Crank Balancing revisited

[Deleted member 12676]

Enlarging those holes from 11mm to 12mm only removes 8.6 grams.
Correctly removing the needed 27 grams is done by making an extra hole 12mm in diameter.
But running the standard CDI requires a bigger hole. And running high compression also requires a bigger hole.
So that little hole enlargement they did was about 1/4th what was needed.

 

[Fabian]

so if that is true, then it should have run with 25% less vibration, but it ran with what appeared to be 500% more vibration than a standard engine.

and what would the optimal rpm be with 12mm holes as opposed to the 11mm holes?

 

[Fabian]

What would the optimal hole size for an (69cc) engine revving at between 3,500 and 4,500 rpm?

 

[Fabian]

Fabian your posts are highly acidic for "DO IT YOURSELF" work on a 99 dollar engine. jaguar is talking about TRYING to move the vibration to a different area. The reason your angry is because YOU spent 99 dollars on snake oil and majic beans in the form crank. congratulations< money wasted.
I for one am going let jaguar rant because as he does that it gets us all ahead. child, senior member my ass!

 

[HeadSmess]

i used to make drill bits for someone...3 inch diameter, machined lip to take carbide insert. takes some doing, sharpening a big carbide drill on a standard greenstone. they wear away really fast. make a mess.

also sharpened a few masonry drills for the cnc lathe a few times. once again, silicon carbide "greenstone". or a non segmented diamond wheel on the angle grinder makes less mess, that one. rougher finish unless you got a diamond file to hone it with. dont make carbide as "sharp" as HSS. even put a 0.2mm flat on the lip. makes it stronger.

i hate drilling hard stuff, especially if the holes already there! ruins drills.

but balancing. pffft.


best option is lightening everything. whole crankshaft to reduce rotating weight. lighten piston to reduce oscillating weight and counterweight required, which also reduces the rotary vibration.

why is a vtwin better balanced? in a single cylinder, piston goes up and down. the counterweight goes down and up. but the counterweight rotates. so it also creates a side to side vibration.

a 90 degree v twin sticks another piston here, at the side. so the side to side vibration is also countered by a piston travelling the opposite way. making for a smoother engine. next option is four cylinders, in line...

reducing total weight will reduce vibration in total.

reducing counterweights/piston weights will reduce oscillatory vibration that shouldnt really be RPM dependent at all, as the g forces increase at basically the same rate in both directions... piston goes faster? so does counterweight. it is rpm dependent though, because the piston travels linearly, and the forces are constantly changing,from dead still to full tilt, whereas the crank experiences a relatively static loading, centrifugal force only...


but even when you get this right, if the counterweights just heavy anyway, its still going to vibrate side to side no matter what you do to the piston, pin, or rod!


reducing weight will allow for faster pick up when the throttles opened. less weight to get spinning.

reducing weight will reduce bottom end torque by removing all that rotating mass!


best to leave it all alone, methinks


has anyone tried turning up acrylic/nylon/alloy weights instead of the stock steel? and looking at the crankpin earlier...theres heaps of space to machine out and press in a new crankpin at a larger offset... more stroke. more grunt

 

[Fabian]

and looking at the crankpin earlier...theres heaps of space to machine out and press in a new crankpin at a larger offset... more stroke. more grunt

Machine one up and i'll make a convenient Paypal purchase along with the associated shipping cost.

How much extra stroke is available in a stroker configuration?

 

[Deleted member 12676]

A 1/2" hole is just right for most 69cc engines using the Jaguar CDI. Hole size need probably differs with a standard CDI but I need to research that with tests.
In re-evaluating what I said at the start of this thread and recalculating everything I found some unsolvable incongruities and so I have forsaken the "old" method completely. My newest method is on my site.
The old method asked you to drill bigger holes for even higher rpm. But I think the opposite is true. Let me explain. More than rpm, the important data is the amount of spark advance and combustion pressure. Engines reving high more often than not have less cranking pressure than "enduro" engines and even less spark advance. The cranking pressure adds to the combustion pressure which teams up with ignition advance (and 2 other things) to cause the only non-linear part of the "equation" that has to be balanced. Both piston inertia and flywheel centrifugal force (due to the imbalance there by the counter balance holes) change equally in response to increasing rpm. Without considering combustion pressure then these two counterbalance no matter what the rpm is. The same counterbalance works equally at 2,000 as it does at 10,000. So the only part rpm plays is that at a certain range of rpm the combustion pressure is more due mostly to spark advance (look at the graph below) and cylinder pressure. That subtracts from the upward piston assembly inertia force.
As revs increase so does dynamic pressure due to rings having less time to lose pressure as blowby past the ring ends. During the upper rpm range the expansion chamber baffle return wave supercharges the combustible mix, so there is that addition to combustion pressure. The delivery ratio decreases at top rpm so that subtracts from the total pressure. So we have a mixture of 4 things, their final curve being the dip you see in the bottom graph. And that is near impossible to predict. So all I can say is here is what worked for me given my peculiar engine setup, and knowing what I do I can predict a greater need for counterbalance with less cranking pressure.

