I’ve already touched on springs a little in previous chapters, particularly leaf springs, but we haven’t really dug into the meat of how springs work or how to select the best springs for a given application. Over the years I’ve taken countless tech calls and e-mails from clients on a quest to make their vintage muscle cars perform like modern ones, but only one single client (a mechanical engineer and SCCA member) has mentioned flat ride frequency, and maybe two or three were interested in changing the motion ratio of their front springs. All of the rest just wanted to know what springs to run in their car to get the ride height and performance they were looking for. With that in mind, let’s to take a look at the fundamentals of springs and spring selection.
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Springs serve two basic purposes. The first and foremost is to simply hold up the mass of the vehicle. Without springs, the suspension would collapse and the car would drop to the ground. The second purpose is to provide resistance to the movement of that mass, vertically and in roll and in pitch. This movement and its velocity are further regulated by the shock absorbers. The shocks actually have more control over ride and handling than the springs, but you can’t properly select any shocks until you’ve chosen the springs.
It’s fair to say that the methodology used to select spring rates has changed quite a bit over the years. Older cars tended to use very soft spring rates (and soft shock-dampening rates), which was just enough to hold the car up at the desired ride height and no more. This was based on the presumption that such a combination would yield a soft and comfortable ride. But that’s only partly true.
The soft rates encourage the suspension to use a lot of travel, which does tend to soak up big bumps, but this travel also allows the car to wallow down the street like a small boat on the ocean. You can forget about performance handling as long as the car is pitching and rolling about like that. The problem is all of that suspension travel isn’t being used effectively and you’re getting a lot more reaction from the suspension than you really need. The new springs help tune this excessive motion out of the car. You only really need to use enough travel to make up for the imperfections in the road.
Theoretically, if you have a 1-inch bump, 1 inch of travel should be enough to soak it up without making the body of the car react. Of course, a statement like that is leaving out a huge number of variables, but the point is you don’t need to use more travel than is necessary for a given circumstance. Some people’s first reaction when they make this realization is to go wild in the other direction, thinking, “If very soft springs make my car handle poorly, then super stiff ones should make it handle great!” So they buy the highest-rate springs they can find in a racing catalog and cram them in. Usually, no thought is given to the shock dampening rates, which become even more critical once you start messing with the spring rates. Does a car like this handle better than it did? Well, sort of; it corners flatter and dives less under heavy braking. But the car also feels much more jittery and unsettled. The ride quality could become down-right poor, and its overall behavior somewhat stiff and erratic.
Consider for a moment that street cars and race cars live totally different lives and drive in very different environments. The race car only needs to perform well on the track for short periods of time, and at 100-percent maximum effort. Race tracks generally are smooth (yes, even Sebring is smooth compared to many public roads) and designed specifically for racing. There are usually gravel traps and large open grassy areas for runoff if you get a little too exuberant in the turns.
A street car spends most of its time idling at stoplights, cruising at low speeds, just driving around, and going for a brisk run down a straightaway or through some twisting turns at, say, 60- to 80-percent effort from time to time. It may occasionally be called on to perform at 100 percent, but usually for a very short burst, then it’s back to tame driving again. No matter how ferocious the street car or how aggressive the driver, it still spends the vast majority of its time driving well below its maximum performance limits. The road conditions will likely run from decently smooth to rough and potholed. Turns are banked—not for best cornering speeds, but for the best water drainage. The runoff area may be a large tree, a concrete barrier, a parked car, or a drainage ditch. Things like this and a little common sense usually rein you in a bit.
Spring rates, like most other aspects of the suspension, have a point of diminishing returns where you get less performance gain and more tradeoffs for each increment of additional spring rate. Once again, more is not better; just right is better—always.
So, what’s just right for a performance street car? Generally, it’s much harder to go wrong with springs specifically designed for your application. If performance street springs or performance touring springs are available for your car, then use them. This applies to coil springs, leaf springs, and torsion bars too.
However, just because you can get stiffer racing springs for your car doesn’t mean you get any more performance out of them. For your type of driving, you may get less performance, and you have to put up with a rougher ride too. The springs I recommend, in rate, generally perform halfway between vintage OE springs and road racing springs.
Performance street springs generally also lower the vehicle some advertised amount. That’s generally good because, compared to modern cars, most muscle cars sit rather tall in the saddle and have a fairly high CG. Lowering them helps overcome this issue and also generally improves the front-end geometry a bit. There are exceptions (such as C5/C6 Corvettes, whose geometry goes to the dogs when you lower them with springs), but for our purposes they’re typically a good thing. Lowering provides several gains: better performance, better stance, and, with the right change, better-than-stock ride quality too. And, happily, there are plenty of good performance-engineered springs out there to do what you need without compromising anything.
I get more calls about this than any other spring-related topic. Everyone is worried about getting the stance of their ride just perfect, and rightfully so. More than trick paint or custom wheels, the stance of the car makes or breaks it. We all want it, but how do you get it? It’s not as hard as you think if you do your homework.
First off, don’t rely on the advertised drop of a given set of springs. “Drop” in relation to what—the car’s original factory ride height from 40 years ago? Do they mean before or after someone put some mystery springs in it in 1983? Yeah, drop is a pretty vague term. And if you can see that your car has old sagging springs, they may already be sagging as much as (or more than) the supposed drop the new lowering springs advertise. On the flip side, many replacements billed as stock replacement springs run much taller than true original springs. In this case, you could get a lot more drop than you were counting on.
