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Crankshaft balance factor?

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Hey guys,

me and a friend of mine were talking yesterday about how the "crankshaft balance factor" affects the engine´s power.

I have an 09 KXf 250 which is claimed to have "a balance factor of close to 60%". In marketing this is made out to be a good thing. This engine has no counterbalancer.

It doesn´t vibrate that much at all. In fact, it´s downright smooth.

How does it affect the power delivery? Does it make it more "tractable"?

If nothing else were to change, how would more or less weight (or balancing factor) affect the engine?

If I was to experiment with this for myself, how do I calculate this?

My friend has a 08 CRF 250. He said he believed that this number was way lower on his engine and I guess he is in the mood for some experimentation...

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Balancing a crankshaft assembly for any reciprocating piston engine presents a variety of challenges and compromises. There are virtually no designs that completely cancel all the competing primary and secondary forces that produce shaft vibration, though some do come fairly close.

Understand that a single cylinder engine produces vibration primarily from two sources:

  1. Rotating imbalance
  2. Vibration induced by the reversion of the reciprocating mass at TDC and BDC

The reciprocating mass is the weight of the complete piston assembly, including rings, pin, and clips. It also includes a portion of the rod weight. A rough estimate of this is obtained by hanging the rod horizontally from two scales. What the small end weighs is reciprocating weight, and what the big end weighs is rotating weight.

The rotating mass is the crank and everything attached directly to it, the rod bearing, and the above mentioned part of the rod's weight.

Balancing rotating assemblies is simple, and needs no explanation here. The problem, however, is that we need to cancel the forces caused by the acceleration and deceleration of the piston. If we have a rotating assembly in perfect balance, you will still have two moments of force on the engine generated by the piston as it changes direction. To counter this, without the use of a counter balance shaft, the rotating assembly is deliberately imbalanced with a percentage of the reciprocating weight. The weight is place in the crank opposite the crank pin so that it generates a force in opposition to the piston's inertia. If the cylinder is oriented vertically, and the crank axis is Z, then the piston produces a downward force along the Y axis as it reverses at BDC, and an upward one at TDC. The counter weighted crank works in opposition to this.

But it can only be partially successful because the piston will generate foce only along the Y axis, while the imbalance in the rotating assembly will generate force radiating from Y out through the X axis, being helpful only at TDC and BDC. So it has to be a compromise, and can never be perfect.

The matter is further complicated by the fact that the forces at TDC and BDC don't match. Because of rod angularity, the piston travels faster in the top 90 degrees of crank rotation than in the bottom, and that means that forces generated by the change of direction are greater at the top. Even if we could totally cancel one force, we wouldn't match the one at the other end of the stroke.

What counter balance shafts allow you to do is to imbalance the rotating mass to the point of near total cancellation of the reciprocating forces, then canceling the rotating imbalance with another imbalanced shaft phased 180 degrees opposite the crank.

More:

http://en.wikipedia.org/wiki/Engine_balance

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Ive heard that like an almost absolutely balanced one can give way more power. Its impossible to have an exactly balanced crank. it just doesnt happen thanks to the laws of physics. But I did read that 60% thing in the magazines. You can lower than number and like a fan that is inbalanced it will sway and whip left and right when going fast enough. What this says is that if they control balance the crank so that on the up and downstroke it is slightly inbalanced then you can get more of a push and pull effect.

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Thanks for the replies. It really is a complex matter, very interesting to say the least.

I wish I was in a situation where I could do more real world testing of engine mods...

To get to the bottom line, does my friend have anything to gain from trying to change the balance factor in his CRF?

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not anything I could see that is worth his time. hes better off to try a lighter flywheel or a heavier one. I like how Kawi makes a lighter one for the 4ts

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To get to the bottom line, does my friend have anything to gain from trying to change the balance factor in his CRF?

It's a different situation with his bike because his uses a counter balancer. The percent of reciprocating mass factor on a counter balanced engine may be much different than one that doesn't use one.

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Ive heard that like an almost absolutely balanced one can give way more power. Its impossible to have an exactly balanced crank. it just doesnt happen thanks to the laws of physics. But I did read that 60% thing in the magazines. You can lower than number and like a fan that is inbalanced it will sway and whip left and right when going fast enough. What this says is that if they control balance the crank so that on the up and downstroke it is slightly inbalanced then you can get more of a push and pull effect.

