The first step is to go over some suspension terms and definitions so you can get a handle on things. Don’t panic—I’m not going to bore you to tears with an endless list of esoteric suspension terms. If you were building road race cars from scratch, you’d need to know every fine point of suspension design, but that’s not the goal of this book.
This Tech Tip is From the Full Book, HOW TO MAKE YOUR MUSCLE CAR HANDLE. For a comprehensive guide on this entire subject you can visit this link:
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You probably know a lot of these terms already and some others ring a bell, but you may not be clear on their exact meaning. The following explanations are intended to be simple but useful. When you understand the concepts, you can start to apply them, then see how they relate to each other, and finally use that knowledge to improve your muscle car’s handling.
Simply put, these are the pivoting points of the suspension. The pivot axis of each bushing, the center of the pivot ball in each ball joint, and the tie rod ends are all pickup points. Invisible lines connecting these points (and also the arcs they swing in) dictate the geometry of the suspension. This forms the foundation the rest of the suspension is built upon. Geometry determines much of the physics, and the physics determine how the car drives and handles. Later in this chapter, I discuss moving some of these pickup points to achieve some big performance gains.
This is the vertical angle of a tire in relation to the ground. Zero degrees means perpendicular to the ground, or straight up and down. One degree of positive camber means the top of the tire is leaning outward by 1 degree from vertical. One degree of negative camber means the top of the tire is leaning inward toward the centerline of the car by 1 degree. The two main aspects of camber are static and dynamic. Static refers to the settings when you align the front end. Dynamic camber changes when the suspension moves as a result of the suspension geometry and vehicle movement. Since tires tend to have the best adhesion when their tread is flat on the road (at lower speeds and slip angles) or at negative camber (at high speeds and extreme slip angles), properly setting and controlling camber is very important.
For our purposes, positive camber (either static or dynamic) is generally not a good thing. You’ll see why in later chapters.
The tilt of the spindle, more correctly called the steering knuckle or upright, toward the front or rear of the car in relation to the ground is its caster angle. If the top of the spindle is tilted toward the front of the car, it’s called negative caster. If it’s tilted toward the rear of the car, it’s positive caster. Mild or negative caster settings tend to make manual steering cars easier to steer, but more positive caster provides better on-center feel, more steering feedback, and much better straight-line and high-speed stability. Guess which one you want to shoot for in your performance car? Note that since caster is measured in relation to the ground, changes in the car’s rake or ride height from front to rear alters the caster. Modern cars tend to run a lot of positive caster, with 7 degrees being common and some cars running as much as 12 degrees. Only specifically designed suspensions work well with such high numbers, but older cars can still benefit greatly from higher-than-stock positive caster settings.
Roll Center (RC)
This one is a little harder to explain because you can’t see it or easily demonstrate it. But it’s still very important. When any car corners, it grips the road with the tires at ground level, but the body and weight of the car are much higher off the road. There is a certain amount of lean (or roll) caused by centrifugal force. Think of the leaning car as an upside-down pendulum. The roll center is the pivot point of that pendulum.
Every car has both a front and rear roll center and they are seldom the same height. In traditional front-engine cars with solid rear axles, the rear roll center is usually higher. A line drawn between these two points is referred to as the roll axis. Think of this as if the car’s body is put on a rotisserie and the roll axis is a shaft going through the car that it pivots on. Why do you care about this? Because the height of the roll centers and roll axis partially determine how much spring rate and roll rate the car requires to handle well. They also determine how body roll affects suspension loading.
In many cars (and traditional muscle cars certainly fall into this category) the roll centers actually move around quite a bit while you drive. If you’re thinking that sounds like a bad thing, you are right! It means that the spring rate and roll rate requirements of the car can change dramatically mid turn. Since it’s rather difficult to swap out your springs or sway bars while your car is moving, you want to have the roll centers in a favorable location and keep them there if possible. Roll center movement is called migration, and it can move vertically and laterally.
When you’re talking to your car buddies about your suspension improvements, mention that you’ve minimized lateral roll center migration and you’ll instantly become the new local suspension guru. Just make sure you read the rest of the book before you say it because lots of questions are sure to follow.
It’s generally difficult to alter roll center locations on a factory-built car, but advancements in aftermarket suspension parts have made it much easier, and I address how and why to change them later on. Note that some race car books have said that moving the roll centers to tune the suspension isn’t the best idea due to its complex repercussions on other aspects of the suspension geometry (such as the camber curves). That’s true in a way, as it is complex and it shouldn’t be toyed with casually. But, they’re assuming you have a very competent suspension design to start with and that it only needs fine tuning.
