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Crank Balancer Calc

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Armilite

Well-Known Member
Joined
Sep 5, 2011
Messages
3,738
Location
AMES, IA USA
Youtube Video
https://www.youtube.com/watch?v=ZRmx0I3k2IY

http://www.dragonfly75.com/moto/crankbalance.html

[FONT=Arial, Verdana]Crank Balance Calculator[/FONT]
CBalancer.png

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Some people will tell you it is impossible to balance the 2 stroke engine for little vibration but I’ve done it successfully and so have others. Some engines don’t need any adjustment as they come very balanced. What is “a little vibration”? It’s little enough that you can see what’s behind you in your handlebar-mounted rear view mirror because it’s not shaking. It’s also little enough that your hands and forearms don’t get fatigued just from the vibration.
In one DirtRider magazine issue Rick Johnson was talking about his 1986 works CR250. He said: ”For example, the magic Honda always had was the cranks. There was something about those cranks- I can’t tell you what they did to them but when you got on and rode there was virtually no vibration to your hands."
A little vibration will always remain but if it is enough to bother you and you’re looking for ways to reduce the vibration at the bars then your engine needs some help. My ’89 KDX200 vibrated so much that I had to fill the handlebars with buckshot to reduce the vibration at my hands. I wish I still had it so I could balance the crank now that I know how.

There are 3 “vertical” up/down forces in-line with the cylinder. They are:
1) upper assembly (piston assembly, wrist pin, conrod around the upper bearing) inertia from changing directions at TDC and BDC.
2) the centrifugal force of the heavier section of the crank wheels (opposite the big conrod pin) that counter-balances the upper assembly at TDC and BDC. (subtracting from it is the centrifugal force of the conrod big end, the bearing, and the rod pin minus the lack of force from the rod pin holes in the crank wheels)
3) [/FONT][FONT=Arial, Verdana]The connecting rod contributes around 70% of the value of the upward force of the piston assembly and [/FONT][FONT=Arial, Verdana]around 83% of its downward vertical force[/FONT][FONT=Arial, Verdana]. That is with my 55cc engine and would vary with other engines depending on the weights of the piston assembly and the connecting rod.[/FONT]
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The 2 "horizontal" forces are:
1) the balance holes in the crank wheels that offset the up/down piston assembly force but have nothing to counter balance in the horizontal plane and so makes vibrations.
[/FONT][FONT=Arial, Verdana]2) the connecting rod contributes around 5% of the value of the centrifugal force of the charnk wheels to the horizontal force. [/FONT]
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The piston assembly force is more around TDC because piston velocity is more there than around BDC due to the geometry created by the crank and connecting rod which causes maximum piston speed around 70 degrees before and after TDC. The upper piston assembly (piston, rings, end of con-rod, wrist pin, bearing) should be as light as possible because the unbalanced weight of the crank wheels can somewhat counter the forces in-line with the single cylinder but then create a fore/aft vibration due to lack of piston movement in that plane. If I’m analyzing an engine and I find that it needs a lighter piston assembly then I look at the wrist pin to see if it’s hefty enough to be able to drill out a larger diameter hole in it without causing it to be too weak for the engine. That lightens it up. Otherwise it’s just a matter of selecting the lightest aftermarket or OEM piston (such as Athena). If that’s not enough then the crank has to come out so balance holes can be drilled in it close to the connecting rod pin.

I have made my own software that displays a graph of the vertical and horizontal forces and their combined result. The goal is to counter-balance so both the average radial force and the variation from maximum to minimum values are as low as possible. The better the balance, the less rider fatigue which means more time of riding pleasure. My 55cc engine vibrated like crazy, and after drilling just two 9mm diameter holes in the crank wheels it was like normal.

