Racecar Science

Jan. 1, 2020
Racing is art, because every race is different. Building a racecar is science, because the cars can be built exactly the same way time after time.

Racing technology is driven by NASCAR's Car of Tomorrow

Racing is art, because every race is different. Building a racecar is science, because the cars can be built exactly the same way time after time. This is especially true in NASCAR's premier Cup Series, where some teams build 50 cars for one season.

Though teams constantly are searching for even the smallest technological advantage, everyone is working with the same rulebook, so they tend to use the same development techniques. Over the years, a huge industry has grown to provide all the development tools and services any team could possibly want. Last year the crew chiefs could duplicate a successful setup on almost any chassis the team built.

But this year, things are different.

This is a watershed year for NASCAR racing teams, because The Car of Tomorrow has become the car of today. Although each team has some experience with the new chassis, even those who started working with it five years ago still are struggling to match the predictability and performance of the old chassis. This is partly because NASCAR keeps "adjusting" the specifications, which is not particularly uncommon. But these adjustments are in response to feedback from the track rather than in response to teams taking advantage of "unregulated technologies," which is very common. In any case, it shows that NASCAR is struggling with the new chassis, too.

The only differences allowed (other than adjustment settings) are bolt-on aerodynamic body pieces, and this is where the greatest development challenges — and the science behind them — are today.

When the cars had different body shapes (Dodge, Ford, Chevrolet, etc.), teams could exploit those differences to gain small but significant aerodynamic advantages. They developed a set-up database for each track that could be repeated the following year.

But the new cars are almost identical, and every team is starting from scratch with a new set of aerodynamics. The front bumper is now the same height as the rear, and it has an air splitter that increases downforce on the front of the car. Rear downforce is increased using a wing instead of the old car's spoiler, and there are endplates on the wing to generate side forces. The splitter and wing are adjustable, and the end plates come in different shapes, but NASCAR will issue the wing to each team at the racetrack. The bottom line is these cars have a lot more downforce available and a lot more aerodynamic adjustability.

A Seven-Post Solution

As the racecar's speed increases, aerodynamic downforce effectively increases the car's weight, increasing the tire's grip in a turn. There's also a drag penalty, so the engineers need to figure out the best balance of drag and downforce. Since NASCAR places severe limits on actual track testing, race teams have found another way to set up a chassis. (See sidebar.)

A few years ago, NASCAR teams discovered a tool that open-wheeled racers have been using for decades: the seven-post shaker. Like the four-post shaker auto manufacturers use to develop production cars, each wheel sits on a pad mounted on a hydraulic ram that can be cycled at varying frequencies and amplitudes. The four-post shaker is used to test for cabin noise, but the seven-post shaker measures forces generated at the wheels using load cells mounted on the pads. These load cell sensors are the key, because every force imparted to the chassis shows up at the tire contact patch. With three more rams (posts) connected to the chassis, those forces can all be simulated on a seven-post shaker: acceleration, braking, weight transfer, downforce and side forces, even shock loads from hitting bumps in the pavement.

The data from this tool is used to figure out how to keep the contact patch firmly on the pavement at all times. The computer that controls the rams can be programmed to produce specific forces, or it can run software gathered from instrumented cars driving on actual racetracks, even through different lines on the track. For instance, it's possible to learn exactly how the tire contact patches distort when the driver keeps the car down low at the entrance to turn four at Daytona.

This may be the most complex (and expensive) tool in motor racing, and the teams using it say it takes a year or two to learn how to interpret the data. Today, only a few of the biggest NASCAR teams own a seven-post shaker, but they've proven the tool's advantage in chassis development. Other teams have begun renting time on the few commercial seven-post rigs around the country. If access to this tool is required to be competitive, then NASCAR's goal of reducing the cost of competition may be in jeopardy. But the setup that comes from the tool's measurements can be repeated in every chassis a team builds.

K&C Rigs

There is a newer tool for off-track chassis development, a special rig for studying Kinematics and Compliance, otherwise known as K&C. This refers to how the suspension moves through its normal stroke (kinematics) and to the stiffness of the springs, bushings and other parts that deform and return to their original shape (compliance), including the tires. If the seven-post shaker can be thought of as a tool for studying a chassis' dynamic behavior, the K&C rig might be thought of as a tool for creating a base line displacement/pressure map as the suspension moves through its arc.