Read on to see that it’s often very beneficial to change more than just the roll center on some muscle cars, so this can really work in your favor.
Center of Gravity (CG)
This is the top-to-bottom, side-to-side, and front-to-back balance point of the car. In other words, if you could hang the car from a chain at this single point, it would be perfectly balanced. That may not seem important, but this point determines the car’s front-to-rear weight bias and also affects how much the car rolls in the turns. The vertical distance between the CG and roll center (RC) forms a moment arm. In physics, a moment is loosely defined as torque caused by load applied some distance from a pivot point. In this case, it’s basically an invisible lever, like a pry bar that causes the car to roll in the turns. The upper end of the moment arm is pushed on by the mass of the vehicle and centrifugal force. Shortening this lever reduces its leverage and reduces body roll. This can be done by lowering the CG, raising the RC, or both. All else being equal, a lower CG is generally a good thing. Lowering the ride height of a car is the easiest way to do it. It can also be done by situating heavy components, such as the engine and transmission, lower into the chassis.
Instant Center (IC)
This is another one you can’t see, but it’s pretty easy to visualize. If you draw an imaginary line through the pickup points of two suspension arms, the instant center is the point where those lines converge. There can be a lot more to it, but that’s as complicated as we need to get for now. This is a handy piece of information for everything from helping to determine tie rod location (for minimizing bump steer) to making a car hook better at the drags.
Plain and simple, roll stiffness is defined as the car’s resistance to body roll. It’s determined mainly by the rate of the springs and anti-roll bars (sway bars), and it plays a big part in determining how many degrees the car rolls on its roll axis when cornering.
Spring rate is a measure of stiffness and is generally expressed in the number of pounds it takes to compress a given spring 1 inch. This measurement is then multiplied by the number of inches the spring is compressed. So, a 500-pound-per-inch (lb/in) spring requires 500 pounds to compress it 1 inch, 1,000 pounds to compress it 2 inches, 1,500 pounds to compress it 3 inches, and so on.
Some springs have a progressive rate. It’s very common on multi-leaf springs, but some coil springs are of the progressive-rate type as well. If a coil spring has its coils spaced much closer together on one end when compared to the other, it’s a sure sign that its rate is progressive. For example, a progressive-rate spring may start out at 500 lbs/in, but the next inch of compression may increase the rate by 550 pounds, the next by 600 pounds, and so on. At 3 inches of compression, this spring’s rate would be 1,650 lbs/in. This is often touted as an advantage, but it has its pitfalls as well. (See Chapter 3 for more details.)
Shock rate is a way to express the amount of dampening provided by a shock absorber. Shock rates aren’t as easy to express as spring rates because, like progressive-rate springs, shock dampening rates vary throughout their travel. Shocks also have different rates in jounce (compression) and rebound (extension). Additionally, these rates can vary with shock piston speed. As a result, accurate shock data is generally expressed in charted curves. Adjustable shocks have different curves for each setting or combination of settings.
Tire Slip Angles
Due to cornering forces and tire distortion under load, the actual direction that a car travels in and the direction the tires are pointed in are different during cornering. The difference between these directions is called the slip angle. Equal slip angles, front and rear, yields a neutral-steering car. Higher slip angles in the front cause understeer. Higher slip angles in the rear cause oversteer.
When a car exhibits understeer, the front end of the car demonstrates some resistance to turning. In other words, the car feels like it wants to go straight when you want it to turn. This is often referred to as push. Understeer is present in most factory cars because people seem to react to it naturally. If the car’s not turning enough they just turn the wheel some more. Understeer can be increased by dynamic positive camber gain and tire sidewall deformation. In general, more front spring rate or front sway bar rate increases understeer. Excessive understeer tends to make a car feel heavy and unresponsive.
The opposite of understeer, over-steer is when the back end of the car feels loose and wants to slide out. Sprint cars and the sport of drifting demonstrate extreme examples of oversteer. To a good driver, oversteer in a controlled drift can be a lot of fun, but it can be very disconcerting to average drivers because they’ll need to counter steer and turn the wheel left to go right and right to go left. More rear spring rate and/or more rear sway bar rate generally increase the tendency to oversteer. Oversteer can be influenced by your right foot as well. As you apply throttle to rear-wheel-drive cars, you can often induce throttle oversteer and use it to tweak the car’s balance mid turn.
This is particularly easy on high-horsepower cars. Sometimes it’s too easy!You may set these cars up with a little understeer as a safety net. All the horse-power in the world is no fun if, every time you get on it just a little too hard, the car spins off the road backward.
