The skinny on rotating weight

By Mike and Tex Powell, Circle Track Magazine
September 13, 2001
12:03 PM EDT (1603 GMT)

Rotating weight is mass. It is basically what the engine has to accelerate. Race engines are not only measured in power, but also the speed at which they accelerate.

Sometimes, that is not always sheer horsepower being used at its best. One reason why, is rotating weight. Obviously, the amount of work an engine has to accomplish from one corner to the next depends on the weight of the parts involved. The lighter the parts it has to turn to get going, the quicker the car will get to the other end.

That is why rotating weight is an issue. Rotating weight includes anything that turns -- not only the parts of the engine itself, but the flywheel, transmission, propeller shaft, wheels, and even the tires.

Many people have come up with formulas to figure out rotating weight and its advantages. There was an old formula when I grew up that stated, "one pound of rotating weight is worth taking 10 pounds of static weight off your automobile." Unfortunately, it is not that simple. But, it is not a bad way of challenging yourself either.

The pursuit of rotating weight does have a value far beyond static weight -- though static weight is important as well. Many times people ask, "Why does static weight matter on a race car?"

The best explanation is very simple. You'll have a minimum weight you have to meet at the track, but if your car is light, then the ballast, or the weight you put back in to meet minimum weight, can be placed anywhere. Most times it is very low or in the corners.

There are certainly good reasons for building a lightweight car. However, the advantages of rotating weight are greater.

Rotating weight is most important starting at the front with the flywheel. It has the largest diameter -- more than 12 inches. Different sanctioning bodies have different rules on their clutches, so there is not a lot you can do there.

Sanctioning bodies know that the flywheel is most important, which means they tend to be the most specific on its use. Components are usually less important toward the rear of the car, the difference being, if we move to the back of the car, the ring gear and the axles are turning much slower.

This is where the math tends to get confusing. If you can find the biggest component on the axle and lighten it up, since it turns slower, it could possibly be in a more useful range. The simple rule of thumb is to always go by diameter: start reducing weight with the largest part, such as the flywheel.

A Tex Racing T101 transmission on the dyno is being checked for noise level and leaks.

We have certain main shafts that we will drill out. Prioritize that last. It is so close to center that you really have not accomplished much in terms of reducing weight for advantageous purposes.

However, after you do everything you can on the larger diameter parts, you will begin to do things that have less of a clear advantage because you have already worked on the larger pieces. Usually this is what happens with a race car -- you have worked your way from the big pieces back to smaller ones.

What is interesting about a race car is there are always some rules or some boundaries regarding driveshafts, flywheels, and transmissions.

With these rules, we have to be very clever in our modifications and disciplined with our money. In other words, we have made it clear to our customers that it would be foolish to try to buck that rule for the tiniest of advantages it would offer, or even be thrown out after a good finish. It is not for us to decide where a person should bend the rules.

To me, one of the greatest challenges in life is to take almost the same thing someone else has and try to beat them with it.

That is essentially the cornerstone of our sport. Just as a pinewood derby car is to a Boy Scout, we make something within the rules like someone else's and beat them with it. The rewards for that are why a lot of us race.

This nine-inch ring gear has been drastically lightened by using a wire EDM process. This process can shave off almost eight pounds and costs around $75.

When speaking of deceleration from a weight standpoint, common sense would have it that if it has lighter rotating weight it will likely stop quicker. In other words, the car will accelerate and stop quicker.

A good example of this is a freight car. It is very difficult to get the train car moving, however, once it starts to roll it is hard to stop. That is simple physics -- once an object is in motion, it tends to remain in motion.

These simple physics can be applied to a race car as well. If it is hard to get the car going due to heavy parts, it will be hard to stop the car. The lighter you make a flywheel, the quicker it accelerates and the quicker it decelerates.

When discussing the quickness of deceleration, there are some variables that come into play, such as what type of track you are running and how much brake you use. If you have a car that accelerates faster than another and it decelerates faster by means of a lightweight rotating part, you could use smaller brakes on it because of the rotating weight being less.

With smaller brakes, you are dealing with a component that is easier to manage, generates less heat, and you can put the weight from those brakes somewhere else. This is just another example of how the uses of rotating weight never end.

The ring gear shown here has been lightened by several pounds. Powell claims that going this route will give you the most bang for your buck.

Rotating weight is usually a lot more simple for short-track cars. With bigger tracks like Daytona, rotating weight becomes more complex because the rpm ranges, the miles per hour, and the amount the cars slow in the turns is different.

If you run on a 3-8 or half-mile mile track, the lighter you make it turn, the better it is going to be. It is not debatable -- when you run on a bigger track, things tend to get more complicated. If you take a car to Daytona, the rpms barely fall in the corner.

