Let's face it: just because we all love to look at, ride in, and even build super cool rods and customs, we don't always understand the intricacies of how every system operates. As long as it bolts on and works well, who cares how each little part does its job, right? Wrong. Understanding the basics of how each system on your rod works will not only help you impress friends and relatives with your vast engineering prowess and knowledge of everything automotive, it will also allow you to build and maintain a safer and more reliable car. Last issue we kicked off a new series called How It Works, which will focus on a different system every month, examining how different parts function and, hopefully, debunking a few myths along the way. Since we tackled the basics of traditional dropped-axle suspension last time, we thought taking a long, hard look at the world of Independent Front Suspension (IFS) might be appropriate this time around.
HISTORY AND THE MUSTANG II
After decades of solid axles pounding across the burgeoning American highway system, Chevrolet introduced one of the first domestic passenger car independent front suspension setups in 1939 (more on this later). Ford came into the independent game a decade later in 1949, when they rolled out their first totally fresh chassis design since the days of the Model T. "The geometry of this design was pretty good," explains Brent VanDervort of Fat Man Fabrications. "However, disc brakes, power steering, and lowering options are very limited. The setup isn't bad, but because most rodders have a lack of experience working on these early systems that combine kingpin technology with IFS, it can be hard to work on for some people and even some shops."
While these early IFS setups worked pretty well on the big passenger cars of the '40s and '50s, street rod enthusiasts were relegated to using variations of the same tried-and-true solid-axle suspension design Henry Ford introduced at the turn of the century all the way up until the mid-'60s. That's when the Chevy Corvair, with its advanced independent front suspension, began to show up in wrecking yards where industrious rodders could get their hands on them. Unfortunately, Corvair IFS was not a perfect solution, as it required hubs and brakes from the relatively rare Pontiac Tempest to be converted over to a five-lug bolt pattern, and the steering system was hard to adapt, as well. "The Corvair parts worked horribly," says Gary Heidt of Heidt's Hot Rod Shop. "The spindles were designed to work with a steering box, and people were putting rack-and-pinions on them. Since rack-and-pinion doesn't have as much travel as a steering box, you couldn't turn the wheels far enough, and it negatively affected the turning radius."
As the Corvair adaptation began to wane in popularity, the AMC Pacer was discovered to have a stout independent frontend that was narrow enough to fit on an early Ford, and it had rack-and-pinion, making it an ideal candidate. However, due to the suspension's heavy construction and the relative obscurity of the car, the Pacer conversion never really caught on. As the '70s rolled forward, Ford's diminutive Pinto and Mustang II (which shared most of their underpinnings) began to show up in wrecking yards. Famed builder Chuck Lombardo claims to be the first person to use Mustang II suspension on the front of a street rod in 1974, and after discovering that the little car's track width and rack-and-pinion placement made it a perfect bolt-on IFS system for pre-war hot rods, the word spread like wildfire. We finally had a combination that would work.
"The Mustang II and Pinto setup became popular not only because the track width was correct, but because they could be modified for Granada rotors and bigger brakes--and it worked so well that it stuck," Chassis Engineering's Eric Aurand explains. "This is the most modern passenger car non-strut tower independent front suspension ever made, because everything built after this design has been either front-wheel drive or a tower-type configuration."
Throughout the rest of the '70s, rodders were relegated to scrounging around wrecking yards in search of Mustang II and Pinto parts, hoping to score enough fresh gear to complete their projects. The IFS situation didn't change much until 1978, when a Ford engineer named Bob Shay started one of the very first replicar companies, called Shay Motors. After designing a fiberglass re-creation of the venerable Model A, powered by Pinto running gear and riding on Pinto suspension, Shay pre-sold 10,000 units and went into production. He actually managed to pre-purchase the suspension for all of his orders but went out of business in 1982 before most of the cars could be built. As a result, an entire warehouse filled with thousands of factory-fresh suspension assemblies ended up on the auction block for pennies on the dollar, and several burgeoning hot rod suspension companies managed to scoop up the loot and start cranking out new IFS kits.
One of these new suspension companies was Heidt's Hot Rod Shop. "What we would do is re-drill the rotors with a five-lug pattern, fabricate our own crossmember, and bolt on the rest of the components," Heidt explains. "The nature of this beast fit the street rod world so well that things just took off. The Pinto kits worked great on '33-and-later street rods and fat fender cars, but it didn't fit the '32 Ford at all, or Model As. The upper control arm would hit the fender, and moving the location of the arm to clear the fender would totally change the geometry of the suspension design so it would no longer work properly."
As an answer to the upper control arm problem, Jerry Kugel's innovative suspension shop came up with their own clean-sheet design that resembled the Mustang II/Pinto but was a fresh take on IFS that would fit under the popular Deuce and Model A. Heidt's quickly followed suit with their own scratch-built setup, called SuperRide, and soon thereafter the aftermarket exploded with different IFS kits composed entirely of brand-new parts.
