Hello, all.

I originally posted this on a different thread, but it fits here as well.
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Gentlemen, I am going to throw some information in here concerning ANY piston connected to a crankshaft with a pivoting connecting rod. AND, this stuff has absolutely no relation to the feces of a male bovine, it is straight fact.

The problem is piston accelleration rates. When going past top dead center the piston experiences much greater accelleration than going past bottom dead center. There have been many attempts to even the rates at top and bottom, but none have been made to operate as well over all as the ordinary pivoting con rod. By lengthening the con rod the difference can be reduced, but not eliminated. In the boxer twin the same differential is there but the forces of the two pistons cancel each other. The remaining vibration in a boxer
comes from the crank itself. Commonly in boxer twins there are opposing counter weights, one at front and the other next to the rear bearing. All by themselves they induce a dynamic imbalance. The manufacturers can vary these to minimize the resultant total vibration, but since the two conrods do not run in the same plane it cannot be eliminated.

To prove to yourself that the accelleration rates are indeed different top and bottom (I heard some references out there to that male bovine) pull the backplate off any of your single cylinder engines. Pull the plug, bolt a cheap protractor on the crank, lining the center up, as well as you can, with the crankpin, bend a short wire to use as a pointer. Now get out your dial indicator, stick it through the plug hole and zero it with the piston at TDC, turn the crank and check the reading at BDC. bend your pointer to get a zero in the middle of the protractor. Now comes the proof. Turn the crank backwards until the piston comes up a bit, let's use 0.15", and note the degree reading. Turn it back the other way, let the piston go back down and come up that same distance again, note the degree reading. See how much crank rotation it took for that much travel? Now go do the same at the upper end. (You'll have to reset your pointer for TDC) Shocked? Usually about 1/2 the angle at TDC for the same distance traveled. With the crank turning at a constant speed that means the piston has 1/2 the time to go the same distance at TDC as it has at BDC. Consulting with Sir Isaac Newton we find that means the accelleration at TDC is FOUR times as great as it is at BDC. Gentlemen, this is simple mechanics, and there is NO POSSIBLE WAY TO BALANCE IT!

If you don't have that dial gauge, and your kids wont let you have their protractor the difference is so great you can use a nail and a ruler to indicate piston position, and stick a prop on the crank. Less accurracy, but the difference is so great you'll still see it.

As a side note, the only piston engine that can be set up with ALL vibration forces internally balanced is an in-line six, or a V-12. Hello, Enzo Ferrari!

If not one of your planes is not a TWIN, no body can say you're not IN!

WLR
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Hope this clears the water somewhat.

When the twins balance, the wings stay on - it soesn't become a lance.

Bill.

....because the ''answers'' only bring up more questions.
For instance....the article in M.A.N. with the Clarence Lee
recommendations states that..."if you balance all of the
conrod and wristpin weight and 1/4 to 1/3 of the piston
weight, you will be pretty darned close to having an
engine run as smoothly as possible".

Well, thank you so much Clarence for that precise
estimate. Yes, precision guesswork is always
better than strict facts, guidelines, or blueprinting. The
article on "Solving engine Vibration" was a joke.

Did you think that the engine tweakers were actually
gonna give up their secrets ? ( yeah right ) Guess again.

Here's another Ditty....(same article)..."Clarence suggests
that you add or remove weight from the crankshaft counter
balance by trial and error to "discover the smoothest run"
within a given rpm range. Isn't that special ! :stupid:

Thank you Dave Gierke, for the article....and Mr Lee for
telling us how to solve our engine vibration problems.
( please note my sarcasm )

Let's look and think about the article posted by my friend
ChuckAuger ( he is my friend ) for a minute, shall we ?

Article titled "crank shaft balancing". In the first place...
there is no such thing as crank balancing with a half-
sided crank, and the rod hanging out in the breeze. Put a
crank in the stand, and you'll see what I mean.

Lightening up the already "light" end of the crank....to compensate for a too heavy piston/rod assy doesn't
make sense for two reasons....( maybe three )

1- William Robison's ( post # 26 ) explanation of accel.
at TDC is fact. So, if the piston/rod assy is heavy, why
would one remove material at the crank to compinsate ?
Doesn't make sense....he's not balancing the crankshaft
at all. He's ''imbalancing'' the already out of balance
crank. I think it's the wrong end to lighten.

Simply put....a lighter weight on the end of a "yank", will
create less vibration than "yanking back" a heavier weight.
K&B engines have holes in the pistons to lighten them.

2- The grinding away of the conn. rod support to lighten
up the top-end weight is silly. The piston assy is the
problem with the vibration...not the crank.

3- I'm not going for the theory (or explaination) that
crankshaft balance is a mystery, and cannot be explained.
Single cyl. engines have been around for quite awhile now,
like over 125 years ! So I guess we'll keep hackin', and
grindin'....till we "discover the smoothest run within a
given rpm range".

Mr. Unbalanced....

Here's a link on what's required for optimum balance that's written by some TRUE experts on engine design. http://home.wxs.nl/~wakke007/fmv/fmv_3.htm
Don't be put off by the fact it was written in the late 70's for 2.5cc diesel engines. The section on bearings is well worth reading too. Heck, the whole article is worth reading!