You could have a perfectly balanced crank, but not a perfectly balanced engine, as per grayracer's first post. Trying to balance the rotating mass of the crank with the reciprocating mass of the piston is very difficult.

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As far as I would take it, that makes it the IDEAL MACHINE, which is impossible. but I could be wrong on my assumption.

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Crankshaft balancing input from Falicon Crankshafts

Can a single cylinder crankshaft be balanced? Does it help?

A single cylinder crank is always unbalanced. The balancing operation we perform moves that unbalance (vibration) to an RPM that is either higher or lower than your engine's RPM operating range. We accomplish this by removing or adding weight to the crank in a location that our balancing equipment has told us to do so. Balancing or "moving the unbalance" really helps a single cylinder crank, especially if it is to be used at sustained high speeds. At Falicon, we balance your crank for its intended use and RPM range.

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I personaly have balanced crank shafts here in my shop using a knife edge system that is levelled i would balance each half of the crank individually by making long shafts and sliding the crank half into them the long shaft kept the flywheel from tipping off the edges .i had used factors from 55% to 68% i never seen any hp gains the lighter the total mass of the crank the higher you could go with balance factor the racing harleys that i did likes 62% .the company in anabour michigan that builds the balance equip ment balanced some bike cranks for us the static balanced cranks were in some cases smoother.

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Thanks for the replies:thumbsup:

My friend has gotten hold of a slightly used crankshaft and is willing to give it a go.

We were also debating how a longer rod would affect things. Especially in combination with a rebalanced crank.

We know that the rod angle is lessened, meaning less sideloads on the piston.

But the rod is heavier too right?

We have heard of some factory (experimental) engines with 5mm longer rods, extra thick base gaskets and longer cam chains.

What would be the gain, and drawback of this?

Does a 450 have the same pitch chain, only longer?

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The length of the connecting rod influences the point in the rotation of the crank at which the rod has the optimum mechanical advantage on the crank pin. Peak mechanical advantage, and peak piston speed, is reached at the point where the lengthwise center line of the rod becomes tangent to the circle swung by the crank pin, or to say it another way, when the rod's length is at 90 degrees to the radius of the crank through the crank pin.

A longer rod delays the point at which this occurs. Since fuel burns at a more or less fixed rate regardless of most other factors, there is a certain amount of time ignition and peak combustion pressure, and this will be fairly constant. At very high crank speeds, the crank mat rotate so far during this time that it passes the point of tangency before the pressure peak. Lengthening the rod works to give the expanding flame more time to work. Conversely, an engine intended to produce power at low speeds will benefit from a shorter rod.

From the standpoint of load reduction, it is also true that a longer rod will reduce the thrust against the piston skirt. As far as balance goes, I'm not sure what effect, if any, a longer rod would have. I think I would ignore it and proceed as usual.

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That's a different approach; cylinder offsetting. On a normal engine, the bore center line runs through the crankshaft axis. On engines like the 2010 YZ450, the cylinder is moved off center to the downward thrust side (forward in this case) while the bore remains parallel with a line that intersects the crank axis. Imagine an engine with a straight vertical cylinder, then push the cylinder forward without changing any of the angles involved.

What this does is to lessen the rod angularity on the power stroke as if the rod were longer than it is. Visualize a cylinder so radically offset that it sits out over the crank pin at 90 degrees ATDC, giving you a rod angle of 90 at 90 degrees of crank rotation. Impractical, but you see the effect it has.

That in turn can let the engineer do a number of things, like use a shorter rod than he otherwise would need or want to so that he could shorten the overall height of the cylinder/head assembly, lower the CG, and save weight, all while having the benefit of a low rod angle during the power stroke.

I should add that offsetting the bore center line, or the wrist pin (same effect) introduces some other interesting factors to the mix. With a centered cylinder axis, a graph of the piston speed will show a uniform deceleration/acceleration from 90º BTDC to 90º ATDC, and another, different curve from 90º ATDC to 90º BTDC. Each of these two segments represents half the crank revolution, and if graphed as a wave, it will show the non-sinusoidal form of piston decel/accel that creates part of the quandary of balancing singles and 360º vertical twins. Offsetting the cylinder center line takes this a step farther because where it reduces rod angularity on the power and intake stroke, it increases angularity on the compression and exhaust strokes. We then have a different rod angle from TDC to BDC than we do going back up from BDC to TDC, which in turn gives the graphed wave form four distinctly different quarters, rather than two different halves. The acceleration from BDC to 90º no longer matches the decel from 90º to BDC, and likewise the decel from 90º to TDC is now different from the accel from TDC to 90º, complicating the balancing issue a bit more.

