What is Adjustable Suspension | Car Suspension System Setup

Another form of suspension takes the concept of adjustable ride height to the next level. Air suspension allows ride height adjustments to be made with the flip of a switch from inside the car. How does air suspension work? The concept is quite simple; an onboard compressor pumps air into airbags or air shocks that replace the car’s coil springs and increase in height when aired up, then decrease in height when the pressure is released. This allows the driver to pump up the ride height for street driving, then dump the car on the ground when parked at the show or profiling on a smooth street.

What’s better when considering air suspension vs. coilovers? There are advantages and disadvantages to both, and the right answer for your car will depend on the car’s intended use and what your priorities are. Coilovers are very reliable. As long as they are mounted properly and stock shock mount locations are sufficiently beefed up to bear the weight of the car, a coilover will perform reliably for miles on the road and many weekends at the track. They are widely used on modified street cars and the go-to for dual purpose cars that will spend time on the street and the dragstrip, autocross, or road course. The only real disadvantage to a coilover suspension is that ride height adjustments are somewhat manual, requiring that weight be taken off the suspension and the adjuster collar cranked with a spanner to move it up and or down.

In contrast, that’s where air ride suspension really shines. That “flip of the switch” that makes major ride height adjustments is hard to beat. But that convenience requires some major complexity to make it happen. Air suspension is a system that includes the air compressor, airbags or shocks, and all the wiring and plumbing to connect everything. Compared to coilovers, air suspension is far more complex, generally more expensive, and inherently less reliable. Is air suspension good for racing? Generally not, unless it’s a dedicated system purpose built for the race type. The added weight and complexity is generally avoided on a race car. But there are street cars out there with thousands of trouble-free miles on air suspension. Just remember that the added lines, fittings, and wiring constitute an extra system that will need to be inspected and maintained.

Adjustable Damping Shocks | How do Adjustable Shocks Work?

Shocks and coilovers with adjustable damping allow for adjustment to the shock’s compression and rebound settings and can be incredibly valuable tools to dial in the car’s performance on the street or the track. Don’t get confused when it comes to adjustable shocks vs. coilovers; coilovers allow for adjustment to ride height, but not all have adjustable valving. Conversely, some non-coilover shocks offer valving adjustment but do not adjust ride height.

While there are many variations, the most common types of adjustable shocks are single-adjustable and double-adjustable. As the name implies, single-adjustable shocks have only one adjustment. That adjustment can be to compression, rebound, or a combination of the two. While there are multiple designs that change how to adjust adjustable shocks, they are most typically adjusted with a knob that clicks as it sweeps, allowing you to repeat the setting from side to side or front to rear. Double-adjustable shocks allow the user to dial in compression and rebound settings separately. This added dimension of adjustability can really be a game changer for a car that spends a lot of time on the dragstrip or road course. Check out our guide to learn more about the best shocks and struts for a drag car suspension setup.

Suspension Pickup Points

Here’s where things get serious. The pickup points on your suspension are the points around which components like control arms, axles, and steering arms rotate. These are the points that determine the suspension geometry and making changes to them impacts things like roll center and center of gravity. These geometry changes can profoundly affect your car’s handling for better and for worse.

It’s important to remember that your car’s suspension is a complex system in which everything has to work together. Hot rodding and customizing is all about messing around with our cars to make them match the vision we have in our heads. When it comes to modifying the suspension, it’s critical to realize that the stock suspension components under your car were designed by a team of engineers to provide safe, predictable handling. Arbitrarily moving any of these to lower a car or add a modern component like rack-and-pinion steering can lead to nasty side effects like bump-steer or roll center changes that yield unpredictable handling. Things that look fine standing still in the shop can cause real problems when put in motion out on the road.

For dramatic changes to suspension geometry, it’s best to stick to an engineered system like Speedway Motors G-Comp performance suspensions. Suspensions like this are engineered as a total package and extensively tested to add desirable changes to your car’s handling while avoiding the negative and dangerous ones. These suspension systems also allow for changes to control arm pickup points to change roll centers and anti-squat (more on this later) to fine tune the car, but are engineered not to allow dangerous extremes.

Sway Bar Adjustment

A sway bar is another aspect of your car’s suspension that can be adjusted to tailor the ideal ride and performance. Sway bars act as a “helper spring” to minimize body roll when the car is pitched into a corner, so bigger must always be better, right? Not always. Instead, sway bars should be selected that work together with the rest of the car and your driving style, just as you would with shocks, springs, and tires.

It should also be noted that a sway bar is also often “adjustable” in the sense that it has threaded end links. These allow adjustment so the bar is either neutralized and not putting preload on the suspension or sometimes to preload one side of the car to straighten out launches or otherwise tune with uneven loading. Always unhook the bars when making adjustments to coilovers or corner balancing, then be sure the bars are neutralized, with no preload, when the end links are reconnected.

Independent Front Suspension Tuning | Instant Center Suspension Adjustment

First, let’s establish “what is independent front suspension?” Independent suspension, front or rear, separates the suspension on each side of the car instead of joining them with a solid axle. Control arms connect the wheels to the car’s chassis and their pivot points, lengths, and angles have a tremendous effect on the way the car will react through suspension travel. These are made adjustable either by an adjustable control arm assembly, adjustable end links, or by making the control arm pickup points adjustable. Is independent suspension better? In the world of street performance, autocross and road racing, and circle track racing, the answer is almost always “yes,” particularly in regards to the front suspension.

As we mentioned above, it’s important to understand the geometry and physics involved and know “what do adjustable control arms do” before making adjustments. Thousands of pages have been written about these principles by engineers with PhD’s. We’re not going to go that deep here, but even a 101-level understanding of concepts like roll center, instant center, and center of gravity can make you a far better tuner when it’s time to make adjustments to your car’s suspension. Instant center and roll center may sound like terms right out of those engineering dissertations, but the concept is not that hard to understand.

