Sway bars are awesome, they are easily one of the most cost effective handling solutions for any vehicle out there even compared to coilovers, but there are some hidden downsides that many companies don't talk about. We are going to give you the good and bad sides to running sway bars on your vehicle.
Sway bars, or anti-sway bars, are a torsional spring that attach to the underside of your chassis and the lower control arm via a sway bar endlink. When entering a corner the weight of the car is pushed outwards towards the outer tires. This compresses the suspension on the outer part of the vehicle and decompresses it on the opposite side. This in turn imparts a twisting force (torque) on the sway bar which then creates a lifting force on the outside control arm. The force counters the weight transfer and the result is a flatter handling experience.
This seems to be a win:win situation because it gives you the same effect as increasing the static spring rate on the vehicle, but without the associated change in harmonics and ride harshness. However, as we learn early in life, there simply is no free lunch.
Newton's third law of motion states that "for every action there is an equal and opposite reaction." This is the case with the sway bars as it is with everything in our universe.
When the sway bar creates a lifting force on the outer control arm an equal amount of lifting force is generated on the inner arm; due to sway bar geometry. This force pulls on the unladen control arm and tries to lift the tire off the ground. This force is resisted by the static spring and compression dampening of the shock itself as the control arm tries to go upwards. This reduces the amount of dampening travel available to the shock if the car switches back quickly, an effect called "jacking" where the suspension continuously shortens due to the shock not being able to properly react fast enough to force inputs. While this is not a guaranteed effect, a larger bar will increase the likelyhood of this occuring.
Another issue is that while driving you will encounter uneven bumps in the road. Without a sway bar the suspension acts fully independent of each side, which allows a single shock / damper to absorb and dampen oscilations without imparting significant loads on the other static dampers. This is an ideal setting for allowing the suspension to do it's job ... dampening oscillations and maintaining tire interface with the road surface. When you install a sway bar you are creating a second, direct load path for the connected corners. When the right front hits a bump, that force is now transmitted to the left front. This creates an unsettling effect compared to the first scenario as the suspension on two corners now reacts sharply to forces. This effect increases as the sway bar stiffness increases.
A third issue has more to do with the fact that many people do not corner weight their vehicles. When you involve a large sway bar the uneven ride heights and static load on the individual dampers creates a static torque in the sway bar. This increases the differential that the right and left side have when it comes to handling. That is to say, turning right and turning left create different handling personalities, amplified by the bar's pre-load.
Lastly, a caution for vehicles that are running big bars on LSDs. Lifting the inner wheel is common with big bars, and if this happens consistantly during moments of acceleration the unladen tire will spin, heating the differential fluid and causing accelerated wear on the limited slip differential. (Added thanks to insight from Brian Burdette)
Now, here's another thing that is commonly misunderstood or simply left out. Sway bar thickness is RELATIVE. There are many things that affect how much force (or how effective) the sway bar is, and they are:
1) Amount of suspension travel.
2) Endlink / bushing compliance.
3) Sway bar geometry.
4) Sway bar thickness and composition.
Suspension Travel - As you increase the amount of suspension travel, e.g. soft spring rates or sticky tires, the sway bar will impart more total force. Torsion members, such as a sway bar, follow Hooke's Law which states that the amount of torque generated is relative to it's spring constant (torsion MoE) times the angular displacement.
Endlink / Bushing Compliance - Rigid mounting due to high durometer bushings (polyurethane for example) and stiff endlinks will increase the effectiveness of the bar. The more the bar is allowed to move in it's entirety, relative to the chassis, the less effective it will be. Notice the presence of travel limiters (bushing stops) on this Eibach sway bar, which limits the side to side slop of the sway bar.
Sway Bar Geometry - As seen above the sway bar is formed with several bends and angles. The key here is the length of the "lever arm" that extends beyond the mainline of the sway bar. The longer this arm is, the greater the torque generated and the more force the bar generates. This is what you are adjusting when you move between the two holes in the blade portion of the bar. Remember Hooke's Law!
Sway Bar Thickness and Composition - Not all thickness is similar! Changes in material can create two different force curves between two 25mm thick bars. The stiffer the material (again, Hooke's Law!) the more force is generated. A hollow bar is a light bar, but is also not as effective as a solid bar. However, you can have 90% of the effectiveness of the solid bar with 25% less weight at the same thickness.
Here's a Cheat Sheet.
As your static spring rate INCREASES, the sway bar thickness should INCREASE.
As your static spring rate DECREASES, the sway bar thickness should DECREASE.
As suspension travel distance INCREASES, the sway bar thickness should DECREASE.
As suspension travel distance DECREASES, the sway bar thickness should INCREASE.
As tire traction INCREASES, the sway bar thickness should DECREASE.
As tire traction DECREASES, the sway bar thickness should INCREASE.
If the car UNDERSTEERS, the FRONT sway thickness should usually DECREASE. (Not always)
If the car OVERSTEERS, the REAR sway thickness should usually DECREASE. (Not always)
We've noticed a lot of companies pushing for thick bars for soft spring rates and thin bars for hard spring rates. This is not inline with how sway bar's work. It's a short-sighted strategy in how sways affect total spring rates.
Sway bars, like every suspension component, comes with some kind of give and take. We recommend that the sway bar fulfill a role of over/understeer correction and fine tuning of the overall spring package of the car. The coil springs should always be your primary body roll resistance force, with sway bars acting to fulfill about 10 - 20% of that force in order to correct any dynamic handling issues or preference.
We love Eibach sway bars for many reasons. They are hollow, which is lighter and just as effective, while giving a superior stress life. They are affordable and made in the USA. They are produced from good spring steel bar stock, and they are adjustable!