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Old 01-09-2011, 05:51 AM   #1
Martian OP
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Balancing Connecting Rods

First, let me say I've never done this. Consider this discussion as a "brainstorming session".

Everytime someone goes into their engine, there is a post telling them they should balance everything. This always leads to a discussion about "static" vs "dynamic" balancing. As I understand the movement of the rods in these engines, the big end moves in two axis while the little end only moves in one. If that is the case, then I can see where a simple weight balance could be misleading. It would be necessary to account for the vertical movement of the two big ends to attain balance.

What if balancing were done with 2 scales? By suspending each rod horizontally from two scales suspended from a horizontal beam and matching the weight of each rod on each scale, wouldn't that account for movement in both axis?
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Old 01-09-2011, 06:22 AM   #2
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Your method would probably work. The usual method is to support the rod on one end with a fixture while the other is supported with another fixture on a scale. One end is balanced, then the other. This generally results with the same total weight.

The difference between static balancing and "dynamic" is that the crankshaft is spun by a motor when being dynamically balanced vs. placing the crank on V-blocks when balanced statically. The rods and pistons are balanced on scales in either instance.
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Old 01-09-2011, 06:57 AM   #3
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Quote:
Originally Posted by Martian View Post
It would be necessary to account for the vertical movement of the two big ends to attain balance.
What you are describing here is 'center of mass'. Weight is one thing, where it is 'centered' relative to the center of rotation (the crank center) is another.
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Old 01-09-2011, 09:06 AM   #4
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Like wrench said - first get the big ends to match by weighing that end first. Leave the small end on a support of the same height as the scale, and make sure they're at the same distance every time. Then match the small ends, and then the pistons. Done right, you will notice a difference.
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Old 01-09-2011, 10:33 AM   #5
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Been balancing crankshafts for quite sometime. While what you propose won't hurt it's not balancing.

You have rotating and reciprocating mass. The crankshaft itself is most of the rotational mass. Now the rod/piston/pin/ring/clip assembly is part rotating and part reciprocating mass. This where the "percentage" comes into play. That's why you weigh the rod ends separately. You weight the rods and other parts and using a formula calculate how much bobweight to use to balance.

If you truly want to balance the assembly contact these guys and they'll help you out.
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Old 01-09-2011, 11:59 AM   #6
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Just a few months back there was a couple of people discussing this issue at length. It seemed to me like one of them new what he was talking about. Rods and pistons are always statically balanced. The hard part is getting a pivot shaft for weighing the small end. A piston pin can be used for weighing the big end. Remember when reading about it that dynamically balancing one type of engine can have very little to do with balancing another. If you want to learn about dynamically balancing an opposed twin with a 180 degree crank be sure you are reading about balancing an opposed twin with a 180 degree crank.
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Old 01-09-2011, 12:08 PM   #7
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Send it here instead... http://www.lindskog.com/
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Old 01-09-2011, 12:30 PM   #8
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It was my thought that dynamically balancing was different than blueprinting. When you make components like conrods of exact equal weight, that is the procedure of blueprinting.

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Old 01-09-2011, 12:36 PM   #9
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The reason for asking the question is becoming pretty apparent.......there's a lot of conflicting information about what is, or is not, balancing, how it is done, and is it worth it. All this has been very informative......keep the thoughts coming.

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Old 01-09-2011, 01:13 PM   #10
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IMO, static balancing the rods and pistons is pretty straight forward. That is once you learn that the rods have the ends balanced with each and not just simply their weight in its entirety.

From what I understand, opposed twins with 180 degree cranks have some unique qualities when dynamically balancing them while others say they don't. It seems to me that they do from what I understand.

Like a LOT of things, when you look into dynamic balancing in depth you will find that it is not a black and white situation.
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Old 01-09-2011, 01:59 PM   #11
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This is shedding a lot of light on a topic. Thanks for asking Martian.

Often, someone says "send the rods and pistons to a shop and have 'em balanced" and I never knew what they really meant (not sure if they know what they mean either ).

I've mentioned before that when I had my rods and pistons out I equalized the weight on each side but I didn't do the balancing suggested here. The bike was considerably smoother after that but a ton of other work (new cylinders, pistons, wrist pins and general tuning of a worn motor) done at the time probably contributed to that result.

