B18C5-EH2

I've done a cursory search here, and didn't see any slam dunks answering my question.

I've got a 1994 Civic Si hatchback (2,300lbs. or so) for which I'm about to purchase BC Racing DR (digressive) coilovers. (<---clickable) Their standard rates are 10K/6K or 560F/336R. I was thinking of going custom rates (for which they re-valve the dampers to match for no extra charge) to have something more like 600/500, and then add a bigger rear bar for more rotation if desired. I'm currently on a stock GS-R 14mm rear bar, and GS-R front bar. I am 100% certain I will stiffen up the rear over the standard 336lb. springs though if only due to my kids riding back there sometimes, and I don't want the car to squat/rub, etc. They're only getting older, bigger, and heavier.

Do I stiffen up the rear by going with a stiffer spring rate, or leave it as-is and go with the 24mm rear ASR bar/brace combo?

My main usage for the car is street driving. Mixed highway, back roads, etc. 90% of the time I'm alone in my car, but for 10% my wife and kids might ride in it on the weekend. I'm used to having rear-biased rates on my old Civic hatch that I built waaay back when, and that car was also street mixed use, but also auto-Xing on the weekends. That car was 1990lbs. and was on 425R/550R rates with no front bar, and a 14mm rear bar. It rode "stiff" and rotated if desired.

Is there any sort of formula one can use to try and see how a rear spring rate vs. diameter of sway bar will affect the roll stiffness/rotation?

Tyson

the ASR rear swaybar has always seemed ridiculous to me. its larger than any other diameter and has short stubby ends. its over kill. the brace is also unintelligently designed and overweight.

that being said, youre better off upgrading the rear swaybar than making the rear springs alone stiffer for the street. you will get the extra roll resistance with more compliance over individual bumps, or even going over speed bumps (straight on).

its easy to calculate the torsion of a swaybar if you approximate the lengths and lever arms and control arms. the math is simple. a simple spread sheet should be easy to configure. but every swaybar will be different so theres no simple formula without all the specific measurements and motion ratio.

comparing the spring rate to swaybar stiffness is apples and oranges tho. they apply at different times. spring rates are in affect all the time, whereas the sway bar only in roll, affecting mostly mid corner. technically, they reduce the effectiveness of independent suspension, and in effect reduce rear grip, but that in effect reduces understeer.