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E: Engine Crankshaft Balance Method


Bill Thomas

Well Known and Active Website 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 Website 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

Well Known and Active Website User
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

Well Known and Active Website User
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

Well Known and Active Website User
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

Active Website User
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
 

Attachments

Bill Thomas

Well Known and Active Website User
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

Well Known and Active Website User
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.
 

greg brillus

Well Known and Active Website User
VOC Member
We always want more power........have you ever ridden a decent 600 Comet with one of Terry's top ends on it, this with the twin plug set up and a good 36 mm Mikuni or Dellorto...........They absolutely fly....... If the factory would have made them like this, they would have sold more singles than twins. This has been my thinking towards the new engine in my racer........92 mm bore x 100 mm stroke..........Should go like a "Raped Ape".......:)
 

Bill Thomas

Well Known and Active Website User
VOC Member
Totally agree Greg, Can't help thinking, The first time the "C" Comet was tested,
I think the "A" was a bit more Frisky ?, Ron's "A" Comet Special, The factory one ! was.
The tester should have said, " Nice Bike But !! ", Needs more Power, And maybe better gearbox ?.
I have not tried a Big One, As I have said, I got my old Comet to go well when I nicked the head and 36mm,
Dellorto of my Twin Special., But I had to give it back :) .
The engine was already Flash Spec'.
If I had another set of cases, I had thought of going Big Bore, But I think I am being Greedy ?,
And I want to see how much difference just stroking makes.
Plus I have been so long trying to build this , My last Bike ?, I have run out of Frames and Wheels etc,
And everything is so much money now.
Plus the Wife has learnt how to count :).
Have FUN, Bill.
 

vibrac

Well Known and Active Website User
VOC Member
Sometimes Greg (especially in the UK) its wiser to select the organisation &race class before you build the bike to a capacity ;)
It would not be the first time I have put a smaller barrel back on to get more points
the BHR series is so complicated nowadays there are 50 yes 50 classes of bikes and 50 'events' .As to the actual races (length generally just 4 laps!) all get jumbled together there are no regulations regarding number plate coulors, different cc within a class get more or less points.the only time it gets separated out is at year end prize giving at the dinner which with 1st,2nd and 3rd must take days. .
We gave up when Girdraulics were banned from the girder fork class (there is a class for Vincent singles but you would run on your own!) I doubt we shall race with them any more, sad after 50 odd years.
 

LoneStar

Active Website User
VOC Member
Greg & Bill,

I chose the radial holes on the rim, instead of enlarging the existing axial holes, specifically because I was worried about either damaging the rod big ends or getting swarf into the bearings - the holes end right at the rods.

The holes I drilled are about 1cm deep at the edge, deeper in the middle because of cone-tipped drill bits. I wanted to get them all the same, so used a syringe to fill them with water and test how much material had been removed, during drilling. This also allows figuring how much weight has been removed. I settled on 4ml each.

With the radial holes being some distance from the big end bearing, so less of a swarf risk, I just wrapped some tape around the rod big ends at the junction with the flywheel.
 
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greg brillus

Well Known and Active Website User
VOC Member
I'll be running in the unlimited class, 525 cc's up to 1300 cc the bike is around 130 kg's so it should go ok i'm hoping......... Dropped that crank in at the machine shop this morning, told my friend to do his best.........It's like using a micrometer, hard at first till you get your head around it, then it all makes sense after a little head scratching.
 

Bill Thomas

Well Known and Active Website User
VOC Member
I know USA Gallons are different to UK,
But are UK Grams the same as Oz and USA Grams ? :) .
I make a standard small end held Horizontal 313 and a Carillo 202 ??,
I don't know if it's my cheap scales, " Salter, I thought they were good ones " ?, Or I might find a different way to hang the rods.
As Greg says, Every time you check, You get a different reading !.
 

Little Honda

Well Known and Active Website User
VOC Member
You can help yourself with cheap scales: Take a real weight, better some, and weigh them with your scale. Note
the results, repeat, list average deviation. Then do your measurements and correct by yr dev. factors.
 

greg brillus

Well Known and Active Website User
VOC Member
Pretty much most things with a digital read out are like that, look at multi meters, they do the same thing. I'll be getting on with that crank this week I'm hoping, so it will be interesting to see how it pans out. When using the scales you just need to duplicate things as you go, so things remain mostly constant. I think it is pretty important to make sure the big end is free of any rolling friction, that is you probably should flush the big end with some clean solvent to remove the drag of the oil within.
 

Bill Thomas

Well Known and Active Website User
VOC Member
You put me to Shame Greg, I am only playing on the bench with the rods,
The crank won't be done till next Christmas :) .
 

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