CHEVY GOES INDEPENDENT
Unlike early Fords, which utilized a C-channel framerail fortified with an extra-stout X-member down the middle for rigidity, '39-54 Chevrolets gain their rigidity through what is called a torque-box framerail. If you look at a cross section of a torque-box rail, it looks similar to a top hat, with a stamped steel upper portion reinforced with a double-thick piece of steel across the bottom. Since these early Chevy frames do not employ an X-member, their strength comes entirely from the shape of the rail, and the frame torques and twists by design as the car goes over bumps in the road, dispersing most of the stress throughout the length of the frame. In contrast, Ford chassis have such a rigid frame that most of the stress is absorbed by suspension. Neither system is better or worse than the other; they're just different approaches to solving the same problem.
In 1967 Chevrolet introduced its answer, the Camaro, designed specifically to take on Ford's Mustang in the ponycar wars. The Camaro has a unitized body construction method, which means that, rather than a traditional body-on-frame arrangement, the floorpan and body are all one piece, with separate front and rear frame clips constructed in a torque box design similar to the earlier cars, only flipped upside-down and welded to the body. The front and rear suspension then bolt to reinforced pads on the frame clips. An extremely popular conversion in the '70s, '80s, and even today is to cut out a Camaro front clip and graft it onto the front of an earlier Chevy hot rod. While this plan can be very economical (there are thousands of old Camaros rotting in junkyards across the country), it is not the safest or easiest way to install IFS.
"The early Chevrolet framerail is meant to torque from one end to the other, so if you cut the rails off and graft this big, solid Camaro frontend on and brace it all in, the stress is still going to have to manifest itself someplace--and now the rails can no longer flex the way they're supposed to," Aurand says. "That stress needs come out somewhere, so what usually happens is it will break the rail right behind where the weld cooled on the frame where the clip was grafted on. The proper way to install independent suspension on these cars is the way the factory did it, with an independent crossmember that attaches to the lower lip of the rail and is either riveted or bolted on."
Camaro subframe clips can be a viable option on Fords, Pontiacs, and other vehicles that utilize a rigid frame with an X-member, but getting the geometry correct can often be more trouble than it's worth. Measurements such as wheelbase, track width, ride height, and tire clearance are critical, and several jigs are necessary to ensure proper tracking. Radiator and bumper mounts also need to be fabricated or swapped out from the old frame, and the Camaro suspension can often be much heavier than stock or Mustang II-style IFS. It is also important to consider that most hot rod applications are rear-steer, meaning the steering input shaft meets the suspension behind the crossmember. Since the Camaro is front-steer, any attempt to change the setup will radically alter the car's Ackerman angle (see glossary for definition), which can make the car highly unstable at speed.
Overall, with the vast plethora of Mustang II-style IFS kits ranging from basic budget setups to fully polished showpieces on the market today, there is very little reason to go with a used Camaro front clip.
PROPER SETUP AND COMMON PROBLEMS
As you set up your new IFS system, the ride height and stance should be roughed out before you install any of the suspension parts. Put the frame on jackstands and position it at the angle and rake you want, then start mocking up parts. Most manufacturers sell pre-cut crossmembers that have a small amount of rake built in, so the member itself sits flat on the frame. Once the crossmember has been bolted or welded in and the suspension pieces are installed, ride height can be estimated by installing the springs and shocks. It is vital to remember at this point that the true ride height of the car cannot be determined until the engine, transmission, and all necessary fluids have been added to the car. Once the full weight of the car is resting on the springs, they will settle over a period of days or weeks. All too often, first-time builders get frustrated when their new IFS system looks like something that belongs on a 4x4 and begin hacking or heating coils, only to find out later that their car now rides like Fred Flinstone's roadster because the suspension has bottomed out.
When choosing and installing IFS, a vital and often overlooked consideration is the location of the rack-and-pinion, which can affect toe change as the suspension moves throughout its range of travel. The lower control arm and the tie rod that connects the rack to the spindle need to work like a parallelogram. If the tie rod is at a different angle because the rack is mounted too high or too low, trouble will follow.
"What happens most often is that a guy will install a rack in the wrong location because it will only fit a certain way, and all of a sudden the car will have terrible bumpsteer," says famed chassis designer Art Morrison, proprietor of Art Morrison Enterprises. "When the suspension is not parallel, every time the tire hits a bump and the spindle goes up or down, the wheel will actually turn." For an in-depth explanation as to why bumpsteer occurs, see illustration number three.