The simplest way to understand instant center is that it is the point around which the wheel or axle pivots. On an independent suspension, that point can be found by extending the angle of the upper and lower control arms to the point at which they intersect, as shown in the diagram above. Where this really matters to us is the relationship between the instant center and the roll center. The roll center is the virtual intersection between the instant center and the tire’s contact patch. Instant center is the center of rotation of a single corner of the suspension and the roll center is the center of rotation of that end of the car.

The relationship between roll center and center of gravity is a critical relationship on your car. But first, where is your car’s center of gravity? A simple rule of thumb that applies to many cars is that the center of gravity is a line that extends through the car at the camshaft. Then think of the distance from the roll center to the center of gravity as a lever arm that causes weight transfer, or body roll, during cornering. A low roll center and high center of gravity equates to a long arm with more leverage, causing more body roll. Conversely, a high roll center and low center of gravity acts like a shorter arm with less leverage. Note that body roll is not always a bad thing. A high roll center and low center of gravity can result in too much sideways force being applied to the tire and a loss of traction. That’s why this geometry is so critical, as it walks the line between flat cornering and traction-inducing weight transfer.

Live Axle Suspension Adjustments | Adjustable Torque Arm

Our description of instant center and roll center above primarily focused on the longitudinal axis of the car and what happens dynamically during weight transfer from side to side. The same concepts can be applied as weight transfers from front to rear.

The concepts of anti-squat and instant center come up often in the conversation about how to adjust 4-link suspension, and for good reason. 4-links are popular because most of them allow for near infinite adjustment to the bar angles and adjustable end links to make changes to the instant center. That’s why those aftermarket 4-link brackets that you see are riddled with holes. The instant center of a rear suspension’s locating links is found using the same principle that we already discussed above. Imagine a line running through the lower control arm extending forward. Then imagine a line going forward through the upper control arm until it intersects the lower line. That intersection point is the instant center.

The relationship between the instant center and the 100% anti-squat line will determine the forces acting on the rear axle during launch and under deceleration. The anti-squat line is another imaginary line that extends from the rear tire’s contact patch diagonally forward to the point where the car’s center of gravity intersects with a line drawn straight up from the front tire’s contact patch. The diagram below will help make some sense of all of this:

When the instant center hits the anti-squat line, as illustrated above, the car will theoretically be neutral and no power will be wasted lifting or pulling on the rear of the car. Instead, all the power will be used to propel the car forward. But in reality, most cars benefit from some mechanical advantage to get the car off the line.

When the instant center moves below the anti-squat line, the suspension will be pushed up and the rear of the car will be pulled down. Generally, this is not ideal because the tires are being pulled up with the axle instead of being forced down onto the track.

Conversely, when the instant center is above the anti-squat line, then the rear axle will be planted and the rear of the car will be pushed up. This is what’s happening when you see drag cars lifting the body and crushing the slicks when they leave the line. As this all applies to your car in the real world, it generally takes some testing to figure out the best setup for your car.

The same general principle applies to other rear suspension designs. Those old slapper bars and more modern lift bars that you see under leaf spring rear suspensions are also designed to adjust the instant center. Same for a torque arm rear suspension. Torque arms are generally more stable through travel than a 4-link because the longer arms minimize the amount that the instant center moves around as the suspension is compressed and extended. Speedway Motors G-Comp Torque Arm rear suspension has proven itself on our test cars to be very stable through travel, on and off the throttle.

Solid Front Axle Tuning

So far, our discussion about adjustable front suspensions has been focused on independent front suspension. We’re sometimes asked “can I adjust a solid front axle?” The answer is both yes and no, depending on what adjustments are being made and how the axle is located. On a stock early Ford, traditional hot rod with split wishbones, or parallel leaf spring frontend, it’s typically only possible to adjust toe. Camber is built into the axle and caster is not adjustable without changing mounting points or angle of spring pads.

Camber will pretty much always be non-adjustable on a straight axle without physically bending the axle. But, decades of hot rod ingenuity have brought about more advanced methods of locating the axle that allow for some caster adjustment without moving the existing mounts. 4-bar front suspensions offer the most adjustment potential by turning adjustable end links in and out of the bars. Even a hairpin radius rod allows for some fine tuning to caster with a pair of adjustable clevises per side. Generally speaking, a straight axle front suspension for the street will work best with 5-7 degrees of caster. When fabricating a hot rod chassis, this angle must be accounted for when installing the front crossmember and locating the radius rods and brackets for suspension pickup points.

Do I Need Adjustable Control Arms?

We’ve discussed a few ways in which adjustable control arms can affect suspension geometry and the dynamics of your car’s front and rear suspension. So, you might be wondering if your car will benefit from the ability to make these changes in addition to the typical caster camber adjustment. Ultimately, you need to be honest with yourself about what you intend to do with the car. Adjustable components that are just there to look cool on a street car will likely cause more complication than they’re worth. However, a dual-purpose car or dedicated race car will certainly benefit from the ability to make changes based on track conditions and driver feel, as long as those changes are being made based on some conceptual knowledge and not random shots in the dark hoping to hit the magical setup.

The bottom line here is that adjustable suspension allows you, the builder and driver, the opportunity to make your car’s stance, ride, and handling exactly what you want it to be. But as they say, with great power comes great responsibility. It’s critical that you do your homework and understand the changes that are being made. But with some research, the right tools, and maybe a little trial and error, the result will be a dialed car that sits right, handles great, and maybe even lands you on the podium at the races next weekend.