Bristol Motor Speedway made its name as a high-banked short track, but Bristol banking has changed over the years.
Bristol Motor Speedway announced that they are grinding down the upper groove of the track to decrease the progressive banking. My sketch shows what I think they are doing, based on the press conference and the tweets (thank you so much Nate Ryan!).
The 2007 re-do introduced progressive banking — that is, three lanes with different banking in each. The lowest lane had 24 degree banking and the upper lane (nearest the wall) had 30-degree banking. I’m guessing that the middle lane was 27 degrees – haven’t been able to verify this, but it’s probably not far off. (My values are from the Bristol Motor Speedway website.)
What Difference Does Bristol Banking Make?
I conclude that the change is to decrease the banking on the upper lane. Assuming they make the banking the same as the middle lane (their graphics seem to suggest this), the new surface would look something like the line in red.
Why does this make a difference? Because banking helps a car turn. A car driving into the page requires a force toward the left to make a left turn. The banking provides additional force, which adds to the force provided by the tires. Force scales like the speed squared, which means that the more turning force you have, the faster you can go. All other things equal, a car goes faster on higher banking.
A uniformly banked track gives you no advantage if you choose the outside line. You have the same turning force, but you must drive a larger distance.
Progressive banking offers drivers a trade-off. The speed advantage from the additional banking can make up for the the longer distance. How effective this is at creating more grooves depends on the relative advantage of distance vs. speed.
There are other factors, of course: How much rubber is in each groove, and whether your car runs better in one groove or the other.
NOTE: This post was spiffied up on Saturday, September 19, 2020. I edited it for length cleaned up the graphic.