Tire Pressure and Brake Bias for the iRacing FR2.0

Tire Pressure and Brake Bias for the iRacing FR2.0

Intro:

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.

http://www.racer.nl/reference/pacejka.htm

Pacejka’s Book:

https://books.google.com/books?hl=en&lr=&id=JCHDuED3WIkC&oi=fnd&pg=PP1&dq=%22Hans+Pacejka%22&ots=cFa_d3smIl&sig=tVmwr355Iw_zd0OjsNY9ryGC8XI#v=onepage&q=%22Hans%20Pacejka%22&f=false

Brush model:

http://folk.ntnu.no/skoge/prost/proceedings/ecc03/pdfs/317.pdf

Essentially:

Which is the basis for the curve you see below:

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

http://www.iracing.com/the-sticking-points-in-modeling-tires/

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.    https://www.hindawi.com/journals/mpe/2013/261582/

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:   http://racingcardynamics.com/racing-tires-lateral-force/

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.

https://www.youtube.com/watch?v=VNYUkRKslEw

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.

Camber/Caster and Toe for the iRacing FR2.0

Camber/Caster and Toe for the iRacing FR2.0

Intro:

In the FR2.0, Caster is fixed and not adjustable. It is most probably affected by other settings. Based on experiments, there is a small increase in positive Caster when increasing the negative value of front Camber.  An increase in positive Caster pushes the inside front tire down which causes weight to be transferred to the outside rear tire and  in most cases this makes the car oversteer more, or understeer less.

The Camber settings on the iRacing FR2.0 are generally lower or less negative than the car in real life, and much lower than you typically see on F1 cars.

Camber adjustments and their effects has a great deal to do with tire characteristics, as the “tire patch” is deformed and moved by cornering forces.

Camber and Toe-In or Toe-Out are essentially settings that affect cornering force, overall stability, traction for braking and power application, and mechanical drag.

Our goal is to achieve the highest cornering force and good stability while minimizing loss to traction for braking and power, and having the acceptable mechanical drag.

Camber

Negative Camber is simply the angle that the tire leans IN at the top.

The photo above is a real test car with an extreme amount of negative camber. Interestingly, the test results confirmed that with “rounded” motorcycle type tires, the more negative camber the better.  On “rectangular” race car tires, the optimum amount is much less because as the tire leans more, the contact patch becomes smaller.

Most race cars use a form of double wishbone suspension. Note that the upper rods are shorter and tilting at a greater angle than the lower ones.

This difference in length and initial angle cause the camber angle (relative to the chassis) to change as the tire moves up and down, whether the movement is caused by aero downforce, bumps, braking, power acceleration, or chassis roll. The particulars of the design, determines how much the camber angle changes relative to the ground.

In the picture below, the outside tire has moved up relative to the rolling chassis. The chassis roll of 2 degrees does not cause the outside tire’s camber to change 2 degrees (less negative) relative to the ground—instead the shorter upper rod causes the camber to only change by one half of a degree.  If the outside tire (left in the picture) had started with negative 1.5 degrees camber, than during cornering it would have negative 1.0 degrees. This is good.

The opposite happens on the inside tire.  The chassis roll of 2 degrees does not cause the outside tire’s camber to change 2 degrees (more negative) relative to the ground—instead the shorter upper rod causes the camber to only change by 1.5 degrees.  If the inside tire (right in the picture) had started with negative 1.5 degrees camber, than during cornering it would have negative 3.0 degrees relative to the ground. One will immediately realize that in a turn, the inside edge of the inside tire will be doing more “work” than the outside edge. This is not good and is really not good for the rear where power is applied to both tires. (That is one reason why camber needs to be limited and rear camber is usually less than front.)