You have some adhesion on tracks like this when the car gets in the corner. This is just one thing that scrubs off speed. You could literally reach a spot at Daytona where you want some heavier parts to keep your momentum going, but that is a very isolated situation.

To qualify, you have to build up speed faster. Many times teams will build a really light car so that it can achieve speeds quickly in order to qualify well.

Ring gears, such as the one shown, are now available "pre-lightened" by the manufacturer.

This will hold true for a short track car as well. When you go up for your qualifying laps you're sitting at an idle, you pull out on the track, and you have to achieve speed quickly. A car that gets to the first corner faster should have a better chance of getting to the next corner faster.

What I mean by that is when you leave pit road to qualify, you reach Turn 3 and Turn 4 still trying to achieve top qualifying speed.

By the time you reach Turn 1, you want to be at optimum speed. I believe the faster a driver gets to Turn 3 and 4, the more focused he is and prepared to begin putting Turn 1 and Turn 2 together on his timed lap.

All of this still has to do with the rotating weight. With lighter rotating components, the car is able to get up to speed at a faster rate.

Cut it where you can. Notice the transmission slider in the front has been lightened using a lathe.

It is important to reduce weight from the front to back. In a recent article I read, an engineering professor states that "it is much more effective to reduce the inertia of parts that spin fast, such as the engine, than it is to reduce the inertia of parts that spin more slowly, such as the rearend."

From a numeric sense, the professor covers this topic very well. From a diameter standpoint, moving from front to back more or less dwindles in diameter.

The flywheel is the biggest part, with the transmission being the next biggest component, then the driveshaft or propeller shaft, and finally the axles are the smallest. Overall, you are dwindling in size. However, there is an exception to this with the ring gear.

With the ring gear you all of a sudden encounter one large-diameter part. We believe you should always lighten the ring gear. Tool-wise and dollar-wise, it is one of the easiest parts to reduce weight.

With some of our ring gears we take five to eight pounds out for $75. Some companies are offering ring gears that have already been reduced. From a business standpoint, whether it is a business or a team owner, I think this is a good way of prioritizing.

Your typical steel reverse gear?only it has been on a diet!

Sometimes you have to say, "How many dollars a pound is this?" The gun-drilled main shaft is one that costs the most per pound to remove. Again, it is close to the center, so it would not be the highest priority on your list to lighten.

Always remember that there are things that you don't want to defy in the pursuit of weight. We should challenge ourselves to not make a mechanical mistake. I am not talking about breakage either. I am talking about a strain, or a power loss, or a vibration.

he driveshaft is always a good example for this topic. There is a specific diameter a car's driveshaft should be. The science and math of what size shaft, in diameter, it takes to do the job is readily available.

As you know, the driveshaft in any rear-wheel-drive car hooks the transmission to the rearend. The rpm you operate that shaft at, the length that it must be, and the wall thickness are a given. The wheelbase of your car is a given as well.

You also need to know the optimal rpm at which your engine will run. There is a given diameter of driveshaft that is required to meet the demands of your specific rpm and length. To defy that would be a bad plan.

The science and math that says how big the driveshaft should be was developed to keep that shaft from vibrating. A larger piece of tubing is stronger torsionally than a smaller piece. There have been many racers who have installed a driveshaft that is too small in the pursuit of smaller diameter parts.

From a speed standpoint, say the driveshaft is too small because a guy wanted to put a real light one in the pursuit of rotating weight. He defied the arithmetic. That driveshaft is now quivering, vibrating, and jumping like a rope.

Friction can drastically slow your car down. The top main shaft features a bearing kit that has been shown to reduce friction and allow the gear box to accelerate faster in all gears.

If he made a mistake with the driveshaft, such as being too small, he won't get as much power to the wheels. Power to the wheels is always very important. The more power you get back to those tires when they are in good condition, the faster that car will go.

When the driveshaft is too small and it quivers, it tears up parts. The parts are vibrating because they are not big enough to do their jobs. Those vibrations are a type that many times a human can't even feel.

The vibrations will literally gobble up horsepower, and the rear wheels never see it. If a person attempts to defy this math by installing a driveshaft that is too small, he has really shot himself in the foot.

If you have a part that you decide is too big and you're going to lighten it up a bit, there will be a point that it will break. When we, as a company, lighten something up, we are really quick to try to set parameters of when we think it will need to be replaced before it fails.

Some of our gears run 30,000 miles before we replace them. Other lighter versions have to be replaced after only 900 miles. The larger or longer the part, the more dangerous it can be. The manufacturers are doing a good job of building flywheels.

Breakages is not as much a part of racing as it was years ago. We have to be smart with flywheels and driveshafts. If you fail you don't finish. Ultimately, racing is about going fast -- you don't want to forget about that. Remember, you can go too far and slow down.

For more stories from Circle Track Magazine, visit the Circle Track Web site at www.CircleTrack.com.