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Thread: 90 degree V twin secondary vibration

  1. #16
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    There tends to be a lot of confusion with the very usage of the word "secondary". I learned from others to keep it clear and refer to "orders" of vibration, i.e. multiples of crank speed. The second-order, or twice crank speed, is the "problematic" vibration in a 90 V-Twin. Although it's actually fairly small in practice.

    If you look up the principle of ("primary", first-order) balancing a 90 V-Twin, which that animated gif above clearly shows if you already understand it (it's also how crossplane V8s are balanced), then hopefully you'll be halfway there. bloodnutt also linked to a cycleworld (Kevin Cameron) article that discusses it, worth reading.


    As the others are alluding to, even the primary balance isn't perfect, because the pistons are not in the same plane. There is a very small "rocking couple" over the distance equivalent to a con-rod's big-end width, which you can immediately cut in half by weighting the crankshaft appropriately (like old-school engines without a balance shaft: "balance factor"). Luckily, rocking couples are often less noticeable anyway, because it twists the engine, rather than heaving it up and down.

    A brief primer on general balance can be found here:
    http://maybach300c.blogspot.com/2012...nder-bank.html

    A deeper practical insight can be gained here:
    https://docobook.com/some-science-of...e-designs.html


    The reason crank-speed (first order) balancing is not adequate in itself, and there is a residual ("secondary") imbalance, is because the motion of the piston is not "sinusoidal". This comes as a result of the con-rod periodically "laying over" and "standing up" again - the nature of this motion, although visually seeming to operate at twice crank speed, is actually infinitely broadband. That means there are an infinite number of harmonics (orders) of sinusoids that contribute to the forces that arise from the mechanism's cyclic motion - it's those forces which cause the "imbalance". In practice, for the balance concerns of small engines, you can generally stop worrying as of the 4th order (there are no odd-numbered harmonics present for the purposes of balance; although they are of significant concern for crank speed fluctuation, i.e. inertial torque and its attendant torsion), leaving only the 2nd order for serious consideration after the primary / first order issue.

    A mathematical approach to the harmonics is outlined here (sections 3.4 & 3.5):
    https://books.google.co.uk/books?id=...page&q&f=false

    You can estimate the magnitude of the secondary imbalance from that, as compared to the primary. E.g. for a rod-to-stroke ratio of 1.5:1 (pretty standard, maybe a tad short for high performance motorcycles), the second order vibration is only about 17% of the first order; 4th order is a paltry 0.25%. My Shiver has a positively sporty 2:1 rod, so the contribution is even less: 2nd = 13% ; 4th = 0.1%.


    Second order imbalance is what gives singles, 360 and 180 twins and flat-plane fours their buzzy feel; look out for pistons starting and stopping together. Note that inline fours in cars often have a pair of balance shafts that spin at twice crank speed to smooth it out a bit; KTM's 690 single now has a lone second-order balance shaft in the cylinder head, which only cancels out at most half of the 2nd order vibration (balance factor again). Inline fours have it hard in that the firing intervals are 180, i.e. also twice every rotation of the crank, adding to the buzz.

    The twin balance shafts are often attributed to one F. Lanchester, and named after him; nice overview here:
    http://www.fordscorpio.co.uk/tech2_3.htm

    The motivation behind the Mitsubishi "Silent Shafts" mentioned in that article also point to why Rotax put a crank-speed balance shaft in the rear cylinder head of the V990 / V60 in the Mille. They deliberately introduce a rocking couple with that second shaft to "undo" the one caused by the main (otherwise single) balance shaft in front of the crankshaft. There often isn't room for two balance shafts proper, although BMW managed it with the new F850s - these have only small second-order rocking couple, too and should be very smooth. The V990's "succesor", the 72 1125 found in Buells, and also KTM's 75 V-Twin show some ingenious use of single, eccentrically weighted "balance gears" dotted around the engine. But none of these tackle second-order vibrations, either.



    So, finally, the second-order motion of the pistons in the 90 V-Twin operate in different planes, just like the primary, and so create a (tiny) rocking couple just the same. However, even if the conrods / cylinder bores could be made coplanar (perfectly in line), there would still be a resultant second-order vibration.

    Clearly, the pistons are out of phase by 90, which is exactly anti-phase for a second-order sinusoid. But the 90 disposition between their motions (cylinder bores) partly undoes that again, and essentially only the vertical component (parallel to the line bisecting the v-angle) of the pistons' second order motion is cancelled out, whilst the rest stacks / adds up in the horizontal direction instead. The total effect is still only equivalent to 141% (square root of two) of the secondary force of just one of the pistons, though, rather than fully double.

    Taking the numbers for the magnitude of second order forces above, this implies a total secondary imbalance about 20% the "size" of one piston's primary imbalance (of course you don't feel the primary imbalance, as it's mostly countered with the crank weights). Still, you'd think it'd be noticeable even at 20%, and I wonder if a 2nd order balancer might turn up in twins eventually. Bearing in mind the hand-numbing effect from an inline four is due to something more like 60% of the primary imbalance of just one of the cylinders it's "constructed" from - so it clearly doesn't take much!

    Then again, as Kevin Cameron relays, it turns out we don't notice horizontal vibration as much as vertical, so maybe that's why the 90 V-Twin can get away with its secondary imbalance? Is that why Ducati's canted "L-Twins" are immediately more characterful?
    Last edited by IndelibleInk; 07-21-2019 at 06:06 PM.
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  2. #17
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    Secondary Balance

    This is a good explanation of secondary balance.


