"Is my crank balanced?" and Target Bobweight Explained

Since different rods and different pistons are different weights, it is impossible to make a crankshaft that is balanced "right out of the box" for any rod and piston combination. All crankshafts must be balanced to your specific rod and piston combination. When an Eagle crankshaft is listed as "internal balance" or "external balance" this is stating how this crank is intended to be balanced. It can be balanced otherwise, but it is much more difficult to do so. Eagle crankshafts are listed with a "target bobweight". This is an approximation (+/-2%) of the bobweight the crankshaft is roughly "out of the box". Because of the tolerance (+/-2%) the crankshaft cannot be considered balanced. For instance, for a crankshaft listed as having a 1800 target bobweight. The actual range of bobweights one of those cranks might have is from 1764 (1800-2%) to 1836 (1800 +2%). It might even be at the high end of that range on one end and the low end of that range on the other! This is not usually a problem because Eagle crankshafts are designed to have a target bobweight higher than most typical rod and piston combinations. Therefore, in most cases you will only need to remove material to balance the crankshaft instead of adding material. The main benefit of the target bobweight is to help the machine shop know what to expect before balancing so that a more accurate price estimate can be made. Eagle will balance a new crankshaft at the time of purchase. You will need to provide the bobweight you want it to be balanced to and the bobweight must be below the target bobweight listed for the crankshaft ordered. Our balancing price does not include addition of heavy metal.

Bobweight Explained

When a crankshaft is balanced, the actual rods and pistons cannot be used in the balancing machine, so they must be simulated. This simulated weight is called the "bobweight".   Once the bobweight is calculated, weights are bolted onto the rod journals to simulate the weight of the rods and pistons during the balancing process. Due to the configuration of a "V" type engine, just adding all the weights together does not work. There are also some dynamic considerations to be made when balancing the crankshaft. Explaining those is beyond the scope of this discussion. If you want to study those topics further, contact a crankshaft balancing machine manufacturer and they can go into greater detail. If you are a machine shop wondering if we take these things into consideration in the balanced assemblies we sell, rest assured we do. To calculate the bobweight of a particular assembly, the following formula and balance card is used:

For example, let's say we are balancing a Chevy 383 with the following component weights:

piston:416g

pin:118g

locks:2g

rings:35g

rod big end:458g

rod small end:186g

bearings:46g

Notice the rod weight is separated into "big end" and "small end". This is necessary because the small end has a reciprocating (back and forth) motion and the "big end" has a rotating motion. This split weight is figured on a special scale fixture that supports one end of the rod while weighing the other end. Given these component weights, the resulting bobweight would be 1770 and the calculation would look like this

There are several things to note about this calculation. The "oil" value used on the left side of the calculation is an approximation of the weight of residual oil "hanging around" on the assembly. The number used here is a matter of preference. There is no solid "rule of thumb" for this. Eagle uses 5g for small block assemblies and 15g for big block assemblies. Since it is impossible to accurately represent this value, it is just an estimate. The actual amount of oil can change constantly and can even be different from cylinder to cylinder! We have found through experience that the numbers we use estimate this property well.

The second thing to note is the 50% value used for the reciprocating factor. This number deals with the geometry of the engine itself. A 90 degree bank angle "V" engine will use 50% here. A V6 or a narrow or wide bank angle "V" engine will use a different value (again, consult the balancer manufacturer). Some engine builders will perform what is called "underbalancing" or "overbalancing". They will use slightly different values here such as 48% or 52%. This is done to help compensate for dynamic effects at extremely high or extremely low rpm operation (again, beyond the scope of this discussion). Eagle uses 50% because this value is required for almost all common street or racing engines.

Eagle Assemblies

Most Eagle rotating assemblies are sold unbalanced so that the machine shop can choose to balance it however they wish. We do offer most assemblies balanced from us. You must specify you want a balanced assembly in order to get a balanced assembly! If you do not specify, an unbalanced assembly will be shipped. All part numbers for balanced assemblies will begin with the letter B. For instance, if you want assembly part number 12006 balanced and in +.030" bore size, you would order assembly number B12006-030. The best way to tell if the assembly will be internal or external balance is to notice how the crankshaft used in that kit is intended to be balanced. All Eagle forged 4340 steel crankshafts are designed for internal balance. You can also notice the contents of the kit. An internally balanced kit will not include a harmonic dampener or flywheel because they are not required for balancing - use whatever brand you like. Externally balanced kits will include a harmonic dampener and/or flexplate as needed. If we provide a harmonic dampener and flexplate it will be a O.E. style replacement, not SFI approved. If you building a high horsepower engine, internal balance is preferred anyway. Internal balance is better for longevity of parts and fatigue life.