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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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Discussion Starter #4
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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Andy said:
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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Discussion Starter #6
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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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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:confused: HUGH:confused:
 

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Discussion Starter #9
Thanks Aris,

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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.
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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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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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Nice one Andy :)

It looks fairly simple design, air cooled and not very sophisticated, so I guess it won't have loads of power.

How about this one then? :D

And since this is the trivia forum, can anyone tell us what kind of counterbalance weights this engine would need? :twofinger

OK I was only joking, but I'm sure you can find which manufacturer who make motorcycles today built more than 21000 of these between 1929 and 1945?

Mind you this is a 9 cylinder radial, they were making a 14 cylinder also at that time!

Aris
 

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Hi Andy,

I see that you are getting really interested in engine vibration :)

I understand your question very well. I suppose you have seen, a BMW boxer shaking at the traffic lights at some point and wonder why. Maybe you have rode one and found out by yourself!
The BMW boxer is a complketely different animal than other bikes (engines). It could be compared only with the Zundapps and the Gold Wing engine.

The boxer engine is balanced for primary and secondary shaking forces, but the characteristic BMW 'shaking' is there because the two cylinders are not located on the same axis and this is how a rocking force is generated that tries to turn the engine back and forth around the vertical axis.
I couldn't find a good picture of the engine to show you what I mean, but it is visible even on the brochure of the R50 below. (see the top view)
Actually as you ride any BMW boxer you can feel this right away, and also it is clearly visible that you have much less room for your leg to the right side as a result of the right cylinder position.

The resulting shake is very different to an inline 4 (like the ZR) and is not a high frequency vibration (like the feeling on the ZR pegs at high rpm), it maxes around 4500 rpm and then settles. That is if the engine runs well, carbs balanced etc.

Another interesting feature of these bikes (and the Guzzis) is that the crank is positioned longitudinaly and there is a torque reaction when you blip the throttle and increase rpms suddenly. The bikes leans to the right if you do that! It's funny but you get used to it quickly. The advantage is that the gyroscopic effect generated by the crank, is at 90 deg to the axis of the bike and thus you don't fight against it when you lean in a curve. The only gyroscopic effect keeping the bike to its course is from the wheels, and this is the reason why the BMW Rs are so flickable compared to the size and weight. You must ride such a bike to feel this for yourself. These bikes are really easy to ride and can sometimes surprise other riders in the right twisties with the right rider!

Aris
 

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Maybe I should have mentioned that this was a reply to a PM from Andy :)
Anyway, I posted here as I couldn't post a picture in a PM.
Here is a nice design showing the BMW boxer (R75 in this case) and how the pistons are positioned from top view:

Aris
 

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Discussion Starter #19
Hi Aris,

Thank you for the clear explanation.

I think BMW should call it a sort-of-horizontally opposed twin. :) Too bad the cylinders are not perfectly opposed to remove all the vibrations.

Regarding the torque action and the bike leaning. I never thought about the gyroscopic effect and how it doesn't fight leaning on a BMW. So when I coast down the mountain on my ZR, the lack of gyroscopic effect is the reason why the bike is more flickable.

Have you heard, the chess club found a location for the Spring Geekdown. It will be in the library. :D

Andy
 

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Andy said:
Hi Aris,

Thank you for the clear explanation.

I think BMW should call it a sort-of-horizontally opposed twin. :) Too bad the cylinders are not perfectly opposed to remove all the vibrations.

Regarding the torque action and the bike leaning. I never thought about the gyroscopic effect and how it doesn't fight leaning on a BMW. So when I coast down the mountain on my ZR, the lack of gyroscopic effect is the reason why the bike is more flickable.

Have you heard, the chess club found a location for the Spring Geekdown. It will be in the library. :D

Andy
Hi Andy,
The BMW R engine IS a horizontally oppesed twin. It 's also called a Boxer because the pistons move against each other. I mean that they are both at the same time at TDC and moving at the same time towards BTC etc.
This is not possible with the rods attached to the same crank pin as in the Ducati for example.
The Ducati has both rods attached on the same crank pin and the distance between them is very small, and the rocking force generated is very small.

I know you use to coast down a mountain (from the other forum:)) but although this is a way to understand the gyroscopic effect of the crank, it's not the best way to go down a mountain fast and safely ;)
Of course I ve done it also, and it's a very good way to race together a group of very different bikes :D A Vespa has proven to be a great contender in tight downhill twisties like that!

If you have the chance to ride an R engined BMW, do it and you will see what I mean. At first you will lough as it leans to the right!

If you ever come to Greece, you can ride mine :)

Aris
 
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