Dynamic Balancing and Beginning Blueprinting


Static of Dynamic balanced fly-wheel, the process we described before is know as static balancing because the flywheel is not in motion when balanced. Static balancing is still favored by many engine builders, but dynamic balancing has gained a large following.

Because it allows computer sensors to assess flywheel balance with the flywheel balance with the flywheel in motion, pinpoint any imbalance, and determine the magic number in matter of seconds, dynamic balancing is significantly faster. Proponents also claim that it is more accurate.

Although a flywheel may appear perfectly balanced on the stand, the story sometimes changes when the flywheels are assembled with the other crankshaft components and the assembly spun up to several hundred rpm. Shaft and flywheel flex can cause high-speed oscillations that throw balance off, as can minute imbalances that are undetectable by static-balancing methods but which increase dramatically with engine speed.
As mentioned in the first story of Bike Engine Balancing and Blueprinting, blueprinting insures that the dimensions of different engine components and their operating clearances fall within a desired range or tolerance. The difference between production-level and custom blueprinting is that tolerances for custom blueprinting are much smaller.

Let's use camshaft endplay, which is the amount of space that exists between the camshaft and the bearings or bushings that support it on the end., as an example. Harley-Davidson lists 0.0254mm -1.17mm as acceptable range for a 1993-94 Softail. Independent engine builders may narrow that to 0.254mm -0.508mm, or even less.

Without sufficient endplay, the end of the camshaft may bind or receive too little splash oil for sufficient lubrication. With excessive endplay, the camshaft can 'walk' back and forth, accelerating wear on its lobes and the lifters. Camshaft endplay is determined by the thickness of a spacer placed between the camshaft and thrust washer, so changing it is easy. Just insert a thicker spacer for less endplay, a thinner on for more. Other dimensions and clearances are more tedious to measure and may require machine work to correct.

Combustion chamber volume, for example, is critical because it determines compression ratio. A difference of 10cc between the two combustion chambers, for example, is enough to cause ignition knock or a noticeable power loss. It's also enough to increase vibration dramatically because of the difference in combustion pressures that will exist between the two cylinders.

To measure chamber volume, the valves must be installed in the head and sealed with grease. Then, a piece of Plexiglas with a small hole is bolted to the head and sealed. The chamber is then filled with a volume of carefully measured fluid administered from a burette, similar to what they use in laboratories to measure and separate known amounts of liquids.

If the chamber volumes of the front and rear head are different, the engine builder can degrease the volume of the larger chamber by removing material from the head gasket surface, we call this decking the head. If the engine builder decides to enlarge the smaller chamber instead, he can machine the valve seats to sink the valves farther into the head, remove material from inside the chamber, or both.

And remember we talk here in steps smaller then a millimeter, so be careful if you going to do it yourself.

Next related article: Blueprinting and Building a Good Engine Case
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