Note added 6/27/12: Kevin Harvick made the point in an interview that one team had figured out a way to use the sway bar for things it was not intended to do. This gave that team (according to the engineers I’ve spoken with) a pretty good advantage on mile-and-a-half tracks. Will we see a difference? We might – who is to say that the team didn’t come with something new that hasn’t been obviated by a rule change yet?
NASCAR issues rules change on Wednesday – one of which extends an earlier rule change to try to improve aerodynamics at intermediate tracks. The other is designed to cut off an entirely new direction of research before it starts.
Many of the problems with passing at 1.5-2 miles tracks are due to aerodynamics. Toward the end of last May, NASCAR revised their rules about the height of the side skirts on the cars to try to change the aerodynamics. NASCAR issued another set of rules changes Wednesday that continues in the same direction.
Left side skirts went from being a minimum of 4 inches from the ground and a maximum of 4.5 inches from the ground to 4.5 inches min/5 inches max from the ground. The right-side skirts went from 4.5 inches/5 inches to 5 inches/5.5 inches. The primary effect of raising the skirts is to make it harder for teams to get the car to seal to the track. This lets more air under the car. That air pushes upward on the car, which decreases the total grip. Additionally, the asymmetry between left/right helps air escape from under the car in the case of a spin. (Thanks to Dennis for noting that I had two left-side skirts there!)
The other rules change is a little more interesting and has to do with the sway bar, a part that people often talk about, but that is rarely seen. Sway bars can be used in the front or the rear of the car: this rule addresses the rear sway bar. The rules change mandates that right- and left-side rear sway bar links must be perpendicular to the ground, as viewed from any direction when the car is at ride height.
Here’s a sway bar assembly. The sway bar itself is the long cylindrical piece of metal running horizontally across the figure. The arms are the two pieces of metal that come out either side. (as Scott points out in the comments, there are no links in this picture. I couldn’t find a decent picture that showed the links, unfortunately.)
Here’s an arm from another angle. One of the arms is attached to the left-side wheel assembly and the other is attached to the right-side wheel assembly. This video from Jeff Hammond shows you the sway bar on a cutaway car.
Using the top picture, imagine that each wheel is raised by the same amount. Both arms move up the same amount and the sway bar simple moves upward with the wheels.
Now imagine that only the right wheel moves upward. The right arm transmits a force to the right side of the sway bar and that force tries to twist the bar. Bars are manufactured with varying amounts of resistance to twisting. A stiffer bar requires more force to move one wheel with respect to the other.
The roll bar is used to minimize body lean – the shifting of the body from left to right when the car turns left. When properly selected, the roll bar helps the car roll through the corner; however, there are a number of secondary things roll bars can do.
Remember the year of yaw? When everyone was trying to get the rear of the car jutted to the side so badly that some of the car looked like they were coming down the frontstretch sideways?
The gossip in the garage is that one team had figured out a way to attach the rear roll bar so that, when the car transferred load, the roll bar would push the rear of the car askew. This is a really clever trick that relies on having very compliant bushings. Anything you can figure out how to do so that the car behaves differently, but only when the car is at speed on the track, is significant, because that makes it much harder for the other teams (or the sanctioning body) to catch the change. Apparently, mounting the roll bar this way requires you to cant the sway bar arms, which is what made the tweak visible to inspectors.
The roll bar is allowed on the car as a tuning device for load transfer – using it to shift the rear end housing opens up a whole new range of possibilities that NASCAR decided they wanted to cut off before things got out of hand.