Balancing crank??

mrfubs

mrfubs

Member
iv been studying this thread http://motorbicycling.com/showthread.php?t=40568

Math is really not my strong point and i cant seem to put all this together. i know i have to weigh the rings,piston,circlips,wristpin+bearing and the reciprocating part of the con rod add all together and pick a balance factor 0*62% reciprocating mass i choose 55% of total reciprocation mass. So say everything adds up to 110g, 110 x .55 = 60.5g then 60.5 - 16.8 g = 43.7g. now did i do this all wrong? if not is that howmuch weight i drill out of the crank?
 
no, I know its tricky to understand, he put alot of jargon in there that just confused me and probably many other people who have never done this thing before.

However math isn't your issue, you're dead on.
So the 43.7g you got, now you have to weigh out a string and stuff to tie onto the string to match that weight. So you can make a wristband out of spare nuts you have laying around, as long as it weighs 43.7g. Then you hang that off the small rod end where the piston connects to the crank. Then you just drill material off the crank's heavy side (usually the side opposite to the piston, whatever side rolls downward is the heavy side) until you can place the crank in any position and it wont roll on its own.

Problem is we dont know how much each crank weighs and where that weight is, they could be very different for all we know, so you might end up removing less than the guy in the original thread did. Just know that 1 CC of the crank is [probably] 7.8 grams, so if you have a 3/8" drill bit, drilling ~10mm into the crank will remove 7.8 grams.
 
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All thanks goes to mew! i appreciate all your help man youve made things a **** of a lot easier for me to do and understand!
 
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Depends on your desire for perfection. It's impossible to static balance a single anyway, so the time you spend balancing it would pay off with an engine that is happy at your chosen rpm range, nothing more. Finding the best compromise is what makes it worthwhile.
Personally, even though I have (had)the skills, I would not bother on my personal ride as I no longer race anything. It takes far too much math for my taste as well.
Buy one from Fred or Neil and mod your brains out. Crank work is best left to racers.
 
I agree I.ve played with balancing pocket bike cranks, but for a HT that I want to do 9000 RPM on I just went and bought another RSE crank today, For the $65 I think he charged me saves a lot of messing around for potentially little or no gain.
Best left to those that know what there doing, my other RSE crank is smooth as.
 
high rpm balance is the limiting factor. i looked at all threads on the subject here and they are a laugh! in a nutshell crank comes from factory balanced for HIGHEST RPM. CRANK DIAMETER IS EQUAL TO A 180CC YAMAHA RT. theres a guy removing 25 grams from crappy 5pc cranks and calling them a hundred dollar bills here. lol

try asking him how it magicaly does not lower case compression?

zpt
 
5 pc crank can be balanced with out losing primary compression, as the outer 'covers' cover the removed metal.
 
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