Crankshaft Balance and Balance Factor

[KenS]

Attached is a spreadsheet to simplify balancing your flywheel assembly. Simply enter you specific information into the yellow cells and the results will be calculated for you, eliminating a mathematic error

With inexpensive material: a 4-foot 3/16" steel rod and 4-foot piece of 1/2" x 1/2" angle iron, each cut in half, the rod tack welded every 6" to the angle iron provide a good low resistance surface to roll the crankshaft on freely in order to obtain the BW. (Balance weight)

In the attached photos, I screwed the angle iron to two scrap pieced of 4" x 2" aluminum beam I had, then clamped them to the outsides of two steel Vee-Blocks which were then clamped into my bench vice and leveled.

[cmfalco]

Attached is a spreadsheet ...

A few more words (and pictures...) might be of help. The spreadsheet calls for the "Conrod small-end bush or bearing" in the upper calculation, but just the "Conrod small-end" in the lower one. To be clear, both are the same. To avoid confusion, it might be better to drop the 'small end bush or bearing', or add the words 'with the' to both. The next photograph shows how I measure this, with the crankshaft rotated such that the rod is horizontal.

Typical instructions show hanging the big end separately from a wire, but that requires taking the crankshaft apart, and it isn't needed for a rod having a roller bearing since the friction is negligible. Also, although my scale has 0.01 g sensitivity, if all the measurements were off by 1 g in the same direction, the error in balance factor only would be 0.1%, which would have negligible effect on the vibration.

The next photograph shows how I hang the weights from the small end.

The plastic cup is part of the total hanging weight and using it, instead of wrapping the wire around the weights, makes the process much faster. Also, the crankshaft doesn't care if high precision balance weights are used, or rusty bolts, washers, or whatever that's handy. All that matters is the weight.

The next photograph shows why you need to balance the crankshaft if you change the piston, even if the new piston is "identical" to the old one.

In the case of my 1928 Ariel, the lighter aftermarket piston I bought (Omega) weighed 81.5 g less than the heavier (Gandini) one. If the balance had been 66% with the lighter piston, and everything else was the same (rings, pin, circlips), it would have been 59.4% if the heavier were substituted without rebalancing the crankshaft.

I was lucky to find two people with original pistons for my Ariel, one of which was NOS. Working backwards, I calculated the original balance factor used by the factory was 64.8±1%. Through a happy coincidence, it was 65.5±0.4% with the Omega, so I was able to use it without having to bother drilling or adding weight to the crankshaft.

[nevhunter]

God advice. How different is the weight of the Carrillo rod? Nev

[cmfalco]

How different is the weight of the Carrillo rod?

For my 1954 BB Gold Star, the weights in grams are:

BSA BB34 Gold Star rod: 508 g
(BSA BB33 rod: 568.5 g)
Carillo rod: 390.0 g

According to the card that came with the Carrillo rod:
Total weight 390 g (which agrees with my measurement)
rotating weight 254 g
reciprocating rate 136 g

[nevhunter]

Pretty light. The tensile figures for the gold star is around 6 tons at about 6,000 RPM' IF I recall correctly. Thanks for the reply. Nev

[cmfalco]

According to the Carrillo card, its reciprocating weight is 136/390 = 34.9% of the total weight. Assuming for present purposes the same is the case for an OEM Gold Star rod, its reciprocating weight would be 0.349×508= 177 g, which is 41 g more than for the Carrillo. With a balance factor of 55% as BSA suggested for a BB Gold Star, that is a difference of 22.6 g.

The relevance of this is that according to an on-line calculator, if 22.6 g were at the rim of a spinning disk of the radius of a Gold Star flywheel, it would result in a out-of-balance force of 140 lbs. at 5000 rpm. Although that isn't quite the same situation as a reciprocating mass, it does indicate that much is to be gained in terms of reduction of vibration by using a lighter connecting rod. But, of course, only if the crankshaft is rebalanced according to the different connecting rod weight.

[KenS]

Attached is a spreadsheet ...

A few more words (and pictures...) might be of help. The spreadsheet calls for the "Conrod small-end bush or bearing" in the upper calculation, but just the "Conrod small-end" in the lower one. To be clear, both are the same. To avoid confusion, it might be better to drop the 'small end bush or bearing', or add the words 'with the' to both. The next photograph shows how I measure this, with the crankshaft rotated such that the rod is horizontal.

Crankshaft_Balancing04.jpg

Typical instructions show hanging the big end separately from a wire, but that requires taking the crankshaft apart, and it isn't needed for a rod having a roller bearing since the friction is negligible. Also, although my scale has 0.01 g sensitivity, if all the measurements were off by 1 g in the same direction, the error in balance factor only would be 0.1%, which would have negligible effect on the vibration.

The next photograph shows how I hang the weights from the small end.

Crankshaft_Balancing03.jpg

The plastic cup is part of the total hanging weight and using it, instead of wrapping the wire around the weights, makes the process much faster. Also, the crankshaft doesn't care if high precision balance weights are used, or rusty bolts, washers, or whatever that's handy. All that matters is the weight.

The next photograph shows why you need to balance the crankshaft if you change the piston, even if the new piston is "identical" to the old one.

BalanceFactor_General02.jpg

In the case of my 1928 Ariel, the lighter aftermarket piston I bought (Omega) weighed 81.5 g less than the heavier (Gandini) one. If the balance had been 66% with the lighter piston, and everything else was the same (rings, pin, circlips), it would have been 59.4% if the heavier were substituted without rebalancing the crankshaft.

I was lucky to find two people with original pistons for my Ariel, one of which was NOS. Working backwards, I calculated the original balance factor used by the factory was 64.8±1%. Through a happy coincidence, it was 65.5±0.4% with the Omega, so I was able to use it without having to bother drilling or adding weight to the crankshaft.

Excellent idea to include the process photos, making the article a one-stop-shop on how to balance a crank. As for the SEW wording, it was an oversight, but in the end SEW is SEW.

[cmfalco]

For what it's worth, the next photograph shows my setup for balancing crankshafts.

It uses a set of Crown balancing wheels, with sensitivity 0.2 g at the radius of the crankpin. The brass sleeve on the right is to give the same diameter to both shafts.

Perhaps even more important than balancing a crankshaft is truing it, and the next figure shows most of the possible issues a freshly-assembled crankshaft might have, all of which will rear their ugly heads during the truing process.

The next photograph shows the truing stand I made with the above figure in mind.

Although all tasks could be shared by a single indicator, having four of them makes things go much faster. Flat "feet" on two of the indicators bridge the width of keyseats and splines, so I can freely turn the crankshaft without an indicator tip dropping into the crevice and catching. I also use dial indicators (as opposed to the dial test indicators in the photograph) with roller "feet," which is useful when a crankshaft has flywheels with rough surfaces. The two (red) indicator holders mounted to the base are in locations that allow repositioning to measure either the rims or the faces of the flywheels.