Let common sense be your guide here. You can also reference original pictures of the cars to get a rough idea of whether your car’s current ride height is typical, higher, or lower than the norm. Weight also plays a big part in establishing ride height.
Here’s a simple method to determine how weight impacts the ride height of a car: Start with the amount of weight you’ll be adding or (hopefully) subtracting. Let’s say you’re shaving off 250 pounds by swapping out your iron 454 for an aluminum LS2. You know your particular springs are 500 lbs/in. The 250 pounds you removed were shared by both front wheels, so each side sees a 125-pound savings. That allows a 500-lb/in spring to compress 1/4 inch less, which is not much. Now, apply the motion ratio of the spring as it’s mounted in the lower A-arm (I call it 2:1 because that’s fairly common) and you’ve got 1/2 inch of wheel movement, which translates to a 1/2-inch increase in ride height. An easier way to express it is to say 250 pounds is half of what is necessary to move a 500-lb/in spring 1 inch, so it moves half that much.
There are a several things you can do to help dial in your ride height. First, take into account any other variables that may come into play. Do you have dropped spindles? Are you using tall lower ball joints as part of a geometry correction package? Are you using after-market lower A-arms with non-stock-depth spring pockets? Are your tires taller or shorter than stock equipment?Take all of these things into account and you won’t get blindsided by a ride height that’s way out there.
Next, with the car all together (that means everything—the grille, hood, antifreeze in the radiator, etc.), put the car on the ground, bounce it up and down a few times, and then roll it back and forth a little to allow it to settle to its lowest natural ride height. This is your baseline. If you chose all of your parts carefully (and you’re a little lucky) it’ll be right where you want it. If not, measure the car at several points and write down the numbers. This is your baseline height.
Finally, decide how much higher or lower you really want the car to sit. To simulate a higher ride height, just jack the car up until it looks right and measure it. When you compare the two sets of measurements you will have some idea how much you need to adjust the ride height to get it where you want it.
The easiest method to use with coil springs is to trim the springs to go down or add spring seat spacers to go up. Mopar guys are in luck up front, as they have adjustable torsion bars, which can be simply cranked up or down to get the desired height. Leaf springs can use different-length shackles and lowering blocks, which can be de-arched for less height or arched more for increased height. Each of these methods carries some caveats. Generally speaking, moderation is the key. Get close with the basic suspension/spring package and only use these methods to tweak it a little if necessary.
Cutting Coil Springs
The main thing to remember is that cutting off some of the spring is easy, but putting it back on is hard. I recommend cutting off only one quarter of a coil at a time. Yes, that means you may have to take the springs out of the car several times before you get it right. And yes, that is time consuming and labor intensive. Remember, unless you have after-market lower A-arms with rotating spring pockets, only the factory-spring clocking positions allow the spring to set properly in both the upper and lower seats. Since the lower seat is much deeper, you want to keep that end of the spring registered properly at all times.
For doing the actual cutting, I suggest using an abrasive cut-off wheel in a 4- to 4.5-inch-diameter grinder. It makes short work of the job and puts hardly any heat into the springs. You can successfully cut them with an oxyacetylene torch without harming the heat treat of the springs, but you can’t dawdle while doing it. Most folks take much too long with the torch and put too much heat into the springs, which pretty much ruins them. Besides, if you have a nice new set of powdercoated springs, the torch really makes a charred, smelly mess of the coating.
There is no reliable ratio of coils cut to the amount of drop; it depends on the individual springs and where the top of the spring is indexed in the helical upper spring seat. Just take your time and your patience will be rewarded.
Adding Coil Spring Seat Spacers
You’ve picked out your parts, put the car together, and excitedly put the car on the ground to see how the new ride height looks. It looks wicked cool and you’re thrilled, until you notice the header collectors and oil pan are an inch off the ground and the tires are hitting the wheel wells. Doh! Spring seat spacers are an easy way to get out of this mess.
Most muscle cars with coil springs that seat in the lower A-arms have roughly a 2:1 motion ratio at the springs. That means that a 1/2-inch solid-aluminum spring-seat spacer between the spring and the lower A-arm raises the ride height about 1 inch. A 3/8-inch urethane spacer may compress to 1/4 inch, which yields a ride height increase of about 1/2 inch. Get the picture? These spacers are available in myriad thick-nesses and for almost any application from companies including Specialty Products (SPC) (which offers aluminum spacers) and Energy Suspension (which offers polyurethane spacers).
Using this type of spacer is preferred over those designed to go in between the coils of the spring. The inter-coil spring spacers affect the rate of the spring by turning some of the live coils into dead ones by restricting spring travel. Inserting a spacer between the spring and its seat changes its overall installed height, but the spring’s rate remains unchanged. Because the rate remains the same, you can even use different thicknesses on each side to balance a car that sits crooked, without harming its driving characteristics.
