E: Engine Crankshaft Balance Method

[Chris Launders]

An example of how far balance can be out and still be ok to use. I built a Brough SS100 replica and fitted it with a crank and rods from 1323cc SV engine, it was beautiful and smooth up past 90mph (as far as it'd been at that time) BUT at tickover the forks the forks flapped back and forward violently so I though I would have it balanced and sent the crank off to a specialist. they came back to me and said it was so far out the motor should have self destructed and my eyeballs pop out, instead of their normal 42% balance mine was 86% !!!!!

All around the rim on the crank pin side were 25, 3/8" blind holes and they said the only way to balance it was to take that much away opposite that they didn't want to do it so I got the crank back.

My solution, clean and tin the holes, pour molten lead into them and tamp it firmly down, then check the balance, I ended up drilling the lead back out of one hole.
Result, lovely and smooth at the bottom end and up to 100mph with a slight tremor around 55 mph.

Incidently I've seen several other sets of these flywheels and none were drilled like mine.

 

[Bill Thomas]

Slightly off subject, I wonder how far out of true the crank can be to be useable,
With the 560 I want to build, Just in case I balls it up !.

 

[timetraveller]

There is something wrong here and it might just be my understanding of what you have written. In #1 you write that the crankpin side was too heavy. If the picture of the flywheels in front of the frame are the ones being referred to then I very much doubt that. Those flywheels have been dramatically lightened compared with the standard ones and I cannot believe that they are too heavy on the crankpin side. I normally balance twin flywheel assemblies for road use at 46%, which is the official factory recommendation. For years that has required drilling a couple of new holes in each flywheel adjacent to the crankpin of about 1/4" to 3/8" diameter. I cannot see anyway that the flywheels referred to above are too heavy on the crankpin side.

 

[bmetcalf]

I have to confess that I would have taken the easy way out and gotten a new assembly from the SparesCo, Maughans, or Terry Prince.

 

[Bill Thomas]

Sorry Norman Those are my light wheels, After all that drilling I did they were fine,
Just trying to show Dave where to drill . Bill.

I took them to Bob Dunn a few years later for a bigend, And he said there was a small crack and didn't want to use them, He then sold me some wheels that were turned down on the outer rim.
But my wheels in the photo were 1/4" more narrow than standard.

 

[greg brillus]

The problem is that on a lot of engines the cranks have come out of race engines and/or used in bikes with completely different frames, perhaps even out of cars like a Cooper or similar. This is where you are taking a complete gamble using the crank. It does take some concentration to get a crank set up........even having any residual oil in the big end can stop the crank rotating "Freely" on the jig. I've known some guy's who only use one rod to balance the crank, suspending the other one with a length of string. I use both rods, that way you treat the assembly like a "Big single" Bill mentions about weighing the rod twice, well you generally weight it on it's own, then with the piston assembly........The variations in figures you get on digital scales is a bit mind numbing, you end up with a different figure every time. Arriving at a balance factor is not too difficult, it is working out how much material to remove that is another.........But as Chris pointed out, you just use lengths of steel rod or similar to work out what is required. Cut pieces up and weigh them..........Once you have done it you look back and see it is not that hard.

 

[Bill Thomas]

One bloke on youtube, Did an assy with piston all set up as ready to fit, And it was balanced where ever it stopped = 100 % ??.

 

[Little Honda]

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.

Don´t do it yourself without experience.

 

[Bill Thomas]

Where's the FUN in that, Got to have a go, They are only OLD BIKES.

 

[b'knighted]

Where's the FUN in that, Got to have a go, They are only OLD BIKES.

OK Bill, if that’s your attitude and you only want to do 70mph, why not fit a Shadow high first gear and lighten the bike by throwing away all those heavy gearbox internals that are no longer needed. You could tell everyone that it is a single gear automatic.