  3. #18
    Honest always, feared often Micah / AF1 Racing's Avatar
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    I can just go down to the garage and pull out a set. Beautiful parts from the Merlin!

    Quote Originally Posted by pete roper View Post
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  4. #19
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    I feel like he didn't really explain the reason the piston moves faster in the top half of the rotation.

    It's because the conrod broadly moves with the piston in the top half, and against it in the bottom half. It "falls over" as the piston descends from TDC, meaning it is "shorter" along the line of the bore, lowering the piston further; then, later, it stands up again as the piston rises to TDC in the last quarter of rotation. The opposite happens in the bottom half of the rotation. After 90 past TDC, the conrod stands up again, slowing the piston down. Likewise, after BDC, the conrod falls over again, retarding the piston's travel up the bore.

    That contribution from the con-rod is not solely second-order (and is not sinusoidal as a result), it has a complex character hidden away in that nonchalant application of Pythagoras (the square root of the difference between a constant and the square of a sinusoid - messy!). You can appreciate this by plotting the sum of the first and second orders and comparing that with the actual piston motion - it will look as though the conrod is changing length. You ought properly to also subtract the 4th order, then add the 6th, subtract the 8th and so on ad infinitum - look at Fourier representations of square waves for a better idea of what I'm getting at (i.e. here).

    Yet another reason to distinguish between "secondary" (i.e. often anything that isn't primary, but sometimes also any rocking, including primary) and second order. What is the fourth order imbalance of the V-Twin? What about sixth, eighth, tenth and so on?
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  5. #20
    apriliaforum expert TRexRacing's Avatar
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    Quote Originally Posted by LUCKY DAVE View Post
    That's a pretty good explanation.


    If we view the engine as a vertical single, when the piston is at mid stroke there is a fore and aft force that tries to shake the bike front to back. Watch a 360 degree twin like an old Triumph at idle and you can watch the front wheel assembly shake front to back, this is the second order vibration at work.
    V twins (and parallel twins with other than 360 degree crankshafts) have another source of vibration due to the fact that the cylinders are offset right to left on the crankshaft and the second order forces do no occur at the same time or in the same plane, the second order forces are trying to rotate the engine around an axis described by a line drawn though the V angle. This is called "rocking couple" vibration. Flat twins such as BMW's do the same thing.
    Thanks Dave means a lot coming from an you. I've spent a lot of time balancing Harley-Davidson crankshafts both static (doesn't help rocking couple) and dynamic (does). Learned a few things along the way.
    oh well nevermind

  6. #21
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    Maybe a dumb question but how does the vibration affect engine durability and are the various types (different V angles) inherintly different from one another is terms of their expected longevity all other things like maintenance being equal?

    I'm asking because some engines that seem like they'd shake themselves apart run forever not taking into account like new performance compared to performance after hundreds of hrs of operation.

  7. #22
    Honest always, feared often Micah / AF1 Racing's Avatar
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    Vibration increases wear on all components and fatigue in a rider. The biggest reasons turbines took over aviation is smoothness. Inline 6 cylinder and V12 motors are also VERY proper smooth and most last a long time.
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  8. #23
    apriliaforum expert plocky's Avatar
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    Quote Originally Posted by Chris Wilson View Post
    None of the explanations so far go close to explaining a secondary on a 90 degree V twin.
    Plocky, why would I look up balance shafts when they are never used on these kinds of engines?
    Do you know of a 90 degree V twin that does indeed use a balance shaft?
    Secondary vibration isn't confined to 90 degree V-twins & so you will find more information about secondary vibration when you look into balance shafts, as that is what they are for.
    The fact that 90 degree V-twins don't use balance shafts is due them being being able to be balanced well enough to negate the need for secondary balancing using balance shafts.

    The video posted above, by Apexer, explains secondary balance perfectly for my understanding.

    As does this one on balance shafts.

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  9. #24
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    Quote Originally Posted by Micah / AF1 Racing View Post
    Vibration increases wear on all components and fatigue in a rider. The biggest reasons turbines took over aviation is smoothness. Inline 6 cylinder and V12 motors are also VERY proper smooth and most last a long time.
    I'd argue the biggest reason is their (turbines) ability to produce the enormous thrust required to propel large airliners close to the speed of sound at high altitude. Piston engines were developed to the point of incredible mechanical complexity and size, yet couldn't come close to turbines wrt power to weight ratio.

  10. #25
    Honest always, feared often Micah / AF1 Racing's Avatar
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    Watch the documentary on Frank Whittle, father of the jet engine on streaming of your choice. At this point we could go back and make very high altitude capable piston aviation motors with modern compound turbochargers but why wouldnt anyone go from thousands of moving parts as with a piston motor to a few with a jet engine. Jet engines truly are amazing at going fast, but only even close to efficient at higher altitudes and with high bypass design. Smooth of course isn’t the only reason but it was by far the first thing early jet test pilots commented on. Powerful back then wasn’t up to piston levels of the time.
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  11. #26
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    Then are there V twin engine designs that are more durable than others because of lesser vibration or other engine types and is it just an inherent part of the design compared to say, in line fours, twins etc? Or is it all really about the quality of the components, better tolerances and so on and V-twin, inline 4 triple, parallel twin doesn't really matter.
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  12. #27
    Honest always, feared often Micah / AF1 Racing's Avatar
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    It’s far more about the engineering, and quality of components. FAR. A well designed and executed single is far more reliable and resilient than a perfectly balanced but shit quality inline six.
    Diminished expectations is the key to happiness in life.

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