Let's review a bit of physics in preparation for Part 2 of our technology transfer series on page 28. All elements in a rotating body have a measure of kinetic energy. Simply stated, this is "energy in motion." If you place a weight on a table, relative to some distance from the floor, it exhibits "potential" energy as a function of the object's weight and height above the floor. At this point, potential energy is at a maximum, kinetic (moving) energy is zero.
The moment you push the weight off the table, it assumes motion (velocity), kinetic energy begins to increase and potential energy starts to decrease. At the weight's maximum velocity (just before it hits the floor), kinetic energy is at a maximum and potential energy a minimum. Upon contact with the floor when velocity becomes zero, so does its kinetic energy. Mathematically, k.e. is equal to one-half the product of an object's weight (mass) and the square of its velocity. Now, let's relate this to rotational moments of inertia (MOI) and crankshafts
Visualize a spinning crankshaft, acting about its rotational axis. At any point on the crankshaft, not located on its axis of rotation but some distance from this axis, we can define the speed of that point. Actually, it's the product of the point's distance from the crank axis times the angular speed of the point. The farther such a point (or points) is located from the axis, the greater its speed (velocity). We can thus compute the total kinetic energy of the crankshaft (assuming it is relatively rigid) as the sum of the kinetic energies of all points on the crank.
Rather than provide a mathematical equation that describes these relationships, suffice it to say that the crankshaft's total kinetic energy would be the cumulative sum of the kinetic energies of all points on the crank. This summation (sum of the products of the masses at each point location by the square of their respective distances from the axis of rotation) is defined as the "rotational inertia" or MOI of the crankshaft about its axis of rotation.
There's one more term we need to introduce: "translational inertia" as applied to a crankshaft. Since crankshafts are required to accelerate and decelerate, we need to consider the amount of power required (or absorbed) in so doing. From a practical standpoint, let's compare two crankshafts of equal mass but with one having a larger overall rotational radius; e.g., its weight distribution involves points located farther from the rotational axis than the other.
Remember, we said that the velocity of points located some distance from the axis is the product of angular velocity times distance. Then, in the kinetic energy equation, we also said that this velocity becomes squared. So even if these two cranks have the same weight at points located a different distance from the rotational axis, those farther away will have a higher MOI. That is to say, they'll require more power to accelerate and absorb more power during deceleration. So, for example, trimming weight from a crankshaft's counterweights reduces its overall MOI, not so much because the crankshaft became lighter, but less mass was located from the axis of rotation
In a sense, you can think of a spinning crankshaft as a flywheel. It requires energy to put into and maintain rotational speed, and it stores energy that must be overcome during times when the throttle is partially or completely closed. If you don't think this is an important aspect of making laps, we refer you back to Part 1 in the February issue that showed throttle on/off times at the Lowes Motor Speedway. Simply put, it's a significant element to good track times.