I can't believe that i forgot one of the most important
aspects of the balance job. There are 2 types of inbalance,
force and couple. Force is the balance of a part that is
easiest described as in a plane 90 degrees to the center of
the crankshaft. This is also the balance that would be
derived if the crank was static balanced. There is also
couple. Most easily imagined for me at least as the in
balance that is 90 degrees to the ends of the crank. For
example you could have a crank that is 10 grams light at one
end and 10 grams heavy at the other. In couple this would be
a 20 gram error in force the error is 0. Yes you have to get
them both right.
Dave
Dave Dahlgren wrote:
>
> Balancing can and will take tons of stress out not hundreds
> of pounds.. And the lighter the parts the less the stress.
> If I was racing an engine with 3 mains i would own the
> lightest stuff i could find..
> 1 gram is common place work now and has been for 20 years if
> the shop is any good at all.
> The trick with the 2 ends and error is to balance 1 end in
> the fixture usually the big end if memory serves me right..
> get them all the same the do a total weight to get the small
> end.. swapping parts around ought to be a last resort for
> me.. the best way to do it is pair up all the big ends for a
> 2 big end weight that is the same. then use the small end
> weight difference to mix and match the different pistons..
> then you can come up with a correct bob weight.
> On the subject of bob weight it does not have to be a 50% of
> reciprocating weight deal and depending on your rpm you may
> want to go to 51 or 52%.. the real beauty of a 51% job is
> later on if you have some new bob weight and it falls within
> 50% to 52% you are good to go with no rebalance on the
> crank.
> The real thing to remember is that the balance job is only
> valid over an rpm range not from 0 to 9000 rpm. The higher
> the RPM the more you have to look at more than 50% recip..
> many slower turning engines are balanced at less than 50%
> BTW.. On some of the small stuff i have worked on that
> really turns up (10 to 13k rpm) i have used 54%.. I spent a
> lot of time with the engineers and field guys at Stewart
> Warner years ago on this very subject for Winston Cup
> stuff.. if you are mathematically inclined there is a bunch
> of good stuff that is written on this subject. I would
> suggest Stewart Warner a good place to start for a suggested
> reading list.
> Hope this helps a bit
> Dave Dahlgren
>
> ardunbill@webtv.net wrote:
> >
> > Hi Folks, recently delved into this subject, one I've always found of
> > interest, because I had to consider it for my 290 Ardun. After talking
> > to various experts and reviewing the literature, I realized it is
> > necessary, at least this time.
> >
><snip>
> >
> > The above is merely what this amateur has been able to learn about V-8
> > balancing up to this point, so I hope the real experts and veterans on
> > this List will point out any errors they see. Cheers from Ardun Bill in
> > the Great Dismal Swamp, Chesapeake, VA
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