Bonneville America Forums Forums Technical Forums General Tech Crankshaft design-yeah we aren't "rocking"

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.

Excellent information there Geoff.
Thank you for posting that.

Thanks Bigbill I was begining to think this note had cranked to a blank response.
Geoff

Excellent post. Really explains why this motor feels so balanced. Thanks. Ah, the Vincents, what a machine...

Thanks for the write-up.

If you're in the neighborhood (BC's a big place), there's a Kootenay Raid in Nelson, July 12/13-15. I don't have it in front of me, but you can "google up" the registration sheet fairly easily.

jh

I posted this same article in response to someones question regarding this about a year ago. Bottom line the 270 engine is an improvement over the the 360 degree engine.

Tom

Very interesting and informative - thanks!!

Thanks for posting this, Geoff! A well written piece.

In it is mentioned the Yamaha TRX parallel-twin, a sportbike based on their TDM dual-purpose bike(360-crank) that was very popular in Europe, but never made many enroads into the American market and was dropped by Yamaha after a few years. The TRX, with it's 270-crank, was never imported into the U.S., which I always thought was a shame, as I believe a more narrow and lighter motorcycle for use "in the twisties" is preferable.

I've had the occassion to meet and talk with motojouralist and racer Alan Cathcart(currently the european correspondent for "Motorcyclist Magazine") a couple of times over the years, who raced the TRX in european and british circuits with much success a few years back, and every time the TRX is mentioned, a smile appeared on his face and he almost swoons...."Ahh, my TRIXIE! How I love her!"

This group is a joy to post to because of the knowledge you all share.
When I was a kid, a life time ago, I would ride into Vancouver and meet other riders at British Motorcycles on a Saturday. This was a great group of riders who would exchange motorcycle knowledge of the day.
This website is worldwide and surpasses anything remotely imagined then. I volunteer in an Environmental Stewardship organization, now so I understand how much time and the dedication must go into building and running this site--so a big thank you to all.

That explains why the first America/Speedmaster engine produced the same torque and power as the Bonneville/T100 with much milder valve timing.

Sorry,I didn't see a need to respond when I first read it. I found it very interesting and informative.

Geoff,
Great article. Thanks for posting it.

Thanks for the post, Geoff!


I love this kind of stuff.

Physics, mechanics, laws of nature harnessed by some fairly bright individuals for all our enjoyment.

We are the fortunate ones....

Geoff,

I went back and found the article that I posted and discovered that it wasn't the same one. It was similar in subject and implied the same conclusion. This is the article that I have. It is long, but interesting.

XS650 Rephased Crank

Motorcycles come with engines of many different designs - some better than others. From singles to sixes they each have their own characteristics. For an engine of a given size and in a given state of tune, the one thing that gives it most of it's character, and it's sound, is it's cylinder arrangement. Take a Kawasaki Z-1300 and a H-D Evo. Both are 1,300cc four-strokes, but they (engines only - ignore the rest of the bike) are nothing alike. Not only do they look and sound different, but they also "feel" different. This is because one is an inline six, the other is a 45 degree V-twin. They both produce enormous torque, but while one is as smooth as a turbine, the other will shake the fillings out of your teeth. Whether you prefer plenty of vibes or none at all is entirely your affair - we're just discussing why engines are the way they are.

Somewhere between these two extremes lies the Yamaha 650. It has a certain feel about it too. It vibrates a lot more than the Z, but nothing like a H-D. Some people like it - the vibes that is, others just accept it as part of an otherwise great little package. As you'd know, the 650's engine is 360 degree, parallel twin. In layman's terms, that means that the cylinders are side by side and the crankpins are in line with each other. Put another way, both pistons go up and down together, but they are one crankshaft rotation apart.
Remember, that a four-stroke engine turns 720 degrees, or two revolutions, to complete a cycle. During this, the pistons go down on intake, up on compression, down on power, and up on exhaust. Though they go up and down together, they are not doing the same thing at the same time. To go through it, as the No. 1 (left) piston is on the intake stroke, No. 2 is running right alongside it, but on the power stroke. As No. 1 is on compression, No. 2 is on exhaust. As No. 1 is on power, No. 2 is on intake. As No. 1 is on exhaust, No. 2 is on compression. Any motor of this type gives an even 1 - 2 - 1 - 2 - 1 - 2 beat from the exhaust.

