## When a Pound weighs more than a Pound

Posted by Kevin Gertgen in Circle Track / Road Racing, Drag Racing, Engine Building, Street Performance, tags: Acceleration, Rotating Inertia, Vehicle Performance, Vehicle WeightSir Issac Newton in the 1700s developed the most important equation in all physics and dynamics, **Force = Mass x Acceleration**. This equation can be rearranged to read **Acceleration = Force / Mass** . Now, as racers it’s all about maximizing acceleration. So to get higher acceleration, this equation says you can either increase the force or decrease the mass (weight). That’s where the term “power to weight ratio” comes from. The higher the power to weight ratio, the quicker the acceleration and the better the vehicle’s performance, and the bigger smile on your face.

Most all racers accept this as common sense, and know they want to keep the vehicle weight as low as the rules will allow. However, many racers may not realize that not all pounds of weight reduction are equal. Removing weight from rotating components like the wheels and tires is more effective than removing weight from the frame. That means that a 10 lb weight reduction at the wheels and tires may be the same as a 11 lb weight reduction on the body. That’s because not only do the tires have to accelerate down the track in a straight line from, say, 30 MPH to 60 MPH, but they also have to accelerate in a rotational sense from, say, 300 RPM up to 600 RPM.

When you look at other rotating components, like the engine, it may have to accelerate from 3000 RPM up to 6000 RPM, 10 time the rate of the wheels. A 10 pound of weight reduction on the engine’s flywheel, could be as significant as a 15 pound reduction on the body, depending on things like gear ratio and flywheel diameter.

How the weights of different vehicle components affect the effective weight of the vehicle is something our Drag Race Analyzer and Circle Track Analyzer programs calculate “behind the scenes”. Then we just show you the total effect on performance, but we don’t really break it down component by component. For that, you can use our Rotating Inertia Calculator.

Pictured below is the main screen for the Rotating Inertia Calculator, showing many rotating components on your vehicle, and how the weight and diameter (which calculates out to a rotating inertia) contribute to the total effective weight of the vehicle. This vehicle weights 3500 lbs on a scale, but has an effective weight of 3875 lbs. That means the effect of the rotating inertia of the components has added about an extra 375 lbs of “imaginary weight” to the car. (Click on the image to enlarge it.)

Pictured below is the Rotating Inertia Calculator’s split screen which lets you compare a baseline, starting condition at the top with your modified vehicle on the bottom. (Click the image to enlarge it.) I’ve made 2 changes, one lightening the flywheel 11 lbs and one lightening the front tires 6.8 lbs each. (The program knows there are 2 front tires for this vehicle.)

The options in the program were set to keep the total vehicle weight constant. That means as you reduce weight to rotating components, the program adds that weight to the body to keep total vehicle weight constant. This way you can see the effect of just reducing the rotating inertia and NOT reducing the vehicle weight.

Figure 2 shows that the vehicle weight did not change, but the weight of the rotating components were reduced by around 25 lbs. Howver the effective weight of the vehicle was reduced by 69 lbs down to 307 lbs, which produced a 1.76% reduction in total effective vehicle weight. We did not change the vehicle weight at all, but produced an effective weight reduction of 69 lbs or 1.76%.

Now how this all shakes out for your car changes significantly depending on your car’s gear ratios, vehicle weight, and size and weight of its rotating component. But, the Rotating Inertia Calculator does show you, **some pounds DO weight more than a pound.**