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| Joined: 03/11
Every driver has felt the effects of an out-of-balance tire. The entire car shakes and shimmies, and the steering wheel almost vibrates out of your hands. Now, just imagine what takes place inside an unbalanced engine. Instead of turning at a leisurely 600 rpm like a tire, the crank may be spinning at over 6,000 rpm. At these engine speeds, the pistons, rods, and crankshaft all become incredibly heavy; a piston whose weight is measured in ounces can exert thousands of pounds of force when it changes direction at TDC. A minor imbalance on a crankshaft counterweight is magnified many times by the centrifugal force of the spinning assembly. A single ounce of metal (28 grams) has a dynamic weight of over 700 pounds when it’s placed on a rapidly turning crankshaft counterweight.
This is why balancing is a vital part of engine blueprinting.
Balancing a tire is a breeze compared to balancing an engine. A tire rotates in one plane; an engine has a crankshaft that is turning, pistons that are moving up and down, and connecting rods that are doing a little of both. This helter-skelter motion produces some very strange vibrations.
The number and arrangement of the cylinders also has a tremendous impact on engine balance. A conventional V-8 engine with cylinder banks spread 90 degrees is a beautiful solution to many balancing problems. Yet if you lop off two cylinders to make a 90-degree V-6—as several automakers have recently done—you have an engine that is a disaster from the standpoint of balancing. To keep such an engine from shaking itself apart, the engineers devise all sorts of ingenious solutions, including offset crankpins, super-soft engine mounts, and special balancing techniques.
There are some engine designs that can never be perfectly balanced, no matter how much time, money, and equipment you devote to the project This is why some automobile and motorcycle manufacturers use complex counterbalancing shafts on inline fours when smoothness is an important consideration. Rearrange those four cylinders into a “V” with the proper angle between
the cylinder banks, however, and the reciprocating assembly can be balanced much more easily.
You don’t have to understand the detailed physics behind these effects to appreciate that some motors will always have a reputation as “shakers,” while others, with a different number or configuration of cylinders, can be silky smooth.
Rotating and Reciprocating Weight
For balancing purposes, the parts of the crankshaft assembly are divided into two categories:
Rotating weight and reciprocating weight. The crankshaft spins, so it is obviously part of the rotating mass. So are the components that spin with it, like the rod bearings. The pistons, rings, and wrist pins, on the other hand, move up and down, so they are part of the reciprocating weight. The connecting rods are a mixed case: the small end reciprocates, while the big end rotates. When an engine is balanced, all the components that are part of the reciprocating weight are matched so that none of the piston/ rod assemblies is heavier than the others. In a well-balanced engine, the weight of one piston is always offset by the weight of another piston moving in the opposite
Before balancing the rotating components, a machinist must first compute the bobweight.
The bobweight is the mass on a single rod journal. It takes into account the weight of the piston, pin, piston locks, rings, the reciprocating and rotating parts of the rod, and the rod bearings.
When calculating the bobweight for a conventional V-8 engine, the formula used by most machinists calls for adding 50 percent of the total reciprocating weight to the bobweight.
The ringer in all bobweight calculations is the weight of the oil on the components. If you dip a piston and rod assembly into a can of oil and then weigh it, the oil will add between two and ten grams to the total weight. How much oil actually clings to the rods and pistons inside a running engine—and how much it affects rotating and reciprocating weight—are difficult questions.
Some engine builders feel that the oil mist in the crankcase clings together to form a rope-like cloud that twists around the spinning crank and rotates with the assembly, which will, of course, affect the balance. Experienced engine balancers usually include an arbitrary amount of weight in the
bobweight calculation, to represent the oil film on the moving parts.
High-rpm racing engines are sometimes overbalanced. When an engine is overbalanced, the percentage of the reciprocating weight that makes up the bobweight is increased from the customary 50 percent to about 51 percent or 52 percent. Although there may be some theoretical advantages to overbalancing, the benefits are difficult to discern.
Advocates of overbalancing point out that bearing life is increased, while disbelievers maintain that the practice makes no difference in either power or reliability. Overbalancing is one of those engine-building techniques that fall into the gray category labeled “Probably Doesn’t Hurt.” For a street performance engine, using the traditional bobweight formula will provide perfectly satisfactory results.
When The Flag Drops,,,
The Bull Chit Stops,,,
P. Engineer, Engine Builder