got a quick question, im balancing a full circle crank shaft for my pocket bike as the one in there at the moment vibrates like hell (which is surprisingly pleasant ! XD) and cant make it rev past 8000 anymore.

how would i balance the thing properly? its just a shaft with a crank and piston attached with threaded m6 holes at either end of the shaft so was thinking i could just make a U shaped jig with the two holes in it sort of like a giant prop adapter.

wondering if id need to attach the piston to the crank while balancing it too..

cheeers
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Complicated Subject !

The short version is, you cant perfectly balance a reciprocating mass with a rotating one since the force vectors dont line up all the time.

There are "rules of thumb" where a certain percentage of conrod big end weight is bolted to the crank pins and spun to get the balance in the ballpark, but I dont know what the numbers are for a single cylinder engine.

Try starting here.
http://en.wikipedia.org/wiki/Engine_balance

If you want to really tweak it, find your local Go Kart Racing track (outdoor two stroke, not indoor hire), and look for the adverts for the race blueprinting guys and ask them how or if they can do it.

Andrew (TotallyRecycled) might be able to give you some tips, he works on single cylinder lawn mower engines, although most mowers probably dont do 8000rpm
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yeah thats what i was curious about. the crank itself is perfectly balanced as its just a lathed out disk of metal, its the piston that throws it off.

im inclined to think the best compromise will be to weigh the piston and then drill some holes out that add up to that weight on that side of the crank. we shall see anyway
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most assembly's are already factory balanced ma bee some one has changed the piston or connecting rod you will have to get it professionally balanced.

Some small engines have a balancing bar arrangement that is connected to the crank via an off set cam and linkage but this makes them very heavy .most are balanced by adding or removing weight from the counterbalance on the crank its self.

If its just a centered disc it will be ballanced once the crank pin is on it its not balanced ad the weight of the piston and conecting rod and wrist pin and it will be a lot out of ballance .

is it a two or a 4 stroke ?
if its a 4 its probaly not deshighned to go any higher than about 4000 rmp as a stock engine

hey andrew its a 2 stroke reed valve engine,



like that one, i figured to balance it id just weigh the piston assembly on the scales then drill out a hole of equivalent weight somewhere on that crankshaft side, just not sure where too.
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Thats a bit simplistic, because the piston/conrod assembly is mostly reciprocating at the piston end), and partly rotating and reciprocating at the big-end.

A race engine building shop I know has a set of scales with a mount point that holds the little end of the conrod up, and weighs the big-end.

Then they plug those numbers into a formula that takes into account the length of the conrod (distance between big and little end centers), and gives them a precentage of the overall weight that they bolt to the crank-pin using special jigs that have the weights mounted around the pin in a particular configuration to simulate the offset weight of the conrod.

Then they spin this up on an electronic balancing machine to various RPM's, the machine measures the vibrations and angles, and tells them where and how much to drill out of the counterweights.

Sometimes, if there have been radical changes made to the factory balance (aluminium conrods, or a super light piston), then they need to cross drill the crank and insert plugs of heavy-metal to actually add weight in some cases and re test several times to get it spot on.

If you're trying to "blueprint" the engine and up its power or rpm's significantly, its probably best left to a pro with the gear to do this sort of job.

Have you changed anything that would require significant rebalancing ?
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Buy a rotary already
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simple question complicated answers it seems

ive put a bigger piston inside with the associated bigger parts (49cc size instead of 47cc) so yeah it will be a bit off balance. but the cranks are usually sold as a "universal" item for all sizes and its recommended you balance them from new.

might give a few friends a ring and see if we cant get it balanced up
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