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The Caterpillar-sponsored No. 22 car. See more NASCAR pictures.

Photo courtesy Caterpillar

Introduction to How NASCAR Race Cars Work

In the beginning, stock-car racing was exactly what it sounds like. Drivers actually bought brand-new cars from dealers and went racing. The National Association for Stock Car Auto Racing (NASCAR), organized in 1947, created a standardized set of rules for stock-car racing and established a system for selecting a national champion based on performance at races across the country.

The original races were run on dirt tracks that got rutted and bumpy. The unmodified cars were not tough enough for this type of abuse, so NASCAR began allowing modifications to the stock cars to increase their durability. Over the years, more and more modifications were made, sometimes to increase safety (see How NASCAR Safety Works for details) and sometimes to improve competition. NASCAR strictly controls all of these modifications, which are spelled out in detail in the NASCAR rule book. Cars are checked for compliance with these rules at every race.

Today, NASCAR race cars have very little in common with street cars. Almost every detail of a NASCAR car is handmade. The bodies are built from flat sheet metal, the engines are assembled from a bare block and the frame is constructed from steel tubing.

In this article, we'll see how these race cars are made, starting with a component that is key to the drivers' safety and provides the foundation for everything on the car: the frame.

The frame of a NASCAR race car before the body is installed

The Frame

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To explore how NASCAR race cars are built, we visited Bill Davis Racing in High Point, NC.

This shop buys its frames prefabricated from a frame supplier. The frame consists of a structure of round and square steel tubing of varying thickness. The bulk of the structure surrounds the driver. This part of the frame -- the roll cage -- is made of the thickest tubing and is designed to stay together, protecting the driver during any type of crash (see How NASCAR Safety Works for details).

The front and rear sections of the frame, called the front clip and the rear clip, are built from thinner steel tubing so that they will crush when the car hits another car or a wall. In addition to being collapsible, the front clip is designed to push the engine out of the bottom of the car -- rather than into the driver's compartment -- during an accident.

When the frame comes into the shop, the firewall (the metal panel separating the engine compartment from the driver's compartment) and floor panels are welded in, along with various mounting brackets for things like the engine, suspension, seat, fuel cell and body.

The next step is building the body and installing it on the frame. This process is amazing -- almost every part of the body is made by hand from flat sheet metal.

Some of the 30 templates used to specify the shape of the body

The Body

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The process of making the body for a NASCAR race car is incredibly labor-intensive. It takes the shop 10 working days to make and install the bo­dy for just one of these cars.

The shape of the car is mostly determined by NASCAR rules. These rules are encapsulated in a set of 30 templates, each shaped to fit a different contour of the car. For instance, the biggest template fits over the center of the car from front to back. When the template is laid on the car, the gap between the template and the car cannot exceed the specified tolerance. Each template is marked on its edge with a colored line. If the line is red, then the gap must be less than 0.07 inches (0.18 cm). If the line is blue, the gap must be less than 0.25 inches (0.64 cm). If the line is green, the gap must be less than 0.5 inches (1.27 cm).

The templates actually allow a little leeway in the design of the car. Because 30 templates are not enough to cover every inch of the body, some areas between template locations are not strictly controlled by NASCAR.

The construction of one of these cars has nothing in common with how a street car is made. With the exception of the roof, hood and deck lid (which are supplied by Dodge), all of the body panels are made by trimming and then hand-rolling flat sheet metal between the rollers of an English wheel, which slowly bends and curves the metal until the contour matches the templates and fits on the car.

An English wheel is used to shape the flat sheet metal into curved body panels.

After the pieces are shaped, they are welded to the car and to each other, using the templates to check their location. The seams between pieces are welded and then ground down so that when the car is finished, it is one smooth, seamless piece. The doors don't even open.

A body that is almost ready to be painted

After the car body is installed and ground smooth, the car is primed and painted. All of the decals are installed, including headlight decals (NASCAR cars don't have headlights), which helps make the race car look more like a street car.

Not all of the cars are built to the same specifications. Some cars are dedicated short-track cars, and others are dedicated super-speedway cars. There are some major differences between the two types.

