People worry that 200-mph laps at Michigan International Speedway could make cars airborne, as can happen at superspeedways. Should they worry?
Average and Instantaneous Speeds
I touched on the difference between average and instantaneous quantities last week with the pit road speeding issue at Pocono. Instantaneous speed is the speed you are going at some particular instant. A radar gun measures instantaneous speed.
Average speed represents speed over a range of times, not one particular instance. Speeding loops measure average speed.
For example: I look at my speedometer five times over the course of an hour. I’m going 55 mph, 62 mph, 67 mph, 52 mph and 64 mph. Adding those numbers comes to 300 mph. Since there were five measurements, I divide by five, making my average speed 60 mph.
But I could come up with many five-number combinations that yield the same average.
I’ve made a rough sketch of speed vs. time (or distance) to the right, with a diagram of Michigan International Speedway in the upper left-hand corner of the picture. I labeled the corners and straights. Those positions correspond to the positions shown in the graph.
The maximum speeds of 218 mph at Michigan happen toward the end of the straightaways. You’re accelerating all the way down the straight, reaching maximum speed just before you have to brake to enter the turn. You slow down going into the turn and speed up coming out of it. (I drew a symmetric graph only because I have limited time and even more limited drawing skills.)
If they’re reaching 218 mph, but the average lap speeds are just over 200 mph, this means there are a significant number of places where cars are going slower than 200 mph. Those places would be the turns, where they are slowing down to a little over 190 mph (Thank you @chrisneville84 and @DRodmanNASCAR.) The solid line in my graph shows the instantaneous speed, while the average speed is shown as a dashed line.
Does that mean we don’t have to worry about lift-off at Michigan?
As I mentioned in my last post, a car becomes unstable against lift off when it a) reaches a high enough speed and b) spins. Cars are more likely to spin in the turns, but the cars aren’t going 218 mph in the turns at Michigan – they’re going much more slowly.
The issue remains that two cars could hit on the straightaway, spinning one or both cars. Hooking a tire on the asphalt/grass transition on the frontstretch amplifies the concern.
Daytona & Talladega
A corresponding picture of speed vs. time (or distance) at a plate track looks like the picture to the left. Cars are flat out all the way around the track and the speed doesn’t change much. At Daytona, they are taking the corners near 200 mph. That’s a far different situation than at Michigan, where the speeds vary around the track. The propensity for getting that perfect storm of speed and angle of the car with respect to the air increases under those conditions.
Why isn’t this a concern for Nationwide?
We’ve heard all week that the Nationwide cars are running wide open all the way around the track, just like they do at Daytona. Their average lap times, however, are in the 190 mph range, which puts the safety concern squarely in the dangers of pack racing rather than cars going airborne.
NOTE: This post was refreshed on 8/17/2021 to make it more succint.