Threaded spring adjusters are available from several circle-track sources. They’re often called “hidden spring adjusters” because they’re used as a method of cheating (i.e., winning) in race classes that don’t allow weight jacks on the springs. At first they seem to be a great idea because they offer a wide range of adjustment and they’re affordably priced. However, they have a number of issues that make them a poor choice for most street cars. These are normally installed in the upper spring pocket, which typically requires some fitting. The seat of the adjuster is either a flat plate or what looks like a slightly over-sized tuna can. The flat-plate type is intended to mount on a flat seat. The “can” type is intended for uneven or helical seats like those found in most muscle cars.
Most folks just jam the adjuster up there and stuff the spring in, which allows the threaded adjuster to rock around and sit crooked. Adjusters can make noise and, because the shock absorber has to fit through the narrow threaded tube, it often damages the shock body. Note that the installation of these coil spring adjusters often precludes the use of larger-diameter high-performance shocks, which is another limiting factor.
To properly install these adjusters, the “can” of the adjuster’s spring should be trimmed to match the shape of the upper spring seat while the suspension is cycled with the shocks (but not the springs) installed. This makes certain the shock body doesn’t contact the adjuster.
Then, the threaded adjuster assembly should be welded into place. Welding upside down is always fun—kidding!Done properly it is solid and reliable. But it still limits your shock choices, and is still way up inside the frame and nearly impossible to access. You’ll need to remove the springs and shocks every time you want to tweak them, reach up into the frame to adjust them, then measure to see how much you’ve adjusted them, and then put it all back together again and see if you’ve hit your goal. If not, the process starts all over again. To me, that’s more like work than fun.
With spring seat spacers, you get a known amount of height change with each shim. Establish your baseline ride height once, insert the appropriate thickness spacers, and you’re done.
The rate of a spring is very easy to understand. A 500-lb/in spring compresses 1 inch for every 500 pounds of load you put on it. In the case of a coil spring, the rate specification is determined by the wire diameter, the overall diameter of the spring, its uninstalled height, the number of coils, and the number of coils that are unable to compress and contribute rate (usually because they’re supported by a spring seat), also known as “dead coils.” A leaf spring’s rate is determined by the thickness, width, length, and composition of the leaf spring pack.
Torsion bars derive their rate from the length and diameter of the bar itself. The spring rates of torsion bars are typically described in lbs/degree, but in practice they almost always use a lever arm of some kind. If a lever is introduced into the system, rates are more commonly expressed in lbs/in.
The rate of the springs used, as well as the vehicle’s mass, contribute to its ride frequency. This frequency contributes directly to the car’s ride quality and behavior, or lack thereof. The shocks help dampen the velocity of the suspension cycle (which is why they call them dampers in Europe) but the natural frequency is set by the springs. These cyclic ride frequencies can be calculated, with some degree of accuracy, for each end of the car. Then, a flat-ride frequency can be determined by using these rates and the wheelbase of the car at a given speed.
Older muscle cars typically have a ride frequency of about 25 to 35 cycles per minute (cpm). Some 1980s-era muscle cars are still in the higher end of this range and others (like the IROC Camaros and similar cars) are even higher at 50 to 60 cpm or so due to their much stiffer springs. Many all-out road race cars have a frequency of 150 cpm or more. That’s more than 2 suspension cycles per second. The higher the frequency, the less travel you probably use as well, ranging from 6 to 9 inches on older cars, to 2 to 4 inches on sports cars, and less than 1/2 inch on some Formula 1 cars.
A ride frequency of 30 cpm means that the suspension cycles through its travel once every two seconds. This gives you a rowboat-like feel. Not surprisingly, this may even give you motion sickness (seasickness). Cars in the 60- to 90-cpm range are widely regarded as very firm, or even somewhat harsh. This frequency replaces the slow undulations of the softer springs with a more jarring, hard-edged feel. This firm feeling is often equated with good handling. It is often present on cars that handle very well, but it is also quite possible to have a car that feels like this and is still a poor handler. Frequencies above 100 cpm are generally seen only in road race cars and the like because they would be brutally harsh on the street.
Weight also has an effect on ride frequency. If you take a car with good ride quality and remove 1,000 pounds of weight, it is now over-sprung for its weight and has a very harsh ride. The inverse is also true. Anyone with a 1-ton pickup truck knows what I mean. Unloaded, the ride is very firm and bumpy, but put 2,000 pounds in it and it rides like a Cadillac. You’ll seldom take enough weight off a street car to make any real difference in ride frequency, but the trend of replacing old-school iron engines with late-model aluminum ones may come close.
The main thing to remember when selecting springs is to let common sense be your guide. Stock springs are much too soft in most muscle cars, so you need to ramp up the spring rate to get good handling out of them.
There’s been a lot going on in the last several years with air springs. Once seen only on 18-wheel big rigs, then on low riders and mini trucks, they’ve made a strong push into the performance market. Let’s go over some basics and sort out some myths and misconceptions.
Often referred to as air bags, or just bags, they really are simply adjustable rate/height springs that use air pressure to provide and regulate that rate/height. This simple fact will probably do more to dispel some of the myths surrounding air springs than anything else. An air spring with its air pressure adjusted to provide a measured rate of 500 lbs/in will, all else being equal, provide exactly the same performance, ride quality, and handling as a steel coil spring with the same 500-lb/in rate. Spring rate is spring rate. An air spring may transfer a bit less high-frequency NVH (noise, vibration, and harshness) to the chassis than a steel spring but even this advantage is minimized if some simple spring seat isolators, usually rubber or urethane, are used with the steel springs.
