Coil Or Leaf Springs: Choosing The Suspension’s Form Of Anti-Gravity


If you stated that the job of the springs is to keep your car’s suspension from bottoming out, you’d be right. But they do so much more than simply determine your car’s altitude. The springs in your car’s suspension have grown to become inextricably linked to all the other components that surround it.

Leaf springs (both parallel and transverse) were initially joined with "knee-action" shocks (left) of various styles before fluid-piston types of shocks appeared (right).

If you were to go back to the dawn of the automobile, you might be okay with such a simplistic view of springs and their job within the suspension. But it didn’t take long before folks realized that they could “tune” the suspension.

Over time, more leaves were added to springs to give them more "load". Some modern springs use small, gliding buttons between the leaves to help with friction while others are a mono-leaf design.

Spring Rate And Spring Load

Soon, the OEMs were finding out that a coiled piece of metal could contain the weight of the car. Space was greatly reduced, and using a coil spring on each corner of the front (and eventually the rear), helped to isolate any road irregularities from being transmitted to the opposing side of the vehicle. This helped ride quality immensely.

Spring & Shock Q&A (Kaitlyn Nelson, QA1)

What is rebound and why is it important?
KN – Shocks can move in two directions. They can extend, and they can compress. Rebound refers to the shock being extended. In a street application, rebound typically controls the wheel as it comes back to the normal road surface after hitting a bump. It can also be used to “tie down” the vehicle when weight is transferring. For example, if you hit the brakes hard and the rearend wants to come up, stiffer rebound can help keep the back end down. When you go hard into a corner, and the inside of the car wants to raise up, stiffer rebound can control body roll.
What is compression and why is it important?
KN – Compression refers to the shock when it is compressing. This is what you feel when you hit bumps. This is also what controls your suspension when you hit bumps. Your shock must be able to compress when your tire hits irregularities in the road. When you do hit sharp bumps in the road, (expansion joints, cracks, etc.), your tire is going to try and bounce off it, like a basketball. Your compression valving in the shock will control that bounce. Ideally, the tire will move out of the way of the bump, but not leave the road surface.
How can enthusiasts decide which adjustments they need?
KN – Many performance applications will benefit from a shock where you can adjust both compression and rebound. Having the ability to adjust the compression gives you a little more freedom to get the car to react the way you want, because it’s just another way to help control weight transfer (whether its front-to-rear, or side-to-side). There are two ways to do this, a single adjustable (simultaneous compression/rebound adjustment) and a double adjustable (independent adjustment).

Even the trusty leaf spring suspensions improved, as did the means of damping any unwanted movement of both types of suspension. One way of limiting the amount of movement permitted through the suspension is by increasing the spring’s rate.

Spring rate is determined by both the material that the spring is manufactured from, and its dimensions. It is the amount of force necessary to deflect the spring by one inch. That means if you put 400 lbs on a spring with a rate of 400 lbs, that spring will compress 1-inch.

How do you find the right rate for your application? The correct spring rate for one application might not be the same for another. That is why a quick phone call to any one of the manufacturers who supply springs and shocks can be such a benefit when choosing the right application. They’ve seen hundreds of configurations, and their wealth of knowledge can be invaluable when trying to get your spring’s rate correct the first time.

Chassisworks’ Chris Alston explains it this way, “Once the correct spring rate has been established, the bulk of suspension tuning is done with the shocks and sway bars, not the springs.” Chris also stated that, “80-percent of the shock problems we see is because people have the wrong springs on their car.”

There are multiple variables playing on the effectiveness of a spring’s rate: The type of suspension, and the amount of friction that it possesses. In fact, whether the spring has multiple leaves or is a mono-leaf design can affect this due to a certain amount of friction between each of the leaves.

Ever notice that many of the early multi-leaf springs had grease fittings? Also, whether your suspension uses heim joints, rubber bushings or durable urethane bushes will affect how easily your suspension will move through its range of motion. This can all have a significant bearing on the proper spring for your application, and should be taken into account when designing or ordering suspension components.

