such a rare crank, with zero info to go on re balance factor, which cylinder to advance etc.

Chris it is the same phase (90 deg) but I chose that setup to make the longest phase between firing (450 deg) the left, primary side, assuming there is a loss of momentum between firing, and a long gap to the RH cylinder would mean greater torque effect passing through the RH web, the L and R of the centre pin and the LH web to primary drive. Doubt it makes much diff, but I had to choose a side, and that's the reasoning.
 
If you wanted to minimise the torque passing through the centre of the crank due to loss of momentum between power pulses, you could have made the firing 90 deg apart with a 630 deg wait between them. It would be almost like a big single.  :D
 
The first I heard of this was a Phil Irving article written in maybe the early 60s or earlier about reducing Parallel twin vibs. I think that was using 76 degs?
 
And that is what my mate Geir did with his SF racer when he twisted it. Apparently one of the US V twins had that solution, with a full lazy stroke. Geir?s bike was/is deadly fast in classic racing, has siginficantly more power than mine and his PBs from the tracks here have not been bettered by any other Laverda, triple or otherwise, so there is no joke where that is concerned. It has torque in buckets, and that is what gets you around the track quickly. From the exhaust sound you couldn?t pick it except at very low revs, which is odd. He has however snapped the center piece twice, using standard splined parts, whether this has anything related to Quentin?s and my reasoning as to which way to twist is debatable. Sandard splined cranks at 360 also break when you get "serious" power out of them. The factory made SFC cranks differently for this reason.
 
Vince said:
The first I heard of this was a Phil Irving article written in maybe the early 60s or earlier about reducing Parallel twin vibs. I think that was using 76 degs?
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I have a vague recollection Phil Irving's thinking behind the not quite 90 deg twist was so that when one piston is stopped at TDC, the other is at maximum speed. Only problem with that is it doesn't work when a piston is at BDC. I don't think it made any real difference to engine vibration though. I can't find my copy of his book at present (must have lent it to someone) so can't confirm.
 
68deg was the original idea, with as you say the advantage of momentum. Apparently it does help vibration just as the 90deg idea but it is just easier with 90deg. Didn?t Red do one at 68 just to compare? These days they phase motors to suit the torque characteristics they need  or want. Xplane Yam for example, and they sound fan bloody tastic. The new Panigale V4 has each side, if you looked at it as two V twins side by side, firing 90deg apart with the 630deg lazy stroke.
 
Yes, the idea was Phil Irvings. And pretty sure the max piston velocity thing came at 76 deg (which red did on one motor). Mine certainly wasn't the first twisted twin, but I believe the first twisted Laverda.

76 doesn't work evenly for each stroke (up or down), but 90 does. Whenever a piston is at TDC or BDC, another piston is moving at close to max stoke velocity up or down, creating a flywheel effect that  pulls or pushes the other piston through its 'stopped' phase. Smart engineer was Mr Irving.

That 630 motor of Geirs sounds bizarre, Lee, but it obviously bloody worked! Splined centres are never going to last with big HP.
 
first run of Twisted Twins I did I made 2 x 270 degree engines and 1 x 284 degree engine, just because I could
I was curious to see if Phil Irving was correct as he wrote that 76 degrees was the perfect solution to counter vibrations on a vertical twin
read his book while travelling to work on a train in Siberia, 42 hours of cold cabbage and Vodka  :o

when they were running you could not really tell any difference between the 284 and 270 engines and same with the vibrations, both felt the same
the 284 degree bike is up for sale if anyone is interested, is set up as a race bike




 
Basketcase said:
Wasn't the precise angle dependent on the rod length to stroke ratio?
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Yes. It's the geometry of the rod length and crank throw that determines the crank angle at which maximum piston speed occurs. But the rod length to stroke ratio doesn't vary a whole lot between engines. It's normally pretty close to 2:1
 
Apparently rod length/angle it is not quite so so simple, it has a major effect on torque and other things, as it governs the angle that the force is applied between piston and crank. Geir with the big bang SF racer, has been seriously interested in building a short stroke big bore 750 motor with which to step up to the million bucks triples which dominate the class. The idea is to use early webs with eccentric ground bigend pins or somesuch to increase stroke. The calculation of ideal rod length has been the sticking point in the plans. It seems to be very difficult to get answers from people. I get the impression that it is a bit of a dark art, like steering geometry or propellor design.
 
  I built an engine with triple rods (shorter) to accelerate pistons away from the high overlap (Large) valves - shortened the barrels and machined head and added a spacer between head and top cover to get engine to fit in frame - for the life of me I can't remember if i actually ever used that motor - or even what spec it was -  Regards - Phil
 
 
That 630 motor of Geirs sounds bizarre, Lee, but it obviously bloody worked! Splined centres are never going to last with big HP.
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Yes I pioneered the "lets do a 180? conversion whilst on the move" idea  ::) :o - Regards - Phil
 
phil37 said:
  I built an engine with triple rods (shorter) to accelerate pistons away from the high overlap (Large) valves - shortened the barrels and machined head and added a spacer between head and top cover to get engine to fit in frame - for the life of me I can't remember if i actually ever used that motor - or even what spec it was -  Regards - Phil
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Well ... that's not much use, Phil!!  :-*  I do know that Steve Elliott had one of your 180 cranks in one of the Palmellis.
 
MotoJouni said:
How many ton press is needed to disassemble and to assemble these crankshafts?
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Not much for a standard crank a 10ton press would easily do it - the later SFC cranks may need up to a 30 ton press - have had a triple take 28tons - SFC could be the same - plus web extremities need more support/care as they are quite thin - regards - Phil
 
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