Balancing a Crank

crawl9

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#1
I found this article about balacing a crank yourself. Will it work For a 125 Crank.

BALANCING SINGLE CYLINDER ENGINES

Firstly, any material that you can remove to reduce the reciprocating mass( piston/wristpin/ring/top half of rod) will make an improvement in engine performance, maybe not noticeable at lower rpm's but certainly at high rpm's plus it reduces stress on the engine parts. This will affect the balance of the engine but not greatly. However , it is a fact is that it is IMPOSSIBLE to balance a single-cylinder engine. The best that can be done is a compromise. The crankshaft counterweights will partly balance the reciprocating mass (piston/ring/wristpin/top half of rod). but this will introduce an out of balance at 90 degrees to the line of the cylinder but we have to put up with that. Simply put, to balance a single cylinder engine to the best compromise, do this if you have LOTS of time.
Use a good quality scales, digital if possible. Take apart the crank assembly, weigh the bottom half of the rod and then the top half of the rod. Then weigh the piston/ring/clips/wristpin as one assembly. Add the weight of the top half of the rod to the weight of piston/ring/clips/wristpin. Now you need to take a percentage of this weight of between 50 to 60%. Lets say we use 55%.
Add this 55% to the weight of the bottom of the rod and machine a piece of brass or steel of this weight to fit over the crankpin. Reassemble the crank with this weight in place and balance the crank on the ballrace surfaces on 2 knife edges. If the crank rotates because its heavier one side than the other( which it will do) you must grind metal off the heavy part of the crank-webs around the crankpin area or wherever until it does balance. At this point you will have balanced the engine to a balance factor of 55%. This balance factor will vary from engine to engine and is rpm dependent to a certain extent but at 15,000 rpm a factor of 55% is not too far off. http://www.prestwich.ndirect.co.uk/technical_balancing.htm




I wasn't too sure about some of the aspects of the article. It seems to say you can balance a crank with just the balance factor.
I have a Rm 125 Dirt bike and Need to have the Crank Balanced. I'm needing it balanced for 8000-10000 RPM. Where Can I come up wit the Balance factor needed to do this? Also, Is it as simple as I think I read, to add the weight to the Pin of all the pistion Etc. and use it like between centers and adjust till one side doesnet always go down(the heavier part) Also, I wasn' sure what you meant by weghing the bottom and the top of the Rod. My bike uses a one piece rod? Also, are their any place I can look to balance a crank?
Thanks
 

EricGorr

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#2
I agree with the article. I looked into crankshaft balancing machines and software at the recent Performance Racing Industry show. Basically the automotive tools balance against each other cylinder and reciprocating mass assembly.
On a 2-stroke single, balance factor is the key. I got a hold of some second-hand notes from Jeff Bratton, probably the most famous crankshaft guy in the motorcycle industry, who now works for a factory roadrace team. His equations could be converted into an Excel spreadsheet and when talking with Phil Schaffer from Crank Works on the same topic, he mentioned that he uses a spreadsheet probably derived from the same information supplied by Bratton. But I've never seen an SAE paper published outlining balance factors for different rpm ranges or applications. I think that the guys who are doing this work (Crank Works, RPM, Poweroll, Falicon) aren't anxious to share it with the world because competitive information drives their businesses.
 

Ol'89r

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#3
Originally posted by crawl9
I found this article about balacing a crank yourself. Will it work For a 125 Crank.





I wasn't too sure about some of the aspects of the article. It seems to say you can balance a crank with just the balance factor.


I'm needing it balanced for 8000-10000 RPM. Where Can I come up wit the Balance factor needed to do this?

Also, Is it as simple as I think I read, to add the weight to the Pin of all the pistion Etc. and use it like between centers and adjust till one side doesnet always go down(the heavier part)

Also, I wasn' sure what you meant by weghing the bottom and the top of the Rod. My bike uses a one piece rod?
Thanks
As Eric said, the "balance factor is the key". But, determining that factor is the hard part. Normally this is done by trial and error. It is impossible to balance a crank perfectly. Reason being, you are dealing with two completly different forces. Rotating weight, (roundy round) and recriprocating weight, (up and down). Therefore you can only reach a compromise. You have to determine what rpm range the engine will spend the most time in. Or in other words, where do you want the engine to run it smoothest? Then you balance it for that range. I don't have the figure that you need, maybe Eric may have a base line figure from Mr. Brattons specs?

