Crankshaft balancing

[Glen]

Hey all maybe its time to strike on this taboo subject?

my gp3 is fairly smooth compared to my last cag and revs extremely fast as it has quite a thin and lightweight crankshaft. unfortunately it came from the factory with the two lobes offset about 2mm which i have just fixed.

but i thought with everything apart i should have a go at balancing it some so i put the whole assembly together and stuck it all in the lathe between a pair of live centers.



as youd assume the piston always comes to rest at the bottom. but by sticking an M10 nut on the flywheel adjacent the crank pin i can cause the setup to rest at any position! the weight of the nut is somewhere between 10-30g.



my question to the geniuses out there is this. is this setup balanced now it can rest in any position! And if it is done - should i add weights to the opposite end or drill out weight from the pin side? i would prefer to remove the material personally.

im aware also that you can never perfectly balance a single piston setup but never the less im sure it can be better than what it was.

im kicking myself i never tested the setup from the dreambike with the stuffed polini crank thats balanced. bugger.

[RD RACING]

the best way of balancing these 2 stroke cranks is to remove the same weight from the crankshaft as the piston weighs. say if your piston weighs 30 grams, you need to remove 30 grams of material from the crank circles, the area for this is located as close as possible to the cranks rod pin [ big end ] and as close to the outer circles as possible. when machining / drilling you must go through both left and right circle sides ie; 4 holes. if you want to keep internal case volume down you can also plug these holes with compressed cork, many guys use reson but i don,t trust anything hard that may come adrift in the case. going by this pic of the crank and piston i would say 8 mm holes would come very close to balance.

[Glen]

cheers,

i weighed the piston earlier and low and behold its the same weight as the nut i used so i guess that dictates that this is how much weight needs to be taken out.

guessing you mean to drill either side of the crank pin? if so ill give that a shot. not much room to remove material from unfortunately. could be interesting..

[RD RACING]

Glen, you will be supprised with how much weight is lost when you drill these 4 x 8 mm holes.

[TheUFO]

Hi there, a couple of things about the crank balancing setup. depending on how free floating your headstock in that lathe is you might not get a very correct reading you are also fighting any balance errors that may be in the 3 jaw. also your centers are dead centers. i must admit i am new to the 2 stroke scene, but usually in the car industry a rule of thumb is bolt the rotating mass (big end of rod and bearing) plus half the reciprocating weight (little end bearing and piston) to the crank journal and balance from there. you dont usually want to have the full weight of the piston hanging from the crank as it does not actually rotate with the crank. apparently there is also a whole bucket of science relating with dynamic balancing (taking the forces of combustion on the journal into account) but thats way beyond me.

hope some of this may help.

[greeny]

you must also consider secondary balancing, that is what effect the crank lobes have when horizontal. A single cylinder engine is always a compromise between primary and secondary balance unless you have some sort of balance shaft.

Secondary balance does not include the piston, only the crank shaft and a sliding proportion of the conrod weight. This is because the bigend of the conrod is further from the centre of the crank than the little end. Infact the little end has no effect on secondary balance as it never moves horizontally.

Be very careful changing the primary balance, as by getting this perfect you may drastically affect the secondary balance and could actually cause worsening vibration.

Personally i would leave the crank alone, but balance other rotating masses seperately (clutch, flywheel, wheels including disc, pumpdrive and sprockets). I think you will get a better result from doing this.

But if you have cranks to burn, give it a go because you might luck out and find a happy compromise.