The primary benefit from Camber is from Camber Thrust.  Read the article at this link for more explanation:     https://en.m.wikipedia.org/wiki/Camber_thrust

Toe In/Out

The primary benefit from Toe Out/Toe In is tire wear and stability.  Read the article at this link for more explanation:    https://en.m.wikipedia.org/wiki/Toe_(automotive)

Testing/Results

We took the FR2.0 to the Centripetal Track and tested five different combinations of Camber:

     Low                              Medium                     High

-0.5F/-0.5R                  -1.5F/-1.0R                  -2.5/-1.8R

-1.0F/-0.5R                  -2.0F/-1.4R

The “Low” settings did not produce the optimum cornering force. The “Medium” settings provided sufficient cornering force to allow a 1.5-2.0% faster cornering speed. 

The -1.5F/-1.0R combination had the best tire wear—more uniform across the face of the tire tread. The -2.0/-1.4R had a marginally higher cornering force.

The  “High” setting produced unacceptably uneven tire wear, although in “high grip” track conditions with higher chassis roll, might be acceptable.

We next checked to see the relationship, if any of camber to top speed—a way of measuring mechanical drag. On to the Talladega Speedway.

With the -2.5F/-1.8R “High” setting, we reached a maximum speed of almost 158 mph. With the -1.5F/-.1.0R “Medium” setting, we were almost 1 mph faster with a 0.5% faster lap time.

Actually, this confirms the “theoretical” mechanical drag produced by the “Camber Thrust” that occurs from tire patch deformation. This Camber Thrust adds to cornering force, but also produces mechanical drag.

Next we checked the relationship, if any of camber to braking power and stopping distance. It is assumed a similar relationship for traction under power would exist.

Running at 155 mph, with threshold braking, the -1.5F/-1.0R “Medium” setting allowed the car to stop 6.5 feet or 1.6% sooner than the -2.5F/-1.8R “High” setting. Running at 100 mph, a similar result with the “Medium” setting stopping in 7 feet or 4% less distance.  

My theory on why the larger difference for lower speeds, is that at the lower speed, there was less downforce and impending wheel lockup occurred over a larger relative part of the braking zone—where traction would be more critical. (i.e. there was “surplus” traction with the high downforce at higher speed.)

Finally, while at Talladega, we tested various Front and Rear Toe combinations.

0/32F/0/32R produced 158+ mph with a lap time of 1:00.749

-1/32F/0/32R produced fastest speed of the day with lap time of 1:00.678 (only 0.12% better)

-2/32F/0/32R saw little or no change from the 0/0 settings.

-2/32F/+1/32R saw little or no change from the 0.0 settings.

-2/32F/+2/32R still achieved 158+ mph and lap time of 1:00.749

-2/32F/+3/32R saw a 0.5% reduction of speed

Essentially the slight toe out of -1/32F offset or cancelled some of the drag from the negative camber.

-2/32F toe out in the front did not produce any significant drag and would be suitable for improving initial turn in according to driver preference.

-1/32 or -2/32 toe in for the rear (each tire) did not produce any significant drag and would be suitable for increasing stability and decreasing oversteer under power during cornering according to driver preference.

Summary

The chart below illustrates the relative grip with various camber settings.

The ideal front setting is in the 1.4 to 2.0 range. With lower settings, you will not be optimizing cornering force that is gained by camber thrust. With higher settings, you will begin to risk overheating the inside edge of the tire, but more importantly will be adding to mechanical drag and reducing traction for braking application. (There are exceptions depending on all the other chassis setup settings, depending on the importance of braking power, and for qualifying vs racing trim.)

The ideal range for the rear is in the 0.8 to 1.6 range. With lower settings you will not be optimizing cornering force that is gained by camber thrust. With higher settings, you will begin to risk overheating the inside edge of the tire, but more importantly will be adding to mechanical drag and reducing traction for power application. (There are exceptions depending on all the other chassis setup settings, depending on the importance of traction for power, and for qualifying vs racing trim.) 