I used to think that the cranking pressure and the combustion force could be subtracted from the upward piston force but now I realize it can't because the same force exerted downward on the piston is also exerted upward on the cylinder head. Since the crankshaft, like the cases/cylinder/head, is basically connected to the frame then they both have the same reference point and the upward force on the head cancels out the downward force on the crankshaft.
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To use my free crank balance calculator just enter in all the required data into the light blue rectangles. The calculator works with whole crank wheels with holes near the con-rod pin for counter balance. The closer to perfect the balance is the smoother the ride will be and the less stress there will be on the crank bearings. Also less nuts and bolts will work themselves lose. Here are the graphs illustrating what good balance is:

[/FONT][FONT=Arial, Verdana]This shows the vertical forces:
verticalF.png


This shows the forces through 360 degrees of crank rotation.
The reference zero point is where the x and y scales cross.
360*forces.png


This shows the amount of radial force at each 15 degrees of crank rotation.
radialF.png


[/FONT][FONT=Arial, Verdana]The calculator can be used to figure out the perfect balance for any type of crank with balance holes. The photo in the calculator is of my crank with holes at a longer radius than that of the conrod pin which gives them more effect for their size. Below is a crank with the biggest holes almost at the same pin radius. The farther away from the conrod pin they are, the least effective they are.

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8holecrank.png
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The best drill bits to use are carbide. Here are some soures:

Grainger carbide drill bit set $13
(use 5/32” to make pilot hole then make the final hole.)

McMaster-Carr has many carbide drill bits of high quality and can be used without a pilot hole.

[/FONT][FONT=Arial, Verdana]Click here to buy the balance calculator for $10, and click here for the usage instructions.
My video about crank balance: YouTube

Questions and Answers

[/FONT][FONT=Arial, Verdana]How does this method compare to the old method of making the counter weight equal to the extra big end weight* plus 50% of the upper assembly weight and conrod weight? They are the close because with my 55cc engine the percentage would of been 46%. But that method only works if you use actual counterbalance weights on the crank wheels opposite of the big end pin and at the same distance from the center of the wheels. The crank wheels have to be whole without any holes in them. Since that style is hardly ever made any more then please just put the old method on the shelf alongside the first Ford cars.

[/FONT][FONT=Arial, Verdana]*The big end weight is the weight of the conrod pin, bearing, washers, and the weight of the conrod encircling the bearing minus the missing weight of the conrod pin holes.
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Does this work equally for 2 strokes and 4 strokes? It was developed using 2 strokes but the same applies to 4 strokes if they don't have an additional counter balance system. If they use the same method of only counter balancing on the crank wheels then yes it does apply.

Can this calculator be used for a crank that relies on true weights opposite the conrod pin? Yes it can. Just calculate with two counter balance holes with zero degree offset and when you have the final result for their right size then make the counter balance weights the same size as the imaginary holes.

Why does this calculator show that the balance is "off" on engines that have very little vibration? Because it allows you to calculate for the best balance, not for the this-is-good-enough balance. Any way within a certain range the vibration difference is very little, meaning you don't have to have perfect balance. That is because perfect balance is only the minimum possible imbalance, the minimum possible vibration. It is impossible to have no vibratory force at the crankshaft.

Why do big bore engines vibrate more? Because the larger piston and conrod are heavier. The heavier they are, the more imperfect even the best balance becomes. To fight that vibration you can put shotgun pellets inside the handlebars if they are hollow. I did that trick and it really worked. But anything smaller than a 300cc engine should not make so much vibration that it tires out your hands. If you use the lead pellet trick instead of correcting faulty counterbalance then you are leaving the crankshaft bearings to be victimized by the extra radial forces on the shaft.

Here's a testimony from a satisfied user:
" I have spent heaps of cash buying "balanced" cranks and "balanced bottom ends" Not cheap when shipped from the USA to Australia. I had two identical "jack-hammer" cranks and then bought this balance calculator. A GREAT investment. I did not trust estimating the con-rod weight, or the big-end or small-end sections of the con-rod for REAL accuracy. I stripped the crank to get EXACT measurements and weights. I cut off the big-end and small end (from a discarded conrod) to weigh them individually. The calculator is GREAT. You need to check your input figures and re-check them. Do not rush it. I found the experience very "addictive" and enjoyable. The crank when drilled with the calculated counterbalance holes has been proven to my BEST CRANK. Jim Baker of OZ Motorised Bikes; a NO-BULL**** facebook Group; where you find my posts about this."

Here's his BEFORE balance graph showing the vertical forces to more than doble the horiz forces:
JBwobbly.gif


Here's his AFTER balance graph:
JBbalanced.gif


If you have an unanswered question then please shoot it to me at [email protected]
After all, health nuts are the smartest people. :)
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