How a Sprint Car Works
Beginners Guide to Sprint Cars
by Al Drake and Craig Smith
This is guide for the casual fan that would like to know how a sprint car works and what the various parts are on the car. Entry level education on the terms will help you at the race track when the announcer says "this broke" or a driver says he had a problem with "that" during an interview.
In this case, the chassis is manufactured by J&J Chassis Company. It is called a "downtube chassis" because it has tubes that run from the top of the roll cage down to the front of the car. This provides stiffness to the chassis and limits flex more than a chassis that does not have the tubes (primarily older cars). The chassis is made of chrome-moly steel tubing. The tubes are welded together in jigs that allow the manufacturers to create a consistent line of cars.
The Drive Line
To start, sprint cars don't have a transmission. They are either in gear or out of gear. The cars used to have an in\out box (sometimes clutch-actuated), but now days, there is a t-handle on the end of a cable that runs down to the rear end. The gears are in the rear end and if you pull the cable, the gears disconnect. The gears in the rearend determine how fast the car can go and how hard the engine has to work to drive the wheels. A plate on the rearend can be removed and the gear sets can be changed to match the length of a given track (and its conditions)
The driveshaft is a solid bar that runs from the engines crankshaft to the rear end. The torque tube is a pipe between the engine and rear end that the driveshaft spins inside of. There are universal joints at both ends that let the driveshaft turn when the rear axle moves up and down. The torque tube is mounted to the rear end and at the engine slides into a swiveling ball housing.
Engines in sprint cars come in primarily two sizes: 360 cubic inch and 410 cubic inch displacement. Some divisions require an iron (or stock) block, while other divisions allow aluminum. The internal parts of the engines in some cases have titanium parts, but require more maintenance. Most divisions of sprintcars use fuel injection (as opposed to carburators). They dont have a distributor like a passenger car, sprintcar engines use magnetos. Magnetos are run with an on\off switch and require the engine to be turning over to start a spark. There are no batteries on sprintcars, they have to be push started. When the car is in gear and pused by a truck, the oil pressure comes up, the on\off switch is turned on which "lights the mag". The engine "fires off" and the car accelerates away from the push truck. The oil in the engine is distributed by a dry-sump system. In todays cars, the dry sump tank is mounted on the front of the engine next to the water pump. The engine is mounted to the chassis in the rear by a motorplate. The fuel pump and driveline stick through the motorplate and reside between the drivers legs! Its a tight cockpit with all the pumps, hoses and pedals. Also in the pump housing is the pump that pressurizes the brakes.
Sometimes the cars do not run mufflers and are very loud. At most races though, there is a sound limit which varies in decibels at 100 feet distance from the car. It's still plenty loud!
Suspension on a sprint car is called 4-bar torsion. There are two torsion bars in front and two in back. One bar springs each corner of the car. The torsion arm is on the end of the bar and supports the axle on that corner. On the other end of the bar is a piece called a torsion stop. It clamps to the bar and rests against the frame so the torsion bar wont spin. While the torsion arms determine the upward movement of the axle, the shock absorber keeps the axle from falling out the bottom. It's attached at the top to the frame and attached to the axle at the bottom. Right to left movement is limited by the panhard rod which mounts one end to the frame and the other to the axle. It's like a hinge, but keeps the axle in place. The other parts to the front suspension are the radius rods. They mount one end to the side of the car and the other to the axle. This locates and squares up the axle so it wont move front to back.
In the rear of the car, the same design technology is used, but instead of a panhard rod to stabilize the right-left movement, a piece called a jacobs ladder is used. It's a "w" shaped piece. The two ends of the "w" are mounted to the chassis and the center peak is mounted to the the birdcage. A birdcage contains the rear wheel bearings that the axle rides in and provides mounting points for the shock absorbers as well.
The shock absorbers on a sprintcar are special items. They are designed with different rebound and compression rates to make the car soft, or hard and keep the wheels on the ground. If the wheels are not on the ground, its bad news!
Sprint cars use disc brakes. In the front of the car, there is one disc on the left wheel and thats it! In the back there is on behind the drivers seat on the rear axle (under the fuel tank) and sometimes one on the right rear wheel. When the car is fast and the driver drags the brakes in the turns, they glow orange, sometimes they throw sparks.
Sprintcars did not always use wings, but in the 1970s they began to show up. In the beginning they were huge! Todays series limit wings on top of the car to 25 square feet (5'x5' in most cases). They as well as the smaller nose wings are mounted to the car with tubes called spuds. Some divisions allow sliders on the top wing which allow the driver to move the wing back and forth using hydraulic levers. In a track is fast and heavy, the wing will be forward, if the track is dry, the wing will be moved back. If a division does not allow sliders, the team has to position the wing before the race to a spot and angle that they think will make the car handle the best in the closing stages of the race.
Driving a sprintcar these days requires power steering. The steering wheel is made of lightweight metal and has a quick-disconnect hub for easy removal. The steering box it mounts to is right above the drivers knees and exits the left side of the car to a lever it rotates called a pitman arm. The pitman arm has holes in it to adjust how much steering input is required to turn the cars front wheels. The pitman arm is connected to a bar called a drag link. The drag link connects to the left front spindle and then another bar goes to the right front spindle that is used to not only turn both wheels at the same time, but also allow alignment adjustments to be made.
Most sprintcars use methanol. Some use pump gas, but not many. Sprint cars carry a fuel cell in the back of the car that looks like a bumble bee's tail. The capacity ranges on the tanks, some use 30-35 gallon tanks, while others use a 24 gallon tank. Less is lighter but if you run out of gas during the race, there is no time to refuel and you're done for the night.
By now, you have a pretty good idea what the environment is like. The driver sits in a custom fitted high-back aluminum seat with 4" wide belts in a 5-point harness configuration. With the belts on and tight, you don't go anywhere. As you sit in the car, the driveline and some pumps are between your legs, fuel lines from the tank come from behind you and wrap around to the right and into the pumps. The brake pedal is on the left and is a swing arm type lever. The gas pedal is a spring loaded stirrup and on the right. Ahead of the steering wheel is the dashboard. On the dashboard is the oil pressure and water temperature gauges. Also on the dash is the mag switch and a fire extinguisher switch. There is also a fuel mixture knob to lean out or richen up the engine. To the left of the steering wheel is the wing slider lever.
Plenty to do! But wait there's more!
The driver wears a nomex firesuit with nomex underwear. Shoes and gloves are also special fireproof material. It doesn't breath very well and the driver is pretty warm and sometimes sweaty by the end of the race (even on a cool night!). The driver wears a nomex hood on his head and then puts the helmet on. Claustrophobic yet? With all this on, and strapped into the car, many drivers put a foam collar on below the helmet that keeps their head from bouncing around. Looking down is not an option. Left and right motion is about it. With the helmet visor down, the driver now has tear-off strips of plastic on the lens he can reach up and remove when the dirt gets too thick to see. Now think about working all those levers on a 3/10-mile dirt oval averaging 95 mph and reaching up to pull a tear-off!