 

[HeadSmess]

A 1/2" hole is just right for most 69cc engines using the Jaguar CDI. Probably needs slightly bigger for engines with standard POS CDI since it causes a huge unwanted early combustion pressure that fights against the pistons upward inertia and so imbalances things even more.
In re-evaluating what I said at the start of this post and recalculating everything I found some unsolvable incongruities and so I have forsaken the "old" method completely. My newest method is on my site.
The old method asked you to drill bigger holes for even higher rpm. But I think the opposite is true. Let me explain. Engines reving high more often than not have less cranking pressure than "enduro" engines and even less spark advance. The cranking pressure adds to the combustion pressure which teams up with ignition advance (and 2 other things) to cause the only non-linear part of the "equation" that has to be balanced. Both piston inertia and flywheel centrifugal force (due to the imbalance there by the counter balance holes) change equally in response to increasing rpm. Without considering combustion pressure then these two counterbalance no matter what the rpm is. The same counterbalance works equally at 2,000 as it does at 10,000. So the only part rpm plays is that at a certain range of rpm the combustion pressure is more due mostly to spark advance (look at the graph below) and cylinder pressure. My bike peaks pressure at around 6500 rpm. I know that because that is the center of the imbalanced range of rpm before I got the counterbalance right. As revs increase so does dynamic pressure due to rings having less time to lose pressure as blowby past the ring ends. During the upper rpm range the expansion chamber baffle return wave supercharges the combustible mix, so there is that addition to combustion pressure. The delivery ratio decreases at top rpm so that subtracts from the total pressure. So we have a mixture of 4 things, their final curve being the most imbalance at a certain rpm range. And that is near impossible to predict. So all I can say is here is what worked for me given my peculiar engine setup, and knowing what I do I can predict a greater need for counterbalance with the standard ignition.

quite well explained theoretically, yes, piston and counterweight should balance out and cancel vibration IN THE PLANE OF PISTON TRAVEL. vertically. they dont. a simple air compressor proves that. ive never found a vibration free air compressor. here we have removed any influence of combustion. take the head off any single cylinder and youll find it well balanced in one plane only. which is rpm dependent to a degree.

what you do overlook is the 90 degree component caused by the rotating counterweight. im just saying. i do agree that the combustion pressure does cause some degree of imbalance. a short con rod will inrease this effect.

lightening the crankshaft as a whole will reduce the 90 degree component. but that can only truly be balanced or cancelled out by use of another piston on this 90 degree plane. be it working cylinder or a dummy. ie, balance shafts etc.


its the 90 degree part that causes the most vibration, and its the hardest to balance. the counterweight will always be pulling radially from the centre, rotating through 360 degrees. the piston weight only goes back and forth. it also "jerks" to a stop, whereas the counterweight always follows a smooth line of continuous, steady force. at 10 degrees, the weight is pulling 10 degrees to the side with just the same force as always, whereas the piston is almost stationary. its decelaration acting at 0 degrees, vertically.

a flat twin can also balance. each piston is the others counterweight. at 90 TDC, where counterweight balance would act in equal and opposite directions on the vertical plane. whilst piston forces are still cancelling on the horizontal plane. theres also little in the way of counterweight to start with. only the crank pins and rod bearings are necessary. (180 crank seperation, not a shared pin)

tis why i like this idea of plastic/ lightweight weights. use that as packer material, with the smallest counterweight possible. tungsten carbide should do the trick nice and dense, right out near the edge. then its a lot of experimenting to add weight, not remove it


FABAIAN. um.... i wish id bought that 5 tonne press the other week when it was on special? cus, im seeing over 5mm of room between conrod and crankcase. an offset of 5mm is a 10mm increase in stroke. whether the crank halves can take that much offset is another matter entirely. but i dont see its being too hard...i havent pressed one apart before? i wouldnt mind doing it to an old scrapper though to see...
whats hard is finding a new rod, cylinder and piston to suit this extra 10mm of stroke... and working out what to do with the ports?

sorry. i gotta go thrash out. bodies. drowning pool. this is not a sit still type song!

 

[Fabian]

what you do overlook is the 90 degree component caused by the rotating counterweight. im just saying. i do agree that the combustion pressure does cause some degree of imbalance. a short con rod will inrease this effect.

I've gone through this discussion with Jaguar before and the requirement of a double balance shaft to properly cancel out the "Y" vector.

 

[Fabian]

lightening the crankshaft as a whole will reduce the 90 degree component. but that can only truly be balanced or cancelled out by use of another piston on this 90 degree plane. be it working cylinder or a dummy. ie, balance shafts etc.

This is the best method for reducing vibration in a single cylinder engine, but with the only downside being an eratic idle or an unreliable idle rpm, which is generally cured by running high idle rpm.

For this reason a good combination is a lightened crankshaft with attached flywheel weights by means of a pull start and centrifugal clutch, to give back inertia to the crankshaft for reliable idle rpm.

Even without a lightened crankshaft, it's surprising to see the improved idle quality with a pull start and centrifugal clutch attached to the engine.