Stock Car Science

Thanks so much to Dave Moody for having me on his (and Suzy’s) show last week. Following that show, I got a number of suggestions for future blog ideas. My favorites ones are the folks who send driver quotes and ask basically if the driver has any idea what he’s talking about. I plan to address a couple of those in coming weeks, so if you find any good ones, send them in.

Thanks also to the folks who came out to see me at Speed Street in Charlotte last week. The most-asked question at Speed Street was who my favorite driver was. Truth is, I don’t really have a favorite driver, although there are one or two I find particularly amusing to listen to on the radio.

Being a techie, I cheer for the teams where I know someone. There was a bit of upheaval at GEM last year, and now there are people on all three of the GEM Cup teams and the No 9 GEM Nationwide team, plus some folks moved to Michael Waltrip Racing and Red Bull Racing. I was really happy to see the No. 84 doing so well–until they lost a wheel. (Careful what you’ve giving wings to there, boys.) I haven’t talked to Josh Browne recently, but I know he’s had his nose to the computer, putting in long hours on suspension simulations. Really big congrats to the No. 9 team for the second week in a row. I know how hard everyone at GEM has been working to rebound from a really disappointing 2007 season. Sunday night’s win is evidence that all the hard work is paying off. (P.S. to Chad and Kiwi– it’s really hard to pick you guys out in Victory Lane when you’re dressed just like everyone else!)

Of course (as someone who works with a company that hasn’t had a lot of success with the “crabmobile” said), the true test is going to come next week when NASCAR starts enforcing the new YRR (Yaw Reduction Rule) announced last Thursday. I covered what yaw is, but why does yaw help the car turn? Remember Newton’s first law of motion? A race car going 180 mph straight will keep going 180 mph straight unless a force makes it change speed or direction. If you’re turning left, that force has to point to the left. When you’re turning in a circle, the force always points out the driver’s side window which is going to be toward the center of the turn, as shown below.

This turning force is called centripetal force (centripetal literally means pointing toward the center). The traditional way of making turning force is to increase the amount of downforce. We give aerodynamic forces names according to the direction they point. Air pushing straight down is called downforce, and air pushing up is called "lift" (although Matt Crossman, a writer for Sporting News, made a really good point when he asked why they don’t just call it "upforce". For racing, that would make a lot more sense). Air pushing along the length of the car, opposite to the direction the car is moving is called drag. Those forces are shown below. The last force is air that pushes sideways, which is called sideforce.

You didn’t hear anyone talking much about sideforce before the new car made its appearance. Everyone was focused on getting more downforce; however, the amount of downforce you can produce from the new car is limited because NASCAR won’t let teams tweak the body any more. They take this restriction very seriously: they suspended the 24 and 48 crew chiefs for six weeks at Infinion and fined them $100,000. They also won’t let you mess with the rear wing (Ask Bootie about that one). Since teams couldn’t come up with (legal) ways to generate significant downforce, they looked for other ways to get more turning force. If you look at the direction sideforce pushes on the diagram below, you’ll see that when you generate sideforce (in the correct direction, which is left), you’re generating turning force.

On the Sirius Speedway show, Suzy Q asked a really good question about how you got sideforce when there was air flowing on the left and the right sides of the car. To explain this a little better (it is so much easier to explain things when you can make pictures!) let’s run a stake through the car’s center of gravity (shown as a red circle below). On the track, the car is moving, but the effect is the same whether the car is moving or stationary. Holding it still for a moment will make it a little easier to explain.

You can think of the pinned car like a spinner from a board game. The force due to the air flowing along line 2 passes straight through the center of gravity and doesn’t make the car turn. If you had a spinner for a game board and you tried to spin it by pushing directly along the arrow (toward the center) it wouldn’t spin very well. Same thing here.

Now look the air traveling along lines 1 and 3. The force from the air coming along line 1 tries to make the car rotate counterclockwise. The force from the air coming along line 3 tries to make the car rotate clockwise. When force is applied at a distance from the line running through the center of gravity, the force creates torque and torque makes the car turn. The further the perpendicular distance from the line along which the air pushing to the centerline, the larger the torque and the harder the air tries to turn the car. Compare the pink line running perpendicularly from line 1 to the yellow centerline with the pink line running from line 3 to the centerline. Line 1 is further from the centerline, so it creates more sideforce than the force from line 3 creates (which is why the pink line on the left is larger than the pink line on the right). Some of the torques counteract each other; however, there are more lines to the left of the centerline than to the right, so the net effect is a torque that tries to turn the car counterclockwise (which is, of course, to the left). The more the car is yawed, the more asymmetry you get and thus the more turning force. Yaw = turning force = speed. By the transitive property, Yaw=Speed.

That’s why teams like yaw. They couldn’t find more downforce in the new car, so they took a different tact and looked for another way to get the car to turn. They found one, and we’ll talk next time about why NASCAR limited the amount of yaw allowed in the cars.