Skidpadman – Effect of the Rear Anti-Roll Bar

by Ray Bryden on September 5th, 2009

When iRacing threw in the free skidpad to the list of goodies in the recent update, I decided it was time to learn things about car handling systematically starting with a car with only a few setup variables – the Skip Barber F2000.

I checked four different rear anti-roll bar (RARB) settings, and at least 3 different spring perch settings.  You may rightly question why I did not measure the effect of tire pressure which has probably the greatest effect on the handling of the car, but I was limited in how much I could do in one experiment and the number of permutations added by varying tire pressures was too much to handle. As well, I had a fairly good understanding of the impact tire pressure: the generally accepted rule (introduced to me thanks to Ian Lake) is to use 23 psi in the rear tires and start with a higher pressure in the front to add understeer, and gradually decrease it to 23 psi when you are more adept at handling the car’s inherent oversteer. For this test I used 23 psi on all tires.

I began with circling at 50m, and found I could only bring on a little throttle before it would start to oversteer to a wider radius before coming back when I backed off, and thus was able to steer the car with the throttle while holding the steering wheel at a steady angle. I would do about 3 minutes of this and then record the lap times (taking special note of the fastest lap), and then also record the tire temperatures and tread wear. Then I progressed to a variety of other radii and repeated for each setup change.

The good news is that my understanding of car control and maximizing the traction limits of the tires has increased greatly with the amount of time I spent on the skidpad. The bad news is that my first few hundred laps of data is useless and will need to be re-done, since I later found I was not pushing the car as hard as I could have. Important as it may be to maintain a  consistent chassis set-up, steady performance by the driver is the most critical element in the testing equation.

My conclusion on the effect of the rear anti-roll bar for this particular test is shown in this diagram.

Rear roll-bar effect on skid pad

Clearly the answer for what anti-roll bar setting to use with this 0/0 spring perch setting (other spring perch settings will be studied in an upcoming article) depends on the requirements of the track. If the track in question has a long straight stretch that is preceded by a tight radius corner, it will be important to have a lower RARB to maximize my speed at the exit and thus gain time down the following straight. On the other hand, if a track has two long straights interrupted by a fast (high radius) kink, a higher RARB will yield best results for me. Like most things in car physics, the idea is to find the best compromise with your set-up, driving style and ability, and how hard you push the car.

Also, keep in mind that the anti-roll bars and perch settings have a big impact on the dynamics of the car handling (weight transfer distribution) as you enter and exit the turns.  The key would be to verify this skid-pad work (which provides steady-state handling feedback) with actual laps to see if the prescribed setting indeed results in a better feel and faster laps. The main thing to keep in mind is a low anti-roll bar setting will make the load transfer slower and the car will feel a little more sluggish but easier to “catch”, while higher (stiffer) settings make the car more responsive but also will be a little more difficult to control including some oversteer on corner exits.