When a car is neutral, it turns exactly where you point it; no more, no less. When a car has neutral steer characteristics it’s said to be balanced. It is easy to tune and easy to drive fast, so this is something you’re shooting for. No car is perfectly neutral all the time but, if it’s close, you can easily make up for the difference with driving technique. When tuning a car’s balance, always start with settings prone to understeer and work gradually toward neutral handling. The closer you get to oversteer, the less room you have for driver error. Take your time—tuning is fun!
More people ask me about this steering term than any other. Bump steer is when the front spindles and tires turn in or out as the suspension moves up and down. When this occurs every time the car goes over a bump or the body leans in a turn, the vehicle steers itself without the driver’s permission. It can also induce understeer or (more rarely) oversteer. Bump steer tends to make cars wander or dart from side to side, which makes them much harder to drive fast and annoying to drive at slower speeds. Under hard braking, weight transfer causes both sides of the front suspension to be compressed at the same time and the bump steer is expressed as toe change, typically toe-out, which makes the car unstable under braking. Bump steer in general is almost always bad. Most muscle cars have a great deal of bump steer by modern standards. (See Chapter 7 for more on diagnosing and fixing bump steer.)
This is sometimes referred to as “Ackerman angle.” Named after Rudolf Ackermann and first used on horse-drawn wagons, this steering term refers to the difference in the angles of the front wheels in a turn. When turning, the inside wheels follow a smaller radius than the outer wheels, so it is often advantageous to have the inside wheels turn sharper to follow that smaller radius. In effect, this dynamically alters the toe of the front wheels. Ackerman can be a touchy subject. In road race cars, the high-speed turns and high tire slip angles can favor parallel or even reverse Ackerman. But since street cars operate at much lower average speeds, Ackerman is generally a desirable feature. Most solid front axle cars with the steering linkage in front of the axle have reverse Ackerman.
Adding Ackerman to cars that have very little can improve low-speed responsiveness. Although some race car components allow Ackerman performance, it’s generally not easy to alter on production cars.
Toe refers to whether the front tires are parallel (zero toe), angled inward toward each other in the front (toe-in), or angled outward and away from each other in the front (toe-out). Toe-in typically adds directional stability, while toe-out can make a car wander or dart from side to side. In most driving conditions, that’s undesirable, so toe-in is most often used. Traditionally, it’s measured at the tire’s outer circumference, front and rear, and at the same height off the ground. This measurement is expressed in fractions of an inch, with 1/8 inch of toe-in being the most common. Newer computerized alignment racks usually express toe-in measurements as degrees. With a 26-inch-tall front tire, 1/8 inch of toe-in equals about .7 degree.
Driving down the road, the tires create drag that takes up some play in the steering components and cause some deflection in the suspension bushings, and this reduces the dynamic toe-in to less than the static measurement. Hard braking also changes the dynamic toe-in due to the same factors, but in most production cars bump steer has a much larger effect. Replacing factory suspension bushings with low-compliance performance bushings reduces the amount of deflection and allow running a bit less static toe-in.
This is a function of the internal gearing of the steering box and any changes caused by the linkage. To determine this ratio in traditional muscle cars, you need to take into account the internal ratio of the box, the length of the Pit- man arm (and idler arm), and the length of the steering arms from the lower ball-joint pickup point to the outer tie-rod-end pickup point. Some cars (like 1967 to 1969 Camaro/Firebird) have interchangeable different-length factory steering arms to alter the steering ratio and the Ackerman. The 1963 to 1982 Corvettes have two tie rod mounting holes in their steering arms to allow ratio and Acker-man adjustment. Others can benefit from aftermarket packages such as the Street Comp-AFX package for GM G-bodies with tall AFX aluminum spindles and redesigned steering arms. These correct the factory bump steer, quicken the steering ratio, and improve Ackerman.
Rack-and-pinion systems are subject to the same stipulations, with the exception of the Pitman and idler arms, of course. Note: When installing a rack-and-pinion into a car that didn’t originally have one, the difference in the length of the steering arms can have a major effect on the final ratio. A fast-ratio rack-and-pinion from a sports car with 4-inch-long steering arms, if installed in a car with 7-inch steering arms, doesn’t perform like fast-ratio units because the final steering ratio is quite slow.
The steering wheel also has an effect on perceived steering ratio. Most muscle cars came with thin, large-diameter steering wheels that wouldn’t look out of place on a city transit bus. A modern sports car has a much smaller, thicker wheel. If the original wheel is about 17 inches in diameter, you have to move your hands roughly 13 inches to turn 90 degrees. Substituting a 13-inch-diameter wheel, you only have to move your hands 11 inches to turn the same 90 degrees. You still have to turn the wheel the same number of degrees to get the same result but, since your hands don’t have to move as much, it feels quicker. The smaller-diameter wheels also yield a slight improvement in steering feel on over-boosted older cars due to the reduction in leverage. At the same time, it makes manual-steering cars much harder to steer at slow speeds.
