DIYbanter - View Single Post

On 27 Nov 2005 22:09:31 -0800, wrote:

I'm having real difficulty finding the answer to what seems a simple
question; I need to know the bob weight formula [balance factor] for an
inline twin [both pistons run in parallel].
Top rpm is 8500, usually runs at about 4-5000. Street use.

I'm in India, and my old 1977 Kawasaki kz400 has a stripped balancer
driving sprocket. This is basically not repairable, but I've had
correspondence with an American who has had good results by removing
the balancer shafts and having the crank balanced.
An old friend of mine is now managing a hi tech machine shop [turbocam
India] not too far away that is about to buy a balancing machine, and
he wants to help.
I need to do the preliminary work at a local shop first, including
making the bob weights; but are they 1;1 in this case [equal to
piston/rod assembly] or does reciprocating weight have to be worked out
separately like on a V8?
I'm a good mechanic and not bad with engineering. There are some local
guys who are good as well, but as I said, no one has ever done this
here.
I'm sure there's a machinist out there in usenet land who knows the
answer!

I bought the old KZ in the US in 1978, and brought it to India in 1982.
It's respected like an old Ferrari would be in the west.
It's getting hard to keep it going now, and it looks pretty rough, but
it still kills any bike in current production in India.
Thanks for your time, I've been mailing and posting this question all
over but have had no replies.
Mark Holden

A parallel twin cannot be fully balanced because there
is no way that a single rotating mass can fully cancel the pure
up and down mass of the two pistons.

The best compromise is to balance out all the rotating
mass plus half the up and down mass - a similar trick to the Vee8
balancing. Balance the crankshaft with an added weight on the big
end equal all the weight of the big ends of the con rods plus
half the weight of the two pistons plus rings plus half the
weight of the little end of the conrods.

Because its's a compromise balance it's not particularly
critical. All that a small error does is to slightly shift the
direction of maximum vibration with little change in amplitude.

Jim