Short-track car

The Right Body for the Track

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NASCAR teams build two types of cars. They build cars for the short tracks, like Bristol Motor Speedway in Tennessee, where top speeds are lower and turns are tighter. They also build cars for the super-speedways, like Talladega in Alabama, where top speeds are higher but engine power is limited.

Short-track Cars

The goal in designing a short-track car is to create as much downforce as possible. Downforce is an aerodynamic force that tends to press the cars tightly against the ground, allowing the tires to grip the track with more force. This makes the cars go around the tighter turns as quickly as possible. Downforce comes with the penalty of increased drag, but on the short tracks, reducing drag is not so important because the engines are able to make their full power output (they are not limited by restrictor plates) and speeds are generally lower.

Extensive testing is done in a wind tunnel to optimize the body design for maximum downforce. The body is mounted as far back on the frame as possible -- about 5 inches (12.7 cm) back from the body location on a super-speedway car. This helps the car create extra downforce.

The front fenders on short-track cars are much more pronounced and curved, which also helps to produce downforce.

Since the speeds are lower on the short tracks, getting an adequate volume of cooling air to the engine and brakes can be a challenge -- especially since the engines and brakes generate more heat during short-track racing. The grill opening on the front of a short-track car is larger than on a super-speedway car, and extra vents duct air directly onto the brakes.

Super-speedway car

Super-speedway Cars

On the super-speedways, the track is much longer and straighter and the banking is high, allowing cars to maintain a high speed all the way around the track. The goal in building a car for super-speedway tracks is to reduce drag as much as possible. These tracks require the use of restrictor plates that reduce engine power from about 750 horsepower (hp) to 450 hp.

Since the engine is not producing its full power, it is critical to make the best use of the power available by reducing drag. The body on a super-speedway car is mounted forward on the frame to reduce drag. The sides and the fenders are less contoured, and the grill opening is carefully tested in a wind tunnel to find the smallest-sized opening that will provide the necessary cooling airflow.

This device is placed behind the grill during wind-tunnel testing to determine the speed of the air entering the engine compartment.

At the higher speeds of a super-speedway track, there is enough airflow for cooling the brakes, and a much smaller grill opening can provide adequate cooling for the engine.

A Dodge NASCAR engine during assembly in the engine shop at Bill Davis Racing

The Engine

The engine in the NASCAR race car is probably the most crucial component. It has to make huge amounts of power for hours on end, without any failures.

You might think that these NASCAR engines have nothing in common with the engine in your car. What we learned was a little surprising: These engines actually share many features with street-car engines.

Dodge provides the engine block and cylinder head for the engines used by Bill Davis Racing. They are based on a 340-cubic-inch (5.57-liter) V-8 engine design that was produced in the 1960s.

The actual engine blocks and heads are not made from the original tooling. They are custom-made race-engine blocks, but they do have some things in common with the original engines. They have the same cylinder bore centerlines, the same number of cylinders and they start out at the same size (they get a little bigger during the building process). Like the original 1960s engines, the valves are driven by pushrods (see this page for information on the different types of valve arrangements).

The engines in today's NASCAR race cars produce upward of 750 horsepower, and they do it without turbochargers, superchargers or particularly exotic components. How do they make all that power?

Here are some of the factors:

  • The engine is large -- 358 cubic inches (5.87 L). Not many street-cars have engines this big, and the ones that do usually generate well over 300 hp.
  • NASCAR engines have extremely radical cam profiles that open the intake valves much earlier and keep them open longer than in streetcar engines. This allows more air to be packed into the cylinders, especially at high speeds (see How Camshafts Work for more details).
  • The intake and exhaust are tuned and tested to provide a boost at certain engine speeds. They are also designed to have very low restriction -- that is, to provide little resistance to the gases flowing down the pipe. There are no mufflers or catalytic converters to slow the exhaust down, either.
  • They have carburetors that can let in huge volumes of air and fuel -- there are no fuel injectors on these engines.
  • They have high-intensity, programmable ignition systems that allow the spark timing to be customized to provide the most possible power.
  • All of the subsystems, like coolant pumps, oil pumps, steering pumps and alternators, are designed to run at sustained high speeds and temperatures.

