Thursday, January 11, 2018
Asymmetrical Setups for the iRacing FR2.0
Asymmetrical Setups for the iRacing FR2.0
Generally speaking, since the FR2.0 is run on road courses, the car needs to be equally agile in both right and left turns. There are however a few courses where lap times can be improved by making the car turn better in right turns than in left turns and vice versa.
When we set up the car to turn better in one direction than the other, the setup is deemed to be called “Asymmetric” or lacking in symmetry.
The art of making a car turn in one direction better, without concern for the other direction is essentially a major part of setting up a car for oval tracks. Note the diagram of the Lotus that won at Indy.
F1 has a recent history of some subtle use of asymmetry in applying aero. See article in link:
And, nothing will teach you more about asymmetrical setups than racing on a dirt oval with limited traction and grip. Remember that we cannot adjust front Caster on the FR 2.0, so that common adjustment used on ovals is not available. But there are many other settings to use.
On road course setups, any asymmetrical setup settings are necessarily subtle as the car still needs to turn well in the opposite direction. While most road courses where an asymmetrical setup would be beneficial run in the clockwise direction, for the purpose of this article, I am assuming that we are trying to make the car left better than right—following the oval anti-clockwise left turn convention.
For right hand turn preference, simply do everything on the other side. (I chose the left preference for this article to be sure that any application by the reader for the more normal right preference in road racing to be only after thorough thoughtful consideration and analysis. This a very advanced technique and should be used only after you have mastered all the other setup parameters.)
Other than driver feedback, the only analytical indication of a “balanced” race car is tire temps and wear. Below is an example of a reasonably well balanced race car where tire temps front to rear are close to equal on the right side, but the left side tires are being stressed slightly more.
Below is a symmetric car. Note the Cross weight: 50%, the Corner weights, and the ARB Preload -0.1 to +0.1.
Let’s assume that we would like to make the car turn a bit better to the left because we have noticed the car is understeering a bit in left turns and the right front tire is running hotter than the right rear. We could change the right rear spring to 900#. This would make the car rotate faster and oversteer more.
Note the ARB preload has been adjusted to 0.0. The Cross weight is now 48.8%. The total weight of the car has not changed, but the LR and RF together are no longer 50%....they are now less = 271+360 divided by 1294 = 48.8%
Essentially we have made the right rear of the car stiffer (reacts faster to weight transfer) and have increased the weight on the LF and RR by 8 pounds each or a total of 16 pounds. We have reduced the RF and LR by the same.
So in a left turn, the right front will have less weight, less heat and tire temp, but more importantly, the left front will have more weight and more grip while the left rear will have less—the result—the car will have less understeer in the left turn.
On Legend cars running on dirt in real life, we would have different spring rates on all four corners depending on the track. Sometimes a bigger spring on the inside rear to give the car more understeer “bite” coming off the turn—sometimes with a bigger spring on the outside rear to make the car rotate faster on throttle in mid-corner.
As shown above, one could accomplish something similar by simply adjusting the ARB preload, without changing the spring, but this is usually not recommended. Mostly because, at least theoretically, it can cause the inside tire (LR) to lose traction.
Rather than changing springs, the most common adjustment is adjusting Pushroad length. In NASCAR, this is called adjusting “wedge”. Indy Cars actually have an in-cockpit adjuster to perform this change. At Indianapolis, the Weight Jacker is often adjusted differently for each corner!
Adjusting wedge or Pushrod Length changes the Cross Weight. In this example to 48.3%, with 11 pounds each added to the LF and RR by increasing the RR Pushrod length by only 0.027in (0.7 mm). This is probably the most you would ever want on a road course setup. This will have a similar effect as changing the spring, but with less change in the handling immediately after throttle application.
One way to make a subtle change is to change the tire pressure, increasing the RR and LF or decreasing the LR and RF. (Increasing tire pressure is like increasing Pushrod length as it makes the tire larger in diameter.) This is essentially what people are doing when they adjust tire pressures on each corner to gain equal 160 kPa pressures, for example.
Another, less common, but still effective way of making the car turn faster is to introduce “Steering” or a “rear slip acceleration” by pointing the rear tires outward in the corner- the inside tire having toe-in, the outside having toe-out. As shown for a left turn bias:
Reducing the negative camber on the inside tire will give that “axle” more relative grip. So to make the car turn better left, by decreasing understeer, simply decrease the negative camber of the inside front tire.
Sometimes, we are not seeking a change in the understeer/oversteer balance, but rather to just give the car more overall grip in one direction. In that case we would reduce BOTH the front and rear camber on the inside of the corner we wish to improve.
Keep in mind that on most road course, what is gained in one corner is likely to be lost in another, so generally asymmetrical settings should be limited to improving handling on “more important” corners that lead to long straights.
From personal experience, asymmetrical settings have provided benefits at Lime Rock and Silverstone for example. Limerock is almost an right hand oval with only one left hand turn. Silverstone’s two longest straights were punctuated at the beginning and end with right hand corners that benefited with slight asymmetry--reducing the weight on the LF helped reduce understeer in these important corners.
Like all setup settings—test, experiment and test some more. (Watch for tire temperatures showing “out of balance” and push the car hard in all corners to test limits.)
Posted by Donald "Wayne" Strout at 9:56 AM