Front Suspension Relationships
Each of the terms above (and many more) form relationships that define how the car handles. Most of the cars you’re dealing with in the muscle-car world have deeply flawed suspension designs by today’s standards. After you understand the relationships between components, geometry, and where the problems lie, then you can fix them and make huge improvements fairly easily.
I focus here on the short/long arm (SLA) suspension, which is common in most muscle cars.
Picture SLA suspensions as seen from the front of the car. SLA systems use a long pair of A-arms on the bottom and a much shorter set on the top. This allows more room for the engine and exhaust and gives it an elliptical camber curve. That can be a big benefit on a street car because the camber change starts out small, which minimizes tire wear, but becomes more aggressive the farther the suspension moves.
In a traditional American muscle car, the lower A-arms usually consist of either a one-piece steel stamping actually shaped something like a capital letter A (as seen on Camaros or Chevelles) or a stamped steel U-channel arm, perpendicular to the car’s centerline, with a separate strut rod forming the other side of the A (like those on a Road Runner or early Mustang). These arms are roughly level with the ground, but with a slight slant downward toward the lower ball joint. The upper arms are usually stamped steel in a true letter “A” pattern. In contrast to modern cars, most of these angle downward toward the upper ball joints.
Here lies the start of your problems. As each arm pivots on the bushings mounting it to the frame, the ball joints at the end of each arm swing in arcs prescribed by the length of each arm. With all four arms in the described orientation, the lower ball joints move outward just slightly when the suspension is compressed (also called bump or jounce). The upper ball joints move outward much farther because the arms are at a much lower angle in their arcs. The shorter arms also have more rapid gain due to their shorter length. Because the upright (often called the spindle) is attached to the upper and lower ball joints, it tilts out at the top and causes positive camber gain.
If the car enters a corner at zero static camber and rolls a few degrees, compressing the suspension on the outboard side of the car, then that tire gains positive camber. Why is that bad? In this situation, the tire contact patch is being lifted off the road and being unevenly loaded. That means much less traction.
The angles of the A-arms also dictate the instant center (IC) and RC. To determine the IC, draw a line through the pivot points of the upper and lower A-arms, as viewed from the front. Next, draw a line from the center of the tire’s contact patch on the ground to the instant center, and you’ve got your roll center. If the lower A-arms are either level with the ground or angled down toward the ball joints, and the upper A-arms are angled down toward their ball joints, then the roll center is below the ground. If all four arms are parallel, the roll center is at ground level. If the upper arms angle upward toward the ball joints, the roll center is above the ground. (Remember, when I discuss these A-arm angles, I’m talking about lines drawn through the pickup points, not the shape of the arms themselves.)
This brings us back to the relationship between the roll center, the center of gravity, and the invisible moment arm in between them. For example, take two cars identical in every way except that one has a roll center below the ground and the other has its RC well above the ground. Due to its longer moment arm, the one with the lower roll center exhibits more dynamic body roll. This is due to its longer moment arm and its additional leverage on the mass of the car. More body roll means the more heavily loaded suspension on the outside of the curve compresses more, and therefore experiences more camber gain.
Since, as you’ve seen above, cars with the upper A-arms drooping downward from the frame mounts also usually exhibit positive camber gain, you also get the tires rolling over toward the outer sidewalls and compromising traction. This makes for a one-two punch that pretty much knocks out any chance of good cornering performance. This type of geometry was very common in the 1960s, and some vehicles built on older platforms continued to use it even into the new millennium. The two-wheel-drive GM S10/S15 pickup is a good example. You won’t find this type of geometry on cars (or even trucks) of modern design, though. That’s why muscle-car enthusiasts are sometimes surprised that their new SUV drives and handles better than their high-horsepower, vintage performance car.
Bump steer. Just a few years ago this term was seldom heard outside of racing circles. That’s remarkable when you consider the dramatic impact it has on how well or how poorly a car drives. Bump steer means that the spindles are steering to the left and right as the suspension goes up and down. This occurs due to misalignments in the steering and suspension pickup points. When it occurs on both sides at the same time it’s often expressed simply as toe change.
Bump steer takes control of the car away from the driver, making the job of driving more difficult. The car is quite literally steering itself. Modern cars as a rule have very little bump steer. Traditional muscle cars, on the other hand, tend to have a lot of it. It’s no wonder that between the poor suspension geometry and bump steer, the vast majority of these cars have been relegated to low-speed cruising and straight-line drag racing.
