Suspension 101

Our Experience with Suspensions is Unmatched and Sets us Apart From Everybody

If you’re thinking about lifting your vehicle, you should have understanding of roll and instant centers, a vehicle’s center of gravity, progressive rate springs, shock absorbers, steering geometry, roll steer, joints and bushings, durability and value.

Roll center geometry is the imaginary point where the body leans in a turn and also where the suspension flexes when riding on the trail. There is a different roll center for the front and rear suspensions. In most solid-axle suspensions the roll center location is set by the geometry of the track-bars. For example, in a late-model solid-axle Jeeps, the front track-bar runs from the frame on the driver’s side to the axle on the passenger’s side in front of the axle. The rear track bar runs from the frame on the passenger's side to the axle on the driver's side behind the axle.

To find the actual roll-center set an imaginary vertical line down the middle of the vehicle and another straight line between the joints at the end where the track bar is connected. (Some manufactures install bends in their track bar but in calculating roll center it has no effect.) The intersection of these two lines is the roll-center. Roll-center is key because it's how you manage body lean and weight transfer in turns. This revolves around the roll center moving from the center of gravity. That in turn causes more lean and decreases handling. Trying to correct the problem with spring / shock tuning will cause a loss of performance somewhere else. This critical piece of geometry must be right in order to lift a vehicle. Again, looking at a solid axle late model Jeep the track bar placement must be properly located to achieve the proper center of gravity. Once the roll center matches up, the real performance gains using springs, shocks and control arms can be used to allow a lifted a lifted vehicle maximize performance. It's impossible to get optimal performance when you have to compensate for poor geometry. The same concept apples to trucks that do not have solid axles except that the control arms are relocated in order to maintain the correct roll center.

In Solid axle control arm geometry, in order to control the fore and aft movement control arms connect the axles to the frame. Just about all solid-axle suspensions have 2 arms on the 4 corners of the vehicle one above and one below. On a non-lifted vehicle, they are just about parallel to the ground. Upper and lower control arms keep the axles from "flipping over" during braking or acceleration forces. Their length and angles relative to the axle geometry points are called "instant centers". It's the same as roll centers in their relation to the center of gravity. This affects what happens during bumps, turns, acceleration, braking, and in real world driving combinations of all these things. Anti-squat is the geometric resistance to the rear end dipping during acceleration. Anti-dive is like squat but refers to the front end during braking and roll-steer that happens when the vehicle leans in a turn. It makes the control arm on a solid axle twist inward in the direction of the turn. This causes vehicle to move without any movement of the steering wheel. All of these are very important to a lifted vehicle's setup.

Correct geometry will result in a stabilizing effect such as understeer. When incorrect there are destabilizing effects such as oversteer. When trying to achieve maximum ground clearance on a lifted 4x4 it can be a challenge to create good geometry. Without it things like understeer, it can make a vehicle hard to handle around curves and difficult to control on mountain roads. Most people report this as twitchiness. A lift kit that increases the control arm angles too much will add a dangerous amount of roll oversteer. The most dramatic effect happens in the rear suspension because there is no way for the driver to control or compensating for the direction of travel using the steering wheel.

Roll steer happens when the control arm angles are so steep they move the axle fore and aft as they move up and down due to body lean. Longer arms are an improvement by reducing steepness and relocating the instant centers but this will only work if the right and left long arms are properly angled towards one another at the chassis at 2°. If angles are not correct the longer arm benefit is wasted. A good example that we often use is geometry correction brackets. They not only improve the approach angle of the front control arms they also change the location of the instant center. This improves anti-dive quality under hard braking.

Progressive rate springs are designed around the concept of frequencies. It’s the speed at which a spring-mass system moves when disturbed. It is measured by frequencies. In the case of a solid axle vehicle like a late model Jeep, the body and chassis are the mass and bumps in the road are the disturbances. Forward and rearward halves of a solid axle jeep both have spring systems and they represent two spring mass systems that must interact with each other.

To Look at the concept behind frequency-based spring rates, imagine a shock-less vehicle driving over a single speed-bump. When the front end hits the bump, it moves up and down. This is the ride frequency. When The rear hits the same bump there is a delay because of the wheelbase and vehicle speed. The rear needs to react faster than the front so that it can catch up at just the right time. To get the best possible combination of ride and handling the front and rear spring rates must be tuned to work in cooperation. There are a lot things to consider in doing this including the sprung weight of the vehicle, load carrying requirements and the different speeds a vehicle travels.