FlyboyDave:I may be off base, but I seem to remember that the big Tigers have a thrust washer with a small & large web in them.Try the big one opposite piston at top dead center, might help? big max 1935

If you go back to the very first of the article, you can't help but notice the name of Rob Metkemeijer...an engine builder who is currently setting the world on fire with his racing engines.

Great reading..some of it is a bit dated (cast iron pistons) but a great bit of reading. I wish (ha ha) these guys would write such an extensive article on contemporary engines.

Best piece of technical data I've seen in ages, Downunder..thanks for linking it up.

I see what you mean.....If the collar needs to be in
a certain location....you think it would be ''keyed''.
I'm gonna see if I can drill out a glow plug, and make
a TDC guage. I'll use a puller, and get that collar off
and see what the story is.

I'm gonna get to the bottom of this "balance thing".
I have a plan.....

Dave.

I just eye-balled those two motors....The newer looking
one is set the about 15 deg BTDC.....the other one is
set at about 80 deg AFTER TDC....
I need to look at the one in the plane....

" Houston....we have a problem ".

Oh boy.....

....the one in the plane. It is set at about the 10 deg
BTDC position.

:stupid:

Any one have a NIB S.T. 3000 they could check ???

The odd one....80 deg....has some marks on it that
"could" be puller marks.....

This is the odd one....It looks like it had been on
3 or 4 positions.....from the marks left from the
split collar. NO KEYWAY....

Flyboy Dave:Now why do you suppose Super-Tiger did that ? After all these years of trying to keep ahead of their carbs. they did this! I was told of this trick by the fliers in Minneapolis. They swore by it. Sure don`t look like enough difference in two webs to help. Sometimes I think they are pulling my leg,not sure of people that eat that Norwegian fish.I still check my props ,mark heavy blade and put it 180 degrees from piston at top dead center. Works for me! big max 1935

I don't know why they have the webs in the collar
like that....but they're 180 degs apart, and the
collar is in pretty good balance. I couldn't get it
perfectly on centers (see the prop nut), but got it
pretty good. I did mark the heavy side, and will use
it.
I don't think the heavy webs will make any difference.

Dave.

With car cranks, if additional weight needs to be added to the heavy side, material is drilled out and "Mallory metal" is pressed into the hole. I don't have any idea what it is or where it comes from, but it is expensive, and heavy!!!

Also, when removing weight from the crank, it is usefull to do what is called "knife edging" the crank. In automotive, there are a couple of reasons:

1.) We have 40 + wt. racing oil dripping onto the crank and being whipped into a vortex.

2.) The knife edge cuts through air (and oil) better than a blunt shape.

Number 2 is the one that will yeild a lil HP for us, especially at the kind of revs that our motors turn..............next

I've been watching ,reading, learning, and remembering this thread. To some of you, this thread may not mean much but there has been a couple of real knowledgeable guys share their views here that others should take note of.

The only thing I would like to add or repeat is that Clarence Lee once said not long ago and that was to try to balance a single cylinder engine is only a compromise.

The thought, I have been having, while reading the part about the ST3000 is, there needs to be a dynamic balancer available whereby an engine could be observed at what crank angles the strongest vibration occurs so that weight could be added or removed to smooth the engine out. This gets back to someone's statement about a Trial And Error Process .

The automobile companies resorted to balance shafts that opposed the vibrations in 4 cylinder car engines. There were natural harmonics formed in these engines at certain rpms that could not be balanced by counter weights on the crank alone. Yet the "A" model Fords had no counter balances at all.

Another important item that is noteworthy is that a vibration of a certain magnitude is less evident at a higher rpm than at a lessor rpm. Therefore larger props can cause more vibration because the engine will run slower? Yet a model engine will idle at a lower speed because of the flywheel effect of a larger prop. Also, this flywheel effect, will serve to smooth out the power pulses and and, therefore, make less vibration?

I'm also an old 'hot rod' guy and I always thought you weighed everything seperately and took all the weight of the big end and half the weight of everything else. But yet again, that was before many toughts of natural order harmonics came along. The problem with natural harmonics is that everything has a natural frequency whereby it will vibrate. There are times that another part in the engine will have a frequency that is a 3rd or 5th order harmonic of the first one and these will amplify each other.

In recent years, there has been a lot of thought about harmonics. They are doing wonders with it on auto bodies in recent years thanks to the computer.

I remember seeing a documentary on the making of a late model jet engine (CF-7?) where in the end they ran it while observing oil lines etc to see if one vibrated. If they found one, they would move the bracket over a little to change the natural hoarmonic frequency of that piece of tubing so it didn't harmonize with the other vibrations.

Trial and error?