With a good understanding of what causes bump steer, and a little help from technology and the aftermarket, you can reduce it considerably, or even eliminate it. That yields a huge improvement in drivability and control for these older cars.
There’s an old street rodder rule of thumb that says if you want to have zero bump steer (a very optimistic term) you simply have the tie rods level with the ground at ride height. Like many rules of thumb it’s technically not true, but it does have a grain of truth in it. Bump steer revolves primarily around tie-rod-end location and the pickup points within them. This is true with both conventional steering-box and linkage systems and also with rack-and-pinion systems. These tie-rod-end pickup points dictate the arcs that the tie rods swing in. If those arcs don’t match the arcs, the spindle and steering arms swing in the spindles will have to steer to compensate for the difference.
A horizontal mismatch in tie rod length causes the arc it swings in to have the wrong curvature, which causes some deviation (bump steer), especially toward the ends of the curves. A vertical misalignment causes the angle of the arcs to change, which causes them to intersect rather than overlap. This causes very rapid divergence between the curves—in other words, really serious bump steer right from ride height. For this reason, 1/8 inch of vertical misalignment can have as dramatic an impact on bump steer as a couple inches of horizontal misalignment.
Let’s consider that many classic muscle cars have 5/8 inch or more of vertical misalignment and you can see how serious the problem is. Right about now you’re probably asking, “Okay, bump steer is bad but how the heck can I tell if I have it?” Well for starters, if you have any kind of older muscle car you’ve got it, and probably plenty of it. I get calls all the time from guys who say that they’ve had this 1967 GTO or 1969 Mustang or whatever for 30 years and it’s never had any bump steer. Those guys are swimming in a river in Egypt . . . in denial. Unless Detroit or Dearborn made that particular car different from all the others (they didn’t) it’s an inherent part of the suspension and steering design.
Some folks have just gotten used to it and don’t think it’s a big deal. That is until they get rid of it, then they always kick themselves for not doing something about it sooner. Remember the AM radio analogy from the introduction? If you’ve only every listened to an AM radio with one speaker you’d think it was pretty great but once you hear a modern stereo system it’s obvious how much the old AM left to be desired and it’s almost painful to go back.
If you’d like to verify that you have bump steer issues on your car before you throw time and money at it, just measure your toe-in at ride height, then jack the car up until the tires just about come off the ground and measure it again. The difference in the two measurements is about one half of your total bump steer. On most muscle cars that half is 5/8 inch or more of toe change. On many older cars you don’t even have to measure it; you can literally see the toe change from 10 feet away.
Okay, so I’ve established that virtually all of the cars discussed in this book have some bump steer issues. What do you need to do to cure it? If you’re lucky, some aftermarket company that supports your car has already done the research and development for you and offers some kind of correction kit. These can take the form of spindles with relocated steering arms mounting holes, specially designed replacement steering arms, different-height tie rod ends, Heim-joint-style ends with height adjustment spacers, offset rack-and-pinion mounting bushings, or even different-height lower ball joints that relocate the spindles vertically and bring the steering arms and tie rod ends along with them. There’s no single “magic bullet solution.” The solutions vary from car to car and I cover them in more detail in Chapters 8, 9, and 10.
As always, caveat emptor, let the buyer beware! Many companies tell you their parts or systems correct bump steer, or that they have “zero bump steer,” even if they do nothing to improve it—sometimes it even causes more bump steer. That’s why it’s important that you know what you’re looking at and what you’re really spending your hard-earned money on.
There are two things to look for in tie rod location—the length and the height of each end, which also dictates its angle. In its most simple form, picture the suspension from the front with pickup points in the center of each ball joint and each A-arm’s bushings where they mount to the frame. Draw lines connecting these points horizontally to find the instant center as discussed previously. Remember that no matter what height the tie rod is at, a similar horizontal line drawn through the pivot points in the tie rod ends must intersect with the instant center. That gives you your angle.
Now picture a vertical line through the inner pickup points where the A-arms mount to the frame, and another vertical line through the outer ball joint pickup points. The inner tie rod ends should fall somewhere on the inner line and the outer ones should fall somewhere on the outer lines. If one is offset 1-inch inboard the other should be as well and so on.
That’s pretty much it. No, really. At least it’s as good as you need to get for general illustration. This basic model works on any SLA front end with arms mounted parallel to the centerline of the car. Some adjustment needs to be made for cars angled in plan view (as seen from above), and for those with a great deal of anti-dive, but this simplified model gets you in the rough ball park.
