Eibach Sway Bar Settings

Natrix

I know I am reviving this thread from the dead, but felt the need to share a few thoughts after reading through all the posts...

Key Points

• For the street, my impression is that the stiffer front Eibach setting is too high. When going over single-wheel bumps, it pulls the car to either side and makes the ride on uneven surfaces a bit rough. Note that I am running 275s all around with the same wheel offset, so this exaggerates the effect. My rear Eibach bar is set to full soft. I have yet to try these settings on the track.
• I tried running no rear bar on the track with the square setup and do not recommend it. It might be because of my spring/damper setup (Swift/Koni), but it felt like the rear was all over the place and transferring too much weight to the outside rear tire. I watched video of a run taken from a friend's dash cam, and I could see the rear end moving around and wanting to break loose. To me, it seems like the car wants some amount of rear bar stiffness.
• If you buy adjustable bars, the safest way to start tuning them is from an understeer bias (stiff front and soft rear). Once you understand how the car behaves in all conditions, adjust from there. Cars are designed to understeer from the factory because it generally makes limit handling more predictable. Changing this balance can drastically affect predictability.
• Some of the forum members who posted in here might be running square tire setups like mine (the same tire width all around), instead of the factory staggered setup. They may also have other modifications that affect how their car behaves. Consider this before putting the exact same settings on your car.

Roll Stiffness
Roll stiffness in the vehicle comprises at minimum four parts:
1. Springs
2. Stabilizer (sway/anti-roll) bars
3. Bushings
4. Tires

Technically, there are at least two more:
5. Mounting point stiffness (how easily the subframe and body attachments flex)
6. Bump stops (only apply at high roll and wheel deflections)

We can leave the last two out for the sake of this discussion, since #5 mainly affects suspension kinematics and compliances, and #6 only applies under high force.

Suspension roll stiffness = #1 + #2 + #3. These combine directly because they act together (in parallel). Springs and stabilizer bars generally have the highest stiffness contributions. Depending on the vehicle type and tuning, their effective rates may be equal. However, bushings can make up 10-20% of the total suspension roll stiffness in a stock suspension. The bushings include the ones in the upper and lower control arms and the dampers. Note that changing control arms to ones with spherical joints reduces overall suspension roll stiffness.

Total roll stiffness = 1 / (1 / suspension roll stiffness + 1 / #4). The suspension and the tire act like springs in series because the suspension is basically "sitting on top" of the tire. Tires are essentially springs made of air, rubber, nylon, steel (and whatever else makes up the tire carcass). Depending on the type of vehicle, the tire can be ~5-10x the suspension roll stiffness. Therefore, the tire can have a ~10% impact on total roll stiffness, up to a point. Factors that influence this include the tire width and aspect ratio, load index and rating (standard or XL), and type of tire (all-season, summer, etc.). Even the same size and type of tire from different manufacturers will have different stiffnesses, because each manufacturer has their own compound and construction. Running a higher tire pressure will also increase stiffness. The tire does not compress as much as the suspension in roll, but it is still a significant part of the equation.

Understeer/Oversteer
There are many factors that affect this behavior, and I would not claim to know them all. My comments below mainly apply to independent suspensions, as on the Z.

One contributor is the front vs. rear roll stiffness balance, which some others have talked about. This is what we can modify most easily on our cars by changing settings on adjustable bars. Generally, the higher the front roll stiffness and lower the rear roll stiffness, the more the car will have a tendency to understeer. Putting tire widths and other variables aside, my understanding of this is that the front "saturates" or reaches its maximum grip first. It becomes the limiting factor. When cornering, the center of gravity or CG of the car wants to keep going straight. It "pulls" on the car and causes it to roll sideways. The suspension and tires resist this force, and if the front is stiffer, it essentially resists more than the rear. What this means is that at the limit, the front will be the first to break traction.

Another contributor is suspension kinematic & compliance behavior.

Generally, the front suspension is designed to toe out under compression and toe in under rebound. As the car rolls, the outside suspension will go into compression, and the inside will go into rebound, so both wheels will "steer" toward the outside of the corner. This is called roll understeer. The rear suspension usually behaves in the opposite way. When a rear wheel goes into compression, it toes in, and does the reverse in rebound. In a corner, this means that the rear tires "steer" into the corner. This is also roll understeer. Why? It helps the rear axle to grip harder in a corner, whereas the opposite behavior in the front suspension causes it to lose some grip in a corner. The combined effect is understeer, which makes handling more predictable.

Finally, many suspensions are designed in such a way that the front wheels "steer" toward the outside of a corner and the rear wheels "steer" toward the inside as a result of lateral force on the tires. This is called compliance steer. The effect is more pronounced under high force, so it may not be as noticeable under normal driving as roll steer. Compliance steer also happens from front to back forces on the tires. If you go for a square tire setup with a common wheel offset like me, you will realize this when braking, as the car pulls to either side, depending on the road surface.

I hope some of this was helpful!