By Tom Cole
Several years ago when we were developing our crankcase ventilation system for the Tecumseh Star engine, I got some seat-of -the-pants experience with the value and need of crankshaft balancing. I was accustomed to driving a 3” bore, 3” stroke test engine, which had one of our billet crankshafts in it. The 3x3 crank had been balanced, and ran very smooth considering it was producing about twice as much horsepower as the Star. The Star’s crank/rod/piston setup had not been balanced, and the difference took me by surprise. As I made 8000 rpm, my vision was so blurred from the engine’s vibration that I had to slow down to see the turn. My teeth and ribs felt like they were banging together and after my 15 lap stint at this ¼ mile asphalt oval, I was not interested in driving any more that day. My body was directly reporting to me the increased pain and fatigue that an unbalanced crankshaft can do to an engine and driver.
Clarence Clark is a friend of mine and he is what I would call an engine-building guru. For many years, Clarence’s company rebuilt the engines for the world’s largest fleet of racecars, the United Parcel Service. He has since traveled the country doing seminars for rebuilding supply companies like Goodson and Cobra Products. When I told Clarence about my experience, he went over to his tool box and produced a metal “H” which was made out of five, six inch long 3/8” metal pipes and two 3/8” pipe “T’s”. He handed me this contraption and told me to gently hold the cross pipe in one hand like an axle and spin it. Everything was fairly in balance and it spun easily making five or six revolutions. He then removed one of the four, six-inch uprights from the “H” and said “now it’s out of balance, spin it again.” I did and it only made one revolution! When I tried to spin it real hard, it only made two revolutions. This was an example of the need of both Force and Couple or “Dual Plane” balancing of a crankshaft. He told me one of the most interesting things about an out-of-balance crank or cam is that this tendency to stop (or inertia) increases with RPM so it requires more and more horsepower to obtain the same RPM as a balanced setup! It is hard to believe that we spend so much time and money on carburetors, valves, porting, flow and displacement and many of us ignore or are unaware of such a power robbing aspect of an engine.
Force and Couple balancing are technical terms used that really just mean top-to-bottom and side-to-side balance and are commonly referred to as “Dual Plane Balancing.” They are easily illustrated with a little history in tire balancing. Many years ago, when car tires were balanced, the rim and tire assembly was mounted on a shaft and then placed on a frame with the shaft resting in a ball bearing V fixture. The tire assembly would then rotate around until the heavy part came to rest at 6 o’clock. A wheel weight was then placed at 12 o’clock on one side of the tire. You kept adjusting this weight until the tire would not move regardless of how you repositioned it in the V fixture. This process is called, Static, or Force balancing and it is the method that is used on most kart and Jr. Dragster wheels today. Later, an improvement was made in this process by splitting the weight and putting half on the inside of the rim and half on the outside. This was the first form of Couple balancing in the tire industry. As years went by the advent of much wider tires came into being, so the need for more accurate Couple balancing increased because the part of the tire that was out of balance was often further from the centerline of the tire. And since one side of the tire could weigh more than the other, it became necessary to be more precise than to just split the weight in half to achieve optimum balance. We now have electronic tire balancers that spin the tire and wheel and calculate the amount of weight needed to Force balance. Then it calculates what amount goes on the inside of the rim and what goes on the outside. This is Dual Plane balancing of a tire.
The crankshaft of an engine has counterweights to dynamically offset the effect that the movement of the reciprocating mass has on the rotating mass of the crankshaft. The reciprocating mass is a percentage of the total mass of the top half of the connecting rod, the piston, wristpin, rings and circlips. The percentage used comes from a chart, which is calculated from expected RPM and stroke length. The rotating mass is the total mass of the bottom half of the connecting rod, the rod bolts, washers and bearings. To balance a single cylinder crankshaft, a bob weight is attached to the journal of the crankshaft that represents 100 % of the rotating mass and the percentage of the reciprocating mass from the chart. This assembly is then placed on the ball bearing V’s of our Stewart-Warner crankshaft-balancing machine and spun up to the expected operating RPM setting. The balancer can read the change in weight on both sides of the crankshaft and precisely tell the operator the amount of weight that needs to be added or removed from the left or right counterweights to achieve optimum dual plane balancing.
Balancing a crankshaft in itself does not produce more horsepower; it improves output potential by eliminating or greatly reducing a non-productive use of horsepower. It also helps prolong the life of an engine by reducing damaging vibration. In any situation where some grinding of the crankshaft or modification of the piston is permitted, it is a largely untapped source of performance gain.