Oak has also suggested that the crank, clutch, flywheel contribute significantly to any buzz and that balancing just the rods is just part of the job.

I'd love to hear more thoughts on this topic.
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Old 01-09-2011, 03:23 PM   #12
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OK equalizing the rod weights is a part of balancing the assembly. You then take the measured weights referred to earlier and calculate the bobweight. Then you attach it to the crankshaft in lieu of the rods ect. then mount it on the machine.

Then you set some parameters such as single or 2 plane,where the bearing surfaces are,where the planes are and adjust the speed and whether you're adding weight or removing. Spin the assembly at 500 RPM while it measures the imbalance. It then tells you where to drill and how much weight to remove. There's a chart that gives you how much a specific diameter hole will remove from the assembly. It pretty accurate but you become pretty good at drilling appropriate holes. Always drill less as it's easy to drill deeper but very difficult to put it back. Repeat until you're within 2 grams. That's dynamic balancing.

Static balancing is the same set up as far as weighing and attaching bobweight. Instead of mounting in a machine you attach a shaft to the flywheels then use a fixture with a knife edge,set the assembly on it and see where it settles. Just like balancing a wheel but you remove weight on the heavy side. Repeat until it will sit wherever you leave it. This only accounts for the vertical axis.

In addition engines with large flywheels,alternator rotors ect. will benefit from balancing as well.

Either is a vast improvement on factory balancing. The factory weighs 100 sets of pistons,pins,clips,rods and rings. They then calculate a nominal to balance to. Better than nothing but cannot account for manufacturing variance. I prefer dynamic because it recognizes 2 axes thus accounts for rocking couple. A smooth rotating assembly will rev easier,higher and live longer doing it. Many engines will make more power as well.
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Old 01-09-2011, 04:48 PM   #13
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Quote:
Originally Posted by Steve G. View Post
It was my thought that dynamically balancing was different than blueprinting. When you make components like conrods of exact equal weight, that is the procedure of blueprinting.

Steve
You are correct when you say that balancing and blueprinting are different. The distinction between the two terms is generally clear, since blueprinting is the term used to describe the checking (and correcting if necessary) the dimensions and running clearances of pistons, crankshaft bearings, cam(s), oil pump, valves, valve springs, cam timing, etc against specifications determined by the manufacturer.

However, the process of matching the weights of conrods and pistons is definitely a balancing procedure, since the specific conrod weight isn't usually part of the factory specs. Matching the conrod weights simply requires measuring the lightest small end and bigend weights, and removing weight off the other conrods to match the lightest one of the set. Matching the weights of the pistons involves a similar procedure for the same reason, namely that there usually isn't a specific piston weight given by the manufacturer.

The process of match-weighting pistons and conrods is a static balancing process, while the process of balancing crankshafts, flywheels, front pulleys, alternator rotors etc (in other words anything which rotates) is referred to as dynamic balancing if the item is spun on a balancing machine as part of that process with the out-of-balance measured by sensors and displayed on a screen. In some ways, it's not unlike an overgrown (and very expensive) wheel & tyre balancing machine. It is entirely possible to balance many rotating items between centres, which then effectively becomes almost a static process instead.

However, in order to balance a crankshaft successfully by either method, the conrods and pistons need to have been match-weighted already, since their combined weight (including wristpins, circlips, piston rings, bigend bolts and bearing shells) is then used to determine the weight of the bob-weights which are attached temporarily to the bigend journals, after the balance factor has been calculated for the particular engine configuration and compensated for. Only a proportion of the conrod's weight is considered reciprocating weight, as part of the conrod bigend weight is included with the crankshaft's rotating weight.
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Max Headroom screwed with this post 01-09-2011 at 04:51 PM Reason: I took so long to write this in-between doing other stuff that Anotherguy had already said much of what i wanted to say!
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Old 01-09-2011, 07:41 PM   #14
Martian OP
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All this balancing stuff sound pretty expensive! Ballpark, what would balancing a BMW airhead engine cost?

The wealth of knowledge in this group is amazing!
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Old 01-09-2011, 08:21 PM   #15
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Unless you need to pull the crankshaft for bearing inspection and/or replacement, I would simply get the rods and pistons balanced. This almost always results in less vibration. Install some lightweight wristpins while you're at it.

Balancing the flywheel and clutch-particularly in '81 on motors is also worthwhile.
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