When these engines are machined and assembled, very tight tolerances are used (parts are made more accurately) so that everything fits perfectly. When an engine (or any part, for that matter) is designed, the intended dimensions of the part are given along with the allowable error in those dimensions. Making the allowable error small -- tightening the tolerances -- helps the engine achieve its maximum potential power and also helps reduce wear. If parts are too big or too small, power can be lost due to extra friction or to pressure leakage through bigger than necessary gaps.

Several tests and inspections are run on the engine after it is assembled:

  1. It is run on the dynamometer (which measures engine power output) for 30 minutes to break it in. The engine is then inspected. The filters are checked for excess metal shavings to make sure no abnormal wear has taken place.
  2. If it passes that test, it goes back on the dynamometer for another two hours. During this test, the ignition timing is dialed in to maximize power, and the engine is cycled through various speed and power ranges.
  3. After this test, the engine is inspected thoroughly. The valve train is pulled and the camshaft and valve lifters are inspected. The insides of the cylinders are examined for abnormal wear. The cylinders are pressurized and the rate of leakdown is measured to see how well the pistons and seals hold the pressure. All of the lines and hoses are checked.

Only after all of these tests and inspections are finished is the engine ready to go to the races. Insuring the reliability of the engine is critical -- almost any engine failure during a race eliminates the chance of winning.

Goodyear provides the tires for NASCAR Winston Cup cars.

Photo courtesy Goodyear

The Tires

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Tires are another critical component on the race car. A high-speed blowout can be incred­ibly dangerous.

Like the tires on your car, NASCAR tires are radial tires, but that is about the only similarity. The tires on a NASCAR race car have some very special requirements. They have to remain stable at very high temperatures and speeds, provide incredible traction and be changed very quickly.

Nitrogen Instead of Air

Most of the teams remove the air from the tires and replace it with nitrogen. Compressed nitrogen contains less moisture than compressed air. When the tire heats up, moisture in the tire vaporizes and expands, causing the pressure inside the tire to increase. Even small changes in tire pressure can noticeably affect the handling of the car. By using nitrogen instead of air, the teams have more control over how much the pressure will increase when the tires heat up.

Inner and Outer Tires

On tracks that are more than 1 mile (1.6 km) long, where speeds are faster, NASCAR rules require that tires contain an inner liner. This is essentially a second tire mounted inside the first tire. It mounts to the rim and has its own separate air supply. If the outer tire blows, the inner tire is still intact, allowing the driver to bring the car to a controlled stop.

Different Compounds for Different Tracks

NASCAR regulates which tire compounds are used on each track. The tire compound is the material the tire is made from -- a softer compound can provide more grip but wears faster, while a harder compound will last longer. Each track causes tires to wear differently, and the inside tires wear differently than the outside tires. Track surface, number of turns, tightness of turns and type of banking are all factors that determine how a tire will wear. Since tires are so critical for safety, NASCAR and Goodyear have determined the best compounds for the inside and outside tires for each track, and these are the tire compounds that the teams are required to use.

Treadless Design

NASCAR tires look completely bald, but that's not because they are worn out. It is by design. On a dry track, tires can generate more traction if more of their sticky rubber is in contact with the ground. Putting a tread pattern on the tire helps in wet weather, but in dry weather it is better to have the whole tire touching the ground. That's why NASCAR races stop whenever the track is wet.

Quick Change

How do they get the tires on and off so fast?

You may have seen a NASCAR pit stop before. In 12 to 14 seconds, seven people manage to completely refuel the car and change all four tires. This requires incredible hand-eye coordination, but there are a couple of tricks the teams use to make things a little easier. When the new tire is placed onto the car, the five lug nuts are already attached to the wheel by an adhesive. The studs are long and have no threads for the first three-quarters of an inch. This ensures that the lug nuts do not get cross-threaded, making it easier for the tire to be positioned.