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E: Engine Crankshaft Balance Method


LoneStar

Active Website User
VOC Member
Hello all,

I'm rebuilding my Series C Black Shadow with new pistons and rods, due to a catastrophic failure last summer. I now have the crank assembled with the rods, and need to check the balance. Based on a consensus of informed opinion, I’m targeting a 50% balance factor.

After doing a lot of reading, I think I understand what needs to be done to reach this figure. But, I’ve made stupid mistakes before and it would be inconvenient to tear down the engine again to correct one. So I’d like to outline my logic, and ask if anyone sees errors or faulty assumptions.

Here are the basic figures:

pistons: 417g each, with rings and pin

rod reciprocating weight: 187g

This gives total reciprocating weight on the crank of 1208g

The idea is to balance 50% of this reciprocating weight, or 604g. This means with 604g reciprocating weight, the crank should be balanced, and stop at any location when rotated on bearings.

To test this, I subtracted the 374g reciprocating weight of the rod small ends, as they’re already in place, and attached 230g to the small ends to reach the 604g figure.

Unfortunately, but not surprisingly, the crank wasn’t in balance at this point: the crankpin side was heavier. This means I need to add weight to the flywheels opposite the crankpin, so I attached weights there until the crank was in balance. This took 113g.

The next question is how to implement this on a permanent basis. The accepted method appears to be inserting plugs of heavy metal, such as tungsten, into holes drilled near the flywheel rim. So, how much tungsten is required?

The weight required will differ with the distance from the center of rotation. My test weight was on the outside of the rim, at a radius of 4.2”. The holes to be drilled for plugs will be c. 3.5” out, so I increased the weight requirement by the ratio – multiply by 1.2. This gives a weight requirement of 135g.

Now in order to insert the plugs, holes must be drilled. This has the unfortunate effect of removing steel, so requiring more tungsten to compensate. The density of tungsten is 19.6g/cc, while steel is 7.85. Based on this, the effective density of tungsten when replacing steel is 11.75g/cc. Therefore the weight of the tungsten plugs will be the required net weight increase x (19.6/11.75), or 225g.

Is this all correct?

The rods and pistons are both Carrillo, and I’m told by the supplier that it’s normal to need to add weight to the flywheels when balancing them.

Thanks!

Dave
 

Attachments

Albervin

Well Known and Active Website User
VOC Member
Oh dear. There is catastrophic and then this. I hope the crankcases survived. There are already holes in the flywheels so you could possibly fill some of them with a less dense material. This will involve physics that I forgot over forty years ago. I would seek the help of a race shop with experience in these things. That is exactly what I did with my old car. Surprisingly not expensive as they have all the machinery, dials and gauges. I wish you good luck in getting the beast back on the road.
 

MartynG

Well Known and Active Website User
VOC Member
Instead of adding opposite the crankpin You could possibly remove weight by drilling into the rims on the crank pin side. Just a thought.

As it is, what is the balance factor??
 

Bill Thomas

Well Known and Active Website User
VOC Member
I am interested because I have to do a Comet.
Not sure of the Sums ??, YouTube gives a few different ways !!.
But if it's heavy at the C/Pin, Drill holes either side of the Pin.
I did it to my Twin 40 plus years ago, But have forgot.
David Bowen told us, The factory drilled out the top 2 outside holes to 17/32 " and the 4 below to 7/8 " when our engines are used in cars with standard crank pins.
Try and keep the swarf out of the bigend bearing !!.
Have a read of "Tuning for speed ".
Good luck, Let us know how you get on, Bill.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Just seen the photo !! How did that happen ?, Never seen that before.

Had to go a bit MAD with mine, After taking 1/4" off the side of each wheel, Looking for more rev's
for Racing.
But it worked !.
 

Attachments

LoneStar

Active Website User
VOC Member
The crankcases, miraculously, survived with very minor damage.

