Optimizing track day performance through car weight reduction.
For automotive engineers concerned with efficiency, reducing mass in road cars has become an obsession. Racers have always understood the advantages of a lighter car. Yes, taking mass out of a car is an easy way of improving acceleration without adding power, but it will also improve braking and handling. Shedding pounds can also improve reliability, and even if your car is already at its minimum weight for its class, you can still see an increase in performance by removing weight in one area and adding it back in another.
Just like your New Year’s diet, the first few pounds are going to be relatively quick and painless, but the more you want to lose, the harder it gets. Car weight reduction isn’t just for pros either. Everyone from a weekend warrior to the very top echelons of motorsports can benefit from counting ounces.
Mass vs Weight: What’s the Difference?
First, weight and mass are not the same thing.
Mass is a measure of the amount of matter in an object. In your garage, on the Moon, in the middle of deep space, the mass of an object does not change. Weight is the force of gravity on an object’s mass. That means, a car that weighs 2,500 pounds in the garage will only weigh 415 pounds on the Moon and will weigh basically nothing in deep space.
So, is your car faster on the Moon? No, because it has the same amount of mass, so it will have the same amount of inertia to overcome. (Also, in 1/6th lunar gravity, you will have 1/6th of the grip you would have on Earth.)
With that said, the terms weight and mass will be used mostly interchangeably in this piece as most of us are involved in racing that takes place in the same gravity well.
Different Kinds of Weight
Taking weight off a car is always a good thing – assuming it doesn’t decrease the function of a necessary system. But some weight is more advantageous than others.
Most people have heard of unsprung and rotating mass (both examples of weight with bonus points). Fewer people consider a car’s center of gravity or polar moment of inertia when decreasing weight.
Want to learn about the nitty gritty of car weight reduction? Let’s look at some definitions:
Unsprung Mass. This is any part of the car not supported by the car’s springs like tires, wheels, brakes, hubs, spindles, etc. The actual suspension components, like control arms and dampers, are considered half- or semi-sprung as some of that mass is supported on the chassis side. The ratio of unsprung to sprung mass is part of what determines how hard your suspension has to work to absorb bumps. Less unsprung mass means it is easier to keep tires in contact with the road.
Rotating Mass. This refers to any part of the car that is spinning around an axis – AND that spinning is relative to the movement of the car. From the wheels to the crankshaft, camshaft, alternator? Yes. The electric fan? No. Not only do you have to accelerate the total mass of a wheel with the rest of the car, you have to spin it up to speed. That equation includes a squared factor for the radius, so it adds up quick. Now think about everything from the input shaft in the transmission and back (by back, we mean towards the engine, opposite to how the power travels to the road). All of that mass has to accelerate through each gear.
Center of Gravity (CG). This is the theoretical point where the sum of all masses exists. If you know an object’s CG, you can balance it in any orientation as long as you support it inline between that point and the force of gravity. It’s also the point we use to calculate the effects of forces from acceleration. The higher your car’s CG, the more leverage it has relative to the tires, so lower is better.
Polar Moment of Inertia. This refers to an object’s distribution of mass. Think of a barbell with 45-pound plates at both ends. It isn’t easy to spin that around an axis perpendicular to the length of the shaft. That is a high polar moment of inertia. Now, slide those weights all the way into the center of the bar. Although it weighs the same, it’s much easier to get it spinning. A car is the same, the more centralized the weight, the easier it is to turn.
Weight Loss Stages
Whether you’re autocrossing or heading out to a road course, consider taking some of these steps to remove weight from your car. They won’t magically turn your Corolla into a Carrera, but with some tailored changes, you can pick up performance in all areas while also doing what you can to reduce wear and tear on your car.
Beat the Bloat
Start with the obvious stuff. If you have any items in your car that aren’t necessary, get them out.
This is as important for safety as it is for weight reduction. Get rid of the floor mats, everything in cupholders, storage cubbies, glove box, and trunk. Next, your spare tire and jack can sit in the pits while you’re driving.
All of this is the first noticeable chunk of weight you’ll remove. Your spare tire and jack are normally pretty far back in the car, so points for lowering your polar moment of inertia, but your mats and stuff in the cupholders are either below or near your CG, and low, so no bonus there.
