E: Engine Crankshaft Balance Method

Bill Thomas

Well Known and Active Forum User
VOC Member
I put new + 0.020" Omega 8:1 pistons in my Rapide last year. They weighed 450.38 grammes each complete with pin, circlips and rings. I had it bored with 2½ thou clearance. It ran fine after a short running-in period and I've two or three thousand miles since including a bit of "spirited" riding.:)

I put together an Excel spreadsheet to calculate what the balance figures would be. I can't upload it to the forum as the extension is not allowed. The link below is to a Google Drive account. If it works it may display the sheet and you may be able to download it and open it in MS Excel. If you have a Gmail account you should be able to open it online using Open With Google Sheets. If anyone has been able to access this could they confirm it? If you can use Excel please tell me if my formulas are wrong. I have left some sample figure in place to play with.
https://drive.google.com/file/d/1pZ9XRJmmoTOnahJpSb1z_VwWQ3_NXmU8/view?usp=sharing
I think I have woke up on another Planet !,
Greens and dark greens ! Your having me on !.
How have you got, Or what does it mean to go from Reciprocating to Balance Req',
I thought the sum of 50% stopped at the small end and complete pistons ??,
Confused of Ruislip.
On a good note, I just clicked on your thingy and your sheet came up.
 

timetraveller

Well Known and Active Forum User
VOC Member
If the spread sheet is only 16 rows deep then it came up for me in Win 10 without downloading anything else although there was a request to move over to another search engine.
 

Bill Thomas

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VOC Member
Sorry, Got it, 634 LESS the weight of the small ends 279 =355 Total to hang on the small end.
Going for a lay down :) .
I have a Twin Crank that blocked the oil feed off after a 1000 miles, I didn't do it !.
Might play with that for some fun.

Green it woz !
 

greg brillus

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VOC Member
Ok so i have checked out this new crank assembly i have here, very similar to David's one with some minor differences but my pistons are much heavier on account of the pistons being 92 mm. Both little ends of rods equal 367 grams, pistons are 446 grams each. David found he needed 113 grams opposite the big end to bring the crank to a neutral balance and I needed 117 grams, so very similar. Only difference now is I'm thinking this crank should be balanced to 60% as it is going into a new featherbed frame. I will need a hung weight of around 388 grams to achieve this.......So I'm going to need plenty of machining at the big end of the flywheels to make this happen. I inquired about the cost of a length of Mallory metal, about $250 AUD for a piece 1/2" diameter and 200 mm long.......... Where's that damned hacksaw Bill..............
 

Little Honda

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Non-VOC Member
Ok so i have checked out this new crank assembly i have here, very similar to David's one with some minor differences but my pistons are much heavier on account of the pistons being 92 mm. Both little ends of rods equal 367 grams, pistons are 446 grams each. David found he needed 113 grams opposite the big end to bring the crank to a neutral balance and I needed 117 grams, so very similar. Only difference now is I'm thinking this crank should be balanced to 60% as it is going into a new featherbed frame. I will need a hung weight of around 388 grams to achieve this.......So I'm going to need plenty of machining at the big end of the flywheels to make this happen. I inquired about the cost of a length of Mallory metal, about $250 AUD for a piece 1/2" diameter and 200 mm long.......... Where's that damned hacksaw Bill..............
U r talking abt radius, or 1"dia, or r u talking abt Firefly tuning?
 

LoneStar

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VOC Member
Greg,

I ended up drilling four 7/8" diameter holes radially into the edges of the flywheels; it removed sufficient weight.

If you do have to use tungsten, I doubt you'll need 1/2" x 200mm of it. Weights of different rod diameters, per cm length: 10mm diameter: 15.5g; 12mm: 22.1g; 15mm: 34.6g. (Of course you have to adjust for the steel lost when drilling a hole for the tungsten plug.)

You can find the rod at reasonable prices on Ebay (mostly out of China), or these people have pre-cut tungsten weights: https://store.mttm.com/crankshaft-weight
 

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Bill Thomas

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VOC Member
Dave, How deep are those holes ?.
I like what you have done, I think it is more easy to keep the swarf away from the big ends ?.

Greg, Are you stroking it as well ?, What size will it be when finished ?.
The more I think about it ,Everything Thrashing up and down !,
I think it's more important, That the wheels are true,
Which is bad news for me as I can't see me boring the big end spot on !.
If I can't get it within 5 thou runout, I think it will be a BIN job !.
I have read lots of people find when they take a crank out , That they have not been running very true ?.
People talk about the balance being anything from 46 to 60 % !!.
The Wife has spotted some cheap scales that Lidl are selling from this Thursday, So I hope we can get them.
Cheers Bill.
 

greg brillus

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VOC Member
Hi there Bill, No this is a stock crank but with Carillo rods, I wish they were ARGO rods made here in Australia, as they are more like the original Vincent rods, these are the type that Terry fits to all his cranks now. I think what you want to try may prove difficult Bill, without some decent machinery and a skilled operator, shame we live so far apart, I have a spare stock Comet crank you could have......... I know it's only a bit of fun, but the run out is pretty important save the main bearing housings.........;)........ Cheers......... Greg.
 
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