Air Spring Myths
Air springs ride much better than steel springs. We covered this previously. Same rate = same ride. If you set the air springs softer than a given steel spring it will have a softer ride, but no softer than a steel spring equal in rate to that softer air spring. Set the air springs harder than a given steel spring and the car will ride harder. Bottom line, where springs are concerned the ride is equal to the rate no matter what provides that rate.
Cars with air springs can never handle well. See above. Rate is rate. Properly implemented air springs can provide exactly the same handling performance as performance steel springs. No better and no worse. There’s a caveat here: Each air spring must have its own air supply. If a transfer line between two air springs is used then as the car turns the suspension on the outside of the turn compresses and simply transfers that air volume and pressure to the air spring on the other side of the car resulting in massive body roll. I suspect that’s where this myth originated.
Slamming a car makes it handle better. We’ve becomes conditioned to accept that lower cars handle better than taller ones. A Formula 1 car is very low, a Hummer H1 is very tall, so simple logic tells you that slamming a car almost to the ground should make it handle much better. The center of gravity is lower, for sure, and there is a quasi-scientific reason to support this theory. However, you know there’s a lot more to good handling than the car’s proximity to the ground. All cars have geometry that changes throughout their range of travel. The same goes for alignment. It’s only at its best at one ride height and it’s almost never when it’s super low.
Air springs throw another wrench into the works—as you let air out of the bags to lower the vehicle the air pressure and the spring rates drop dramatically. You may recall that earlier I talked about increasing spring rate and dampening to control suspension movement better as you lower a vehicle and lose available bump travel. Air springs have you going in the opposite direction so as the car gets lower, it also gets softer and handling begins to go away. There’s good reason why the low rider’s mantra is “low and slow.”
Air springs will let me drive my car at any ride height I want. Okay, air springs technically let you drive your car at any height, but you may not want to. Due to your car’s camber curves, bump steer, and alignment changing throughout its travel, the car will only drive really well at one ride height. One. Every other ride height is a tradeoff of some kind. It’s not a huge deal if you’re cruising the fair-grounds at a big car show. But if you’re thinking of flying down the freeway or doing any corner carving you really need to bring the air pressure back to your baseline ride height setting where the alignment was done.
technically let you drive your car at any height, but you may not want to. Due to your car’s camber curves, bump steer, and alignment changing throughout its travel, the car will only drive really well at one ride height. One. Every other ride height is a tradeoff of some kind. It’s not a huge deal if you’re cruising the fair-grounds at a big car show. But if you’re thinking of flying down the freeway or doing any corner carving you really need to bring the air pressure back to your baseline ride height setting where the alignment was done. compromise the rate to get the ride height, or sacrifice the ride height to get the rate. Here’s where knowing the rate at a given pressure and installed height comes in. If this is starting to sound complicated, then you get the idea.
Since air springs are adjustable I can get them from anywhere and put a system together myself. I’ve only touched on a few of the critical factors in setting up an air-spring system and I think you can already see that it’s not a simple plug-and-play deal where you can get a bunch of stuff helter-skelter and expect it to work well. You want to deal with a company that really knows these systems. Talk to them and if they only use words like “cool” and “bitchin’,” and can’t answer any technical questions, then take your money elsewhere. Better companies have complete systems for specific vehicles. They may cost a little more than piecing a system together but they usually perform a lot better. If there is no system available for your car, then a good company should be able to provide you with charts of spring rates and various pressures and installed height as well as information on compressors, air tanks, line routing, etc. You really do get what you pay for.
Air springs are not reliable. Air springs in general are extremely tough and long lasting. Many years of use in industrial applications and trucks have them down to a science. There is a 1965 GTO that comes into our shop that’s had the same set of coil-spring-insert rear air bags since 1967, and this car runs 10s! Reliability issues almost always come from the controller system or the installation. Get top-quality parts; don’t skimp.
Tips and Caveats
Systems that use separate air springs and shocks sometimes relocate the shocks to undesirable locations that compromise their function and cause tire-clearance issues.
In order to get a good-performance spring rate at a lowered ride height, many air spring systems take advantage of drop spindles or air-spring-specific lower A-arms with lowered mounts to achieve lowered ride height, but allow the air spring itself to run at a higher pressure and higher installed height. You can save money by not running these components as a complete package but you may forfeit a lot of performance. Before you buy, consider all of the geometry, clearance, bushing, and alignment issues just as you normally would and make sure it’s the right system for you. Just because a package involves air springs doesn’t exempt it from every other aspect of building a good suspension system.
Cars with rear coil springs can take advantage of old-school separate air bags that install inside the coil springs. These are very inexpensive, durable, and are a great way to easily increase spring rate and ride height. They’re generally filled using a garage air compressor through a simple Schrader valve much like the ones in tires. Use a tire gauge to check the pressure. They can also be filled with an inexpensive 12v car air compressor plugged into the cigarette lighter. You can preload the right-side bag to help a car launch flatter at the drags; firm up both sides if you’re going somewhere with passengers in the back seat, etc.
Don’t skimp on or rush the plumbing. An air spring system is only as good as its poorest connection. Even a small, slow leak will drive you crazy over time so be very deliberate in the way you route and mount your lines and make your connections. A little extra time spent here will save you a lot of grief later on.
