The "Latest" Rig

The "Latest" Rig
Bodnar Wheel w HPP Pedals (Added Rift in Summer 2017)

Tuesday, January 9, 2018

Tire Pressure and Brake Bias for the iRacing FR2.0

Tire Pressure and Brake Bias for the iRacing FR2.0


The “science” of tires very complicated.  A little background (which I will provide) is interesting, but the most important thing to remember is that the iRacing FR2.0 is a “Spec” series, which means, we have one tire supplier---iRacing.  The characteristics and performance of these “Spec” tires is entirely determined by the “Tire Model” that iRacing has provided.

There is much debate as to just how close to “reality” the iRacing tires perform. Like, beauty, the performance is subjective and “in the eye of the beholder”.

The complete details of the iRacing “Tire Model” is probably as secret as the formula for Coca-Cola.  But, we really only need to focus on the things we control—in this case Tire Pressure, as well as Camber, Toe and other settings that affect handling.

Keep in mind that iRacing regularly revises and adjusts this “Tire Model” so conclusions we draw here need to be re-validated each time a revision is made.

The Science

Newton’s Three Laws of Motion and Einstein’s Theory of Relativity were important and profound “breakthroughs”. In the science of tires, the Magic Formula (really..that’s the name they call it) put forth by Hans Bastiaan Pacejka, a Professor at the Delft University in the Netherlands, is equally as important. After, this formula was presented, another called the Brush Tire Model was postulated by a couple of Swedish engineers.

Pacejka’s Book:

Brush model:


Which is the basis for the curve you see below:

Every iRacer interested in the “Story” of tires used in iRacing must read:

Written by Dave Kaemmer, Co-Founder of iRacing, it gives invaluable insight. He calls the zones in the drawing above: Linear No Slip Zone, Limit Zone (Driver’s “Office”), and the Scary Zone.

Opinions regarding iRacing’s Tire Model for the Linear and Limit Zone are generally high. What happens in the Scary (Exceeding the Limit) Zone is where most of the controversy comes from—particularly when it comes to how the tire’s performance varies with track temperature. (See end of Article)

Another article for you engineering types is in the following link. I lifted the next diagram from their article.

In addition to the issues regarding grip, the “model” of the tire requires one to think of it as a spring and damper in “series” with the race car suspension.

So, we have a basic relationship between grip and slip angle as illustrated by the “Magic Formula” curve. But, this curve is radically and profoundly affected by the tire compound (soft vs hard) and tire pressure and track temperature. And, the performance of the tire is different during transition from steady state to loading and unloading.

Added to the changes in grip is the change to the overall car’s suspension that occurs with inflation pressure changing the “tire-related damping and stiffness”.

So, do you really think the simple Rule of Thumb, “Just set tire pressures so the hot pressure is 160kPa (23.2 psi)” is adequate?  It might actually be close—but’s let’s do some analysis.

In closing the discussion about “theory”, here is an excellent article that will provide some useful info:

So the contact patch, that little patch of vulcanized rubber touching the racing surface is squirming and moving all the time.  A portion of the contact patch may be in the “limit” zone while another portion is in the “scary” zone. All of this is changing as the car goes around the track.

The way the car feels and tire reacts in the “scary” zone depends a lot on temperatures. While vulcanized rubber does not burn, it does get more “slippery” when hot—and the temperature “when hot” depends a lot of the tire compound. The way the car feels and the tire reacts in the “scary” zone also depends on the road surface.

A word about the spring/damper concept. When you put springs end to end or in “series” the “overall” spring rate changes a bit differently than most think.  Two 800#/in springs in series have a combined or “overall” spring rate of 400#/in.  400#’s compresses each spring by a half an inch, but the total movement for the 400# force on both springs is 1 inch, so the overall spring rate is 400#/in.  Adding tire pressure will increase the spring rate of the tire, but the overall % increase in stiffness of the car will be less than the increase in stiffness of the tire. Increasing tire pressure will reduce the overall damping of the total combination.  

In all cases, changing tire pressure will not only affect grip, but will also change the transitional handling characteristics of the car—just like changing springs and dampers.