On the existing holes, I would need more of them - for which there is not much room. Filling them would have the opposite effect to what's needed. Martyn's idea of drilling the circumference near the crankpin might work, although I suspect it would take too much drilling to remove the necessary weight. Pretty much all area near the crankpin where a hole can be drilled, it already has been - see pic. I'm not sure if that's the factory treatment or if some holes were added later.

I don't know the balance factor on the old assembly, as pictured. The crank and rods were apparently redone decades before I acquired the bike, based on receipts I inherited - original Vincent rods, and an Alpha big end. The pistons were Omega cast 8:1. I can't check the previous factor now, as the crank is together again with new rods.

Carrillo rods are heavier than Vincent (660g vs. 620g with races), although the reciprocating weight is lower (187g vs. 226g). This seems to result in the need for extra weight opposite the pin. Coventry Spares tells me that when they have cranks balanced with Carrillo rods, they always need plugs to add weight across from the pin.
 

Attachments

LoneStar

Active Website User
VOC Member
Bill,

I don't really know! I was riding along around 70 mph in third, minding my own business, when it died. It looks like one gudgeon pin boss broke away from the piston, causing the bend in the rod. The pin stayed in the small end for a while, as you can see the marks its end made when crashing into the bottom of the piston crown. The piston shows some signs of seizure, but not that severe - and in any case, a seizure is supposed to drag you gracefully to a halt, not break the piston apart...

Dave
 

Attachments

Bill Thomas

Well Known and Active Website User
VOC Member
I would open out the top 3 holes.

I think you were lucky !!, The rods snap there and the bit that is left bangs a hole in the cases.

What make piston was that ?.
 

LoneStar

Active Website User
VOC Member
Bill,

True, there's some room for that. I'll investigate. Perhaps a risk of weakening the flywheel too much, though?

Dave
 

Bill Thomas

Well Known and Active Website User
VOC Member
I think it will be strong enough, I would start with the very top one, But check the sums first ??.
 

MartynG

Well Known and Active Website User
VOC Member
Hello all,

I'm rebuilding my Series C Black Shadow with new pistons and rods, due to a catastrophic failure last summer. I now have the crank assembled with the rods, and need to check the balance. Based on a consensus of informed opinion, I’m targeting a 50% balance factor.

After doing a lot of reading, I think I understand what needs to be done to reach this figure. But, I’ve made stupid mistakes before and it would be inconvenient to tear down the engine again to correct one. So I’d like to outline my logic, and ask if anyone sees errors or faulty assumptions.

Here are the basic figures:

pistons: 417g each, with rings and pin

rod reciprocating weight: 187g

This gives total reciprocating weight on the crank of 1208g

The idea is to balance 50% of this reciprocating weight, or 604g. This means with 604g reciprocating weight, the crank should be balanced, and stop at any location when rotated on bearings.

To test this, I subtracted the 374g reciprocating weight of the rod small ends, as they’re already in place, and attached 230g to the small ends to reach the 604g figure.

Unfortunately, but not surprisingly, the crank wasn’t in balance at this point: the crankpin side was heavier. This means I need to add weight to the flywheels opposite the crankpin, so I attached weights there until the crank was in balance. This took 113g.

The next question is how to implement this on a permanent basis. The accepted method appears to be inserting plugs of heavy metal, such as tungsten, into holes drilled near the flywheel rim. So, how much tungsten is required?

The weight required will differ with the distance from the center of rotation. My test weight was on the outside of the rim, at a radius of 4.2”. The holes to be drilled for plugs will be c. 3.5” out, so I increased the weight requirement by the ratio – multiply by 1.2. This gives a weight requirement of 135g.

Now in order to insert the plugs, holes must be drilled. This has the unfortunate effect of removing steel, so requiring more tungsten to compensate. The density of tungsten is 19.6g/cc, while steel is 7.85. Based on this, the effective density of tungsten when replacing steel is 11.75g/cc. Therefore the weight of the tungsten plugs will be the required net weight increase x (19.6/11.75), or 225g.