This is where a progressive spring rate will be beneficial as they keep their frequencies closer and consistent within the load range but there is more to determining the spring’s ideal rate then just weighing the vehicle and adding on some extra capacity for passengers and cargo. When we install a performance suspension system we carefully select the correct springs that are constructed using a frequency-based methods because you want a vehicle to level out quickly after a bump. That way the shocks are not overtaxed with trying to control body position motion. Instead of their real purpose which is controlling the up-and-down movement.

There are two functions of shock absorbers to damp out body motion and limit the downward and rebound (upward) of suspension travel. Shock tuning should be undertaken after geometry, spring design, and stabilizer bar sizing is complete. Shock absorbers are all about refining the ride not correcting bad geometry or incorrect spring rates.

Actual shock tuning involves varying the internal valves to get the dampening forces matched to spring frequencies. We carefully select our shocks from those that have been tested over thousands of miles. This allows our builds to cruise over uneven services, turn corners with complete confidence and unmatched ride comfort.

Steering systems dramatically change when suspension height is increased, the track bar and drag link must be parallel to avoid bump steer but that’s only the beginning of the puzzle


Understanding steering geometry starts with the concept of roll steer that happens when the steering linkage doesn’t pass through the role center of the suspension. Every time a vehicle leans or articulates there is steering input that that the driver did not initiate. It’s a lateral shift of the axle relative to the Pitman arm on the steering box. Since the steering wheel didn't move but the axle did, the steering knuckles must rotate to make up the difference. This creates unwanted steering. On twisty and bumpy roads roll steer along with rear suspension oversteer can keep the driver busy trying to maintain the intended direction. It quickly leads to Driver fatigue and frustration.

It’s situations like this where we use a high steer kit which re-positions both the track bar and the steering drag link to flatten out the operating angles insuring that the drag link passes through the role center of the suspension geometry. A lifted vehicle with these corrections will greatly reduce driver fatigue, improve safety and allows for very precise steering response.

Joints and bushings may seem simple but these rubber bushings are just as important as springs, shocks and stabilizer bars. Their hardness is a science in itself. For example, track bar bushings that are too soft will result in vague steering and a tendency to shimmy (“death wobble?). Bushings that are too stiff can cause the bar or brackets to fail. Control arm joints are another issue entirely because oftentimes lift kits use hard plastic or all metal joints that result in a harsh ride or bracket failures. It’s been found that there is no reason to eliminate stock suspension’s inherent bind at-large degrees of articulation. A common pitfall is replacing bushings with joints that seek to eliminate the bind all together. This is extremely shortsighted because they ignored the fact that the binds were actually observing part of the impact forces from bumps which is why that are there in the first place. This is why in the majority of cases we recommend using the factory upper and lower control arms. In reality, the amount of flex needed for 95% of jeep drivers can be had by keeping the stock Jeep JK arms they are long and strong enough for even hard off-road use and contain factory-tuned and durability-validated bushings.

Durability and overbuilt is not the same but the latter is typically used when there’s a lack of technical resources, time or patience. What happens is that a heavy accessory causes a lot of new problems usually not in that accessory but in those around it that must now cope with the extra weight. This is how 5000-pound Jeeps fail on the trails much more often than lighter rigs on the same trail.

Making a bracket or part means more than just the design of the bracket or part, it also requires a full understanding of and managing the forces that apply to it from the overall system. For example, a track bar bracket should not cause the failure of the stock track bar it’s bolted to because of the excessive leverage it causes. The recipe in creating an optimal suspension set up is picking the right parts in relation to the others that are going to be integrated and matching those to the factory components that are going to be retained.

There is a falsehood that the recipe for one vehicle applies to all for example:
The “long-arm legacy” from Jeep TJ to JK Wranglers. Long- arm-based suspensions were an ideal geometry solution for TJ. because the stock short-arm geometry degrades rapidly with lift height. Contrast that with the 5- link/solid-axle JK suspension similar to the TJ in basic concept but with numerous improvements such as 40% longer boxed-section arms and longer track bars. This is the reason, we don’t recommend long arms to correct geometry issues in Jeep JK’s unless they are lifted more than 4.5 inches.

Value is not a technical issue. it’s what you get for your money. There is so much miss conception regarding the right way to do a given lift height for a given application. A bargain hunter might dismiss a complete kit as having too much fluff. The reality is that’s more than just buying something cheaper or listening to the opinionated crowd. In the long run, those that do that often end up paying more than they should have and even worse is the suffering after they discover the design, durability or performance shortcomings on their lifted vehicle. Done properly getting everything right the first time is the basis for a safe, enjoyable and highly versatile suspension system.

This is why you can rely on us with our more than 20 years of experience in suspension applications and design to provide you with the right information to ensure you have an enjoyable experience on your next adventure whether it’s the mall, the Rubicon trail or anywhere in-between.