Shock Absorber Technology
Shock absorbers are absolutely critical to ensuring proper suspension function. They have a profound effect on how the car drives and performs, yet they’re almost always underemphasized. Most hot rod builders buy their shocks near the end of a build, almost as an afterthought. That’s a real shame, because they’ve usually blown most of their spare cash by then and the shocks get downgraded. Once bolted onto the car, the driver is blissfully unaware of all the potential performance and drivabil ity that is missing. It’s always amazed me how someone spends a fortune to get another few top-end horsepower that they barely notice and hardly ever use, and then cry poverty when it comes to buying good shocks. Bottom line? Don’t cheap out on your shocks!
What makes shocks such a big deal? Of all the suspension components on your car, the shock absorbers are the only thing that controls the velocity at which the suspension moves, dampening all of the unwanted oscillations and sharp jolts that would normally unsettle the suspension and the passengers. This simultaneously improves ride comfort and helps to keep the tire contact patches firmly in touch with the ground. On an old-school, non-computer-controlled car, the shocks are the smartest component in the whole suspension system. Take the most exotic sports car in the world, remove the shocks, and it is instantly transformed into a bouncy, uncontrollable pig. Yes, they’re that important.
History of Shocks
Shocks were invented by C. L. Horock in 1901 and first used in 1902 by the French automaker Mors to get a competitive edge on arch-rival Panhard. The shock absorber proved to be a huge step forward in suspension development. Those first shocks, both in knee-action and linear tube format, were nearly all adjustable in rate. People ask every day, “Do I really need adjustable-rate shocks?” Well, it made perfect sense in 1902 to have them, and modern race cars all run them (if the rules allow). So while you may not need them, if you want to get the best performance out of your car you certainly do want them.
Tube-type linear-action shocks have been available widely in America since 1932, when they were introduced as standard equipment by Hudson. All U.S. muscle cars were equipped with them. Unlike the first shocks, the standard-equipment shocks on these cars were not adjustable, and unlike the tremendous amount of effort expended by race teams (then and now) on performance shock tuning, little thought was given to high performance of any kind.
Today, performance shocks are available for many common applications. These typically have somewhat firmer dampening than stock units and are direct-fit replacements. They range from being a waste of money to being major players in getting the best performance and ride out of a car.
First, let’s address the basic misconception that shocks only affect ride quality for rough roads and bumps. This thinking is a big part of why many folks tend to minimize the importance of shocks. Performance enthusiasts are usually not pursuing a smooth ride. I often have clients tell me “I don’t care about having a good ride. I only care about the handling,” as if it were impossible for a car to handle and also have good ride quality. I’ve got news for you: That good ride quality can also be indicative of a car that still sticks to the road like Velcro when the road gets rough. Because the impact of shocks on performance is often undervalued, many folks reason that they can run stiff performance springs and cheap out on the shocks. That couldn’t be further from the truth. Firmer spring rates require firmer dampening rates to control them, and the shocks actually have a huge impact on handling, even more so than the springs.
Let’s turn from the influence shocks have on ride quality for a moment and consider how they factor into weight transfer and tire loading. I am not just referring to them in regard to drag racing, either. Drag racing is a good example of longitudinal weight transfer and tire loading, but you see all of the very same issues come into play while cornering as well.
Consider a car in a left-hand turn, for example. Weight is transferred from the left side of the car on the inside of the turn, to the right side of the car on the outside of the turn. The suspension on the right/outside is then in compression and receiving more weight on the suspension and tires, while the left/inside is in rebound, shedding some of the weight from its suspension and tires and transferring it to the other side. A shock with firmer compression dampening on the right/outside of the turn allows the suspension to compress at a slower rate, which transfers weight more quickly to the tires. Firmer rebound dampening on the left/inside of the turn resists lifting that side of the car and slows the transfer of weight to the right/outside suspension and tire.
The same car taking a right-hand turn reverses these roles side for side. If the weight transfers too fast and the tires on the inboard side unload and lose traction, then the tires on the outboard side are overloaded and unable to keep the car on line, and you get loss of traction and severe understeer. This can and does happen even if you have lots of spring rate and big sway bars. A few laps around a track in a car with adjustable shocks set to zero can demonstrate this in a hurry.
Under heavy braking or turn entry, the front suspension is in compression and the rear suspension is in rebound. All the same rules of weight transfer and tire loading still apply; I’m just applying them in a fore/aft format. Now weight is transferring rapidly, back to front. When you combine these two modes of action, fore/aft (longitudinal) and left/right (lateral), you find yourself heavily loading the right front tire and suspension and unloading the left rear tire/ suspension. While all of this is going on, the shocks are the primary mechanism for regulating the speed of the weight and energy transfer. (See the tuning section for more detail on how to use the shocks as a tool to tune that weight transfer.)
Shock Terminology and Construction
After my harping on how important it is to get good shocks and explaining a little bit about why, you’re probably wondering how to tell a good shock from a mediocre one. Modern shocks fall into several basic categories; all u se viscous oil and some type of gas (usually Freon) in some manner to achieve their dampening.