Jim

Hi guys, this thread is really great, agree with w8wye and also agree on the harmonic vibrations issue - if you are unlucky your modified engine may well turn out to vibrate excessively at your normal operational rpm and that is hard to calculate in advance I guess - hence Clarence Lees note on trial and error - he should now..!
Holeshot , yes I think you are right that shaping the crank should decrease "oil mist resistance" somewhat, wonder if the effect can be measured..? I have not tried measuring but still do shape cranks as shown on attached pic.Feels good at least..!
Bill Robisson , I was intrigued by your explanation on piston acceleration and my subsequent question is the following:
What limits the possibility to lighten the con-rod, is it tension or compression/bending that will normally break it...??Same thing for 2- and 4-strokes..?? Conrods in titanium were available some years ago but I have not seen such ads lately.
OK guys, sack time in Sweden now........Cheers/Harald

Harald:

It's time for the evening frolics to start here.
Compression and tension, given reasonable strength, never cause a rod failure.

There are two sources of bending load, bearing friction and distortion of the crankcase and crankshaft.

Bearing friction is obvious, so long as the rod shank is wide enough to withstand this load no further consideration is needed.

In a well designed crankcase thermal distortion is kept to a minimum, the crankshaft stays at a 90 degree angle to the cylinder.

The load of the piston and crankpin, pushing against each other, caused the crank to flex. this is load is greatest when the cylinder fires, and is the time of greatest flex. This flexure is minimized by having bearings on both sides of the crankshaft throw. In a model engine this is seldom used because of assembly considerations and weight.

This flex causes the big end bearing of the conrod to have much higher bearing load on the edge next to the web of the shaft, and if too high can wipe the oil film out, then the bearing runs dry and fails, leading to a broken rod. This failure is almost never seen in a model engine.

This constant flexing is the cause of most crankshaft failures, however.

At the same time this off-center load on the bearing will cause the conrod also, to flex. This happens constantly while the engine is running, and can cause a fatigue failure.

All this is supposing the engine is in good condition, with correct clearances in the big end bearing. Extended running at elevated rpm makes the bearing loosen, bringing another set of problems.

With a loose big end, centrifugal force tends to throw the big end outwards, then when the cylinder fires the big end is hammered down on the crankpin. This can cause the metal of the conrod to crystallize, leaading to a fatigue fracture. This is a common failure in the engines used in model racing cars. It also aggravates the tendency to break the crankshaft.

Do not "Airfoil" the rod, this will lighten it, but it weakens it at the same time. Drilling holes will lighten the rod, but unless carefully polished after drilling scratches are left, these scratches are places for fatigue cracks to start.

All this leads to my way of lightening a conrod, which I (obvoiusly) think is best, the greatest lightening while retaining its strength.

Leave the edges approximately square, and polish to remove all tool/filing marks.

Then, leaving lands about 2/3 the width of the edges, cut a groove in each side leaving the metal in the center at 10% to 15% of its original thickness. Be very careful to polish all marks out of these grooves, again, a scratch is a focus for the start of a crack.

You have now made the rod into an "H" section, which except for a tubular rod, is the strongest possible, and the lightest. It is also the hardest to do well. I have never broken a rod done this way, but I have broken crankshafts.

Yes, exotic metals, such as titanium, can be used, but I don't think there is any gain in their use. And they can cost a lot more.

Hope this answers more questions than it raises.

Alloy rods do fine, for titanium get a gold mine.

Bill.

Read an article on the profile of the Corsair. the pilot said at economy cruise rpm everything was just a blur. Resonante speed of engine to airframe. Lucky we don't have to fly in our planes!

Not really..this is one vibration that can't be balanced out because it comes from the variation in torque applied to the prop and not from any inherent imbalance. If you could check the revs of a prop at any point of its rotation you'll find that it varies, in other words it's constantly accelerating and decellerating. This variation in revs can amount to several hundred rpm over a period of about 90 degrees of rotation. Weight of the prop has no affect on this vibration as it's purely a torque reaction. The only thing that smooths this out is more cylinders so that the pulses can overlap which is one reason why an 8 cylinder is smoother than a 4 cylinder.

Hey guys
tell me what you think of this device http://www.balancemasters.com/ a spinner back plate could be turned to mount it in similar to automotive engines and it will adapt to all RPMs automatically

PTooey, Dactyl:

Where's the spitoon? Remember, sir, snake oil and BS come in many forms.

A liquid that moves itself AWAY from the heavy side? Come, now.

My twin and snake oil? I'd rather have Olive Oyl.

Bill.

Bergen makes a canfield balancer for his heli's that uses a very similar principle. It uses balls that run in a race that seek there own position. It is also the same as the liquid balance that you can use in your car tires. I believe that this is a proven design.

Alan Angus

Interesting concept. It sure would be cool to see if it worked on some of the larger engines out there, like 100-150cc. They're only 75 bucks for the Rotax.... I'd pay that to reduce vibration on my 35%, even just a little.

A friend told me that 350 Chevies have a liquid
balanced harmonic balancer. I don't know, I'm a
Ford Man !

Gentlemen, We in the trucking industry have been using two wheel balancing products for some time! The first is called Centramatic wheel balancer. This is a metal disk placed between the hub and wheel. It has a tube around the perimeter of the disk filled with ceramic beads. and the other is "Equal" a heavy powdered material that is placed inside the front tires. Both products rely on centrifugal force to distribute the balance weight, effectively re balancing the wheel every time the vehicle comes up to speed!


Regards
Mark