A simple pull-down rig maps suspension displacement as the chassis is pulled down. On a K&C rig, the chassis is held firm as hydraulic rams move up to compress the suspension, which simulates cornering forces at the tires. The wheels rest on six-axis load cells similar to those on a shaker rig, and as they measure forces at the contact patch, optical sensors measure suspension displacement. The result is a pressure map of the contact patch at all suspension displacements.

The goal is to learn how weight transfer and suspension settings affect the contact patch as the suspension moves and vice versa. This lets the crew chief develop a basic setup for that chassis before the car goes to the track.

Looking Through Wind Tunnel Vision

On any racecar with adjustable aerodynamics, the goal of wind tunnel testing is to minimize drag and maximize downforce. In this particular kind of racing, the task is complicated by the way they race; often two cars in trail are only inches apart, and the resulting "dirty" airflow reduces downforce on the rear tires of the leading car. We've seen this happen when the lead car's rear tires lose grip and the tail swings out during a turn, even though there's been no contact. The new cars' wings and splitters mean aerodynamics plays a much bigger role in the car's handling, so NASCAR teams will be spending lots of time at the wind tunnel.

There are only a few wind tunnels around the country big enough to test a full-size car at simulated racing speeds. One of them is owned by Lockheed Martin in Marietta, Ga., and NASCAR teams have been renting time there since 1968. NASCAR itself often uses this tunnel for post-race inspections. It's a closed-loop system, meaning the air is recirculated, so big heat exchangers are needed to maintain an air temperature of 75 degrees Fahrenheit. The 9,000-hp electric motor turns a 39-foot fan at only 250 rpm, but it can generate wind speeds of 200 mph in the test chamber. The car rests on a platform with sensors that measure, in three dimensions, how aerodynamic forces on the car show up at the tire contact patch. Another aircraft wind tunnel in Langley, Va., has a test chamber large enough for three cars, and it's used to study the effects of "dirty air."

Two new wind tunnels are under construction near Charlotte, N.C., that are specifically designed for full-scale automotive testing, and they boast the most advanced capability in the industry: the rolling road. When a car sits on a wind tunnel measuring platform, air moving under the car creates what's known as a boundary layer, a layer of static air that doesn't exist in real life as the car moves over the road. With the car on a moving belt, that boundary layer is removed and the measurements are far more accurate.

A rolling road also shows the aerodynamic and gyroscopic influences of tire rotation. The WindShear, Inc. rolling road wind tunnel, owned in partnership by Jacobs Engineering and Hass CNC Racing, has a belt that looks a little like a giant inverted belt sander. The stainless steel belt is 1 mm thick (0.39 inch) and can reach speeds of 180 mph, and it's mounted on a turntable that can yaw (rotate) ±8 degrees. Sensors under the belt can measure pressure at each contact patch, and the belt can accelerate from zero to maximum speed in less than one minute.

Wind Tunnel eXtreme, another rolling road wind tunnel in Salisbury, N.C., is under construction now and scheduled to open in late 2009.

There Is No Finish Line

NASCAR has two main objectives for this new chassis: increase driver safety and reduce the cost of competition. Safety has been improved by enlarging, strengthening and padding the driver's cage, and by moving it back and closer to the center of the car to increase the crumple zone. The fuel cell has been relocated, the bumpers are all the same height and the exhaust system has been rerouted to prevent hotspots in the floor pan. On the old cars, drivers actually have suffered second-degree burns to their feet during a race.

Reducing the cost of a racecar will make it possible for smaller teams to participate, which should make racing more competitive. The cost will (eventually) be reduced because teams will need to develop only one racecar. In the past, different chassis/body sets were built for different tracks — short tracks, super-speedways and even specific tracks. Under the new rules, every team will use the same NASCAR-spec chassis and bodywork for every race at every track, even for road racing.

NASCAR has been publicly criticized for racing with "outdated" automotive technologies like carburetors, solid rear axles and steel tube construction. But off the track, the level of science and technology that goes into developing these racecars is nothing short of state-of-the-art. The Car of Tomorrow may still have a carburetor, but it's driving the science of building racecars into new territory. And this is just the beginning.

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