Okely,
Sway bar stiffness is based mostly on two things:
1) diameter of bar
2) the torque arm (some bars are adjustable)
1) Changing the diameter only will increase the the stiffness by the diff. of the two bars to the 4th power. So, the stock rear bar on a '09 Civic Si sedan is 17mm. Increasing it to 18mm will get you a 25% increase in stiffness. BTW, go with solid when ever possible, it might be heavier, but when you go with hollow, you have to calculate a solid bar with the OD, a solid bar with the ID, and subtract the ID bar from the OD bar. So it is lighter by a few lbs. but you are losing out all that stiffness.
Roll bar stiffness (in %) for increase in diameter only:
Tantiswaybar
Usually replacing the anti-roll bar will get you a diff shape/size/etc. than stock. If it is going to bolt up to all the same points, etc. then you might want to consider it the same bar, just a larger diameter. So, lets say you buy that Blox bar, 21mm, do the calcs, you get a 133% increase in stiffness.
2) If the bar is a diff shape enough where you want to calculate it, use the equations at the site below.
Sway Bar Rate Calculator | Build A Faster Car
I dropped the info in a spread sheet, makes it easier to do.
In the diagram on the webpage, the torque arm is -A-. For some anti-sway bars the mounting point is fixed, for some, it is adjustable. In the equation, A is squared, and it is on the bottom of the equation for stiffness. So, if A is a small number, when you square it, you get a smaller number, and then you divide by it, which increases the answer. In math terms; the stiffness rate decreases by 1/r^2. So, the smaller r is, the better. For example is the mounting point is 3 inches away from the bend in the bar (the torque arm = 3"), you will be dividing by 9 in the equation. If you move your mounting point to 2 inches away (torque arm = 2"), you are now only dividing by 4.
Confused? Sweet. No really, simplify it as much as possible. Assume the bars are the same (they can't be that far off if they are mounting in all the same locations), and calc the % increase in stiffness (the first calc.). If you want to some adjustability, get an adjustable bar, you can change the stiffness if you want.
More rules to follow:
-anti sway bars cut down on body roll, but basically do not change ride quality.
-stiffer shocks cut down on body roll, and changes the ride quality (usually not too bad).
-stiffer springs cuts down body roll and lowers your center of gravity, but will change the ride quality, usually to the point of verily harsh.
Cheers!
Jim
'09 Civic Si 4 door