Bob weight toal for Balancing a crank

[81cr450]

Hi I need to know the bobweight figure for balancing a kx500. I'm a machinist engine / assembler by trade & have a balancer that I run but its all automotive & industrial innillators. I can do the work but I dont know the math figure for the bobweight. I realize this is info most machinist's dont want to share but... Please. I figure its got to be total weight on the rod, bearings" upper & lower." piston, locs, rings, pin. I just need to know thats right, where on a v-8 you split the second rod on the throw by eliminating the pin end half of that rod. Or where on a opposed inline you use no bobweight. Oh great machinist wiseone please teach young grasshoppa. Input on crank truing in a v block would help also.

[byron909]

wouldn't you just weigh everything on top of your rod?? your stuff might weigh different than my stuff.

[greencannon]

the machinist in my area that i use from time to time is very reputable in my area. His work is top notch and have had a lot of referals by many enthusiasts.
The shop he runs specializes in custom drag bikes and really high hp engines. I had him true and weld my K5 crank that came brand new right from Kawi.  a worth while mod in my opinion.
 He told me that you can never truly balance a crank of that type. He wouldnt disclose too much info on why you couldnt. I figured he was protecting  his trade secrets, but he did say there were also too many other variables around the crank that didnt allow true balancing. 
If you dig around this sight you can find a bit of discussion on crank mods, I recall someone here that had pics of a crank that had material drilled out of it....maybe an attempt to counter balance all the "weight" bolted to the small end  of the crank.

[Hillclimb#42]

I'm just gonna throw in a couple things that I think I picked up while having mine balanced. I think its about having equal crank halves to start. Installing the two halves correctly is important too. The rod and piston are too far away from center to ever get it to be perfectly balanced, but engine builders are able to get it balanced in a certain rpm range. Modifications that lighten the piston and rod help the "balance." Also alot of cranks are not true in their circles that they make while spinning. If either half of the crank is bad on it tolerances, I don't think you can fix it. I'm no machinist, and could not grasp everything that was explained to me, but I did understand that it is really tough to get there without good matching crank halves.

[jonny500]

i have just had my crank dynamicly balanced, it is come back monday. the guy said he is going to send some pics and a vid of how it was done. as said above you cannot truely balance a single cylinder properly so they balance it to a certain rpm. as soon as i get the info i will post it on this thread

[Dutch-K5 Fan]

This PDF file is a what I found in englisch, whanna make a simple version so we all can understand it.
Till now we can use this so you don't have to wait on me.

Dutchie

[jonny500]

thats gold dust dutchie, many thanks

[Dutch-K5 Fan]

Thank you. Give me till the end of next week to post a new topic with "al the secrets".
Want new topic for easy finding with minimun discusion to ceep it simple and clear. Because it's a difficult subject.
Post links in al topics ware crank balance is talked about.

Dutch

[81cr450]

Awesome, Thanks Dutch, That seems too easy. I'm not quite sure why he's is splitting the weight on the pin end & the piston , must be RPM related . I thought I'd need the machine to do it but the static versus dynamic does make sense as the width of the crank is so narrow the dynamic doesnt really come in to play. I thought for sure youd use the whole weight of the combined piston pin rod bearings combo. All else fails I figured I could weigh a stock piston & rod & bearings to try to see what the factory did Thanks again.
Look forward to your simplification

[RoostDaddy]

Where is everyone sending their cranks to get balanced?

Here's a little caption that I've found,
there are many different balance factors you can use, the most important part is how the cranks are balanced. To do it right it is very time consuming. The crank must be disassembled and put back together with nothing on it. Then after weighing the appropriate parts you create a formula using rotating mass and reciprocating mass to establish what?s called a bob weight. Keep in mind now that all these cranks are different .
After spinning in the balance machine, a specific amount of weight is added or removed at a specific rotation of degrees (different on each crank half). This completes the balance process and the crank must be disassembled and reassembled in the same manner.  Dead balls true!! Crank width perfect!!