Another area that will affect the proper spring rate (as well as a shock’s damping), is the installed angle relative to the axle or suspension. The more vertical they are mounted to the axle, the more effective they will be. The less vertical the spring is to the axle or suspension, the more leverage will be given to the suspension component as it affects the spring through its motion of travel.

Early Fords and Corvettes since the mid-'60s have used transverse springs like these aftermarket units from VanSteel (left), which tie each side of the suspension to the center of the vehicle. Today, F1 cars and some modern street rod suspensions like this Heidt's unit (right), also use this design but with coil over shocks.

This means that more spring rate is necessary to keep it from compressing. If the spring angle designed into the suspension is too severe and the spring rate unnecessarily high, it can create enormous forces in the area surrounding the spring, and can cause component failure.

Another aspect determining the proper spring design is spring loading. Whereas spring rate explains how much weight is needed to compress the spring 1-inch, spring load is the amount of weight a spring is designed to carry when deflected to a certain height. This takes into account the weight of the vehicle and how high/low you want it to sit in the suspension’s travel. This is a major consideration when designing springs for any vehicle, especially with leaf springs.

If the vehicle rides perfectly, but you want to raise/lower the vehicle, you wouldn’t change the spring rate, you would change the spring load. A sagging leaf spring hasn’t lost any spring rate, it’s simply lost its spring load. Conversely, spring rate affects a car’s weight transfer characteristics, while spring load determines how much weight a spring can support at a given height.

When installing springs and shocks, the angle relative to the movement of the suspension will dictate how efficiently they control movement.

Variables Of Spring Rate

Spring rate is spring rate, no matter what kind of spring it is (i.e. coil, leaf, torsion), but there are a few different ways that spring rate is quantified. You will hear spring rate identified as “linear” or “progressive”, but what do those terms mean? A linear spring rate merely means how much force (weight) it will take to compress the spring 1-inch. It will take the same amount to compress the spring another inch.

The nice thing about leaf springs is that they help keep the suspension geometry. Even this semi-elliptical spring (center) acts as a lower part of a four-bar suspension as well as support the vehicle (note the bolt for adjusting the ride height). Coilover shocks require a means of controlling the suspension as it moves. This musclecar rear suspension from QA1 uses a variety of bars and braces to keep the proper geometry.

Progressive springs will build spring force progressively the more it is compressed. It may take 150 lbs to compress the spring an inch, but then may take 500 lbs to compress it the second inch.

A dual-rate spring (also known as variable rate) will be two spring rates wound into the same spring. Half the spring may be 130 lb/in, and the other half 250 lb/in. Once there is enough weight to compress the 130 lb/in coils into bind, the 250 lb/in rate takes over.

Shocks & Springs Q&A (Gary Nelson, Aldan American)

How do you know whether to firm up or soften the suspension?
GN – The adjustments revolve around your driving style. If the shock is adjusted too heavy, the ride quality will suffer. Too soft, and the car will have poor handling and feel sloppy. Typically, a sway bar is added to control the body roll. A heavier car, such as a fat fender car, has more weight higher up. If you add more spring rate and shock dampening to control the body roll, the ride quality suffers. The spring must first be correct to support the car, then by adding an anti-sway bar, you can stay with a softer spring and “tune in” reduced body roll.

How can you reduce wheel hop?
GN – Chassis design and chassis’ geometry play a role in reducing wheel hop. Selecting the proper shock and shock damping along with spring rate also helps to reduce and eliminate wheel hop when selected and tuned properly.

Can you tune to enhance weight transfer?
GN – Typically for drag racing and weight transfer, you use a long, soft spring up front that stores a lot of energy, and a shock that has low rebound force. In the rear, you need a spring that will support the transferred weight without squatting, and a shock to dampen the movement of the spring.

In general, the bulk of performance and regular street applications benefit most from a linear spring rate.

Springs and support components

Consider that everything your car does is transmitted to the road through the small contact patch of each tire, and the springs and shocks are major components in the link between them. Their relationship is so entwined, that in many instances, the shock and spring have become part of the same design.

