Rules Changes…Again

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.


  1. anti roll bar “arms” are roughly “parallel” to the ground, the “links” are “perpendicular” to the ground. I would be interested if the links are solid with heim ends or chains. There is no suspension is the front, so all roll is adjusted in the back of the car, which is driving me crazy because there is no wiggle room in the handling adjustment, you are a rocket, or dogmeat (reference: DW).

  2. Just FYI, the rule is actually specific to the sway bar arm links, which you do not have pictured. The arms cannot be perpendicular to the ground, because if they were body roll would put the bar in shear, not torsion as the anti roll bar is designed for. The issue with the links and their attachment to the frame comes in when you vary the angle of the link relative to the frame, when if done correctly can incuce a jacking or de-jacking force to effect pitch as well as roll. By mandating that the links be perpindicular to the ground they are basically isolating the rear anti roll bar so that it can function only as a rear anti roll bar, and not be used to induce other handling effects. To answer Henry’s question, the links can be solid stubs with heim joints, or chains, or one of each. It really comes down to team/driver preference. With two solid links (heims) the anti roll dynamics are common to any basic anti roll bar. Depending on which side the chain is run on the bar can also be configured to act more like a torsion spring, than a standard anti roll bar. The only thing that can move the rear axle laterally is the rear track bar, and is dependent on the swing arc of the track bar, in the cup cars this cannot be influenced by the rear anti roll bar in any configuration. The application of jacking or de-jacking force, however, can speed up or slow down the lateral translation. This can be desirable in a variety of situations, and again would fall outside of the intent of the rule to isolate the rear anti roll bar effects as such.

    • Thank you, Scott! The engineers I spoke with are pretty convinced that hooking the roll bar to the rear axle housing could have an effect on skew during a heave event.

  3. i have been posting for years get the cars up off of the track to get rid of the aero down force so we have more passing on the tracks and not in the pits. look what bill elliott car looked like when he went 212 MPH. NASCAR knows when the aero down force goes away there will be more wrecks and the TV people don’t like having expensive TV time taken up by wreck cleanups so NASCAR will shorten the races to keep the same time frame wrecks and all

  4. Ok, I knew I was missing something. I guess I should send back the trophies and rings. Might I suggest Bill Milliken’s book Race Car Vehicle Dynamics and a Rule Book. I was just trying to help out. As usual no good deed goes unpunished. Maybe next time get one of your engineers you spoke with to write the article.

    • Scott: I was sincere in my appreciation of your clarifications. It is a fine line trying to reduce something as complex as suspension geometry to explanations that are simple, but not wrong. Sometimes I’m successful and sometimes not. I always appreciate input from well-information people like you. It’s additionally difficult because the engineers in NASCAR often purposely get vague at times because they a) don’t want to be identifiable and b) don’t want to accidentally give anything away. I need to be careful with what I say here because I don’t want to get any of these guys in trouble with their bosses.
      Keep the comments coming. They are appreciated.

  5. Another great writeup…might be worth pointing out for those that don’t know that sway bar is actually short for anti-sway bar which is really a poor name for the anti-roll bar which is the technically correct term. As you indicated above, it limits body lean or “roll”.

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