# crankshaft balance factor

• posted
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
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
• posted
This might help:
• posted
The principle is very easy: Compensate all rotating masses fully Compensate translating masses partial (50%, depending on application)
Rotating masses are: Masses on the crank shaft that are out of center. -> crank + crank pin rotating part of connection rod
Translating parts: translating part of rod piston, piston pin, piston rings
How to get the rotating mass of the rod: It is very easy. put the top end of the rod (right where the pin is going through) on a knife, the other end on a balance. What you read is the rotating wheight. Reverse the setup (top end on balance) to get the translating wheight.
Every mass that is out of center of rotation has to be compensated with a mass that has the same distance*mass product. Say the crank pin weights 0.1kg and is 25mm off center. The product is 2.5mmkg. You can compensate it with a mass that is 50mm (CG) off center and weights 0.05kg Do that with all rotatning masses. The translating masses are calculated with a distance half the stroke and 50% of their mass.
The 50% can be varied. It depends on how the cylinders are arranged (horizontal/vertical) and if you prefer vibrations to be horizontal or vertical. Bikes normaly have vertical vibrations. Compensation is going up to 70%. Assuming translational masses are 0.3kg, stroke is 50mm and you want a 60% compensation, you get: 0.3kg * 50mm/2 * 0.6 = 9mmkg.
Now if you add all compensation masses you have to select a mass who's CG offset and whos mass is the same product. Counterweights normaly do have a D-shape. It takes some math to find the CG. Better to use a CAD for that.
HTH. If I was to confusing, feel free to ask. Nick
• posted
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
• posted
Best bet then is to cut off the two cylinders and weld them on the sides of the motor. This makes an engine with perfect primary balance.
:^)
Actually the best bet is to contact a bike salvage yard and get the part that's busted.....
Cheapest and simplest in the long run. Anything else is going to result in an investement in time and money that *won't* get paid back. If Kaw could have built 'em that way, they would have.
Jim
• posted
Since this is what I call an inline engine {not a V} and the engine is a 4 stroke means both pistons come up at the same time. Almost impossible to balance perfectly, the counter balance method would be the best for drivability. That being said, you'll have to balance it as if it were a single cylinder. Probably have to add a lot of weight to the counterweights I am sure. Will make for a rough idle I am sure if you balance it for higher RPMs. Will free up a little HP maybe.
• posted
I was curious about this specifically, for different types of IC motorcycle engines. I bought a copy of Fundamentals of Internal Combustion Engine Design, i think by Heywood?
anyhoo, it is an engineering text that covers all aspects of engine design, not too deep for a layman, I recommend it.
• posted
This particular engine has both pistons going up and down at the same time. I am pretty lame in motorcycle knowledge. What well known parallel 4-stroke engine type use the opposite, one up and one down? Sound must be quite distinct.
O=={o] Hot Rod
• posted
"Mike" wrote in news: snipped-for-privacy@enews2.newsguy.com:
Early triumphs had both 180 ( pin 180 apart ) and 360 (pins together alternate firing). Some laverda 500 racers had 180 cranks. Yamaha 850 twin and Triumph Bonneville america have 270 cranks where the crank pins are 90 degrees apart..... they sounf like a V twin
• posted
Are you sure about the early Triumphs? My '56 pre-unit had a 360 crank. I've never heard of any Triumph with a 180 crank, but I've never heard of a lot of things. You're absolutely right about the Laverdas though. I seem to remember a model test done by a bike magazine on a street model parallel twin Laverda back in the '70's that stated that it also had a 180 crank.
Garrett Fulton
Garrett Fulton
• posted
Two words ebay
• posted
thanks for all the info, guys. so the answer seems to be 50% to 70%; quite a spread. Just to be clear; crank is 360 degrees, that's what I meant when I said both pistons run in paralel. Jim; The Bike is 27 years old; used parts will be the same age. shipping costs to India will cost more than the entire job, machining and all. you can't put a different crankshaft in without new undersized bearings and a crank grind. finding new undersized bearings will be as much work as getting the crank ballanced, aside from that, techincal items will get stuck in customs for months; if I ever get them at all. you don't mail things like this to countries like this, you make do with what's available. A full days wages here is about \$3.US, 5 for a skilled machinist. that's why we do these things instead of just throwing the bike away; and because there is no new bike like it to replace it with anyway. Daniel; ebay is not two words; when you're in this part of the world, it isn't even 1. few vendors will even ship out of the US. information flows cheap and good though! regards, Mark
• posted
If the skills are there, and labor is inexpensive, maybe they could simply fabricate a new sprocket and braze it on to the end of the crank?
Jim
• posted
For me, British Motorcycles (note: respectfully I didn't say "brit bikes") were the classical parallel twins (360°). But I would't bet more than a beer or two. Or ten ale. But we were talking about beer.
Nick

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.