For no other reason than ...


...I reposted part of the aussie sight somewhere else and it was ready to paste




The late Phil Irving could hardly have wished for a more loyal disciple than Ron Valentine, best known in motorcycling circles as designer of the highly successful Westlake racing engines. More than 30 years ago nobody was quicker than Valentine to recognize the merit of Irving's proposed cure for the engine vibration that, in varying severity, had long plagued Britain's big four-stroke parallel twins, condemning their riders to mobile vibro-massage without the option.

The essence of Irving's brainwave was to replace the standard crankshaft with its crankpins in line, by a shaft with its pins staggered so that when either piston is at top dead centre (where it primary inertia force is greatest) the other is approximately at midstroke and generating no primary force. This arrangement considerably reduces the engine's maximum inertia force, and thus vibration - though not by so much as the 50% one might suppose, as we shall see later.

A second benefit stems from the fact that when either piston is stationary at tdc, the other is moving at or near its maximum speed and thus contributing to the flywheel effect, so that the flywheels themselves can be a bit lighter for the required level of smoothness. It was this consideration that dictated Irving's original choice of 76° for the spacing of the crankpins. For with that angle - given that the centre length of most connecting rods is near enough four times the crank radius - when either piston is at tdc, the "big-end" angle (con-rod to crank) in the other cylinder is a right angle, hence piston speed highest.

To their shame - and perhaps because of an irrational horror of uneven firing intervals? - the manufacturers of even the most vibratory parallel twins showed not the slightest interest in Irving's proposal despite the relative ease with which they could have converted an engine for test by twisting the crankshaft and camshaft, adjusting the spark timing and fitting a second carburetor where necessary to obviate a mixture bias from overlapping induction phases.

Following the demise of the British industry, it eventually fell to Ron Valentine (and his assistants, including mathematician Tom Oliver) to prove the soundness of Irving's scheme when (three or four years ago) they completed their second 76° crankshaft for Steve McFarlane's 952cc (80.5mm x 93.5mm) BSA parallel twin classic racing sidecar outfit. Stretching both bore and stroke of the original A65 engine had aggravated its vibration to the point where the crankcase was in danger of disintegration.

Machined from a solid bar by Dave Nourish, Nourish Racing Engines (NRE), in his Oakham workshop, the new shaft proved to require balancing (to a factor of 50%) as if it were two separate flywheel assemblies joined together. Once that was done, the engines character was transformed. Gone were the frantic shakes. Instead, said McFarlane, there was a slow and lazy throbbing sensation as - to the accompaniment of a pleasant off -beat exhaust lilt, reflecting the 436°/284° firing intervals - the revs soared to 7,000 RPM and the more powerful 1000cc - 1200cc Imp engined outfits were humbled as the BSA won its heat in the Snetterton Race of the Year meeting in 1990.

Encouraged that his and Nourish's sacrifice of valuable time and effort had proved worthwhile, Valentine decided to follow his hunch that a 90° pin spacing would give even better results. True, the instantaneous contribution of the descending piston to flywheel effect, while the other was at tdc, would be slightly reduced because it would be just past its maximum - speed position (big-end angle only76°, not 90°) but there would be two overriding benefits - one to mechanical balance, the other to the smoothness of the flywheel effect.

Balance would be enhanced because the top and bottom dead - centre positions of either piston (where the secondary inertia forces act upward) would coincide with the midstroke positions of the other, where the secondaries act downward. Thus those forces would counterbalance one another at the cost of a small rocking couple.

As to the moving piston's contribution to the flywheel effect, this would be the same whether the stationary piston was at tdc or bdc (the big-end angle being 76° in both cases). With the earlier 76° pin spacing the ideal "big-end" angle of 90° was achieved only when the stationary piston was at tdc. When it was at bdc and the moving piston was rising, not descending, the angle was only 62°, so the effect was not constant but fluctuated at high frequency. Of these two benefits in favour of 90° pin spacing, the absence of unbalanced secondary forces is clearly the more significant.

When the subject of "cranky cranks" was discussed in Motorcycle Sport three years ago Charles Bulmer suggested that a 180° crankshaft would be even better provided its primary rocking couple were eliminated by means of a crankshaft-driven contra-rotating balance shaft. Quite so, for an engine so designed from scratch by a manufacturer, as with some Hondas.