By doing some tedious calculations, you can determine a rough “estimate” of the spring rates of the car.  By adding fuel weight and checking changes in ride height, one can estimate that the “overall” spring rate of each “corner” of the car is around 450#/in. (79 N/mm). 

Then one can change tire pressures and note changes in ride height. And, one can change springs and note changes in ride height.

Some needed info is missing to be exact, but with some trial and error and other knowledge, one can estimate that at 21.5 psi (148 kPa), the spring rate (fspring1 above) is approximately 1125#/in (197 N/mm).  Reducing pressure to 20.0 psi (138 kPA) reduces the estimated tire spring rate to 1046#/in (183 N/mm) or almost proportionately to pressure.

What this means is that with 100 pounds of extra load on the tire (aero downforce, cornering or braking weight transfer) the tire is compressed by about 0.080 in. (2 mm). More tire pressure, less compression. But, it also means that changes in tire pressure make relatively small changes in tire compression. Changing from 21.5 psi (148 kPa) to 21.0 kPa (145 kPa) only changes tire compression by 0.002 in. (0.06 mm).

Still, as a rule of thumb, changing the tire pressure by 0.5 psi (3 kPa is like changing the suspension spring by 20#/in (3.5 N/mm).


Practical Considerations

First, let’s test the car to see what effect changing pressure has on lap times, under varying track temperatures. For this, let’s take the FR2.0 to the Martinsville Oval—probably a good “model” for tire pressure effects in high speed corners as we are cornering at 100+ mph.

I ran many laps. Each run was designed to test lap times and hot pressures as well as tire temps.

I could find no advantage in differing pressures front vs rear.  Despite the fact the FR2.0 has different size fronts compared to the rears, it appears that pressure front and rear should be the same.

Starting off at Time Trial Conditions (Warm track) with 21.5 psi (148 kPa) cold with tire warmer, I achieved a good lap time of 18.6 seconds. Hot pressure was the magic “160” kPA.

Taking the pressure down three clicks to 20.0 (138kPA) cold, Hot pressure was 150.  Lap time was off by 0.2 seconds or 1%.

Starting at 21.0 psi (145 kPa) cold (one click lower than the 21.5/148) I achieved the same good lap time of 18.6 seconds. Hot pressure was 158 kPa)  The car felt like it had more understeer at corner entry, but hooked up better with less oversteer on corner exit as compared to the 21.5/148 setting.

Starting at 22.0 psi (152 kPa) cold (one click higher than the 21.5/148) I also achieved the same good lap time of 18.6 seconds/ Hot pressure rose to 165 kPa.  The car felt like I was 4 wheel sliding a lot thru the entire corner and the tire temps showed signs of slight over-inflation.

Based on these results I concluded that the 21.0/145 cold and the 21.5/148 cold settings were very close in performance.

Next I tested these two settings at an extreme Hot track (116F) and again at an extreme Cold (74F) track.

At the Hot track with 21.5/148 I ended with 162 kPa. With 21.0/145 I ended with 159 kPa.  Both settings produced the same lap times with same difference in understeer/oversteer “feel” in regards to mid-corner, except the 21.5/148 setting produced a feeling of more overall sliding. Times on the Hot track were  0.5 seconds, or 2.6% slower than the Warm track!

On the Cold track with the 21.5/148 I ended with 159 kPa. With the 21.0/145 I ended with 155 kPa. Here the 21.5/148 was marginally faster (18.19 vs 18.20) and simply “felt” better. The lap time with the 21.5/148 setting was 0.4 seconds quicker than the Time Trial Warm track conditions or 2.2% quicker.

In all the tests, the rear tires were gaining slightly less pressure and temperature. I ran the tests by boosting cold pressure on the rear to get the final “hot” pressure closer to the “magic” 160 kPa figure—equal to the front. It did not make any significant improvement. I then instead, tried adjusting the rear downforce, which did result in balanced/equal front to rear “hot” kPa, but in this case, lap times improved slightly. 

The testing at Martinsville took into the handling characteristics in transition during the corners as well as grip. To check grip alone, I took the car to Talladega for 155mph  threshold braking tests.

Interestingly, the difference in stopping distance for the 20.0/138, 21.0/145, and 21.5/148 cold settings were negligible. 