Coilover shocks have not only made each wheel independent of suspension inputs from other corners, but they have also allowed for an unprecedented level of tunability and adjustment.

Even among those immersed in a world of spring rates and shock adjustments, there are some differences of opinion, but they all agree that the proper suspension is a sum of all the proper parts. We asked Kaitlyn Nelson at QA1 about the process of choosing the right spring/shock, “Most people worry more about spring rate, so that is what they want to solve first. But truthfully, for coilovers, you need to choose a shock length before you get into spring rate, because a longer spring can be a lower spring rate. Since the shock length will dictate which spring length you can use, choosing the shock length first will allow you to alter the spring rate for the length.”

Companies have brought the adjustability of coilover shocks to enthusiasts by offering kits to retrofit into earlier suspensions like this Fat Man Fabrications '49-'51 Ford chassis (left) and '66-'67 Fairlane Pro Touring suspension (right) from Total Cost Involved .

Why would you want a lower spring rate? Remembering that spring rate affects a car’s weight transfer characteristics, a stiffer spring increases a suspension’s roll stiffness, it also degrades ride quality. Conversely, softer springs will allow the tires to follow the road surface more closely. That is why some experts suggest using a lighter-rate spring and increasing the stiffness of, or adding an anti-sway bar. The bar will prevent unwanted body roll, while the softer spring/shock setting will allow the tires to keep better contact with the road surface, which improves handling and is safer.

Some springs are complete with the shocks while others may be a hybrid to match the original coil spring pocket and mate with the bottom of the shock, like the ones from Flaming River (left).

Aldan American’s Gary Nelson explains how to know what your suspension is telling you, “If you’re on a race track, you can quantify your results by lap time. On the street, you have to find a series of road conditions to evaluate how the car feels to you, and make an adjustment and repeat. Sway bar tuning can be determined by how the car reacts. If the car wants to spin out on a corner, it may not have enough anti-roll control in the front or too much in the rear. Sway bars may also be necessary if the shocks are turned up or the spring is too stiff. A sway bar can be added, and the shock/spring softened to find a happy medium.”

Abounding Adjustments
We have come a long way from those simple suspensions found during the early days of autos. Now, enthusiasts have a myriad of choices when building their cars, and there are even several options after everything is bolted in! QA1’s Nelson put it best, “With great adjustability comes great responsibility.”

These coilovers by RideTech show many reasons why enthusiasts choose coil over shocks. Vehicle height is adjustable via the threaded nut on the bottom of the spring, the springs are interchangeable for different rates, and adjustments to the shock’s damping is easily done through the valve at the top.

Where ride quality is the priority, a rebound-only adjustable shock with a fixed compression valving is very popular. As the suspension compresses, so does the spring. The further it compresses, the more spring force you have. Thus, a relatively soft, fixed compression valve works well here.

The rebound adjustment will then help control how the car comes back to ride height. Stiffer rebound valving will return to ride height slower and more controlled.

Gary has some of the best information when starting the process, “If you are doing a shock, chassis, or coilover conversion project with a mix and match of manufacturer’s components, don’t be afraid to pick up the phone and call the manufacturer’s tech line.”

We went and asked a few questions from each of these experts (see sidebars) to bring some of this great information to you, our readers.

While this may not be a comprehensive list of FAQs, we think that it will help you greatly in understanding how these systems all work together, and it’ll get you well on your way to choosing what combo might be right for you.

Article Sources

About the author

Andy Bolig

Andy has been intrigued by mechanical things all of his life and enjoys tinkering with cars of all makes and ages. Finding value in style points, he can appreciate cars of all power and performance levels. Andy is an avid railfan and gets his “high” by flying radio-controlled model airplanes when time permits. He keeps his feet firmly grounded by working on his two street rods and his supercharged C4 Corvette. Whether planes, trains, motorcycles, or automobiles, Andy has immersed himself in a world driven by internal combustion.
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