For cars that do have arms angled in plan view or a lot of anti-dive you can rough in those adjustments fairly easily too if you have one very high-tech tool: a yardstick. Say you’re checking a GM A-body with lower A-arm mounting points that angle inward toward the front of the car in plan view. Get under the car and lay a yardstick along the axis of the lower A-arm bushings. Now notice that the inner tie rod ends sit about 5 inches forward of the lower A-arms. Where the yardstick intersects that line is roughly where your vertical line between inner pickup points should end. Likewise, if the car has a lot of anti-dive (the upper A-arms are angled upward at the front in side view) you can lay a straightedge on top of the cross shaft to extend this line fore or aft over the steering linkage to fine tune your vertical pickup points.
Steering misalignment has predictable results. If the outer tie rod ends are too low or the inner ones too high, the front wheels will toe out when the suspension is in compression. This means when the front end dives under heavy braking, you lose toe-in and sacrifice straight line stability. Certainly a less-than-ideal circumstance. Conversely, the wheels toe in when the suspension is in droop. Scrubbing off speed under acceleration is also less than ideal. In a drag car that carries the front wheels off the line, the right front wheel generally touches down first. If it’s toed in, the car steers to the left. You don’t want that either.
If the outer tie rod ends are too high or the inner ones too low the car will be unstable under heavy acceleration when the wheels toe out and will jerk to the right after a hard launch. As you can see, be it a street car, road race car, or drag car, bump steer has ill effects on all of them. You can also troubleshoot bump steer by simply checking toe change at different heights. If you use vertically adjustable tie rod ends or similar components to correct bump steer issues you can use these measurements to rough in their adjustments. You’re shooting for as little toe change over the full range of travel as possible. With this method you need nothing more complicated than a tape measure. Of course, it won’t be as accurate as a true bump steer gauge but it’s easy and free.
If you become afflicted with severe “Bump Steer Correction Disorder” (BCD) and decide to buy a professional bump steer gauge, I suggest the Long-acre style with a single-dial indicator. They are much quicker and easier to use than dual-indicator gauges and usually less expensive as well.
My last word on the subject of bump steer gauges (because I think we’re getting a bit beyond the scope of this book) is to make certain that it is absolutely, positively rock solid before you start taking readings. The very smallest amount of movement at the base of the gauge will throw all of your readings way off and drive you nuts. That said, for the vast majority of muscle car owners, buying a bump steer gauge will only result in a) eventually hurling it across the garage, or b) endlessly obsessing over bump steer minutia. So if you can locate a well-designed bump steer correction package for your car, go for it. Don’t say I didn’t warn you.
SLA vs. Strut
When I first sat down to write this book I have to admit that I didn’t give much thought to MacPherson-strut-equipped cars. I had always planned to cover the G-body GMs from the late 1970s and 1980s since their suspension basically trickled down from the early A-bodies, but an IROC or Mustang GT still doesn’t seem like a vintage muscle car to me. I suppose that’s because I remember when those models were introduced and how thoroughly new and modern they seemed at the time. Of course some of them are nearly 30 years old, and they are certainly muscle cars, so they do have a place here.(I’ll touch on the differences in their strut front suspensions versus a short/long arm suspension and then go into a lot more detail in Chapters 8, 9, and 10.)
There are some fundamental differences between these cars and their older counterparts. One of these differences is a switch to the MacPherson strut instead of keeping the more familiar (at the time) short/long arms, or SLA, suspension. In a MacPherson front end, the upper A-arms are replaced by what is essentially a coil-over shock that has been designed specifically to tolerate side-loading forces. These are used to locate the top of the spindles laterally. This format is simple, light, and often less expensive to produce than an SLA front end so it has become the standard in most modern production cars.
Determining the geometry of a strut suspension is similar to determining that of an SLA suspension with a few variations. Rather than using upper A-arm pickup points, you draw a line perpendicular (90 degrees) to the center axis of the strut to help define the instant center. Horizontal lines are still drawn through the lower A-arms pickup points and from the center of the tire contact patch to the instant center to determine roll center location. This format, combined with the packaging constraints of the tall struts and their large strut towers, can impose some limits on the range of geometry a strut suspension can achieve, but it’s also hard to design a strut suspension with poor geometry. They tend to have very efficient spring-motion ratios, which means lighter spring rates than SLA cars, so don’t try to compare the rates to each other.
