From Chris @ Redshift Motorsports
Rear tire pressure is more complicated because it's a balance of ultimate grip, rotation, and how easy the car is to drive. I hear many people raising rear tire pressures to get the car to rotate better, and this is a very bad idea in my opinion. A tire with higher pressure will grip really well until it starts to slide, and then it's hard to control. I am a strong believer in running lower rear pressures. Why? Because it provides rotation without the tire sliding (which allows the car to rotate in a more controlled manner). You see, the lower the tire pressure, the more "slip angle" the tire will provide (because it's soft and flexes more). Slip angle is the angle defined by 1) the direction the tire is pointed and 2) the direction the tire is actually travelling. A good diagram is located here: Slip Angle Explained ...note the diagram is good, but I don't like the explanation they have much because a tire's slip angle isn't necessarily "sliding" as that page suggests... slip angle from a softer tire is simply the tire "working". Sliding is what happens when the contact patch cannot grip anymore, and slip angle actually decreases when the tire is sliding due to lack of grip (and this is where you spin or at least loose control).Stay with me on this.... so, with a softer tire, we get increased slip angle when the tire is working... and that increased slip angle is effectively "dynamic toe". Meaning, it's the equivalent of the rear tire toeing out a bit to let the car rotate better. Also, a softer tire is easier to control; so you get good rotation without risking a loss of control. I regularly run 24-27 psi in the rear tires on nationally prepped FWD cars and have run as low as 12 psi in back to see how a particular car would act (although I DO NOT recommend you try that). If I were to take a guess at what would be the fastest tire pressure setup on your 2006+ Civic, I would guess 35 psi front and 25 psi rear. Start at something more like 35 psi front and 32 psi rear, but then lower the rear pressure in steps and see how you go faster.ONE MORE POINT.... a softer tire provides additional slip angle well before the limit of the tire. This means that the rear tires are providing rotation for the car well before it's limit, and that makes it faster. For those who increase rear tire pressure to make the car rotate better, you are causing reduced slip angle in back and the car will feel "tight" and not want to rotate until the tire actually starts to slide, and that lack of rotation hurts your speed everywhere on course. Still, it is true that the fastest car will not have too low pressure in the rear tires because that will reduce rear grip; so some balance of tire pressure, spring rate, swaybar, shocks, toe, camber, and what you eat for breakfast is needed to win at Nationals. And if you've read this far, welcome to car development. "