Mono-Tube Shocks: These use a single internal tube charged with a gas-and-oil emulsion, or gas (usually nitrogen) and oil in individual chambers separated by a floating piston. The pressure of the gas helps to reduce aeration (bubbles) in the shock oil, which would compromise the dampening rate of the shocks. These shocks are typically inexpensive because of their fairly simple construction. Both styles of mono-tube shocks exhibit their internal gas pressure by always pushing in the fully extended position. This can amount to an additional 30 to 40 pounds of spring rate measured at the shock. This is called “nose pressure,” which may be a small bit of unintentional help for a car that’s too softly sprung. It’s not a huge deal but keep it in mind.
Twin-Tube Shocks: These have a tube within a tube. The inner one is the compression tube. The outer is the reservoir tube, which holds oil displaced by the shock shaft during compression. Oil flow from the inside tube to the outside tube is controlled by carefully metered valves, which can be made externally adjustable to change this flow and alter the shocks’ dampening. Better twin-tube designs efficiently circulate all of the oil volume between the inner and outer tubes to aid in heat management. That is important because shocks basically dissipate kinetic energy by converting it into heat energy.(Remember, energy can’t be created or destroyed; it can only change form.) To better manage heat, the best shocks use thick aluminum bodies because of aluminum’s conductivity. Hardcore racing shocks, especially those intended for off-road racing, may be fitted with remote reservoirs to add even more fluid capacity and heat dissipating surface area.
Piston and Valve Assemblies: Regardless of the number of tubes used, the major shock action is controlled by a piston-and-valve assembly at the end of the shaft. These are either small spring-loaded individual valves or deflective valve discs that meter oil passage and control dampening. Dampening can be very precisely controlled in compression or rebound, and that dampening can be altered with the velocity of the shock piston as well. If you plot out the dampening of a shock throughout its travel and at various speeds, it then gives you its dampening curves. These can be linear, progressive, or digressive. These terms refer to resistance in relationship to piston speed. As piston speed increases, a progressive shock yields progressively more dampening, a linear shock yields the same amount, and a digressive shock yields less dampening. There is some debate on this subject as to which type of shock provides the best blend of performance and ride quality on high-performance street cars.
- Progressive-valved shocks are the easiest of the three to construct. A simple disc piston with holes for oil passage is naturally harder to push through the viscous oil the faster you push it, hence there is a progressive increase in rate with higher piston speeds. Progressive-valved shocks are very seldom used in racing anymore.
- Linear-valved shocks came later, with advances in shock piston design, and allow the shock to maintain the same dampening rate at various piston speeds. Many shocks for both street and racing use linear curves.
- Digressive shocks are the latest development and the most difficult to engineer and build. Dampening decreases as the piston speed increases.
How does all this affect performance and ride? Given that small, sharp bumps and jolts create high piston speeds, and large suspension motions (such as body roll under hard cornering) generate very slow piston speeds, this means that a progressive-valved shock is stiff when hitting sharp bumps and softer when cornering. Poor ride quality and poor handling all wrapped up in one! No wonder they’re not popular in racing.
A linear-valved shock treats sharp bumps and cornering the same. It’s not the most comfortable ride in the world but it’s not bad, and, all else being equal, handling is generally improved.
Digressive valving has softer dampening at high piston speeds to help soak up sharp bumps, expansion cracks, etc. But at slower piston speeds, when cornering or braking, it is firmer and controls weight transfer better than a comparable progressive- or linear-valved shock, all else again being equal. This means that a really good linear-valved shock may handle and corner better than a mediocre digressive type, but at equal price points the digressive usually offers a superior balance of ride and handling for a performance street car.
So, which architecture is better: mono-tube or twin-tube? Both designs can work very well when properly executed. Both can work poorly if they’re not. Bilstein has made a great name with performance high-pressure mono-tube gas-charged shocks, often with digressive valving. Koni has done the same, usually with linear valving. Both have applications that perform very well on a mild street car with regard to ride quality and handling. You could do a lot worse than going with one of these proven performers on a mild-performance car.
Externally Adjustable Shocks: When you get more serious about performance, you really should look into externally adjustable shocks. Most entry-level adjustable shocks are single-adjustable and only on the rebound side of the valving. These are usually steel body mono-tube shocks such as Koni Classics. They generally have a fairly small range of adjustment and usually have to be removed from the car to adjust them. They are still a useful tuning aid, but shocks that adjust both compression and rebound with a single knob are even more so.
The QA1 Stocker Star and VariShock QuickSet 1 are both good examples of shocks that adjust both compression and rebound simultaneously. Both have a billet-aluminum body, and are twin-tube, rebuildable shocks with external knob adjustment. The QA1 has 12 settings and the VariShock 16 settings over a slightly larger range. The QA1 shocks are linear valved, while the newer design of the VariShocks is digressive. In this case, the digressive valving also yields a few extra useful settings on any given vehicle because the compression and rebound curves diverge. That means when you set the rebound softer at the drags for better weight transfer, it softens the rebound proportionately more than the compression, so you can go one or two settings softer and still not smack the headers on the track when the front end comes down after launch. The opposite is true when you firm them up for an autocross event, where extra proportional rebound dampening can be a bonus.
Shocks that allow independent adjustment of compression and rebound dampening are considered to be double-adjustable, and a fantastic tool to get your car dialed in where you want it. Since the compression and rebound can be individually set, you can do very precise adjustments.
