What does the 3rd Spring Do?

On the IndyCar in iRacing, there is a "third spring" in the front and rear.  Pictures are worth a lot of words so:

Pic of Front Suspension

Pic of Rear Suspension

Here is a link to article that provides some interesting data. Pics are from that article.


Article
http://www.lolachampcar.com/3rdSpring.html

Bottom line is that the "3rd Spring" essentially provides resistance to pitch and squat relatively independent of the four "corner" springs.  Note the rods connected to the suspension rockers that go to third rocker supported by the "3rd Spring" assembly.  If the car rolls to one side in a turn, with one suspension rod going up while the other moves down, then the third rocker simply rotates and the "3rd Spring" does not move.  If the car dives while braking or is lowered by aero downforce, than both suspension rods move closing the "gap" and then compressing the "3rd Spring".

Keep in mind that IndyCar aero downforce can nearly double the wheel loading at very high speed, so with the "3rd Spring" it is no longer necessary to carry that aero load with the "corner" springs, allowing the car to handle with more compliance in turns.

The "gap" is the distance that the rocker will travel before engaging the "3rd Spring".  Choosing that setting is an important part of chassis setup.

The actual "3rd Spring" may be a form of dense foam bump-stop progressive rate spring rather than the coil spring in the photos, but the principle of action is almost the same.