Finally, the experts all warn about the use of power steering on pre-WWII hot rods. While fat fender cars and later-model customs have plenty of mass to move around, early Fords are very light up front and are not heavy enough to put the necessary load on a standard power rack-and-pinion system. As a result, over-correction becomes a problem, and the car will dart around with little or no road feel. Most people do not realize that rack-and-pinion steering requires far less effort to turn than a standard steering box, so power usually isn't necessary anyway. If you absolutely must have assist, there are a few safe options available, including retrofitting a late-model Fox-platform Mustang ('79-93) power rack-and-pinion setup or finding a shop that can reduce the line pressure.
Proper alignment is always vital on any car, but things can get tricky if you aren't prepared when you walk into the alignment shop, especially with an IFS under the fenders. "When swapping suspensions, the heart of the idea is to make the spindle think it's in its original home," VanDervort says. "Believe it or not, I've heard of shops trying to use '40 Ford axle alignment specs on a Mustang II front suspension just because it's in a '40 Ford!"
By using the manufacturer-supplied alignment specs for the spindle, your new IFS should work as well as it does in a brand-new car, if not better. The kit manufacturer should supply all alignment details, and the following information should be used as a loose guideline only. Camber on an IFS-equipped car with radial tires is generally set around 0 to 1 degree positive, which puts the top of the tire slightly outboard of the bottom. This creates a tendency for the tires to turn toward the vehicle centerline, providing straight-line stability.
Toe-in keeps the car tracking straight as it moves down the road, and with radial tires somewhere between 1/16- to 1/8-inch toe-in works pretty well according to VanDervort. While the car is underway, this is usually reduced to 0-inch as any slack in the steering system comes under the pressure of driving.
"Caster is where things get interesting," VanDervort says. "By leaning the kingpin angle back somewhat, an effect is created where turning the wheels raises the car. Therefore, the car's own weight attempts to push the wheels straight again. As you can imagine, more caster provides increased straight-line stability at the expense of harder steering. Bonneville cars often run up to 15 degrees positive caster to gain the stability they need at ultra-high speeds." For a more in-depth look at caster, see illustration number seven.
As you can see, there are quite a few different ways to choose and dial in an IFS system, but with the proper research and a little time planning, just about any rod or custom can benefit from modern engineering with better handling, a nicer ride, and an improved driving experience. We would like to thank Art Morrison, Katz Tsubai, Eric Aurand, Brent VanDervort, and Gary Heidt for their invaluable assistance with the making of this article.
Shown here is a fairly standard...
Shown here is a fairly standard aftermarket IFS system based on Mustang II/Pinto geometry and design. Tubular A-arms, coilover shocks, new power rack-and-pinions, and five-lug disc brakes differentiate new aftermarket street rod IFS from the factory pieces that rolled off the Ford assembly line.
Here you can see the basic...
Here you can see the basic components necessary in an aftermarket IFS setup.
A steering system is also...
A steering system is also necessary, and while some early factory setups utilize a standard steering box and drag link arrangement, most aftermarket Mustang II-based setups come equipped with a rack-and-pinion that bolts either in front of or behind the crossmember.
This is a typical Mustang...
This is a typical Mustang II-type IFS arrangement as viewed from the top. Improper rack-and-pinion installation height (other than the manufacturer's recommended setup) will result in having to change the length of the tie rods to reach the spindle. With typical MII spindles, adjusting the tie rods 0.200 inches out will steer the tire almost a full inch with 25-inch-tall tires as the suspension goes through its travel. This means that the toe can go from 2 inches in to 2 inches out as you drive down the road, which is called bumpsteer. This is why proper rack-and-pinion mounting height is critical.
Corvette suspension is far...
Corvette suspension is far less common on street rods but can be an effective choice in certain applications if properly set up. Notice that the factory control arms are visibly longer than Mustang II-type arms, which increases stability. The newest generation of Corvette suspension (C5) is significantly improved over the older C4 setup, which didn't allow for much travel and had a relatively harsh ride.
You may have seen IFS systems...
You may have seen IFS systems with upper control arms that angle up in the center, as shown on the left. If designed correctly, this type of IFS can have good straight-line stability but will have problems going around corners. Compare the camber angles between the two illustrations. The IFS on the left tilts the tires significantly more than the IFS on the right. This not only hinders traction, it also accelerates uneven wear of the tire tread.
When you drive on rough pavement,...
When you drive on rough pavement, this is what happens to your front tires with IFS. When a tire hits a bump or a rut, road surface applies a force to the tire-contact patch. Scrub radius (see glossary for definition) acts as leverage and turns this force into torque, which will try to swivel the tire around. As soon as the tire goes back on a flat surface, the force (therefore the torque) diminishes, and caster trail will rotate the tire into the straight-ahead position and maintain it until it hits another road irregularity. This cycle can be felt as kick-back through the steering wheel and can be decreased by reducing the scrub radius on your rod by using positive-offset wheels (like a new car).
