Every ask yourself why Triumph moved from a 360 opposed crank design to a 270--was it just the sound?
Well Bill Denton,PA, thought a lot about it and tried to design a 650 Yami to a 270. Here is a selective copy of his thoughts some few years ago.
It all started when I read an article in Cycle World entitled "TRX Tech,,, Motorcycle or Myth Maker?" (June 1995) about the engine in the Yamaha TRX 850. This engine, an 850cc upright twin, is built with 270 degree crankshaft timing (see following discussion about 90 vs 270 degree timing). This idea made way too much sense to ignore, offering all the advantages of a V-twin; a wider, flatter torque curve, and less (destructive) vibration by minimizing both secondary imbalances and rocking couple forces, while maintaining the small,compact engine profile of an upright twin. In addition, the sound of the engine note would be too unique to ignore, especially coming from an XS! I thought, would it work in an engine without balance shafts? I had no idea.
So, I set out on the information trail to get some opinions on whether it would work or not. I started by writing a letter to everyone listed in the Int'l 650 Society Newsletter as a 650 rebuilder, thinking that their opinions might have just a little more weight. The responses I received ranged from "it will vibrate apart, don't do it", to "It will have (much) less vibration than the stock engine, and I'd be glad to help you build one." I also became aware of two gentlemen in particular who had already done 360 > 270 deg crankshaft conversions; one on a 500cc Triumph (Dick Cookson in England), and the other an XS-650 (Dave Rayner in Australia). I wrote letters to them as well, telling them of my interest and asking for information. Both responded by mail (send me a message if your interest runs deep enough to read these letters, and I'll snail you photocopies of them).
I also discovered along the way that the original idea for a 270 deg crank in an upright twin came from an Australian fellow named Phil Irving (co-creator of the pre-war Vincent engine design) in his musings in a series of articles he wrote in the 1940's. Apparently, he initially tried to convince Triumph of the virtues of this crank arrangement, but they would not have anything to do with it, choosing to stick with the 360 degree design. Apparently, there was a bias or prejudice against uneven firing engine designs that stuck with BSA, Triumph and Norton until their respective bitter ends.
As you know, in the XS-650, both pistons move up and down together (aka 360 deg crankshaft), with plugs firing left and right on alternate strokes. In this arrangement, both piston/conrod combos achieve maximum velocity TOGETHER twice per revolution (once on the way up and once on the way down). In addition, they both come to a complete stop TOGETHER twice per revolution (once at TDC and once at BDC). As you can imagine, as RPM increases, so does the vibration coming from this Primary Force Imbalance (PFI). This phenomenon is made worse by the laws of Physics, which dictate that doubling the RPM quadruples the forces (and thus the associated vibrations) involved. Flywheel weighting added opposite the throw of the crankpin minimizes PFI, but a compromise has to be achieved between cancelling out PFI and exacerbating a Secondary Force Imbalance (SFI). This is the centrifugal force of the weighted portion of the flywheel trying to move the entire engine fore and aft as it spins.
Now then, the basic idea with a "quartered" crank engine (and design advantages associated with it) is to never have both pistons at maximum or minimum velocity at the SAME time (as you know, this is exactly what DOES happen in a 360 degree upright twin). More specifically, the idea is to have one piston moving at MAX velocity while the other one is at MIN (zero) velocity, in order that inertia about the crankshaft is preserved. In other words, the first swiftly moving piston/conrod combination helps "pull" the second piston/conrod combo through it's deceleration & change in direction, a point at which it has little or no inertia. In effect, the quickly moving half acts as a inertial flywheel for the other, and visa versa several times per revolution, resulting in relatively constant inertia (stored or potential energy) in the rotating parts in the big end of the engine. See the URL
http://www.interlog.com/~lcl/tdm/tdmpower.html for more info on crankshaft inertial torque and it's effect on perceived torque feel in a 270 degree crankshaft engine design. (or copy of same graph on local site)
This "inertial torque smoothing" effect is achieved by separating the crankpin throws. Maximum velocity occurs at 74.1 degrees before & after TDC in an XS-650 with a 74mm stroke and a connecting rod length of 130mm (#447 rods). This is the point at which the crankpin throw and the conrod are at right angles.
However, at a 90 (or 270) degree angle, the piston/conrod assembly is still pretty close to maximum velocity, so a high degree of inertial torque is still achieved. But (and this is a big "but") at 90 degrees, the PFI is less than at 74.1 deg, and the Secondary Forces, which have a frequency twice crankshaft speed will have a phase difference of 180 deg., thus cancelling each other out entirely (credit to Brian Woolley; The Classic Motorcycle, 2/92). According to Woolley, this should result in a 43.5% improvement in the balance of forces within a 90 degree engine, when compared to a conventional 360 degree upright twin.
Because of a combination of better balance and conservation of momentum (inertia), the "flywheel" portion of the crankshaft may now be lightened, further improving balance by reducing rotational weight while increasing throttle response without sacrificing drivability (a common problem when "race" engines with lightened flywheels are driven on the street).
So there you have it. An engine with more torque, more perceived "pull" at lower revs, quicker throttle response, less vibration, and with an "Italian" exhaust note. What more could a man want out of his mid-life crisis project!
Bill in Yardley, PA
Blue--A Vincent design no-less now there is something to brag about--Yeah I'am so old my buddys rode new Vincents and they would balance a full wine glass on their gas tank without a ripple. Yes at eighteen and an apprentice machinist I was duly impressed. And now I have the Triumph BA with the same principal as the Vincent which balanced out mechanical forces. PS: rotating counter-balancing shafts were not invented back then.
Old-Blue and still riding.