For example, you could add just rebound to the rear shocks to keep weight on the rear tires just a bit longer on turn entry under heavy braking, without stiffening the compression side and causing the car to get loose on corner exit. Or, you can soften rebound for much better weight transfer at the drags and increase compression dampening to prevent the suspension from bottoming out after launch, like a 90-10 drag shock.
The next steps up are the exotic three- and four-way adjustable shocks offered by Penske. Three-way (triple-adjustable) shocks have a single adjustment for rebound dampening and two separate adjustments for compression (one for high piston speeds and one for lower piston speeds). Four-way (quad-adjustable) shocks use Formula 1–derived dual-bleed shock pistons to allow separate high-speed and low-speed adjustment for compression and rebound.
Do you need shocks like these for your performance street-based car? No, of course you don’t. You don’t need 600 hp either, but it’s really cool to have it. Typically, shocks at this level are hard-core racing units with remote reservoirs that require custom mounts and look more like a warp drive component from Star Trek than a car part. Prices are in the thousands of dollars—each. Rumor has it that by the time this book is released, Chassisworks/VariShock will have direct-fit quad-adjustable shocks available for most muscle cars. That will be a real revolution in the shock world—quad-adjustable shocks that mere mortals can afford!
In summary, shocks are very important, so get the very best ones you can afford. For mild street machines, good-quality fixed-rate performance shocks from a reputable company are adequate, but externally adjustable shocks are better. For more serious cars, externally adjustable shocks are almost a must. You give up a lot of performance without them. Single-adjustable shocks are a big help, and are probably your best budget bet. If funds allow, or if you plan to autocross or road race the car and drag race it, then double-adjustable shocks will be worth the extra investment. Exotic remote-reservoir, triple- and quad-adjustable shocks are awesome, but at the time of this writing, so is their price tag.
Coil-Over vs. Coil Spring
Coil-over shocks were once reserved for race cars only. In recent years though we’ve been seeing more and more of them on the street thanks to the availability of many coil-over conversion packages. More often than not they’re installed to give the car adjustable ride height. There can be some merit to running coil-over shocks, but also some pit-falls to be aware of.
So what makes a coil-over shock different from a conventional shock and spring? Primarily, the coil-over wraps them both up into a compact, lightweight package. They use an adjustable threaded lower spring seat to allow for different-length springs, different preloads, and some ride height adjustment. This would seem to be the ideal package. Properly selected and adjusted for a specific application they are hard to beat.
Coil-over conversions for muscle cars come in two basic flavors: true coil-over shocks and coil-over hybrids, which are actually coil-over half breeds. The first are the familiar traditional format with both ends of the coil spring seated on the shock itself. Remove the shock and the spring comes with it, still registered in both seats. The second type, the hybrid, is only coil-over format on its lower half. That gives it an adjustable lower spring seat, but the top of the spring sits in the factory upper spring seat in the frame. Remove the shocks and the spring comes loose.
There are advantages and disadvantages to both systems. The first is preferable where overall length constraints allow because their motion is completely linear. That’s very smooth and efficient. They also accept standard-size racing springs, which are available in a dizzying array of rates and lengths.
The downside to this format is that compared to a stock spring configuration (where the top and bottom spring seats are as far apart as physically possible), the upper and lower seats of the coil-over are both about 3 inches from the original spring seats, on the shock body, which yields a spring about 6 inches shorter than stock. In general that leaves a 7- to 9-inch spring versus a 13- to 15-inch-tall stock spring. Performance springs are generally a bit shorter than stock ones, but even so the coil-over format is giving up quite a bit of spring travel. A 7-inch coil spring generally has about 3.5 inches of available travel. Taking the motion ratio of the coil-over in the lower A-arm into account, this still yields acceptable travel at the wheel, but it reduces the range of available adjustment and you need to be sure you don’t put too much preload on the spring and cause it to coil bind in bump. If that occurs you need to go to a higher spring rate. As a result coil-over conversions of this type tend toward relatively firm, even very firm spring rates, especially on heavy cars.
There is also a sub category of true coil-over shock conversions, which are those that require fabrication. Generally, the factory upper shock mounts are cut out and welded in a new raised upper shock mount that is coil-over specific. They are often used with either heavily modified or aftermarket tubular lower A-arms as well. These conversions allow a longer coil-over and spring to be used, which opens up more choices and gives them a larger range of adjustment. The downside, of course, is that you have to be comfortable with cutting and welding mounting points for critical weight-bearing suspension components. Also once done, they’re difficult to undo if you or a future owner ever wants to bring the car back to stock.
Hybrid coil-over shocks are a whole different animal. At first blush using the stock upper spring-seat seems like a great idea, because it puts the spring loads where the factory intended and you can run a longer spring than a true coil-over can, given the same basic envelope size. It’s not without its issues though.
One is that the upper spring seat is seldom perpendicular to the vertical axis of the shock, especially as viewed from the front. The shock is usually not even perfectly centered inside the spring. Both of these cause some amount of undesirable side loading to be incurred by the shock. It’s undesirable because you’re dealing with shocks, not struts here. Struts are designed to deal with radial as well as axial loads, shocks are not. Tube- type shock absorbers are designed to take only axial or in-line loads. That means that the increased side loading causes accelerated wear on seals and guide bushings and a decreased shock life. Once a shock sustains damage and begins to leak, it starts the transformation from precisely tuned instrument to oil-filled slide hammer, with predictable results. Close attention to detail in the configuration of the springs can help offset these issues to some extent.
