Dave Moody is My Muse (aka Why Stock Cars Slow Down When They Turn)

The new car comes with a Sirius/XM Radio, which is perfect timing since my long-standing insomnia finally seems to be succumbing to the wonders of modern medicine (which means I can’t drive in NASCAR). But I also haven’t been lying awake in bed listening to the replays of NASCAR shows on channel 128. Since I was tooling around the city today catching up on errands like getting plates for the new hot rod, I got to listen in real time.

That actually got me in a bit of trouble when I mentioned to my husband how odd it was to hear Dave’s voice and not be in bed. I think we have that cleared up now, but if you’re in Dallas in November, Dave, I’ve hidden all the ammo just in case. Speaking of Dave, he challenged me some time ago to write a science blog in verse. I haven’t quite gotten to the level of a whole post yet, but I did write a limerick about Brian Vickers at the Nationwide race at New Hampshire. (Yes, I know I’m behind.)

The story of Loudon is writ
Speed and Vickers collided a bit
Scott said Brian slowed down
As the corner he tried to round
And Brian said, ‘Yeah, no (expletive)‘

Brian was simply expressing his understanding of Newton’s First Law, albeit in words Isaac Newton probably wouldn’t have chosen. (This, after all, is a man reputed to have died a virgin.) But it is what my colleagues in education call a “teachable moment”.

Newton’s first law says (among other things) that when a car is going straight, it’s going to keep going straight unless something makes it change its direction. Sort of like if you’re lying down watching TV, you’re going to keep lying down watching TV until your spouse tells you to get your butt off the couch and take out the trash. If Newton had married, he would have come up with that one as well.

The force needed to make something turn is the centripetal force, which is proportional to the mass of the car (m) times the speed of the car squared (v²) divided by the radius r of the turn.

\text{Centripetal Force}= \frac {mv^{2}} {r}

It takes more force to turn faster. A lot more. If you double your speed, it takes four times as much force to make the same turn. Similarly, it takes more force to make a tight turn. That’s one reason cars slow down going into the corner.

Depending on the turn and the speed of the car, it can take as much as 6 or 7 tons of force to make a stockcar turn. All that force comes from the friction between the tires and the track. How much friction you get is a combination of the stickiness of the tire, the track surface and banking, the mechanical downforce (the weight of the car pushing on the tires) and the aerodynamic downforce (the force of the air pushing down on the car, which pushes down on the tires). All those forces added up have to provide a force equal to mv²/r. If they don’t provide enough force, you don’t turn. You crash.

Track banking helps the car turn because the banked track actually pushes the car in the direction it should be turning. The more banking, the more centripetal force the track provides and the less force has to be provided by mechanical and aerodynamic downforce. The angle of the banking actually changes as you take the turn, so you get different amounts of help turning in different parts of the corner.

The other two factors – aerodynamics and mechanical force – push the car’s wheels into the track, which creates more friction. The problem is that they don’t push each tire with the same force, and the force changes throughout the corner. I’ve talked in previous blogs about the fact that weight ‘shifts’ when you change direction or speed. To review:

• When you brake, the rear wheels lose grip and the front wheels gain grip.
• When you accelerate, the rear wheels gain grip and the front wheels lose grip.
• When you turn left, the right side wheels gain grip and the left side wheels lose grip

The important thing to remember is that you can only turn as fast as your least grippy tire. So if you have a lot of force on your right rear and very little force on your left front, you are limited by the left front.

When a driver talks about ‘rolling through the corner’, he’s talking about the part of the turn in which he’s traveling at roughly constant speed. The car’s already shifted load to the front wheels and he’s off the brake. The load’s shifted from left to right and he’s just waiting for the right moment to get on the gas.

Two cars going into the corner one behind the other are experiencing the same force from the track. The cars have the same mass, so the things that distinguish how one car turns from how the other car turns are the aerodynamic downforce and the rate at which the load shifts. That’s why springs, shocks and swaybars are so important. These components can’t change how much load shifts in a turn, but they can change the rate at which it shifts.

The next couple posts will address the coyote hood ornament they were discussing on Tradin’ Paint and Suzy Q’s tummy gurgles. And why the switch to Fuel Injection is just for show…