Shaking forces of twin engines - Sportbike Forum: Sportbike Motorcycle Forums
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post #1 of 22 (permalink) Old 03-06-2002, 11:06 AM Thread Starter
 
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Shaking forces of twin engines

Primary force is easy to understand (shown by the blue arrow).

Secondary force is not as easy to understand (shown by the red arrow).

What causes secondary force?

Link source:

http://www.mecc.unipd.it/~cos/DINAMOTO/indexmoto.html
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post #2 of 22 (permalink) Old 03-06-2002, 12:09 PM
 
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This is the X-axis force we discussed in the counterbalance shaft Thread.
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post #3 of 22 (permalink) Old 03-07-2002, 05:29 AM
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Hi Andy,

Very nice graph, and this site you found has all the 2 cylinder configurations analysed! I really enjoyed it.


Primary and Secondary forces of inertia depend on the 1st and 2nd moment of inertia.
They are caused by the reciprocating and rotating masses.

Aris
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post #4 of 22 (permalink) Old 03-07-2002, 06:42 AM Thread Starter
 
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Hi Aris,

Would I be correct to say that if there was a counter weight on the crankshaft above, the primary balance would be perfect and the blue arrow would go away?

Andy
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post #5 of 22 (permalink) Old 03-07-2002, 07:38 AM
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Quote:
Originally posted by Andy
Hi Aris,

Would I be correct to say that if there was a counter weight on the crankshaft above, the primary balance would be perfect and the blue arrow would go away?

Andy
Yes Andy, you would be 100% correct.
This is possible for an engine with the cylinders at 90 degrees.

Take a look at the V60 engine (Aprilia) and see what happens. It's the opposite than the V90 really. The primary forces can't be completely balanced as the graph is an elipsis, but the secondary can be easily balanced!

It's the result of the function of sin and cos of these angles.

Aprilia opted for the V60 in order to create a more compact engine, but it's not easy to balance this engine.
It has two counterbalance shafts, one countershaft that turns in the opposite direction than the crankshaft, and one smaller one which is in the rear cylinder head and balances the moment of the first countershaft!
Their plan was to remove the 2nd one from their racing bikes, I don't know if the did it.


Aris
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post #6 of 22 (permalink) Old 03-07-2002, 08:20 AM Thread Starter
 
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Thank Aris,

Now that we have removed the primary shake (blue arrow), is it true that on a 90 degree twin like the Ducati, that the shake from the secondary inertia is completely linear, or is it somewhat circular?
I know the diagram shows it as linear, but maybe it is just for clarity.

Andy
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post #7 of 22 (permalink) Old 03-08-2002, 02:02 AM
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Hi Andy,

The formula for the components of the 2ndary inertia forces given in the link you posted is correct as far as I remember.

The magnitude of the 2ndary force for each cylinder at any moment depends on the cos of double the angle between the piston of that cylinder and the crankpin.
The X and Y components of this force make up the final magnitude and direction of the force.
It seems correct to me that the result for the Ducati is a force whose magnitude changes , and the final vector lies on the axis which is vertical to the line dividing the 90 degrees angle. (I don't know if I expressed thjis correctly, I mean the line that divides the 90deg to two 45deg angles).
It's direction changes according to the change of the angle.

It would be interesting to make the calculation for the Guzzi engine and see the results. I would do it if I wasn't at work
The interesting thing with the Guzzi is that it's exactly like the Ducati geometrically, but positioned differently.
The result in the graph ios the 2ndary force is lying on the X axis, meaning that the Y component is 0.

Aris
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post #8 of 22 (permalink) Old 03-08-2002, 02:56 AM
 
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HUGH
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post #9 of 22 (permalink) Old 03-08-2002, 08:31 AM Thread Starter
 
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Thanks Aris,

-----------------------------------------------------------------------------
The magnitude of the 2ndary force for each cylinder at any moment depends on the cos of double the angle between the piston of that cylinder and the crankpin.
-----------------------------------------------------------------------------

Yesterday, I went to my local Ducati shop and the salesman started a 748 for me. I was very impressed of the lack of vibration when I revved the engine. I only felt a little vibration in the fairing at higher RPMs.

Andy
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post #10 of 22 (permalink) Old 03-08-2002, 11:12 AM
 
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Now, do you think it would be possible to make an engine composed of 2 V-twins. One shown the same way it is on that graph and 2 more pistons rotated 180 degrees from the ones that are there, making a cross shape? I realize that they could just put them side by side and make a V-4 but would there be any advantages to having it in a cross shape? One disadvantage would obviously be size. It would be tough to make something like that compact.
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