But what Phil Irving was proposing was the least possible alteration to already established mass-production lines to overcome a serious deficiency in British parallel twins. Given a clean sheet of paper, he had long since shown his own preferences for twin cylinder four strokes: designed just before the second world war, his 600cc Velocette Model 0 vertical twin was a model of smoothness; like its racing stable mate, Harold Willis' 500cc super-charged "Roarer", it had contra-rotating geared crankshafts and shaft drive. Later, his postwar 50° V-twin Vincent Rapide ranks as one of the industries greatest designs.

Again drawn by Ron Valentine and machined by Dave Nourish, the 90° crankshaft has four flywheel discs and is a replacement for the conventional (360°) shaft in one of Nourish's Westlake powered classic racing 500cc NRE Triumph based pushrod parallel twins. Since the total upward inertia force with the standard crankshaft occurs when both pistons are at tdc together, it might be supposed that separating the tdc positions by means of staggered crankpins would halve the force and double its frequency, regardless of whether the stagger is 76° or 90°. Not so as Mr. A Archdale was at pains to confirm in the original "cranky cranks" discussion, although the individual tdc forces in each cylinder remain one half of the total for the 360° shaft, the total upward force occurs when both pistons are level and their cranks equally disposed each side of tdc, i.e., 38° before and after tdc for the 76° stagger and 45° for the other. The point one must grasp here is that - although one piston is moving upward and the other downward when they are level - their inertia forces are both upward, as can be seen in the accompanying curves, where both points are above the base line. Note too that the 45° points are slightly lower on the curves than the 38s, indicating a slightly lower force in favour of the 90° pin spacing.

The net result, as calculated by Valentine and confirmed by Bulmer, was that whereas the 76° shaft reduces the maximum upward inertia force by a useful 32%, the 90° shaft is an altogether better proposition with a reduction of almost 45% (see Tom Oliver's graphs)

The first tentative outing for the "90" was at Mallory Park last May, when Martin Smith found the engine considerably smoother than standard. It was also 4-5 bhp up in power as a result of new cam profiles - Valentine had replaced the Triumph type radiused tappets by experimental roller cam followers, since they are less dependant on copious lubrication, and had taken the opportunity to up rate the cams.

In the Classic TT Robbie Allen was in the hot seat and finished 10th in the 500cc race, eighth in the unlimited. However, in common with most classic racing participants, Robbie is well over the win-or-bust age range and the object of the exercise was not to win but to compare the modified crank with the standard one. Riding apart, the fact that Robbie's bike finished well ahead of a brace of standard NRE's reflected the cam changes rather than the pin spacing. Subsequent bench tests at Oakham gave a repeatable 57-58 bhp at 9,000 RPM and a one off flash reading of 62.5 bhp at 9,800 RPM.

Since riders with an analytical flair of a Geoff Duke, John Surtees or Peter Williams are as rare as elephants teeth, Nourish had intended to try the machine himself at Jurby airfield but was foiled by atrocious weather. Before the final outing (to the Manx GP) however, he took it to East Kirby airfield, in Lincolnshire, for assessment and was delighted by the extent of improvement. A few other invited riders were equally impressed. Alas in the Island, Allen was troubled by a few unrelated problems - valve float and missed gears - before an ignition failure in one cylinder brought about his retirement in the third lap. But Nourish is now so hooked on the new crankshaft that he is planning to market complete 90° NRE engines.

Conversion kits for other engines, however, would not be a commercial proposition. Each 90° shaft, says Dave, is suitable only for the precise reciprocating weights of its particular engine - and con-rod weights, for example, vary enormously (the top portion is the critical end), so the balance holes in the flywheel discs must vary too. There are other relevant variations and the net result is that machining a shaft to suit a particular engine would take too much costly time.

Engine vibration is unpleasant in touring machines as well as racers. Some tourists may dismiss a mild case as inevitable provided they don't make day long trips - or try a BMW boxer and realize what they are missing. In racing, however, it is unforgivable, especially in long distance events such as TT races. It can cause not only metal fatigue - breaking anything from brackets to engine plates - but also rider fatigue. It can impair engine performance by seriously upsetting carburetion. And, at best, it must absorb a modicum of power. Phil Irving now has another disciple in Dave Nourish. Only 10 more to go for a full apostolic set!


And a hearty thanks to the xs 650 club of Australia for the info as it was lifted directly from their site
a great resource and I highly recommend you all visit and contribute .

