Crank balancing

[RichardL]

I must admit, I am confounded by and doubtful of the significance of the % values for crankshaft balancing. In a quick search, I did not find a definitive engineering article written on the topic, though there must be hoards. By "confounded" I don't mean that I don't get how the number is derived, that has been explained clearly enough. My problem in understanding this is that it only considers the weight of the moving components but not the uneven forces that occur when the piston is in varying positions in the stroke. It seems to me that at exactly TDC and 180 deg BTDC all applied forces are rotational because there is no reciprocation whatsoever. After TDC, in one cyclinder, forces of friction are being overcome by combustion, while in the other, downward motion pulls in air and fuel via the short path through the air filter and carburetor. On the upward stoke, one cylinder is subject to forces of friction plus the resistance of compression, while the other is pushing out burnt gases through the impedance of the exhast pipe. My point being, forces during downward travel are different than forces during upward travel and, therefore, prohibit exact balancing in the inline twin.

Given the above, it would seem to be enough, and all one can do, to balance the rotational weight and assure that rods, pistons, rings and pins all match each other in weight.  

If there is a balancing approach that considers the forces in a running engine, I would be very interested to hear of it.

Also, please don't send the BSA police if I am miles off the mark, though I have tried to think about this from an engineering viewpoint.

Regards,

Richard L.

[beezalex]

Richard, balancing a parallel twin is impossible, it is always a compromise between fore-aft vibration and vertical vibration.  Both increase in amplitude linearly with rpm.  And yes, since the rotational velocity is not really constant under power, this adds another term to the vibration.  Ever notice that you get more vibration at full throttle under load than while cruising?  All these forces then come together to excite vibrations in the chassis.  The difficulty lies in the large number of vibrational modes of all of the components of the bike and their various resonances.  But what it comes down to is that some modes are stimulated more by the fore/aft forces and some are stimulated more by vertical forces.  All of these interactions are so complex that, even by today's standards with finite element analysis, it is difficult to calculate even the primary resonances.  So, it then comes down to experimentation, and that's exactly how these balance factors were determined.  What balance factor (compromise between vibrational directions) stimulates the least offensive vibrations is thus arrived at after a series of guesses and tests.  Change the conditions, though (the chassis, the state of tune, etc.) and the result can change.  So, it's a loaded question and it really comes down to tapping into the knowledge base and what has worked for other people with similar setups...and that's what I'm trying to tap into.

[RichardL]

Oh, sure, go all empirical on me.  

Actully, I really appreciate your response. Funny, it seems modern motorcycles have gone to rubber suspension for their engines. Can you imagine such a thing?

Richard L.

[olev]

Richard, as Alex says, all parallel twins vibrate.
They also have a "sweet spot" or rev range where they vibrate less.
This sweet spot can be moved up or down the rev range by changing the balance factor.
If you do all your riding at 60mph you do not want to have the sh*t shaken out of you at this speed while its smooth at  75mph.
Normally the balance factor is not a problem unless you want to alter it for racing (move the sweet spot up) As the man says: It's better to shiver from horror instead of being shaken to death by the engine.
or .. if you want to get your engine dynamically balanced.
Dynamic balancers run at a fixed speed (about 500rpm??) and you have to tell the operator the balance factor you want.
I always mistakenly thought they could run the crank up to the speed you wanted and balance it there. They can't.
The problem here is if you give them the wrong balance factor you could to be worse off than before you started.
If you get it right the thing is a joy to ride.
That is why I for one would like to know the factory figure as I intended to get my crank done.
And even here we have a problem, the factory balance factor for an A7 plunger could be different to an A10 plunger which could be different to a road rocket which could be different to .....
think I'll retire gracefully from the discussion, cheers

[beezalex]

Sorry to dork out on ya, Richard.  I deal with dynamic analysis on a daily basis so I may have lost my bearings a bit.

Olev, unless you're worried about the crankshaft flexing, there's no difference in balancing a crank at 500 vs. 1000 rpm except that the vibration will be four times bigger at 1000 rpm (frequency squared).  The reason the balance factor changes the "sweet spot" is that it changes how you excite the chassis of the bike.  Problem is, there are so many other things that change it, too...or make it go away...like how things are bolted together, that it's hard to figure out why you're vibrating...hence my dilemma.  I hope I'll have time to fiddle with it this weekend...so many bike...so many problems...so little time.

Cheers.

[RichardL]

Alex,

In a perfect world, engine in perfect balance means no exciting energy to affect tubing and tin, except the road. Makes you wonder what would happen if all the tubing were pumped full of some dampening material. No problem on the dork factor, my days are often chock full of acoustics and noise control discussions, though, architectural in nature.

Richard L.

[MikeN]

Richard,
  I think of it this way. At the 12 oclock position ,the crank and centrifugal force is trying to send the complete con rod and piston assembly up through the cylinder head.At 6 oclock its trying to fling it down into the road.
 BUT, at 3 and 9 oclock ,the crank and centrifugal force is trying to send the big end, MINUS the weight of the rest of the rod and MINUS the weight of the piston (because they are now at 90(ish) degrees and are not affecting it) fore and aft respectively.
  So the weight to be balanced is constantly changing and that is the compromise that has to be achieved.
What do you think?
Mike