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Stopping a stockcar

In NASCAR, it's about more than just speed.
Monday, March 1, 2010 - 01:00

Jeff Eicher, brake specialist for Joe Gibbs Racing, works on the new Alcon design calipers.

“Racing in today’s NASCAR circuit is more competitive than ever. With the field so evenly matched, an advantage like better braking can easily make the difference between winning and losing.” So says Jimmy Makar, senior vice president for Racing Operations with Joe Gibbs Racing.

What are the challenges in designing a NASCAR brake system? A vehicle in motion accumulates kinetic energy proportional to its mass and to the square of its speed. So to stop that vehicle or to even just slow it down, that energy has to be transferred somewhere else. The easiest method is to convert that kinetic energy into heat, and that’s exactly what braking systems do through friction between the pad material and the rotor. The vehicle we’re designing for is a 3,400-pound, 800 hp rocket ship capable of speeds of nearly 195 mph. That’s a lot of energy to convert!

And not every track is the same. In fact, many engineers designate NASCAR venues as super speedway, intermediate tracks or short tracks. Road courses like Watkins Glen fall into the short track category. Each has its own braking challenges. Super speedway cars, for example, use pad and rotor combinations similar in size to conventional passenger car systems. Other than avoiding the “big one,” these brakes aren’t called into use very much. Their biggest challenge is slowing that 195 mph car down to pit road speed in just a few seconds.

This shows (in red) how much more downforce is exerted on the COT (foreground) versus the old style stock car (background).

Because the brakes are cold when applied, it’s not unusual to see cracks in the rotors from the thermal shock load incurred. Speedway systems are built as light as possible to reduce unsprung weight and improve handling. They also are designed to produce near zero drag when the brakes are off, using special caliper seals and pad retraction springs. This adds to overall speed and increases fuel mileage, a factor that has determined the winner of more than one super speedway event.

Working With a ‘Paper Clip’
On the other side of the spectrum is Martinsville, better known as the “Paper Clip” to the teams that compete there. It is a half-mile oval shaped like, well, a paper clip, with two 800-foot long straights. Cars run up to 120-plus mph before stomping on the brakes and slowing down to 60-plus mph. On a 500-lap race, the brakes are hit more than 1,000 times, and the drivers are on the pedal nearly 35 percent of the time.

Brake fluid leaks and hot rotors equals fire!

This generates tremendous heat, and if you’ve ever watched a short track event on television, you’ve seen the undercar camera shots of disc rotors glowing cherry red. Those rotors are about 40 mm thick, and the pad linings start out approximately 32 mm thick. That’s each pad!

When the Car of Tomorrow (COT) first was introduced on these demanding tracks, it soon became obvious that the braking systems in use at the time couldn’t handle the new car’s needs. Because the new car design has the front spoiler so close to the ground, and airflow across the front is so different, insufficient air was getting to the braking components. That heat is going to go somewhere, and it did. It went into the front suspension components, wheel bearings and even the tires. Caliper piston seals were melting down, and even a newbie in the shop understands what that means: instant loss of pedal and an introduction to the wall.

 

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