Note that the rear range is lower, and the “average” difference between front and rear is generally 0.4-0.8 degrees. This is partly due to the individual car’s particular suspension design, but generally, rear wheel drive cars are using the rear tires for braking, cornering and power application, and therefore having both tires with maximum rubber on the ground is the goal. Too much rear camber (which rises on the inside tire during cornering) can result in lost traction on the inside tire during corner exit.

Toe Out on Front should be -1/32 to -2/32 each tire to suit driver preference on initial turn in.

Toe In on Rear should be 0/32-+2/32 each tire to suit driver preference for reducing “power on” oversteer at mid corner and on corner exit. (Throttle application can be theoretically more abrupt.)  

Higher Camber will result in a larger difference in tire temperature and wear from inside to outside. More Toe Out will increase this tendency. Toe In will decrease the tendency. Look for an ideal where the Inside and Center Tire Temps  and Wear are close to the same with lower Temps and Wear on the outside edge. And, of course, as mentioned in earlier articles, a balanced race car exhibits similar tire temps and wear on the front and rear tires on the same side of the car. (You will often see a difference between the right and left sides of the car—that is dependent on the track design.)

Please remember that these settings all are interdependent on other chassis settings and driver preference. Test, make minor changes, and retest. Each driver/car/track combination will be different. 

Setup Training for the iRacing FR2.0-Differential Setup

Differential

Differential—Preload and Ramp Angle

Intro:

When writing about race car dynamics, I often comment that the most mis-understood aspect of setup is front caster settings.  In the FR2.0, the caster is fixed, so it is not an issue.

But, for this car the most mis-understood setting is probably the Differential.

Race cars generally use one of four basic types of differentials:

1--Open

2—Spool or Locked

3--Limited Slip

4--Locker

The Open differential was developed to allow the tires on each side R-L to turn different speeds—helpful when turning. Torque is applied equally to both tires, with the torque applied being determined by the tire with the LEAST traction.

The Spool differential is essentially where the tires on each side are locked and always turn at the same speed. Same torque applied to both tires.

The Limited Slip differential is an Open differential with a mechanism that allows to act like a Spool differential, up to a limit, afterwhich the tires on each side R-L may turn at different speeds.

The Locker is a special device that acts like an Open differential when “coasting” or decelerating and a Spool when accelerating. (e.g. Detroit Locker)

The FR2.0 uses a Limited Slip Differential.

The following Youtube video’s are quite informative.

https://www.youtube.com/watch?v=SOgoejxzF8c&feature=youtu.be

https://www.youtube.com/watch?v=PEdnH7_7_yc

The above drawings illustrates the Ramp Angles very well.  Often it is confusing that a lower Ramp Angle produces more locking and a higher Ramp Angle produces less locking, but from the video and the drawings, you will see that the lower angle is essentially a “sharper” wedge that produces more sideward force against the clutches.

The Diff Preload is created by springs that push against the clutches all the time.

The Ramp Angle creates additional force against the clutches during “coast” and “acceleration”.

The more force applied to the clutches, the more locking force===the more the differential acts like a Spool.

The less force applied to the clutches the less locking force==the more the differential acts like an Open Differential.

The following link will take you to an excellent writeup by a group of Sim Racers in Denmark where I found the drawing.

 http://www.intothered.dk/simracing/differential.html

Another link that you will find useful was written by my friend, Craig Taylor at Taylor Race Engineering in Plano, Texas.  Craig and I raced against each other in SCCA FF1600 during the 1980’s. at Texas World Speedway. He was really fast and like me, was an airplane pilot. He later went on to win the US F2000 Championship.

https://taylor-race.com/sites/default/files/DIFFERENTIAL%20ESSAY%20W%20PHOTOS2.pdf

Oversteer (Loose) and Understeer (Push)

Determining exactly how the car will react to changes in the differential is much more complex than most people realize. So much is “going on” with the suspension, brakes and steering, that the answer as to specific cause and effect of changes in diff settings is: IT DEPENDS!