Compare that to the sound of, say, a V-twin. Why do they sound the way they do? The crankpin of both cylinders are in line, as on the 650 (usually, it's one pin), but because the cylinders are splayed, the pistons don't arrive at top and bottom together. In fact, when one is at either end of it's stroke, the other is somewhere around the middle of it's. This gives an uneven 1 - 2 - - - 1 - 2 - - - 1 - 2 beat. V-twins typically have Vs of anywhere between 45 and 90 degrees. The narrower the V, the more compact the motor and the easier it is to fit it into the frame. Imagine a H-D with it's motor widened out to 90 degrees. It simply wouldn't fit in the available space. Why then does Ducati, and now Yamaha, Suzuki and Honda make 90 degree V-twins when they could just as easily make 45s. They choose to sacrifice simple frame-design and easy engine-fitment for better engine-dynamics. That's a fancy word for the physical forces that are at work inside a motor. There are many, but the particular force that concerns us here is primary balance. This is not to be confused with the balance factor and the matching of weight for each reciprocating assembly, which is what you're referring to when you say "I've had it balanced".

At either end of it's stroke, a piston must stop momentarily to change direction. Of course, the speed at which it travels from one end to the other is not constant, even though the RPM of the crankshaft may be. At any given but constant rpm, and beginning at 0 degrees (or the pin at 12 o' clock), it starts off slowly, gains speed, is going fastest at mid-stroke (with the pin at 3 or 9 o' clock), slows down, and stops again (with the pin at 6 o' clock). We've said that when one piston of a V-twin is at top or bottom, the other is somewhere around the middle of the stroke but, depending on how wide or narrow the V is, that can be almost anywhere. Does it matter? Yes it does and, in this regard, a 90 degree V-twin is unique. With a 90, the other piston is exactly at mid-stroke and travelling at it's fastest. Thus, the kinetic energy of that piston overcomes the inertia of the other piston which is stopped. This gives perfect primary balance.

Now imagine the inside of a 650. Think of two pistons going up and down together. All that weight going up, stopping, then going down, stopping, then up again. Also, two sets of flywheels, heavier on one side than the other, going around in unison. It might as well be a big single. The only reason a 360 degree, parallel twin feels any smoother than a single is because there is a small power-pulse twice as often instead of one big one half as often. Splitting a motor of a given size in two, and firing each half alternately does smooth out the power pulses, but it does nothing to eliminate the vibration caused by all that weight going up and down in the same direction at the same time. The only way to do that, and have both pistons at top and bottom together (if that's what you want), is to have the cylinders opposed as BMW has done since the beginning of time. The sound they give is the same as a parallel twin but, because the pistons are going in opposite directions to each other at the same time (even though they are going up or down in their respective cylinders together) vibration is cancelled by the fact that each force is equal to, but opposite the other. With the 650, the forces of each piston are equal to the other, but combined and with no opposing force. This gives exceptionally bad primary balance. Actually, the 650 has no primary balance whatsoever. To have any kind of balance at all, one reciprocating assembly must act upon another. Since the two reciprocating assemblies of the 650 are as one, what can it act upon? It has the primary balance of a single - none.

The forgoing is basic stuff to most motorcycle riders but it had to be said before we can move on. Move on to where? We all know that our bikes vibrate and if it's because of the engine's basic design, how can we do anything about it? We certainly can't move cylinders around to get an opposed or V-twin, so why even discuss it? Over the last few years, a few people have been investigating a little-known operation called rephasing. We've seen how a 90 degree V-twin works and while it's true that we can't separate the cylinders of our 650s, we can separate the crankpins to give the exact same result. This idea was first mooted back in the 1950s by one Phil Irving, designer of the Vincent engines and the Repco Brabham engine that took Sir Jack Brabham to his formula 1 championship. He came up with an extremely complex formula for calculating the degree of separation based on the stroke and the length of the rods. Unfortunately, he never actually modified a motor like a Triumph or Norton to test his theory, and it remained that - a theory - for many years. Around ten years ago, a fellow named Lee Kernich of Adelaide in Australia, who races a post-classic Triumph, came across Phil's writings in an issue of Classic Bike that was a few years old at that time. He decided to go with the idea, did the calculation, and came up with 76 degrees. That will always be the answer if the rods are twice as long as the stroke which, on a Triumph, they are. The conversion required a crank from a Norton Atlas with the center section made from scratch. With a Norton crank, it's a fairly easy job. Anyway, the bike was featured in Streetbike magazine about seven or eight years ago. It's as ugly as sin but goes like a rocket by all accounts. That particular issue of Streetbike was read by David Rayner who was about to give his '77 a complete rebuild. He contacted Mr Kernich and they had a long chat. Lee said that he had never seen the crank of an XS-650 and so had no idea if it would lend itself to such a modification, but the dynamics should be the same. David asked, assuming it could be done to a Yam, if it was worthwhile. Lee replied that it was the greatest thing since sliced bread and had transformed his bike completely. He lent David a copy of the formula.