Is this all correct?

The rods and pistons are both Carrillo, and I’m told by the supplier that it’s normal to need to add weight to the flywheels when balancing them.

Thanks!

Dave
Have a look at OVR #46 in the OVR archives

 

Bill Thomas

Well Known and Active Website User
VOC Member
So who has the right balance factor ??, I hate books !, So it's like the old days, Suck it and see :) .
My special was very smooth at 6000 revs But not good at 70 ish, Don't know what the balance factor was.
By putting lead balls in the handle bars, As Big Sid told us, My ex L/ning is nice now at 70 ish, I don't go much above that now, Well not for too long anyway :), To old and too many Police.
 

MartynG

Well Known and Active Website User
VOC Member
Yes Bill, its a conundrum. For as I understand it the 'correct' balance factor can vary from bike to bike as there is a definite relationship to the harmonics of the overall vehicle. The vibrations within the frame and even within the wheels. Race bike with lightened frames and other tweeks apparently need different balance factors to road bikes.

All this confusion is apparent in the quotes in the OVR article. The only thing I can definitely say is that the balance factor for a road going SINGLE recommended by Phil Irving is 66%.

I wonder what some of our 'professional' engine rebuilders think about this.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Dave I am rubbish !!! at maths, But if you need 113 grams on the far side, Wouldn't you only need half that
Taken Off near the pin ????, Sorry if I am completely wrong. Bill.
 

Chris Launders

Well Known and Active Website User
VOC Member
A good helpful start is to weigh a piece of steel, say 3/8" diameter and the same length as the flywheel is wide.
This would give you an idea of the volume that would need removing.
I personally think you could remove most of the webs between some of the existing holes, just leaving say 1/4" in the middle
 

greg brillus

Well Known and Active Website User
VOC Member
To arrive at any actual balance factor is completely Do-able........however, "the" actual balance factor is a myth subjective to the actual engine and how it is mounted in a frame (if any) I'm pretty sure the factory spec for road twin cranks was 47% or thereabouts........perhaps as high as 50%...........How the engine is mounted is quite important to the whole idea........for example, I can guarantee anyone that if you mounted a Comet engine with the cylinder vertically, the engine would vibrate differently to normal. This is why a Vincent twin mounted in a full loop frame (Featherbed) needs a different balance factor to a stock Vincent. I had a new crank assembly for an "A" twin that was supposed to be set at 60% (it was actually 61%) I lowered it to near 55% by drilling 4 x 3/8" diameter holes opposite the big end, two of in each flywheel. These wheels were made from a much higher grade of steel than the originals which were basically "Boiler plate" and much softer. This has resulted in a much smoother engine in the lower rev range..........with that said, the same (or very similar) engine and crank combination mounted in a post war frame, that is, conventional UFM and RFM as we know, and again that engine is not to bad, this with the 60% crank balance factor .............I know you are all thinking, what the hell am i up to.........Anyways.......In the example of this discussion, the pistons are quite light verses the originals, generally around 450/460 grams....... I would start by removing some material from the top three holes as suggested by Bill, and then look at adding some tungsten or mallory metals opposite the big end. You would be surprised at how far out the balance factor can be, and the engine still be quite smooth........The flywheels on my twin racer, picture on the left, were actually out by 520 grams...........The engine was quite smooth, but it did vibrate quite badly at low engine speed. I did not set it up myself, but I did check it when the engine was rebuilt a couple of years back........I nearly fell off my chair when I found this out........The flywheels had to have four holes bored through the wheels just opposite the big end near one inch diameter each.........that is a lot of metal to remove.
 

Bill Thomas

Well Known and Active Website User
VOC Member
My understanding is you balance it to what speed you use most, As you can't have it spot on all through the rev range ?.
But what are the maths Greg, Has Dave got it right ?, Some youtubes weigh the rods twice !!/,
Then do maths that are too much for me !.
 

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