For those interested, read the paper of research at GM over many years authored by Bundorf, who actually created a form of analysis for studying dynamic understeer/oversteer.  Bundorf Analysis where understeer is measured in units of degrees of additional yaw per g of lateral acceleration.

Following is the link.

http://www.millikenresearch.com/VehicleDynamicsAtGMByRTBundorf.pdf

The above illustration shows what the driver is sensing is changes in relative slip angles on the front steering tires vs the rear powered tires.  Here is the link to the entire article:

https://sweetmfg.biz/uploads/files/tech-04understandingsteering-4.pdf

Practical

OK, lots of theory and engineering stuff.  Let’s again do some real world testing and practical info of how to make the IRacing FR2.0 go fast.

Here are the screens for setting the Diff Preload and Diff Ramp angles. What many  don’t realize is iRacing provides a HUGE range of adjustment in Diff Preload from a low of -100 ft-lbs to a high of  +100 ft-lbs. That is a total range of 200 ft-lbs—theoretically in the amount of torque that it would take to rotate one wheel while the other is stationary. (I assume -100 is zero and +100 is 200 ft-lbs total but I have no way of knowing that for sure. The most common settings I have seen for Diff Preload is in the 0-25 range.)

For the Diff Ramp Angle, there are only two choices: “70 coast/65 drive” and “65 coast/70 drive”.

The 65 is the angle of “wedge” mentioned earlier and provides more locking force.  So, generally, unless you love driving karts with Spool-like solid axles where you throw or wrestle the car into the corners, generally “70 coast/65 drive” will produce the best handling car as it will have less locking and understeer during Corner Entry and Mid Corner. (Mid Corner understeer is generally not fast.)

Minimum Locking= -100

Maximum Locking = +100

“Typical” Locking =  +10

To determine the effect of changes, I tested the car at all three of these setting examples at several tracks. Actual engineering data was hard to collect, so here is some “anecdotal” evidence.  Try it for yourself. 

In T1 at Silverstone, with Minimum Locking, I got a lot of trailing throttle oversteer on corner entry—the car felt a lot like a Pro Mazda with an Open Diff. My steering input was quite low at the apex. The feeling of oversteer disappeared as I tracked out and the car felt a bit “understeery” on Corner Exit.

In the last hairpin at Sonoma, the car with Maximum Locking seemed to have a great deal of understeer on Corner Entry and then felt a bit “oversteery” on exit.

Hmmm—Minimum Locking a bit like an Open Diff—like the Pro Mazda. Maximum Locking a bit like a Spool---like a front engine GT3.

Keep in mind this was testing at the extreme settings.

I really wanted data, not “feelings”, so I tried the car at Milwaukee.  A flat oval with big radius corners. Then at Martinsville—a flat oval with relatively sharp corners.  Ahh! Eureka!!

See the iSpeed Telemetry snapshot:

Here is a link to the replay of the test at Martinsville in the FR2.0

https://www.youtube.com/watch?v=Us6-ofZjNYI

More steering input on Corner Entry and Mid Corner (A-B) would indicate understeer.  Less steering input on Corner Exit (B-C) would indicate oversteer or almost neutral. This is what I found.

The more locking force (mostly from Diff Preload setting) the more the car pushed or understeered in Corner Entry and Mid Corner. Lots more, like 20+ degrees more steering angle required to maintain the same arc. Because of the additional steering input at Mid Corner with more locking force, the car has a bit more rotating momentum or inertia as it accelerates and hence needs less steering on exit.

Keep in mind that even with -100 Diff Preload, because of the 70 coast Ramp Angle, there is still quite a bit of locking force on coast. 

Discussion

A Spool Diff works OK on a dirt oval. I have driven one in a Pro Dirt Legends Series in Central PA. It can be made to work on a paved oval. But, it does not work very well on a road course. Why?

