<VV> Rod caps and balancing

[JVHRoberts at aol.com]

Two banger opposed have TWO crank throws, just like the opposed pairs on a  
Corvair, VW, Porsche, etc. 
The only flat engines I am aware of that have a common throw for opposed  
cylinders are flat 12s, like Ferrari, etc. 
 
 
In a message dated 6/11/2008 6:08:59 PM Eastern Daylight Time,  
tibbitts at qx.net writes:

Eric  makes a good point, except he is mistaken about the rods on a Corvair.  

The big ends do not travel together.  Each connecting rod has its  own
journal on the crankshaft and the paired rods actually travel in  opposite
directions.  When #1 is at TDC for compression, #2 is also at  TDC for
exhaust. #5 and #6 will both be at BDC and #3 and #4 will be  somewhere in
between. 

My blown 140 is sitting on the bench and I  just looked to confirm my
suspicions. 

Cliff Tibbitts
Lexington,  KY

<snip>
I don't buy that. Think of a two cylinder opposed  engine, the big ends go
around together. The small ends going back and  forth are going the same
direction, so they are not canceling each other,  they are helping each
other. What keeps it balanced are the pairs of  cylinders canceling each
other out.
The total of 1&2 needs to  equal the total of 3&4 which needs to equal the
total of 5&6.   The counterweights are the two other pair. Big end & small
end/piston.  That way as two pistons are going to the left at full speed, 4
are going to  the right at reduced speed (one of each pair is leaving BDC,
and the other  is leaving TDC).

Long story short... balance ALL  rods & pistons.

Eric

<Snip>
The way a 'Vair engine is put together is pretty  much tantamount to three
2-cylinder engines sharing a common crankshaft.  The opposing pairs of
rods/pistons simply don't interact with other  opposing pairs... unless
something is VERY MUCH different as regards weight  and even then it's not
gonna amount to THAT much vibration. 

This is  why a 'Vair crank gets away with having no counterweights.  
<Snip>







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