Using centers does not work because there is too much drag on the centers to allow the crank to move freely. Knife edges or rollers are the best way. But, you are correct in your assumption that you simply grind or drill the heavy area away until the crank will stop in any position.

To determine the weight of the small end of the rod, simply dangle the rod, using a peice of string from the big end and using a gram scale, weigh the small end. Then tie the string to the small end and weigh the big end. When weighing the rod' keep it horizontal. Sounds crude, but it works. The weight of the top half of the rod is the recriprocating weight and the bottom end of the rod is the rotating weight. Be sure to include the big end bearing and shims with the rotating weight.

I would recommend using an old crank pin that is polished down to slip into the crank halfs fairly easily. This makes it easy to push the pin in and out of the crank to test the different counter weight. Also, when machining the counter weight for the crank pin, make it a little lighter than the formula may call for. You can always add strips of solder to it inorder to add weight to it, but you will have to take it apart and machine it to remove weight.

If you need the formula for balancing, PM me and I'll send it to you.

Good luck. :thumb:

BTW. Eric, clean out your message box.
 
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#4
When I did the balance on the Husky crank, I was too lazy to machine a weight, so I used solder for the entire thing. Lots of it! If I remember correctly, I used a 53% balance factor. The Husky 360 revs to about 8500 max, however it spends most of it's time around 7500RPM.

I believe in a previous post of mine, I explained how I externally balanced the Husky using helicopter equipment. Of course, adding weight to the flywheel was easy because it has drilled and tapped holes in it. So I just threaded in 2 bolts of a given weight in the proper location.

During the last overhaul, I removed the weights form the flywheel and machined the crank instead. I can detect no difference in vibe level.

Chris
 
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crawl9

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#6
I was wondering how Changine the Shape of the Crank Would Effect it? I'm thinking about Clearing part of my Stroked Crank For Clearance for the Piston Skirt. I've attached a pic Of What I'd Need it to look like. I'd have to take Apporx. .150 off the one side. How much would this affect Crank Case pressure? What would be the Results? and Would this effect flow any in the crank case? or create Resistance?


Pictrue is kinda crude, but i think it show's what I'm talking about.
 

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#7
The increase in volume is minimal. The difference in shape will not make much difference either. However removing that much metal will affect the balance big time.

Chris
 

crawl9

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#8
I'M NEEDING TO GET THE CRANK BALANCED ANYWAY, WILL IT STILL BE ABLE TO GET A GOOD BALANCE. ( OR WILL THIS AFFECT IT TOO MUCH)
tHANKS
 
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#9
You might want to try the guy at Falicon Cranks. Click on the link for his website.
http://www.faliconcranks.com
I think they could add back the weight you removed if they necessary to balance the assembly. The weight could be added to the sides of the crank near where you removed the metal (as not to interfere with piston)
 
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#10
Please ignore the word 'they' which is next to necessary. I missed the typo and the system won't let me edit my post. oops
 
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Ol'89r

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#11
Originally posted by crawl9
I'd have to take Apporx. .150 off the one side.
Crawl9.

Have you considered removing the material from the piston skirt instead of the crank. If you only have to remove .150 and you cut it in a radius the same as the flywheel, it may not shorten the face of the piston skirt too much.
 

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#12
I agree that the balance factor method only works if you know what your engine needs. But that info isn't available so I discourage people from trying to use that method. Here's my video comparing that method to a modern spreadsheet method of mine:
 

jaguar

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#13
The reason one balance factor doesn't work for all cranks is the percentage change in the up/down force to change in RPM is not the same as the centrifugal force (opposite side of the holes) change of the crank. So the higher the top RPM is the lower the balance factor needs to be, and not by a little but by a lot. For one engine data I have the correct balance factor at 6300 RPM is almost twice the correct balance factor at 10,400 RPM. The only good way to precisely calculate the needed balance is with a computer. Unfortunately only my spreadsheet painstakingly calculates the contribution of forces that the con rod makes every 15 degrees of crank rotation. All the other programs just add percentages of the con rod weight to the piston assembly and the crank. That's a very inaccurate method.