A Spool resists the turning forces on a car and causes understeer, UNLESS, you can adjust the tire size or “stagger” so that the inside and outside tires rotate at different speeds, and unless you can adjust what is called the “bite” and “crossweight” as the car "rolls thru" Mid Corner and accelerates to Corner Exit. So on ovals, you can setup the car to turn left better than it turns right. (An Asymetrical Setup for ovals.)

An Open Diff does not resist the turning forces on a car. Most cars with an Open Diff have some trailing throttle oversteer—not because of the differential, but because of all the other setup parameters. The Open Diff simply does not offset any of the ovesteering tendencies. (Although it does seem to have a bit of oversteer induced immediately as you lift the throttle and the diff sees engine braking resistance.)

Because an Open Diff will stop delivering torque when the inside wheel loses traction, it often tends to accelerate less powerfully off the corner. And, when trail braking, it is easy to lock up the inside tire, which often produces snap oversteer. 

A Limited Slip differential is a hybrid, that acts a bit like an Open Diff and a bit like a Spool. The Limited Slip Diff will introduce understeer during corner entry, offsetting some or all of the trailing throttle oversteer tendencies. A Limited Slip differential will allow much more trail braking than would be possible with an Open Diff.

A  Limited Slip differential does not have as much corner exit understeer as would be evident with a Spool, and because at the limit, it allows the tires to turn at different speeds, while still transmitting torque, it will allow for significantly more corner exit acceleration.

As one realizes from driving IndyCar or F1, even a Limited Slip differential will allow the car to be loose when applying power if traction is lost or even limited by the inside tire because of springs or ARB settings that are too stiff.

Also, as oval track drivers know, there is a phenomenon known as Tight In/Loose Out caused by driver input overcompensating for understeer at Mid Corner. Here the driver enters more steering input at Mid Corner to make the car rotate, and then the car becomes loose or oversteery on corner exit. 

Finally, the sharper the corner—the lesser the corner radius, the less locking you will want.  The larger the corner radius, the more locking you will want.

My advice

Set the Ramp Angle at “70 coast/65 drive” and forget it unless you really love a lot of understeer on Corner Entry. Use the Diff Preload for “fine tuning”.

Initially set up the Diff Preload somewhere in the -10 to +25 range depending on your preference for understeer during Corner Entry and the amount of trail braking you prefer.  The higher the more understeer you will get, but the the more trail braking you can use to get the car to rotate. This intial setting will be your “neutral” or “baseline”. Experiment to determine your “preference” in this range.

Then, AFTER you think you have everything else (Downforce, Springs, Dampers, ARB’s, Camber, Tire Pressure, Toe) just the way you want it, go back and experiment to see if any changes improve your lap times.

One final note. Keep in mind that iRacing provided the huge 200 ft-lb range in Diff preload for a reason. They seldom provide a range that is completely irrelevant, so do not be afraid to experiment with settings that you might not often see or have seen in the “shared “ settings.  I found the car surprisingly drivable on many tracks with both extreme settings—I achieved identical lap times at Milwaukee, Silverstone and Sonoma—by just adjusting my driving to the car’s limits. When I set the preload at +100, I expected the car to "push like a snow plow" but it did not.  On tracks with sharp corners, like Martinsville, the low Diff settings allowed for significantly faster laps.

BTW--the little 210hp FR2.0 just about matched the XFinity car times at Martinsville!


NOTE:  You will find that many drivers will run a much "stiffer" rear--with 900# springs and quite a bit of ARB. This makes the car quite "loose" or oversteery----so they offset this by increasing Diff Preload to 40+ and change ramp setting to 65coast/70 drive. These same drivers tend to run much less front brake bias.

My personal view is much of this driver preference has to do with how they trail brake. Normally, higher Diff Preload can be run with a heavy trail braking technique, but lower front brake bias makes trail braking more difficult. So this "stiffer" set up preference is generally a desire to rotate the car during corner entry without trail braking.