That fact that people are even talking about restrictor plates for Cup racing at Michigan International Speedway indicates a lack of understanding of the issues that give rise to concerns about cars getting airborne.
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.
For example, let’s say I look at my speedometer five times over the course of an hour. I note that I’m going 55 mph, 62 mph, 67 mph, 52 mph and 64 mph. Adding those numbers up comes to 300 mph. Since there were five measurements, divide by five and my average speed is 60 mph. The measurements could vary much more widely and still produce the same average. Any five speeds that add up to 300 mph would give you an average speed of 60 mph.
I’ve diagrammed to the right a rough sketch of speed vs. time (or distance). A diagram of Michigan International Speedway is in the upper left-hand corner of the picture, with corners and straights labeled. Those positions correspond to the positions shown in the graph.
The maximum speeds of 218 mph being reported happen generally 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.)
Pre new left-side tire, cars were reaching a maximum of 218 mph with average lap speeds a little more than 200 mph. This tells you there must be a significant number of places on the track 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, the big concern is that the 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. This concern is heightened because of the possibility that a spinning car can hook a tire on the asphalt/grass transition in the frontstretch.
Daytona or Talladega
A corresponding picture of speed vs. time (or distance) at a plate track would look like the picture to the left. Cars are flat out all the way around the track. The speed doesn’t change very much through an entire lap. At Daytona, they are taking the corners near 200 mph. That’s a far different situation than at Michigan, where the speeds vary considerably 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.
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