Speed Secrets: Physics For Gearheads
By Ross Bentley
May 06, 2015
Randy holds a Ph.D. in mechanical engineering, and for 30 years has been an automotive engineer at General Motors, where he is a technical specialist in noise and vibration (N&V) at their proving ground in Milford, MI. He has published numerous papers on powertrain N&V, and holds three patents. His first assignment at GM was the enviable task of developing Corvettes for minimized “rough road shake.” Randy sent this week's feature article on a Saturday evening, right in the middle of driving his Cayman in a 2-day track event at Putnam Park. So you know that he not only knows the theory, he drives the theory. –Ross
A couple of years ago, I finally took my wife, Tara, to the Kentucky Derby. She has always appreciated the strength and beauty of the thoroughbreds and the skill of the jockeys as they manage their race and position their ride. It rained all day, never heavy, but when the Derby ran, the track was officially listed as “sloppy.”
During the rain, most everyone was under the stands, trying out the mint juleps. Our son, Aric, had come along, and found that rain doesn’t dampen the mood so much in the infield. And the mud troughs are handy for doing belly slides (fortunately, he came back clean).
Under the stands, Tara and I started talking with someone from a large city, and the subject turned to jockeys. In a while he said, “I don’t get the big deal about jockeys. All they do is ride the horse.” I replied with something profound like, “Oh?” Then he said, “Yeah, it’s just like race car drivers. All they do is drive the car!”
Back before you could see what went on inside the car (say, in the 1970’s), it was easy to think that the Daytona 500 was a leisurely 200-mph Sunday cruise. From the outside, the cars smoothly tracked around the course, as if on rails. It looked so easy that it made you wonder why anyone ever drifted wide, or spun. Drafting - new back then - was discussed more than driving technique.
That changed when I saw video from an in-car camera. This “cruise” around the track involved constant steering corrections of what looked like 30°. It seemed like moving the wheel that much would send the car veering into the wall! Why was the driver so busy?
But as you’ve heard, “If everything seems under control, you're just not going fast enough.” To corner harder, the tires must produce a higher lateral force – through having a larger slip angle. How responsive is the car now? If the tire starts off at 3° slip (and producing 1025 pounds of lateral force), the quick 1° correction to 4° produces only a 60-pound increase. Turning the steering wheel has become more like “fine-tuning” the cornering force. And you can feel the tire is “saturating.”