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MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Balancing a Crank
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[QUOTE="Ol'89r, post: 545351, member: 17212"] 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. [/QUOTE]
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MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Balancing a Crank
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