Another effect is the slight reduction in engine compression braking.

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So, does this make the engine smoother?

What about cylinder bore and piston wear? Do the reduced angles contribute to the longevity of the piston, rings, and cylinder bore?

Another thing - they say the new YZ-450 is hard to start...that it requires a "procedure," similar to the old YZ-400/426. Why do you think it's hard to start, and is this related to the unconventional design?

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No, it doesn't make the engine smoother, and it's not done to accomplish that.

The bore is offset to achieve a reduction in rod angle on the power stroke while still using a short rod. That reduces piston skirt thrust loads, angular loads on the crank, reduces wear, and lets the engine perform better at high RPM.

All engines require a "procedure". They're hard to start because people don't do it right, and the issue is related to the EFI setup, not the cylinder layout.

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Another thing - they say the new YZ-450 is hard to start...that it requires a "procedure," similar to the old YZ-400/426. Why do you think it's hard to start, and is this related to the unconventional design?

The "procedure" is nothing more than a couple of extra kicks to build enough power to operate the fuel pump and injector. All of the non-battery-powered EFI systems have this in common.

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The "procedure" is nothing more than a couple of extra kicks to build enough power to operate the fuel pump and injector. All of the non-battery-powered EFI systems have this in common.

Not what I've read...and I have a couple of buddies who have FI'd KX-450s, and they start first kick virtually every time.

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Not what I've read...
Maybe not, but if you do it the right way, the YZF will light up quickly, too.

This sub-discussion is drifting well away from the thread topic, but I'll address it. The only two things required for starting a 2010 YZ450 is to, 1) push it up against compression, reset the lever and kick it, and, 2) ignore the stern and emphatic warning to never open the throttle. In almost every case, they start pretty immediately with the throttle opened just off idle, or with the idle set a little higher than called for.

The fact is the even the carbed YZF's start better if kicked from the start of the compression stroke than from some random point in the cycle, anyway. The idle/throttle cracking thing is just EFI calibration.

You can discuss this more if you like over in the YZ450 forum any time.

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That's a different approach; cylinder offsetting. On a normal engine, the bore center line runs through the crankshaft axis. On engines like the 2010 YZ450, the cylinder is moved off center to the downward thrust side (forward in this case) while the bore remains parallel with a line that intersects the crank axis. Imagine an engine with a straight vertical cylinder, then push the cylinder forward without changing any of the angles involved.

What this does is to lessen the rod angularity on the power stroke as if the rod were longer than it is. Visualize a cylinder so radically offset that it sits out over the crank pin at 90 degrees ATDC, giving you a rod angle of 90 at 90 degrees of crank rotation. Impractical, but you see the effect it has.

That in turn can let the engineer do a number of things, like use a shorter rod than he otherwise would need or want to so that he could shorten the overall height of the cylinder/head assembly, lower the CG, and save weight, all while having the benefit of a low rod angle during the power stroke.

I should add that offsetting the bore center line, or the wrist pin (same effect) introduces some other interesting factors to the mix. With a centered cylinder axis, a graph of the piston speed will show a uniform deceleration/acceleration from 90º BTDC to 90º ATDC, and another, different curve from 90º ATDC to 90º BTDC. Each of these two segments represents half the crank revolution, and if graphed as a wave, it will show the non-sinusoidal form of piston decel/accel that creates part of the quandary of balancing singles and 360º vertical twins. Offsetting the cylinder center line takes this a step farther because where it reduces rod angularity on the power and intake stroke, it increases angularity on the compression and exhaust strokes. We then have a different rod angle from TDC to BDC then we do going back up from BDC to TDC, which in turn gives the graphed wave form four distinctly different quarters, rather than two different halves. The acceleration from BDC to 90º no longer matches the decel from 90º to BDC, and likewise the decel from 90º to TDC is now different from the accel from TDC to 90º, complicating the balancing issue a bit more.

Another effect is the slight reduction in engine compression braking.

Always wondered why cylinders weren't placed off centerline for a better power stroke, when Yamaha did it on the YZ I looked up the subject and found Honda had done it to a car engine, which begged the question what's the down side and I think it's the extra angularity on the conrod on the compression stroke

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