Hey, how boring is that...
This thread was way more entertaining until 2 2 posts ago...

No, honestly. Good to see that things are smooth again.

Here is my understanding of 2 stroke crankshaft balance factors...

I have used these theorys for adjusting crankshaft balance for 2 stroke racing motorcycle engines. They apply equally to r/c engines.

The best way to support the crank, is like Edward said. Being that the crank has only one side journal, there is no other practical way. His way should get you very close.

In my experience, I have had the best luck with balance factors between 58-62%. It's amazing how much difference the engine runs with different balance factors. But like most 2 stroke modifications, nothing is written in stone. It's always a trial and error game.

On the other hand, 4 stroke singles, balance well between 48-54%. Go figure!!

Here is a simple diagram for my technique that may help you guys visualize the balance process.

The formula is very simple: B/A=%

where A= approx 1/2 the rod weight + the piston + the piston pin + the piston pin clips

where B= The weight of A (Piston assy, and 1/2 rod)counteracted by the crankshaft counterbalance weight

where %=the resulting balance factor

Hope this helps!!

BK

There are a couple other things that come into play.

1. which way the engine is mounted. from the side/from the bottom.
2. how stiff it is mounted. rubber mounted Ie. boat, solid mounted Ie. car

Now what and why the balance has such a significant effect and why it is not set in stone.
The engine as it is reving through its rpm range goes in and out of phase (periods of intense vibration and periods of calm diametrically opposed in the horizontal/vertical axis's) picture a sine wave that crosses two direstions.
why is that significant? because fuel and air vapor has physical mass. Even though this mass is significantly small.
At an rpm level of 30000 rpm the ports are opening 1000 times a second. given their only about 2.5 mm tall you can infer they are only open a few milliseconds at best.
if the vibration of the engine, that can reach levels of 20 G's is creating a shaking force the opposite way the fuel is trying to flow, (at the wrong time)up the port in the engine case it can literally short circuit the fuel transfer literally acting like a governer on the engine. its like you stuck a cork in the carburetor.
Most 12 engines are close enough on balance from the factory and have smaller levels of g force vibration so this is less of a problem. as the engine gets bigger in displacement it can be a significant problem if its not properly addressed.

there you have it
the rest of the real dirt.

The balance also relate to the high of exhaust port. When the exhaust port closed by piston,the combustion chamber start to get compression. It is a force resistance to the inertia. I don't know how to explain it with my poor english....sorry...

Interesting ! You must do a lot of grinding and testing.

I assume in Fig -1, the crankshaft is also supported with a cutaway crankcase with no lube in the bearings?

yes dino is referring to secondary order firing pulse

Dennise you are absolutly right. On 12 engines from factory with stock P/S sets it is pretty good balanced.
You have mentioned about soft mounting. I did use those on tether hydros and I can tell is very important ( 60 size) that were si center of rubber mounting located-over the center line of crank, on the line or under. In those points it was realy important pretty good ballancing-the vebration point was changing power, blew glow plugs, blew pistons,blew cranckshafts etc.
One more thing about different balanging in hobby-it is control line FAI speed and combats-motor is on horizontal position with head out of circle. That one is another b... ch to work with. There centrifugal forces changing whole story. It was a lot of tryes to put motor vertical with head up, but aerodynamic was bad.
Edward

yes I had a CMB 90 pre grand prix motor. Remember those? steel rod with needle bearings and a non stacked crank.(mechanical overkill on construction but indestructable) That thing shook so bad that it would not rev past 18000. we threw 20 degrees more exhaust timing at it and it literally did nothing cept piss us off. LOL It popped plugs and did all kinds of wierd shit. we literally had to take the flywheel and remove almost a half ounce from it and index it to the crank to get it to run right. Once that was done with no other changes bam 26000.

I will tell you exactly the method we used to balance at that time.(crude but it was effective, glad the crank was solid as it had to be bending alot) This was before we reversed the engineering to determine what we considered good balance numbers. In those days balance was closely held and anyone with any experience just would not talk about it. It was a don't ask deal.

we mounted the engine in rubber mounts like it was installed in the boat. we cut two 28 inch pieces of .020 piano wire and mounted one to the engine in the horizontal plane and one in the verticle plane mounted at the engine mounting lugs of the case. we spooled the engine up close to the 16000 rpm window and watched the wires. the one in the horizontal plane was shaking like crazy. like 3 harmonics down its length with an amplitude of 3 inches. so we just started drilling holes in the flywheel on the crankpin side until the wires vibrated evenly. crude but effective. Their was no way I could add a half ounce to the counterweight or remove that much from the piston. we had already gone there.

what prompted all of this was we noticed that an engine would run good in a boat with a solid one piece mount but when you hung it in the new technology rubber mounts the engine was a turd.

Multiple engine boats twins and triples were a entirely new nightmare. Compounded vibration. When the harmonics line up watch out lucy. My friends triple 45 hydro would literally shake the engine mounting lugs right off the engine case in 4 runs. he had picco engine cases stacked like cordwood

There can be alot of force generated

Very interesting method, relly never heard or use this kind.
In our case, ones engines became much more RPM ( I am sure you remember general theory from using torq and use RPM was all over the world) and it was implemented noise reduction rull 80 Db, if I remeber it right -it was earlier 80 then vibration became really big problem-it was extra generator of noise. So we put engines on rubber mounts then vibration start really killing everything-transmission rods, pipes. We tryed a lot of different ballnacing and nothing realy work. We still was able to make Km/h, but after every run we will need to fix something-those couple of years was really hell. Then some guy developed self balancing flywheel. It was look as follow-it has empty space in the walls-double walls around and it was filled with very small balls by 60 %. Ones flywheel is spinning balls destributed arround and make ballancing almost perfect, and the thing is there was no more problem with low or high RPM-flywheel was ballanced instantly by itself. Since that all problems got away. So everybody easy got 80 Db and speed got back.
Unfortunately we can't use this thing in the RC cars-flywheel is toooo heavy. I wish we can use it here, but weight will be unacceptible.

ed I remember that. in fact here in the usa someone marketed a wheel balancer for cars based on the same theory. it sandwiched between the wheel and the hub. it worked but occasionally would explode shooting out ball projectiles flattening the tire. LOL The idiots made the housing for the balls out of plastic and in those days plastic took a dive in a real hurry. well enough telling everyone what an antique I am.
LOL D

PS a mercury filled version of that is still used on diesel engines for torsional vibration reduction.

Necessity, The mother of all innovation.

Dennis, are you comming for Airtronics? I got to show to you that Bi-metall set. Sh..t I was expecting some performance gain, but didn't expect that much right of the box. The tapering is realy different and still not sure if it is the best for that set up-will know after one of the set will get loose.
Edward

no will be at rd race in vegas

Salutations!
Is there any chance that Palmaris Engines will make a .21 buggy engine?
-alexnander

Hi Foampevert.
No, we are not planing work on off-road engines any time soon. Sorry about it. but we are only now for on-road.
We can make custom parts per request ( on-road, off-road, marine, airmodeling), but there is minimum order quantity. As regualr production basis we are offering only .12 and .21 on road.
Best regards
Edward

Got a lot of catching up to do. Did a little research and found these statistics interesting. You complete 74laps in 30 minutes in the Emain averaging 17.72mph and Peter Breton driving the palmaris p/s set was in the C-main. His average lap was 27.46mph completing 54 laps in only 14min 58 seconds until a oneway bearing froze.