For 2018 S1 iRacing version of FR 2.0 Tire Model

Choose either A) 21.0 psi/145 kPa Cold or B) 21.5 psi/148 kPa Cold

At colder (below 105 F) tracks, the 21.5/148 will probably provide better handling and grip.  (The car will however tend to oversteer more on corner exit, so on tracks with hairpin turns and slow chicanes, consider the 21.0/145—this has nothing to do with grip per se, but rather the difference in how the tire transitions from the mid corner to corner exit.)

At hotter (above 105 F) tracks, the 21.0/145 will probably provide better handling and grip.

The 21.5/148 will provide somewhat the same relative “feeling” as a small increase in spring rate.

Additional testing is merited for tracks with hairpin turns and slow chicanes as running the lower 21.0/145 rear and higher 21.5/148 front might be good for some drivers.

Rather than “chase” the “magic” 160kPa hot pressure goal—use the cold setting you like best, and make final adjustments to balance the car using other suspension and/or wing adjustments.  With the current tire warmers, starting with “warmed” cold tires, the difference between setting and final “hot” pressure is less significant.

Warning: All of this could change with one iRacing update.

Do not fall into the trap of self-fulfilling prophecy, where you change tire pressure and see an improvement in lap times--then conclude that you have discovered a profound tire pressure to best grip relationship. If you are convinced that a tire pressure change resulted in an improvement--go back and test the previous setting AND be sure you drive the car just as hard. A confident driver is a fast driver, and often we grab hold of settings where we have become confident, and simply drive the car harder with those settings and higher confidence. 

The biggest aspect of tire performance with the iRacing FR2.0 is the change in grip with change in track temperature.   Affected less by ambient air temperature. Affected dramatically by sun heating of a dark colored track.  It can change from one section of a track to another. It can change with sun angle (morning vs late afternoon) as well as cloud cover and location. Dramatic changes in grip will be evident.

As mentioned earlier, this has been a "bone of contention" for some time and the consensus is that it has improved. (While we all know that drivers at Indy tend to pay attention to sun and clouds during qualifying--some think iRacing has overdone it a bit.)  Related to this is science that teaches us that at the limit, a portion of the tire contact patch is in the "scary-exceeding the limit" while the adjacent portion is not--so if iRacing's Tire Model places a high degree of grip variability in relation to temperature, this may explain why some feel the "scary" zone is more scary than racing tires in real life. 

Brake Bias

There is no perfect setting for this. The best setting depends on many factors.

Lots of weight is transferred from the rear to the front tires under braking.  This decreases the braking power available to the rear tires and increases the chance of rear tire lockup during heavy braking. This is less of an issue with higher downforce from high wing settings and higher speed.

This weight transfer occurs more quickly with stiffer springs, higher compression damping in the front, and higher rebound damping in the rear.

Past a certain point, higher front brake bias is counter-productive as it will lead to pre-mature front brake lockup and increased stopping distance. This is less of a problem on the FR 2.0 because of the unique monoshock front that tends to be less likely to cause inside front tire lockup during corner entry trail braking. (This premature inside tire brake lockup is most commonly noticed in other cars like the PM in the entry to the corkscrew at Laguna Seca and most famously in  the entry to Turn 10 (Bico de Pato aka Duckbill) at Interlagos

With a limited slip differential, higher preload and lower numerical coast ramp angle setting may reduce rear brake lockup of the inside rear during trail braking—calling for LOWER front brake bias. If you feel the coast differential settings are causing too much corner entry understeer, try decreasing front brake bias and using more trail braking. 

Range for the Brake Bias Setting is 55.0 Low to 57.0 High.  56.5 is manageable for most drivers and settings, however drivers that are highly skilled with fast reflexes tend to use lower settings. Lower settings tend to allow faster lap times as long as the driver does not suffer rear wheel lockup. Lower settings allow for more braking in power from the rear brakes and hence a shorter stopping distance---all of which depends on track conditions, car settings and driver skill regarding brake force modulation.

Start at 56.5, and try lower to see if stopping is improved. Raise it to a higher number if “objectionable” rear wheel lockup occurs.


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