Lacking a degree in advanced calculus, David enlisted the help of one Ian Irving, Australia's best-known bike hot-rodder and great nephew of Phil's. Ian was quite familiar with this but, like Phil, he'd never done it. Being a Triumph nut however, he'd always wanted to and, Triumph or Yamaha, here at long last was his chance. Using a stroke of 74mm and a rod of 136mm, Ian worked through the formula and came up with a number that David no longer remembers, but that doesn't matter.

Not long after that, David came across an issue of Classic Bike that contained Phil's story. Not the same one as Lee had read though, a much more recent edition that had a little something extra. This one also gave an account by a Brian Wooley, who stated that Phil's theory was fundamentally flawed. It involves, while one crankpin is at 12 o'clock, rotating the other crankpin to the point where an imaginary line drawn from the centre of the crankshaft to the centre of that pin, and then up through the middle of the rod forms a right-angle. You can see how the rod/stroke ratio would affect this. The shorter the rod or the longer the stroke, the lesser the angle (X) will be. Having the crankpins placed so is one thing, but the result would be pistons that have the same relationship with each other as those of an X-degree V-twin. He says that placement of the pins is irrelevant - it's where one piston is when the other is stopped that's important. He says it must be a maximum speed and mid-stroke is where that occurs. Therefore, 90 degrees is the optimum, regardless. David showed this to Ian and asked for his thoughts, and who Brian Wooley was. Ian said that Brian was an extremely well respected engine designer and that anything he said could not simply be dismissed. Never mind blood being thicker than water. Ian thought about it and decided that Phil was wrong and Brian was indeed right. Most would not have heard of Ian Irving but, suffice to say that he's spent several years in Japan working for the various factories as a mechanical engineer and he's on call to several 500cc GP teams. He is currently putting the finishing touches to the engines of the Hunwick-Harrop. His credentials are beyond question. His personality is another matter. Thus, Ian was to build for David what may have been the first rephased XS-650, or at least the first one done at 90 degrees. You see, David wasn't the only one who'd read that issue of Classic bike.

Tony Hall of Halco Tuning had too. He liked the idea, but couldn't think of a way to get 90 degrees or whatever else would result from Phil's formula, given that the 650's crank is in two halves - a male and female with thirteen splines. Thirteen - why not twelve or sixteen? Anyway, for the sake of experiment, he simply separated the two halves, then rotated one by three splines and pressed them back together. This gives a separation of 83.076 degrees. He then cut the cam in half and rotated one side by 41.538 degrees and welded it back together, and moved one set of points around by the same amount.

Tony told David this over the phone late one night. Needless to say, David asked him the same question he'd asked Lee. Tony said that it had drastically reduced the vibration and increased the torque by about as much. Same story from two people. This was great. A while later, David met a fellow who races a Triumph outfit in post-classic. You guessed it, this one also has a rephased crank. David asked him the same again. He said that it was, without a doubt, the greatest thing one could ever do to a parallel twin and that he would not have one any other way. Like Lee's, his was 76 degrees.

At about this time, the most promising sign of all came about. Yamaha released the TRX-850 onto the market. unfortunately, it's not sold in the US but is reputed to be a quantum leap in parallel twins. It's a cafe racer that looks a lot like a Ducati Monster, uses the same basic engine as the TDM-850, but with it's crankpins set at 90 degrees.

Now that the idea had been endorsed by a major manufacturer, David needed no further proof that the idea was sound and worth pursuing. Oh, and he says that to hear a TRX at full-noise can induce an orgasm. Most embarrassing in a public place. Sometime during all this, David received a letter from Bill Denton of Pennsylvania. Bill had also read about Phil's theory and had heard somewhere that David was attempting to apply it to an XS. Thinking that David's engine was well advanced if not running, he wanted to know about all kinds of things related to the project. David answered his letter and the two have talked on the phone many times since then. Bill runs a 650 discussion group on the internet and everything David wrote was posted there for all to see. Debate on the subject has gone on ever since. Whether 90 degrees or some other number is correct, and even if it was worthwhile at all.

