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Rotating Weight, Moment of Inertia & Reality

How important is rotating weight? How important is horsepower? If you feel horsepower is important, then rotating weight is equally important, if not more. Your engine’s horsepower (or torque – horsepower is a mathematical factor of torque) carries your racecar’s total physical weight to its maximum velocity. The time it takes your racecar to reach that maximum velocity is the acceleration time. Most short track racecars spend a much larger portion of their total lap time accelerating to maximum velocity than running at maximum velocity.

One lap around the track can be divided into 3 functions with 6 individual segments.

Function 1: Acceleration
Function 2: Maximum Velocity
Function 3: Deceleration

These three functions are repeated once each lap. Let’s assume that acceleration consumes 50% of the overall lap, 25% each half. Maximum velocity consumes 30% and deceleration consumes 20%.

If more of your lap time is spent accelerating, then increasing your acceleration speed will decrease your acceleration time, thus dropping your overall lap time. Using this principal, a car with equal handling, less horsepower, yet faster acceleration will produce lower lap times. Faster acceleration allows the car to reach maximum velocity quicker, which could lead to a higher final drive selection, which will allow a lower horsepower car to run at a maximum velocity equal to a higher horsepower car with a lower final drive.

Enter Mass Moment of Inertia, or MOI. MOI is a calculation of an objects resistance to rotate. An object with a lower MOI, will accelerate to it’s maximum velocity faster. MOI is not just about weight, it is about the balance of weight and diameter. An object with a heavier weight and lower diameter can accelerate faster than an object with much less weight, yet larger diameter. MOI is measured by figuring how far the weight is from the axis of rotation. An object with equal diameter, yet double the weight, doubles the MOI. However, an object with equal weight and double the diameter increases MOI by 4 times!

If you reduce the rotating MOI of a racecar, specifically in the driveline, it will accelerate faster. At the same time it will decelerate faster, MOI works both ways. They key is to focus on MOI, not just weight.

Now reality. You can analyze every component of the driveline until your head spins, yet if you don’t have an overall components and set-up package, you’re not going to win. Use common sense, focus first on the largest diameter objects in your driveline. Flywheel, Flexplate, Clutch, Differential, Ring Gear, Brake Discs, Hubs & Wheels. Try to reduce the diameter first, then the weight of these objects.

When you are making decisions regarding the components you put on your racecar, don’t just assume if a part works, it is just as good as another. More races are won during the construction process than you may think!