The rephase distributes rather than concentrates forces . With the exception of side thrust near all of the 360 degree cranks forces are in a vertical plane . There is a secondary rocking couple induced by the alternating left right firing sequence but that is actually an order beneath the primary forces .
The rephase was initially used to increase available traction , in steam locomotives . A timing arrangement that allowed for application of force to one side of the locomotive and then delaying the other side for 90 degrees with respect to the primary and then allowing for 270 degrees "rest" to allow recovery of traction .
Fast forward to the English and their venerable vertical twin . Their cranks were almost exclusively bolt together although tapered fit and key'd . As modern fuels crept in to the mix higher and higher compression engines became possible due to advanced alloys . The crank became not only the weak link but a source of annoying vibration .
Enter an old idea with a new purpose . I'll spare you the force diagrams .gif and leave those as a homework assignment or for some member to post following this monologue . To reduce the inherent vertical impulses one piston was to be at TDC whilst the other was to be at maximum acceleration which is when the connecting rod and piston are perpendicular to the stroke (90 degrees) . This happens to be near our 277 degrees . With the reduction of vertical forces by way of distributing them throughout the cycle and consequently having portions of those forces happily cancel each other out we have by subjective definition a "smoother" rotating and reciprocating mass .
Does it make more HP and Ft/Lbs ? Not necessarily . The bore and stroke have not changed nor have the combustion chamber dimensions . So what is the real performance benefit ?
Honestly it accelerates quicker because it isn't acting against itself throughout its cycle . It acts like a much lighter crankshaft and well it should .
Longevity . It isn't beating itself to death just to idle . It isn't prone to the wild imbalances of a secondary rocking couple that is at the mercy of combustion processes rather than mechanical timing .
Mean repeatable maximum BEMP . meaning the ultimate breaking strain of a rephased engine is much greater than that of a 360 or even 180 degree engine .
Specifically the limiting factor in our engines is the center crank pin . Anything we can do to keep stresses off this one area will add to the longevity of the rotating and reciprocating mass .
Traction . Back to the original reason for the phasing . The reason I used it was specifically to keep up with a well known 45 degree V-twin. There were tire sizes and sprocket ratios unavailable to us with a 360 degree crank because we were hitting on the same spot on the tire every time around !
This uneven firing order also allowed the tire and chassis to recover in between power impulses allowing subsequent impulses to gain traction .

Since I'm sure you have all had enough of this ...
More Ft/Lbs ? maybe a little
More HP ? Again maybe a little
But both due to the reduction in vertical forces
A fair impersonation of a lighter crank ? Absolutely
More reliable ? Apples to Apples , yes
Able to leap tall buildings in a single bound ? YMMV

The animations are from DINAMOTO, the Italian Motorcycle university.
They show 'normal' single cylinder or twin with 360 degree crank then the 90 degree re-phase (Yamaha already use it in TDM 850)
Its a really interesting site, I used to spend a lot of time going through stuff there (and there is a LOT of stuff)

Make sure you hit the British flag if you return to homepage (unless you can read Italian ;) )

Check out the Victoria
I always wanted a 4 bar link rear suspension, ever since I saw one in the early 70's
Now this is a topic about which I know little but would love to know more...

If I understood the gist of what was written, the purpose of the rephase is to sacrifice a little primary balance for a good deal of secondary balance? Or is primary balance also maintained by the multiple flywheels on the crank?
Primary balance is through flywheels as normal. the reasoning behind the 76degree crank was to have one piston moving at maximum velocity while the other was 'stationary' so negating the need for heavy flywheels.
I'm not an engineer but it just seemed simple to me, I could 'see' the principle.
Everyone thinks it was done for reduction in vibration, originally it wasn't , it was done to improve acceleration.
Because the flywheels are 'offset' in a parallel twin, the secondart rocking couple is reduced compared to a 180 or 360 twin. I've built one, but crank is completely stripped at present for work on flywheels (has been about 2 yrs now ;D )
really, it still needs a counter rotating balancer shaft
Different engine with different stroke and rod length will have a different optimal angle, I think the XS650 with CR500 rods was 79 degrees, 83 is easiest re-phase though(without having a second crank and new center pin made)
OK... so having a 360° crank you get a lot of up-and-down movement and a good deal of strain on the crank (especially the ends)?
With 180° you get a lot of side-to-side rocking and still a considerable amount of stress on the crank?
76° would be the happy medium? How did they arrive at 76°? 90° would have been my first logical choice and if I understand the article correctly, the 90° is a superior choice after flywheels have been taken into account...

