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Old 08-17-2010, 06:19 AM   #1
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Default Resize crank counterweights with lighter rods?

I would be interested to hear if anyone resizes their crank counterweights to minimize crank deflection and bearing forces or do you just throw in a set of rods ignoring the weight difference compared to stock. Theory calls for 100% of rotating mass (big end etc) and ~50% of reciprocating mass (small end+piston) to be counterbalanced. And please lets not get into static and dynamic balance, naturally any crank needs to be in balance no matter what size counterweights are used.

Some theory to support the discussion can be found here:
http://www.modelaengine.com/13.html
http://www.oldjohnno.id.au/page10.html
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Old 08-17-2010, 09:36 AM   #2
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Quote:
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or do you just throw in a set of rods ignoring the weight difference compared to stock.
^^this
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Old 08-22-2010, 06:49 AM   #3
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Just make sure the rods are equal weight to each other and its all good. the crankshaft is balanced individually, adding rods and pistons is no reason to change that.

This is coming from a noob though, please correct me if i am wrong.
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Old 08-22-2010, 06:52 AM   #4
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Just make sure the rods are equal weight to each other and its all good. the crankshaft is balanced individually, adding rods and pistons is no reason to change that.

This is coming from a noob though, please correct me if i am wrong.
^^this

The crankshaft itself will always have the same balance. As long as the rods and pistons weigh the same amount, all is well.

Hmm, actually, upon further thought, maybe not.

No, never mind. I was right the first time I think. You always have two weights and two rod/piston assemblies 180* out from the other two, so as long as the crank is balanced, which it should be unless someone tampered with it, and the rods and pistons weights are close, all should be well.

A V8 crank on the other hand might be different, since they usually have a less uniform, 180* setup like a 4 cylinder crank. Dynamics probably play more a part in that though.

Last edited by NA6C-Guy; 08-22-2010 at 07:06 AM.
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Old 08-22-2010, 11:47 AM   #5
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The crank weights balance the con rod inner race on the other side of the crank. The rod and piston assembly on cylinders 2&3 balance out the rod and piston assembly from 1&4.
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Old 08-22-2010, 05:02 PM   #6
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has anyone gotten thier crank lightend? (knife edging, ect...) did it help much?
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Old 08-22-2010, 05:58 PM   #7
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I'm not sure many people on this board have. At 200+ horsepower, I'd imagine it'd be difficult to feel the difference, especially since we struggle with traction and breaking drive trains already. Once you have a turbo, adding a psi of boost is much cheaper than knife edging the crank and all the labor involved. I know it's not the most solid argument, but you get my point.
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Old 08-22-2010, 06:35 PM   #8
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ya thats probably true.... but if you can make more "efficient" power i think thats better... your just having to make more power to turn the crank... just like the effect of putting on a light weight flywheel....

if you have the motor apart, are doing a crazy build and have the money why not.... when i build my motor up i think i will do it... but im still thinking about it lol...
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Old 08-22-2010, 08:38 PM   #9
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ya thats probably true.... but if you can make more "efficient" power i think thats better... your just having to make more power to turn the crank... just like the effect of putting on a light weight flywheel.... .
that's why a lot of us have lightened flywheels. To be honest though all shaving weight out of the rotaing assembly does is make it harder to drive on the street. i have a 12lb flywheel and it's a lot more work in the city.
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Old 08-22-2010, 09:04 PM   #10
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I run a 13lb flywheel. It's a bit more work in stop-and-go, but once you get used to it, it's cake
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Old 08-22-2010, 09:11 PM   #11
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my festiva is so light weight you barly need to touch the gas when you let out the clutch... the torque of the 1.8L is awesome so i guess im lucky cause im getting a 8.8lb flywheel soon
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Old 08-23-2010, 12:39 AM   #12
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For quicker, easier, and cheaper lightening, when you change your clutch, have the machine shop who's resurfacing it lathe off the weight ring. I did that at work, and my engine shop ground the PP a bit to balance the assembly, since you normally drill holes in said weight ring. I think it's good for 4 lbs.
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Old 08-23-2010, 02:22 AM   #13
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Had my crank completely worked over. Adding 2 extra oil feed ports on the crank for each set of rod bearings. Having the factory oil gallies plugs removed on each counter weight(they look like ball bearings)
The oil gallies where also cleaned and enlarged by drilling them out for more oil flow.
Counter weights PROPERLY had weight removed from the correct side of the counterweight, NOT knife edged.
Crank along with every single rotating piece was balanced to the dot.
The fruits from the machining should result in free'd up torque all across the board.

I wish i could get to my other laptop with more detailed pics of the amount of work i had done, This should show the juicy stuff though

[URL=http://img710.imageshack.us/i/crankn.jpg/]
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Old 08-23-2010, 03:36 AM   #14
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I see many are discussing balance though I specifically asked not to go there. The issue is crank deflection and peak main bearing loads as explained in the links I provided. Anyway, it seems most are getting away with less than perfect setups. Which is what I wanted to hear, I guess. Interesting work, railz. So they removed weight from the big end rather than the counterweight? That would mean the Miata has too small counterweights as stock, assuming you installed lighter rods. I have not had the chance to open up my bottom end to measure and model it in CAD. Will do if I put in new rods at a later date.

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Old 08-23-2010, 04:07 AM   #15
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Quote:
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The issue is crank deflection and peak main bearing loads
The peak loads on the main bearings wasn't something i was to worried about. It was the peak load on the rod bearings during TDC that was stressing. Scared I would end up with another ruined motor because of a spun rod my machinist added 2 more with total now of 4 oil feed ports for each rod bearing. Also Calico teflon coated bearings were used for even more protection.
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Old 08-23-2010, 11:08 AM   #16
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I got a price for a knife edged crank and it was going to be around $850 total I believe which is rediculous considering there are only a few members actually running more than 300hp so it's highly unnecessary
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Old 08-23-2010, 11:24 AM   #17
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Is the BP noticeable smoother after having things properly balanced?
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Old 08-23-2010, 01:52 PM   #18
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I got a price for a knife edged crank and it was going to be around $850 total I believe which is rediculous considering there are only a few members actually running more than 300hp so it's highly unnecessary
that is just about how much i spent but i got so much more then just plain old knife edgeing
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Old 08-23-2010, 02:31 PM   #19
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that is just about how much i spent but i got so much more then just plain old knife edgeing
Nice but I couldn't justify spending that much more considering that I had about $1200 already wrapped up in the head total
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Old 08-23-2010, 06:26 PM   #20
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Seems pointless to me for the minimal gains. I'm in the boat with the add a pound more to the boost. Plus spending that much on stuff like that would be wasted if you had internal failures and trashed rods, pistons or crank.
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