The newest generation of strut suspensions sometimes use two lower ball joints, which form a virtual pivot point that allows for more dynamic camber gain when the wheels are turned, but that’s well beyond the scope of this book. It should be noted that 1993 to 2002 F-bodies, Camaros/Firebirds, are not strut-equipped cars. They use very small upper A-arms and large lower A-arms (think Tyrannosaurus Rex) and a long, non-adjustable coil-over shock that is often mistaken for a strut. Lastly, MacPherson strut suspensions don’t leave much latitude for geometry modification so it’s a good thing that they’re generally pretty decent to begin with. Rather than tweak their geometry you usually just tune a strut front end with the alignment settings, spring rates, and dampening.
SLA Suspension Fixes
Now that you know a little about the problems, you’re probably wondering what you can do about them. Well, you can ignore them and hope they go away, or you can move the pickup points that define the geometry and you can fix it. There are a number of ways to do this.
If you were building a chassis from scratch, you could simply move the mounting points of the upper and/or lower A-arms to alter the geometry to your liking. That’s easier said than done when working with a production car, though. This is especially true when working with an original SS 396 Chevelle or K-code Mustang or any other rare car where you’d rather not ruin its potential collector appeal or value.
There are a couple of modifications that can be done to the frame side-mounting points that don’t require major surgery. One is the Guldstrand mod (or G mod), named after racing leg-end Dick Guldstrand, for 1967 to 1969 Camaros and Firebirds. Another is the so-called Shelby mod for 1964 to 1965 Mustangs. (I address each of them in Chapters 8, 9, and 10.) Generally speaking, modifications of the frame side-mounting points require extensive cutting, welding, and fabrication beyond the scope of most hobbyists and even most professional garages.
This leaves you with the option of modifying the outboard side of the suspension. These components are all bolt-on pieces that can easily be changed back to stock in case some poor, misguided individual ever wants to put the car back to 100-percent original and make it drive poorly again.
The most common parts to get changed are the A-arms. That’s because they look cool and advertising implies that they’ll improve the geometry. This is only true to a very small extent. A-arms only connect pickup points; they don’t define them. They’re only capable of making changes in static alignment, which is important, but it’s not the kind of drastic geometry change you’re looking for. (See Chapter 2 for more information on aftermarket arms.)
Upper and Lower Ball Joints
These are the two outer pickup points that affect the camber curves, roll center location, etc. If you’re dealing with a car on which the lower A-arms are more or less in the proper orientation, but the upper ones are drooping down to meet a pair of short spindles, then it’s easy to see how a taller set of spindles might be a big improvement. A taller spindle moves the pivot point of the upper ball joint higher in relation to the frame mounting points, changing its angle and offering potential geometry gains. The same is true of taller ball joints.
Although circle track racers have been using variations of this trick for years, SC&C was the first to use it to reengineer the front ends of muscle cars with purpose-built components. In the past, the degree of improvement has been severely limited by the selection of factory ball joints that would both bolt into the application and give some kind of gain. But, with the introduction of CNC-machined modular ball joints (from Howe Racing, mainly) offered with interchangeable studs of different heights, much more profound changes are possible. Caution should be exercised with these because it’s possible to create as many (or more) problems as you solve with them due to the thousands of possible combinations. Properly integrated into a complete package (such as the Street Comp packages I have developed and offer for sale through my company), they can yield serious geometry gains. Why does it have to be a package? Why can’t you just slap a pair of tall spindles or some tall ball joints on the car and go? Because any real change in geometry requires some other changes to go along with it.
As the upper ball-joint pickup points are raised by taller spindles or ball joints, the upper A-arms have to swing upward and outward in their arcs. If the car is lowered (with springs or coil-overs) they swing up and out even farther. This adds positive camber, which has a negative effect on handling. Since the stock upper A-arms were designed with poor alignment specs in the first place, there’s no chance of putting in enough alignment shims to fix the problem. Also problematic because the shape of the stock arms is also intended to mount the ball joints at the proper angle to maximize suspension travel, a lowered ride height and taller spindles (or ball joints) can put the upper ball joints near the limit of their travel, even while resting at ride height. When the suspension is compressed, it can over-travel or bind the ball joints, putting extreme stress on them and the upper A-arms. This can lead to component failure and having a very bad day. Nobody wants this, so don’t try to cut corners. Do it right and do it once.
Also known as steering knuckles or uprights, spindles are the next most common parts to replace. These fall into three basic categories: stock (or stock reproduction), dropped spindles, and tall dropped spindles.
Stock or stock reproduction spindles are self explanatory. They’re the baseline from which the other types evolved.