One way to extend the life of the shocks is to use softer spring rates that induce less side loading. Some of these rates are lighter than those the cars originally came with, so if you’re looking to improve handling, beware. Do your homework or you may end up with less performance, not more.
Both formats of coil-over conversions typically use an OE-style crossbar on the bottom of the coil-over, as opposed to the double-shear mount and cross bolt usually seen on racing coil-overs. This is usually much more beefy than a stock shock crossbar, and it should be because it’s now holding up the weight of the whole front end of the car. That simple fact is the reason why these crossbars are almost always mounted on top of the A-arms rather than on the bottom. On top, all of the load can be transferred directly from the bar to the arms, and the bolts only locate the crossbars. Mounted underneath, the full weight of the vehicle and all of the dynamic load are hanging on two tiny bolts on either side of the car.
Many manufacturers make coil-over-specific tubular A-arms. There are several good reasons for that. One is that the stock A-arms were designed to take load over a fairly large area and the coil-over localizes those loads in one small area of the thin sheetmetal stampings. The other is that you generally like to use the longest coil-over you can in a given application, and the stock A-arms often have a raised shock mount stamped into the arm. In stock format this helps to center the coil spring, but with a coil-over it raises the lower mounting point and makes you run a shorter shock or a higher ride height.
When we get cars in the shop that have been exhibiting coil stacking with a coil-over conversion, the use of stock lower A-arms is often a contributing factor. Many of the coil-over-only arms actually drop this part of the arm to allow for a longer coil-over and spring to be used. That yields an increased range of usable coil-over adjustment. Some food for thought.
Regardless of whether a front coil-over conversion uses a true coil-over or a hybrid configuration, these packages seldom change the motion ratio of the shock and spring, so spring rates are usually an apples-to-apples comparison with OE-configuration springs and shocks. Now some of you, who are a bit ahead of the curve, are probably asking yourselves, “If there’s no change in motion ratio and we have some pitfalls to watch out for and we can get conventional springs and adjustable shocks for our cars, then what’s the advantage of using a coil-over conversion?” Good question. The coil-over conversions can still give you some ride height adjustability and they are lighter than a conventional spring and shock. Of course let’s not discount the cool factor either, after all we’re talking about hot rods here.
Coil-over conversions are also available for the rear of many coil spring cars. These are a lot different than the coil-over-equipped four-link conversions and so on that I’ve already talked about for leaf spring cars, in that the general format of the factory suspension is retained by these packages. They simply exchange the coil springs and separate shocks for a pair of coil-over shocks and new mounts.
In the rear you have a lot more vertical room to work with than you do in the front so the packages become less “fiddly” and easier to work with and tune. Spring rates are easy to choose because nearly any sensible spring rate will work; it’s just a matter of selecting the one that works best with your particular car. Some of these packages are direct bolt-on kits and others require some welding and/or light fabrication.
It’s important to note that it’s a very bad idea to simply bolt a coil-over into the stock shock mounts. Those mounts were engineered to take shock dampening loads only, and with very soft factory shocks at that. Mounts for a coil-over must support the weight of the vehicle, all of the dampening loads, and any kinetic energy passed through the coil-over turning hard cornering, acceleration, etc. A broken rear coil-over mount means a bad day! I’ve had to repair cars that broke subpar coil-over mounts already and the bill for chassis and body damage can be impressive, so don’t take any chances.
Look for a dedicated coil-over mounting crossmember or at least a highly reinforced mount with a double shear bracket on the top and a beefy double shear mount on the bottom as well. Single-shear mounts may work but they need to be extremely beefy and properly designed. Unlike front coil-over conversions, rear ones sometimes change the motion ratio of the springs in roll. Vertical spring rate only changes with different springs that are actually a different rate, but the farther the spring and shock are from the longitudinal centerline of the vehicle, the more effective they will be in controlling body roll and wheel movements in general when cornering.
A lot of rear coil-over conversions were originally intended for drag racing applications and so I recommend moving the coil-over shocks inboard to allow for big wheel tubs and larger drag slicks. This may be desirable for packaging in some extreme Pro Touring/g-machine applications running 335/345 or even larger cross section rear tires, but keep in mind that while huge tires are undeniably cool you may actually pay a penalty in handling if you compromise the suspension to fit them under the car. That is not to say that this type of kit can’t be used in a performance-handling application; just that you should mount the coil-overs as far outboard as you can realistically get them.
You can retain good handling by running higher rates if the motion ratios have been compromised but ride quality will suffer. The cant of the shocks in rear view also alters the motion ratio, so watch out for this as well. Canting the shock lets the designer get more travel with a shorter shock and can help in packaging the system but it also results in some effective loss in dampening and spring rate. It’s generally wise to keep the angle of the coil-over shock to less than 30 degrees. Fifteen or less would be even better.
Rear coil-over kits often allow for extreme amounts of ride height adjustment. That’s cool, but keep all of the geometry lessons from earlier chapters in mind too. Just because you have a huge range of adjustment doesn’t mean that it’s a good idea to use it all.
Written by Mark Savitske and Posted with Permission of CarTechBooks