One subscriber who had more than a passing interest in all this was Terry Gliddon of Victoria, Australia. He contacted David and they too have spoken many times about rephasing. All the while, the three believing that David would soon be riding such a beast. However, that was not to be. Ian and David had a major falling-out, and the motor was never finished. Bill has since ridden a 650 that's had it's crank altered the same as Tony Hall's, and says it's unbelievable. There is no doubt in his mind that he wants one. That's four people who are convinced, not counting Yamaha. But how to get 90 degrees? With Ian's wealth of engineering expertise no longer available, David had to carry on alone. He came up with an idea that was workable, but very expensive. It involves cutting up two No. 2 flywheels and making one out of the resulting pieces. Then came a major breakthrough. Bob Bertaut hit on the idea that if one could get hold of an extra No. 3 (the female) flywheel, one could simply have a shaft made that's splined at both ends, one end being 90 degrees out of phase to the other. This would replace the centre section of the crankshaft, where the sprocket is. One would then discard the No.2 'wheel (the male), and assemble the crank using two females instead of a male and a female.
Brilliant. David and Terry discussed this at length, and were so keen to try it that Terry has since contracted a company to make the shafts. Bob has built one or more motors the same as Tony Hall and Bill Denton's friend. That is, he's rotated one half of the crank by three splines. They all seem over the moon about the difference it's made and full marks to them for doing it.

As we've said though, that's not all there is to it. The cam must be done likewise and so must the ignition. Instead of modifying the points, Bob has used an electronic ignition from Boyer-Bransden and simply moved one of the triggers. Recently, a fellow in Victoria did the same but used the electronic ignition from another bike, moved one of the pick-ups, and runs it on the crank. He says he has no wish to ride a 360 degree XS ever again. He works as a mechanic at a Suzuki shop and says that his bike feels similar (engine only) to an SV-650 - the TL-1000's little brother. Not to take anything away from these bikes because everyone who's ridden them is mightily impressed, but the fact remains that some of the brightest engineers around say that 90 degrees is perfect. Could you tell the difference? Is it worth worrying about? We can't say, as no-one has yet built
a 90 degree 650. As for the shafts, it's far more cost effective to have a number of them made and there are now enough people who are willing to spend the extra money to find out once and for all. Terry wants one, David needs four, Bill Denton and Bob Bertaut each want one. That's enough to get the order done. The reason we're telling you this is not because the project has hit a wall until more people express an interest - it's going ahead regardless - but so you can have one if you'd like. Whilst not wanting to exaggerate the importance of it, we believe that this is your opportunity to correct the most basic flaw in the 650's design.

As for the expense, most of it is the same no matter which way you go. You'll need to have a cam and ignition modified, and the crank taken apart and put back together. Bob is happy to handle that and it applies even if you're simply turning it three splines. Actually, if you're modifying or just rebuilding the motor anyway, you would have done most of that. Here then, is where the added expense begins. You'll need another No. 3 flywheel but there are any number of cranks at the wreckers. Condition is unimportant so buy the cheapest one you can find. You'll need to have a small amount machined off the centre-boss of one of the two No. 3s you'll be using. Not expensive. You'll need to buy the shaft from Terry. It looks like these will cost somewhere around A$160 - 180 (US$100) each. As we've said, several people have been wanting to do this for some years and now it's so close they can taste it.

Are these people crazy to insist on 90 degrees when 83.076 is pretty close? Consider that most V-8 engines are 90 degrees. That the flathead Ford of 1932 was may have been coincidence, but the motor that comes in a Ferrari F-50 being 90 degrees is certainly not. Sure, there are some that aren't, like there are some V-twins that aren't. All the Japanese manufacturers now make cruisers with motors of around 45 - 60 degrees. That's so they'll look, sound, and feel like a Harley. But their V-twins that are meant to go fast - the Suzuki TLs and Honda Firestorms are 90s. The Yamaha TRX is a 90, and so is the Triumph Bonnieville-America. In case you missed that - the latest Triumph (the bike that Phil had in mind when he first thought of this) twin is 90 degrees, NOT 76 or 83.076 or any other number. All of Yamaha and Honda's V-4s are 90s. Dr Fabio Taglioni never considered anything else for his machines and nor would his successor, Dr Massimo Bordi. If, by some chance, Phil Irving's theory is correct, you should know that 83.076 degrees is not the magic number for a 650 anyway. It's a lot better than 360 degrees, no doubt about that, but it's not perfect either. It's just the most convenient and easy to do. If that's good enough for you, then fine, you save $100. If 360 degrees is good enough, even better, you don't have to do anything. But if you want the sweetest-running 650 possible, an overwhelming tide of evidence suggests you make it 90 degrees.