Simple dropped spindles share the same overall height as the stock spindles, but have the actual spindle pin raised (usually 2 inches) in order to lower the car. They’re usually installed just for looks and their claim to fame is that they drop the car but don’t change the factory suspension geometry. Well, considering that most muscle cars could benefit from some thoughtful geometry changes, I wouldn’t consider that a selling point. It’s also technically not true. They do alter the geometry, and can even provide a few benefits, albeit small ones. The lower ride height lowers the CG a bit. The new stance raises the RC on some cars just a fraction of an inch, shortening the front moment arm only slightly. The difference is so small, though, that it’s only academic. The camber curves are unaltered and lateral RC migration can either go up or down, depending on the car.
There are also some downsides, though. Lowering a car without increasing the spring or shock dampening rates is often a recipe for the headers or other low-hanging components to bottom out on the road. For example, it may take the stock springs and shocks 3 inches of travel to absorb a bump and keep your exhaust headers 1 inch off the road surface. Drop the car 2 inches with no other changes and things may get ugly when it tries to move that same 3 inches! Also, most of the cars in this book see more geometry improvement from a set of performance rate-lowering springs that further reduce body roll. If you combine a rate-lowering spring with a 2-inch dropped spindle, you may end up with a ride height that’s too low and you’re bottoming out again.
Raising the spindle pins has another consequence as well. Moving the pin up moves the wheel and tire with it, and puts the wheel rim much closer to the outer tire rod ends. This can limit the amount of wheel back spacing you can run and ultimately limit the front tire width. It’s important to take things like this into consideration so you don’t end up with a really cool set of new wheels that won’t fit on the car!
The last type is the tall dropped spindles. The term “tall” refers to a unit that’s taller than the original spindles for the purpose of correcting/improving the geometry. The actual height increase is generally between 1 and 2 inches. Most are still designed with a 2-inch drop, with the same pros and cons of any 2-inch dropped spindle. This class of spindles is most prevalent for GM A-body, G-body, and first-generation F-body cars. These tall spindles should be used with an appropriate set of upper A-arms that complement their non-stock geometry.
The improvement in geometry and performance can be very impressive with spindles of this type. They’ll all improve negative camber gain, raise the roll center, and help stabilize it laterally. The degree to which they do so depends on the individual spindles chosen, the car they’re used on, and the rest of that car’s setup. Some improve bump steer by relocating the factory steering arms or by supplying new steering arms. But some actually make the bump steer much worse. Most are based on stock spindles, are made of cast iron, and accept stock brakes, bearings, etc.
A notable exception is the American Touring Specialties (ATS) AFX tall spindle. It’s a forged 6061 T6 aluminum spindle designed to retrofit modern geometry, modern C5/C6 Corvette brakes, and massive C5 hub/wheel bearing packs to older cars. It’s not adapted from anything; it’s a clean-sheet design that allowed its designers to optimize every aspect to the last detail. This unit is a 1-inch dropped spindle, which is still enough to yield geometry gains but with better tie-rod end clearance than a 2-inch dropped spindle.
Which spindle best suits your car depends on what brakes you intend to run, your target wheel size, and your budget, of course. (I address the specific attributes of the various spindles in Chapters 8, 9, and 10.)
It’s always a good idea to ask questions before you buy spindles or any other aftermarket suspension part. How much of a change in negative camber gain can you expect when running your tall drop spindle of choice with 1-inch-lowering springs? Roughly, what will the new roll center height be? If your supplier of choice doesn’t know the answers, or if they’re the wrong answers, you might want to keep looking. If they’re not willing to take the time to answer your questions, they don’t deserve your money.
How does installing a tall spindle package, tall-ball-joint package, or doing the G mod or Shelby mod affect the relationships of different aspects in the front suspension? As you change one thing, you know you’ll also be changing a lot of other things in a sort of chain reaction. As you change camber gain from positive to negative in bump, the instant center locations change as well, and the roll center is raised. This shortens the moment arm between the RC and CG. The front view swing arm (FVSA) is shortened as well, and must be prevented from getting too short. In conjunction with a higher roll center, it could induce too much suspension jacking and make the front end porpoise during heavy cornering. A well-designed system won’t let that happen, but it is one example of why more isn’t always better; just right is always better. The combination of changes results in a car with a lot less body roll, much better lateral grip, and much more precise and predictable behavior. Provided you also use the appropriate tires, springs, shocks, and alignment, it can now drive and handle like a new performance car.
All suspension systems are a compromise between what’s ideal and what’s practical to build. In years to come, we’ll probably see dynamically adjustable suspensions that can alter not only shock dampening (as we have now on some high-end cars) but also spring rates, suspension arm length, camber and caster, and roll center location to fit the ideal requirements for a given situation at millisecond intervals. But that’s way beyond the scope of this book.
Written by Mark Savitske and Posted with Permission of CarTechBooks