How Champ Cars Work

Suspension and Tires
A close-up of the hub
A close-up of the hub

The front and rear suspension of a Champ Car is known as a double-wishbone suspension. This type of suspension has the advantages of light weight, impressive strength and a well-controlled ride. The racing surface can be surprisingly bumpy -- for example, at the Cleveland track the surface changes several times along the course, with a bump at each transition. The purpose of a Champ Car's suspension is to keep all four wheels glued to the track despite these aberrations in the pavement.

Two wishbones hold the hub and allow it to move up and down in a plane. The diagonal piece connects to the shocks and springs (which are located under a panel just in front of the driver). The horizontal tie-rod provides the steering input to rotate the hub.

A Champ Car's suspension also has to be lightweight and compact. The front suspension consists of the triangular supports (wishbones) that mount to the front hubs, along with the springs and shocks. The springs and shocks, as well as the equivalent of the anti-roll bar, are all mounted on top of the car just in front of the driver.

The rear suspension is similar to the front. The main differences are the lack of the steering mechanism, the addition of the drive shaft and the greater weight that the rear suspension must carry. The springs and shocks follow an arrangement similar to the front suspension, but they are larger and fold alongside the transmission.

An important part of tuning the car for maximum performance is done in the suspension. The team controls the toe-in and toe-out of the tires as well as the positive or negative camber of the tires and the caster. These settings help adjust understeer and oversteer. The team can even shorten and lengthen the wheelbase by changing the wishbones!

Because the suspension components are so important to the reliability of a car and its ability to travel in a straight line, CART rules are quite specific on how they must be built. For example:

9.8.2. All highly stressed steering and suspension components shall be made from SAE 4130 steel or an alloy, specified by its manufacturer as having equivalent physical properties. Front and rear uprights may be made of magnesium alloy or an aluminum alloy. All such parts must be heat-treated, including stress relieving, normalizing, annealing and hardening when applicable, after forming and/or welding as recommended by the manufacturer of the alloy being used. All such parts that are electroplated must be oven-baked at a temperature of 375 degrees Fahrenheit, plus or minus 25 degrees, for not less than three hours after such plating. Parts that have been stripped of plating must be similarly baked unless they are to be reprocessed within a three-hour period. Parts are not to be joined by brazing, soldering, or by dissimilar metals. Shot peening is recommended for highly stressed parts.
Unlike a normal car, which may have 4 to 6 inches of clearance to work with, Champ Cars ride extremely close to the ground.

As you can see, the center of the car is perhaps 2 to 3 centimeters off the pavement. The suspension system therefore does not need to offer tremendous up/down movement. With less than 1 inch of up/down travel available, the ride can get extremely stiff.

The tires on a Champ Car are incredibly important -- they keep the car on the track and translate the driver's inputs from the steering wheel, brakes and accelerator into reality. Especially in the street course events, the driver continually tests the boundaries of tire adhesion -- the limit to how fast the driver can accelerate, brake or take a corner is determined ultimately by the tires.

To handle the incredible forces applied to them, the tires of a Champ Car are much different from the tires you find on your car.

The three biggest differences are:

  1. The tires on a Champ Car are much wider -- 12 inches wide in the front and 16 inches wide in the rear. A normal car's tires are only 8 inches wide.
  2. The tires on a Champ Car are completely smooth to maximize the amount of rubber touching the track surface.
  3. The rubber on the face of the tires is extremely soft. It is more like a soft rubber eraser than anything else, and very unlike the hard rubber found in a normal car's tires.

Between the size of the contact patch of a Champ Car tire and the softness of the rubber, the tires have incredible adhesion.

The tires on a passenger car are meant to last 40,000 to 60,000 miles, while the tires on a Champ Car are designed to last 60 to 70 miles! The CART rules allow a racing team to use up to 60 tires during a 500-mile race.

Besides the incredibly soft rubber used on the surface of the tires, the other thing that makes Champ Car tires last for such a short period of time is the fact that they are very thin. Rubber conducts heat well and retains heat, so the tires have a very small amount of rubber on them to avoid heat build-up. If the driver locks a tire just once during hard braking, it will create a noticeable flat spot on the tire and expose the tire's cords, severely affecting the tire's performance. This limitation is especially important during qualifying, because each team only gets two sets of tires for qualifying runs.

Tires are mounted on magnesium rims and attached to the car's hubs with a single locking bolt.

Firestone provides all of the tires for all of teams at every race and practice session. The teams bring their rims to the Firestone area in the race paddock, where technicians mount and balance the tires.

Each tire is stamped with a yellow CART seal once it is certified.

Given that there are 25 cars and each team gets as many as 60 tires per race, Firestone is mounting up to 1,500 tires per event!

Because the tires are so important to the team's success, each tire's pressure is monitored by its own radio sensor.

This small cylinder, which is mounted on the rim opposite the valve stem, contains a 0.25-watt, 900-MHz radio transmitter and a centrifugal switch. Once a tire starts spinning, the radio is activated and begins transmitting the tire's pressure to this antenna, located just behind the driver's head:

All four tires transmit separately. If the driver runs over something (like debris from a wreck), he can talk to his pit crew, and they can check the tires immediately to see if any of them are leaking using the telemetry stream from the car (see the section on telemetry for details).

The teams pay a good deal of money for this peace of mind -- each tire's radio costs thousands of dollars, and each of the teams' 60 rims needs to have one! That gives you a good idea of how important the tires are.

The CART rules specify everything about the tires and the rims. Here are some of those specs:

  • Rear rim diameter - 15 inches
  • Rear rim width - 14 inches
  • Rear rim minimum weight - 14.7 pounds
  • Rear tire diameter - 27.0 inches
  • Rear tire width - 16 inches
  • Front rim diameter - 15 inches
  • Front rim width - 10 inches
  • Front rim minimum weight - 13.48 pounds
  • Front tire diameter - 25.5 inches
  • Front tire width - 12 inches
  • Pressure - 35 PSI for oval courses, 20 PSI for road courses

The rims are tested by X-ray and dynamic tests before they are allowed on the race course.