Next up, think about weight that may be bolted down, but still not a necessity. Your backseat might be relatively simple to remove and could represent a decent chunk of weight. One caveat, however: if you have a seat back that doesn’t fold down or is held in with huge latches, it may have a structural function and might be worth keeping in the car for handling purposes.
Next, we’re not saying you should drain your washer fluid, but maybe don’t top it up right before race day. Gas tanks are a point of contention amongst amateur racers. Some swear by ‘the lighter the fuel load the better’, while others want a full tank to stop the fuel sloshing around which makes the car less stable. You could pull out the passenger seat also, but if you’re a beginner, an instructor or driver coach in that seat can be the most valuable weight in the car.
A Note on Daily Drivers: If you’re racing a car that you also drive on the street, then it’s important to decide just how serious you want to get about weight reduction. Some upgrades won’t ruin the drivability of your car, and others may even improve it. Either way, check the rules to see if these modifications will bump you up a class.
Make Some Lifestyle Changes
One of the biggest changes you can make is wheels and tires. Yes, they are both unsprung and rotating mass, but more importantly, having a dedicated set of track tires and wheels is life-changing. You can run a semi-slick tire for track events (which will be one of the biggest performance upgrades you can make); you can buy wheels that are considerably lighter than stock; and you won’t have to drive to work on tires with worn-out shoulders or giant flat spots from a big spin.
Car batteries are heavy. You can buy a lightweight one and while you’re at it, move it to a better location. You want it lower and more centralized. Most front-wheel drive – i.e., nose-heavy cars – will benefit from moving it to the trunk. Something like a lithium battery from XS can sometimes drop as much as 40 pounds.
Some race classes will let you switch out a cat-back exhaust without penalty. One of the reasons is that you aren’t likely to see big power gains. However, what you may see is a loss of 50 or 60 pounds when taking out a heavy OE exhaust system and using something from Borla or Corsa, which can be a third of the weight. The weight is coming off the lowest point on your car, but you will sound way faster.
This is a judgment call, but car stereos are generally pretty heavy. Some are tied into other systems of the car and can’t be removed. If it isn’t essential to the car’s operation or the operator of the car though, you can save anywhere from 30 to over 100 pounds by removing the head unit, amps, speakers, and subwoofer.
For a daily driver, you may not want to sacrifice too much more in terms of removing parts. But if you have a car that is popular amongst racers and performance enthusiasts, you’ll likely be able to find lightweight versions of some components. First, lightweight clutch and flywheel kits are common for most cars and save rotating mass. Lightweight starters and alternators are also fairly common. Some parts will have a bigger percentage of weight than others, but if you’re replacing something and you have the choice, all the pounds will eventually add up.
Getting Serious
Building a dedicated race car is outside the scope of this discussion, but you may get to a point where racing is the car’s primary use, but you drive it to and from events, so it needs to stay street-legal. You can still get pretty serious about weight reduction on a “streetable” car.
Removing more non-functional mass will be step one. Interior components like trim panels and the headliner are easy targets. You may want to fabricate new door panels from aluminum just so you have a place to mount door handles and window switches, but they will likely save several pounds each. Removing pillar trims will likely also result in removing airbags, so give that some thought before pulling the trigger. The same with seats. Some passenger car seats can weigh 100+ pounds each. Lightweight one-piece racing seats can weigh as little as 15 pounds, and offer far more support and a better connection to the car.
If you have a sunroof, especially a big glass panel, removing it and the sliding mechanism can be a huge weight savings at the car’s highest point. There are composite panels available off the shelf for popular cars, otherwise some fabrication may be necessary.
Replacing steel body panels with fiberglass or carbon fiber can provide big weight savings, but hardly ever fit as well as factory and the price can get pretty high for quality components (especially if you want them painted by a pro). The same is true for swapping out glass windows for plastic. You have to decide if the performance advantage is worth the trade-off in durability – and sometimes, some level of body rigidity.
Racing Is A Game of Percentages
All things being equal, a lighter car performs better in every metric. How much better is hard to estimate.
In acceleration, power-to-weight ratio is pretty straightforward and there are several calculators online that allow you to input your data and estimate everything from quarter-mile ET to trap speed. These will allow you to virtually shave off weight and see the results.
Handling is far more difficult to estimate. By now, you know the location of the weight might be nearly as important as the amount. For this, real-world testing is about the only option for amateur racers. The takeaway is – if you can remove weight without harming another area of performance, then you will see performance gains.