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V-Twin 90 degree = harmonically balanced?

Discussion in '24-7 Lounge' started by Pspice, Mar 5, 2019.

  1. Chris Wilson

    Chris Wilson Just got it firing!

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    What I find weird is the claim of perfect primary balance in an operating 90 degree V twin IC engine. I would accept the claim in a theoretical model that is driven by a constant speed from the crankshaft but in practice the pistons do the driving and cause an acceleration that is uneven throughout the stroke.
    The secondaries or vibration that occurs twice per revolution is not twice the amount gained from 1 cylinder but the square root of 2, or on other words 1.44 times 1 cylinder.
    And so a 90 degree V twin will never be as tingly as any other style of twin.
    Worst tingly twin that I have ridden is a BMW 800 GT, loved the bike, hated the engine.
     
  2. flong

    flong Just got it firing! GT Famiglia

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    I'm amazed at how often I've read that a 90 degree V twin has perfect primary balance. This is NOT correct. The 90 degree V twin CAN be balanced for a perfect primary balance but is not inherently balanced.

    From P. E. Irving's Motorcycle Engineering:
    "In the 90 degree form, perfect primary balancing is obtained by counterweighting to 100 percent of one piston plus the small end."
    As earlier mentioned, the secondary forces will be 2^1/2 (about 1.4) times the force at twice piston speed. There is also rocking forces if the conrods are offset.


    Boxer twins have perfect primary and secondary balance but a rocking couple.

    An inline triple (my Triumph Trident T160) has perfect primary balance but a rocking couple.

    A mirror image symmetrical straight 4 cylinder crankshaft has perfect primary balance but a high frequency secondary balance (twice crank speed) and no rocking couple.

    Frank
     
  3. Godfrey

    Godfrey High Miler GT Famiglia

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    Consider a single cylinder engine.

    Primary balance is the amount of vibration along the direction of the piston's movement. The counterbalance weight on the crankshaft is there to reduce primary imbalance.

    However, as the counterweight and the big end of the rod inscribe a circular motion and swing out at approximately 90 degrees from the direction of the primary balance, a secondary balance vibration is induced. In the single cylinder engine, you set the balance factor (the percentage of primary balance weight defined by the piston and connecting rod masses) to split the difference on this secondary imbalance so that the two vectors happening at right angles are minimized.

    In the 90 degree V-twin, because the primary balance of one cylinder is exactly at 90 degrees from the primary balance of the other cylinder, you can set the balance factor to be 100% of the moving piston/rod weight of one cylinder and achieve perfect primary AND secondary balance.

    These are not all the balance factors in a piston engine. There is tertiary imbalance created in a single cylinder engine because the big-end and counterweight masses have different inertial moments that happen not at exactly 90 degrees around the crank rotation. This is because the inertial moment of the piston and rod are displaced differently when the crank is north of the 90 degree line vs south of the 90 degree line. This tertiary imbalance is not entirely resolved in a 90 degree Vtwin, but it's usually very small .. that depends to some degree on the length of the rod. The longer the rod, the smaller it is, the shorter the rod the larger it is.

    Another imbalance is created by the fact that the two cylinders, even on a single crank pin, are slightly displaced side to side from each other unless they are a fork-and-blade rods such that the centerline of the two cylinders is identical (like in a Harley-Davidson V-twin). This imbalance imparts a twisting moment to the entire engine around a line that bisects the center of the V.

    (Flat twins like in the BMW have perfect primary balance, slightly off secondary balance, and a larger twisting moment because of the necessary larger displacement between the cylinders required for a two-pin crankshaft.)

    And so forth. There's a lot to this. I recommend the classic book "The Design and Tuning of Motorcycle Engines" by the late great Phil Irving if you want to understand all the different forces and considerations when designing a motorcycle engine.

    Now, with all that said, the difference between the vibration moments felt in a Ducati 90 Vtwin vs a Guzzi 90 Vtwin come from the different arrangement of the engine in the frame. The Ducati engine is oriented such that the crankshaft rotates transverse to the frame and the frame's structure—along with the suspension, wheels and tires—is designed such that the various vibration moments of the engine are broadly distributed into the frame and damped out. The Moto Guzzi engine is mounted such that the crankshaft rotates longitudinally to the frame and in this direction, the frame cannot be designed to absorb the vibrations the engine makes as effectively, particularly at low rpm and idle, when the engine's imbalances (due to rotating masses and power pulses) are greatest. As the Guzzi engine spins up, just like the Ducati engine, the inertial imbalances reduce (due to timing) and the package becomes smooth, but at low rpm it's just impossible to resolve those imbalances completely. (And it would be impossible even with a balance shaft, since much of these imbalances are the pulse generated by the power stroke...)

    Hope that helps!™ :D

    G
     
  4. Chris Wilson

    Chris Wilson Just got it firing!

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    Nice read, the problem that I have with the tertiary imbalance thing is that the main vibe I get from my Bellagio is very low frequency - once a revolution not twice or three times like a tertiary issue would give.
    So this leaves me to believe that the vibes are deliberately designed in.
    If perfect primary is possible (I don't personally believe it) and secondaries are resolved my signature rumble on over run is caused only by tertiaries?
     
  5. john zibell

    john zibell Moderator Staff Member GT di Razza Pura

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    OK, What does it really matter on balance? The darn engines run and they run well. Be happy you have a motorcycle and just ride the darn thing.
     
    scottmastrocinque likes this.
  6. scottmastrocinque

    scottmastrocinque High Miler GT Famiglia

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    You said it my Brother. Amen John!
     
  7. Godfrey

    Godfrey High Miler GT Famiglia

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    There are also vibrations that come from the power pulses, which are asymmetrical in a 90 degree vtwin. They are part of the design, for sure.

    As John said, just enjoy and ride. :)

    G
     

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