Thoughts on my new suspension setup

Thanks, now when I get the suspension done, I'll know where to go!

 

It's rare to see a fellow create a thread with a a lot of research already done. You were definitely looking in a lot of the right areas and have ended up with some good hardware. RCE Yellow + D-Spec is a nice performance package.

As for sway bars, realize the rear spring rate of the RCE spring set is relatively speaking (compared to chassis weight) stiffer than the front. This will allow the car to oversteer and even now without sways should be noticeable on throttle let-off into corners. You have a sizable amount of stiffness and damping to control the chassis well, so any slides are quite gradual and easily controlled. When picking sway bars, you have the option to stay pretty light, think stock light, if you so choose. You already have a lot of roll stiffness with the springs, so how much you need is dependent on what you want from the car. Understand a couple things about the sways.

First, big sways at a LOT of spring rate during lateral roll. Roughly, it's somewhere on the order of 50 lb/in per mm, but realistically it's not linear. Model years, turbo, non-turbo, STI, non-STI all use different geometry, so the effective spring rate at the wheels varies some. There are some calculators out there that can give you a reasonable approximation if you don't mind hopping under the car and taking a few measurements. The data should also be floating around on various Subaru forums. Don't feel like you have to go big.

Second, understand that sway bars are spring devices that are not damped. This means big sway bars will make for a very springy car during side to side transitions. This can be quite problematic with softer spring and damper setups as big bars are so minimally damped in motion that the car can be quite twitchy, and you can get into snap oversteer situations pretty easily with the wrong setup. Even with a reasonable setup, you will find a car running bigger sways rather than stiffer springs will be a little more wild around the limit. Some of the choice of spring stiffness versus sway bar size comes down to how much damping you want during dynamic transitions.

I also want to bring up the idea of suspension independence. Sway bars link both sides together and try to move both sides up and down together. You will find that you can run quite stiff springs with small sways and get a lot of rough terrain compliance despite the high spring rate. Some may argue that the big sways are used for handling responsiveness. We certainly like to reduce roll in order to improve turn in and side to side transitions. However, this is also achievable through other forms of resistance, like damping. High damping will also resist the roll and can allow excellent turn in while still offering the ability to keep moving after you've generated a response from the car and allow compliance where needed. Even on a setup that may generate more overall body roll, high damping can still allow you to create tire loading force immediately and get you that quick response. Damping follows spring rate, so in order to run higher damping, you would be running higher spring rates. If you do big sways but stay with soft springs, you're stuck with low damping. You get spring stiffness but not motion resistance. It's an entirely different feel and behavior.

The short of it all is don't feel too eager to go big with sways. You likely would still prefer some, but don't think you have to toss on massive sways for no reason at all. This can be especially true if you intend to rally-x the car. Unfortunately I would also suggest a taller spring for rally-x as adequate bump travel is important. RCE springs are a bit too low, fine for asphalt, not so fine for rough terrain. You will just bottom out a lot.

 

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