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Thread: 90 degree V twin secondary vibration

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    90 degree V twin secondary vibration

    Hi all,
    Can anyone please point me in the right direction as to where I can find an explanation of what exactly is a secondary vibration in a 90 degree V twin?
    I know that secondaries tend to happen at 90 degrees or perpendicular to primaries and therein lies the issue, anything at 90 degrees to one primary tends to be the other primary.
    Thanks in advance.

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    apriliaforum expert plocky's Avatar
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    I'd look up why balance shafts are used in some engines.
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    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?

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    apriliaforum expert bloodnutt's Avatar
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    But Ducatis are V-twins, but their front wheels sit still at idle. And Ducatis–well, they may be V-twins, but they don't sound quite like Harleys. The first thing is that Ducati twins have twice as much V-angle, or 90 degrees. This uniquely makes it possible to balance a 90-degree twin with counterweights of 100 percent of one piston's shaking force. Here's how it works. At the front cylinder's TDC and BDC, the heavy crank counterweights cancel the front piston's shaking force. But at 90 degrees and 270 degrees, those same heavy counterweights now cancel the TDC and BDC shaking force of the rear piston. In other words, both primary piston shaking forces are 100 percent balanced, leaving the engine smooth.
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    apriliaforum expert TRexRacing's Avatar
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    At mid stroke of the the crankshaft the pistons are not at mid stroke due to rod/stroke geometry. They are a bit below and the corresponding conrod angle combined with acceleration rates causes the shake. Happening twice per revolution makes it twice the crankshaft speed hence second order. That's as close as I can get in into words. Help?
    oh well nevermind

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    apriliaforum expert LUCKY DAVE's Avatar
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    Quote Originally Posted by TRexRacing View Post
    At mid stroke of the the crankshaft the pistons are not at mid stroke due to rod/stroke geometry. They are a bit below and the corresponding conrod angle combined with acceleration rates causes the shake. Happening twice per revolution makes it twice the crankshaft speed hence second order. That's as close as I can get in into words. Help?
    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.
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    Quote Originally Posted by TRexRacing View Post
    At mid stroke of the the crankshaft the pistons are not at mid stroke due to rod/stroke geometry. They are a bit below and the corresponding conrod angle combined with acceleration rates causes the shake. Happening twice per revolution makes it twice the crankshaft speed hence second order. That's as close as I can get in into words. Help?
    Thanks for the thought out reply.
    I like it!

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    Honest always, feared often Micah / AF1 Racing's Avatar
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    On a 90 or 180 twin knife and fork rods allowing truly inline cylinders with no offset...would be interesting
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    apriliaforum expert LUCKY DAVE's Avatar
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    Quote Originally Posted by Micah / AF1 Racing View Post
    On a 90 or 180 twin knife and fork rods allowing truly inline cylinders with no offset...would be interesting
    Harleys have had them forever. Along with rolling element bearings, which have only been obsolete for 50 years.....
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    apriliaforum expert LUCKY DAVE's Avatar
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    Micah, just thinking about it, the "fork" rod would have to be really heavy if the system was designed to use plain bearings. Add up the width of the center rod, add stiffness to reach round it, and......
    A finish is a win! Moderation is the key! More wine!
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    Honest always, feared often Micah / AF1 Racing's Avatar
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    I’m just thinking it would be interesting. They are old tech but, lots of old stuff is finding a place in modern engines
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    apriliaforum expert LUCKY DAVE's Avatar
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    It would be great in a cruiser engine, very smooth.
    A finish is a win! Moderation is the key! More wine!
    And never lend a faggot your hat

    Proud Member of Team Punisher and The Texas Mile 200 MPH club
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    Quote Originally Posted by Micah / AF1 Racing View Post
    On a 90 or 180 twin knife and fork rods allowing truly inline cylinders with no offset...would be interesting
    A 180 degree twin with knife and fork rods would have a common crank pin, worst design ever for vibration as all of the forces add together.

  14. #14
    Honest always, feared often Micah / AF1 Racing's Avatar
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    You are totally correct. Long week, didn’t think that one through, twin cranks would be required I guess for a 180 twin.
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