Also... how do the counter-rotating balances work? I understand the theory, but not the mechanics.
76degree only works for Triumph 650.
I may not be explaining this too well but I know what I mean ;D
I've forgotten the math, it was about 8yrs ago I did it.
The 360 crank stresses the bearings and journals at TDC and BDC, doesn't have much of a rocking couple from equal firing
With or without center bearings, the center can (and does) separate (on XS650, around 100bhp, 9,500 rpm)
180 crank does the same thing but at only half the amplitude (only one piston 'vibrating')
There is less ultimate bearing stress but it puts more stress on the center journal as that becomes the pivot point (like a see-saw)
90 degree is better for balance but not ultimate crank accelleration.
Counter rotating balancer has weights that counteract forces in opposite directions at all times.
"correct' way to do it would be place the weights between flywheels (KZ400)
CB/CM400t has chain driven balancers rotating same direction as crank.
The vibration is reduced if you use a single counter balancer but it does stress bearings a lot (Hayabusa, GSXR1000)
Sven, Honda made it work for several years.
CB/CM400 has dual chain driven balancers, bit of a pain to time correctly but they actually work quite well to reduce vibration.
Crank is in center driving 'outside' of chain, outer sprockets turn opposite direction (draw a 'smiley' clown face with circles each end and 'nose' touching center)

Pretty sure you could Google Lanchester shafts/balancers, they were developed in the 1930's

McGraw-Hill Science & Technology Dictionary: Lanchester balancer
(′lan·chə·stər ′bal·ən·sər) (mechanical engineering) A device for balancing four-cylinder engines; consists of two meshed gears with eccentric masses, driven by the crankshaft.
Main reason as far as I remember was to reduce material use.
On the XL250, weight dropped more than 23lbs by adding a 4 lb balancer, all components were lighter/thinner wall and frame didn't need to be as heavy to reduce stress cracking
crazypj said:
Sven, Honda made it work for several years.
I bet, but with reverse rotating countershafts.

Pretty sure you could Google Lanchester shafts/balancers, they were developed in the 1930's
Sorry, I'm afraid you mix up two different things:

the countershafts in the Honda 400 engine are designed to balance primary forces,
hence both spin with engine speed but (both) in the opposite direction,


the Lanchester balancer is designed to eleminate secondary forces, so its counter-
shafts spin a twice the engine speed, one in direction of the crank, one against it.

Crank is in center driving 'outside' of chain, outer sprockets turn opposite direction
Yep, that's right! ;) (you just claimed the contrary in your last post)

Best regards
The Lanchester balancer was originally designed for straight eight as far as I can recall. (my father explained it to me at least 45 yrs ago, I was only 8 or 9 ::) )
Because the crank was so long they had problems with them breaking so yes, it was designed for secondary forces.
Honda used a variation which was easier to explain as a Lanchester shaft
I really thought it would be easier to find information on it but there isn't an awful lot online that I could find (probably using wrong search terms?)
BTW, I realised I had posted wrong information, which is why I changed it.
Had to think about the way balancer shafts were driven for a minute ;D
One thing that has surprised me when drilling camshaft on CB360 for oil passage, the amount of vibration from cam lobes was higher than expected
crazypj said:
The Lanchester balancer was originally designed for straight eight as far as I can recall.
Maybe, but an inline 8 cyl. engine can be fully balanced without any additional countershafts just
by using the proper crank design (one "bigbang" 4 cyl. crankshaft like the new Yamaha R1 has
(like crossplane V8 crank) connected with another mirror inverted one). Normally these long crank-
shafts (also inline 6s) suffer/break from torsional vibration.

Honda used a variation which was easier to explain as a Lanchester shaft
Well, apart from the frequency a major difference between the Lanchester balancers
and the ones used in the Honda twin (and a lot of other 1 and 2 cyl. motorcycle engines)
is that the Lanchester produces/compensates unidimensional forces, whereas the others
balance rotating forces.

I really thought it would be easier to find information on it but there isn't an awful lot online that I could find (probably using wrong search terms?)
Tried it but I couldn't find anything but brief overview on wikipedia.
Easier to find some lectures on it in German...

One thing that has surprised me when drilling camshaft on CB360 for oil passage, the amount of vibration from cam lobes was higher than expected
You mean when you machined it in your lathe?

Best regards

Hi Sven,
Yep, I only have a small lathe and I had to reduce speed as there was a noticeable vibration from cam.
My re-phased XS840 cam was even worse
Unfortunately I don't read German as I would like to find out more, maybe babelfish or google can translate into english?
(although I've never had too much success with technical terms)
I'm going to bump this as I've been getting a bunch of questions about rephase recently

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