This illustration should demonstrate...
This illustration should demonstrate the effectiveness of camber. Anyone who has ever pushed a shopping cart knows that the front casters will swivel to the position shown below and stabilize themselves as soon as you start pushing the cart forward. This self-aligning effect is generated by "caster trail." As you increase the caster on your car, you increase the caster trail as well.
You also know how unstable...
You also know how unstable the swivel caster is when it is at the position shown below, as it will try to swing back as soon as you start pushing the cart. That is exactly what happens when your rod has negative caster angle. While the tires won't actually flop backwards (the tie rods retain them), you will not be able to take your hands off of the steering wheel for a second, as the tires keep hunting for a stable position.
IFS SUSPENSION GEOMETRY GLOSSARY OF TERMS
With a perfectly set Ackerman Angle, the inside front tire will steer more than the outside tire by just the right amount so the tires don't scrub during turns. An example of incorrect Ackerman Angle is when your tires screech as you turn into a parking lot, a result of the tire's dragging.
Anti-dive is a suspension's self-rising effect generated by brake torque applied on wheels. It reduces nose-dive under hard braking, making a vehicle more stable. The amount is determined primarily by the car's center of gravity height, wheelbase, and side-view swing arm (see below for definition).
Bumpsteer is the self-steering effect caused by conflicts among the arcs in which tie rods and A-arms swing. If the lower control arm and the shaft coming out of the rack-and-pinion are not parallel, the incorrect geometry will cause that wheel to turn when a wheel goes over a bump or through a dip. Simply lowering the rack-and-pinion 1/2 inch will drastically alter bumpsteer. It can also occur when the rack-and-pinion is not matched to the rest of the components.
Camber is the angle of the wheels when viewed from the front. You have negative camber when the top of the wheel is tilted toward the center of the vehicle. The opposite condition is positive camber. For most street cars, anywhere from 0 to 1 degree negative camber works fine.
Caster angle is the side-view angle between vertical and the virtual line that connects the upper and lower ball joints. On dropped-axle suspension caster is measured by the number of degrees the kingpin and front axle are laid back from vertical; on IFS it is measured by the lean of the spindle. Increasing positive caster (when the spindles lean toward the back of the car) will increase stability at speed as well as the tire-contact patch during cornering. However, too much caster will increase steering effort. Stay within the IFS manufacturer's recommended setting, as deviating from design spec will change the tie rod height and cause bumpsteer.
Front-View Swing Arm (FVSA)
FVSA is the horizontal distance from the instant center (see definition below) to the center of the tire-contact patch. This length determines the rate of camber change and how much side scrub the suspension will have. Too short an FVSA will make a vehicle feel very unstable on the road. FVSA length should be at least 100 inches for street cars.
Instant Center (IC)
The instant center is a virtual intersecting point of suspension links, including the angle of the control arms and the centerline of the contact patch. This is the point around which the spindles rotate. This rotation point itself keeps changing its location as the suspension arms cycle up and down and change their angles (that is why it is called "instant").
King Pin Inclination (KPI)
KPI is the angle (number of degrees) between vertical and the imaginary line joining the center of the upper ball and lower ball joints. This is typically not adjustable as it is built into the spindle. The steeper the angle, the greater the self-centering effect. Too much KPI will result in high steering effort and poor tire-contact patch during cornering.
Roll Center (RC)
RC is the geometric virtual point that the body rolls around during cornering. Too low an RC will result in too much body roll. Too high an RC on IFS will result in self-rising (known as "jacking effect"), which unloads the tires and may result in loss of traction and control.
The scrub radius is the horizontal distance between the tire centerline and the intersecting point of the ground and the King Pin Angle. The lesser the scrub radius, the better, as long as it is not zero. Too much can cause instability under braking on wet surfaces, as well as increased steering effort and higher stress on suspension components. Scrub radius can be improved by using deep backspacing, which is why most new cars employ that kind of wheel design. Minimum backspacing for the front wheels on a properly functioning IFS system is half of the rim width plus 1 inch (5-inch back spacing for 8-inch wheels, for example).
Side View Swing Arm (SVSA)
SVSA is the horizontal distance from the side view Instant Center to the spindle centerline. This length, as well as the height of the SVIC, determines the amount of anti-dive. SVSA length and SVIC height also have an effect on ride characteristics.
Side scrub is the tire-contact patch's lateral movement as a result of the suspension's cycling up and down. The less side scrub the better, as too much will make the vehicle feel very unstable. It can be minimized by carefully dialing in the car's camber during alignment.
Toe is the angle of the wheels when viewed from the top. When the front of both left and right tires are pointing toward each other, you have toe-in. The opposite is toe-out. Modern radial tires need zero to very slight toe-in.