As with the front suspension, the rear suspension is very important when you’re building a performance Restomod. These cars are supposed to be driven, and driven hard. This chapter will go over different factory and custom rear suspension systems for your Restomods. It will also explain how to get the best all-around performance out of your rear suspension.
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There are two typical types of rear axles: live axle and independent rear suspension (IRS). The live axle consists of a rigid housing that contains the axle shafts and differential, with wheels mounted solidly on both ends—this is the rear end that most of us are used to seeing under cars. Any travel or motion from the left tire directly affects the right tire, since they are both attached to the axle. A live axle is attached to the car’s frame using links, bars, or leaf springs. There are many different link configurations. When it comes to locating the live axle, the concept is basic. The axle needs to have limited fore and aft movement. It also needs to have limited travel from side to side. All this is needed, while still allowing the axle to move up and down. From those basic principals, live-axle rear suspension gets complex.
Pinion Angle
The pinion angle is simply the angle between the rear end’s pinion shaft and a true horizontal line. The transmission angle is the angle between the transmission’s tail shaft and a true horizontal line. Together, these angles form the driveline’s phase angle. Pinion angles can make the difference between a smooth ride, or a noisy and shaking ride down the freeway. Correct pinion angles are also very important to the life of your U-joints. Over time, the angles can change and become incorrect due to loose factory tolerances, body and frame alignment, and changes in spring rates due to wear. You should check and correct pinion angles any time you change the ride height or modify the rear suspension. Even changing leaf springs can change the pinion angle.
Hot rod shops and seasoned backyard mechanics commonly overlook pinion angles. When I was doing research on this subject, I found out why it seems like there is very little information about it, and the little bit of information available seems to be contradictory. After a lot of research, I decided to enlist the help of Kyle Tucker (an ex-GM suspension engineer). It turns out that pinion angles are not a Jedi secret, though they differ for each application. Family trucks, production sports cars, and Restomods are not built for the same type of driving. I am going to cover pinion angle settings for Restomods built for street and road course racing, since the book isn’t strictly covering drag racing, Baja racing, swamp buggy racing, or grandma cars.
Here are two types of angle gauges. The analog gauge on the left can be purchased from most hardware stores. The digital SmartTool is more expensive. Both show that the rack is very close to zero degrees, which means the suspension is loaded correctly
This is another way to lift the car up for measuring the pinion angle. Make sure to do this on a flat, safe surface. Place jack stands safely under the rear axle to compress the suspension as if the car were on the ground.
Maybe you’re building a full-tilt tube chassis to slide under a Fairlane shell. The best chassis designers determine the ride height first by positioning the rear axle, the correct size of tires, and the wheels under the body. Once they do that, they set the transmission height and angle, and then build the rest of the car around that. If your car is finished or you are in the middle of a build-up, don’t worry. These angles can be adjusted later, but it would be better if the car were designed around the correct angles to begin with.
Checking Pinion Angle
Checking pinion angle correctly is important. Start by getting an angle gauge. A few types are available. The most common is an analog magneticbase protractor gauge (pictured in photos checking angles). These are available for about the cost of a meal at your local restaurant. Other types, such as digital angle gauges, are far more expensive. They basically do the same job, but they’re much more precise.
Next, find a place to check your angles. Using a four-post rack or a pit is the most accurate way. The car should be on a level surface and at ride height. To get the ride height correct, you should fill the fuel tank. Fuel can add a lot of weight and change the ride height of the car. If the car is not level on the rack and resting on all four tires, your readings will not be accurate.
If you don’t have access to a four-post rack or pit, you can use jack stands and/or ramps to simulate the fully loaded ride height. Do this on a hard, flat surface and make sure the car is level. You should place jack stands safely under the rear axle tubes. Do not use a floor-jack to support the front of the car. This is very unsafe. The front of the car needs to be lifted exactly as much as the rear of the car. If you are using jack stands under the front of the car, placement is very important. If you place the stands under the frame in front of the centerline of the spindle, you will be placing moren of the vehicle’s weight on the rear suspension. If you place the jack stands under the frame behind the spindle centerline, you will be moving some of the load off the rear suspension. This factor will cause incorrect measurements.
The most precise way to set your car up for measuring pinion angle, without a four-post rack, is to use two sturdy car ramps and two sturdy jack stands. Start by driving the front of the car up on the ramps. Safely place jack stands under the left and right rear axle housing tubes. At this point, the front and rear of the car have to be raised evenly. If the ramps raise the front tires up 9.75 inches off the ground, the rear jack stands should support the rear axle so the rear tires are 9.75 inches off the ground. This will ensure you have lifted the front and the back of the car evenly for accurately measuring pinion angles. Don’t forget: safety comes first.
The transmission angle can be measured off the yoke or off the tail-shaft seal. Here, a straight bar was pressed up against the seal with the magnetic-base angle gauge attached to read the transmission tail-shaft angle.
This picture shows phasing angles that should be used for most leaf-spring Restomods. Notice the pinion and transmission angles are both down.
This picture shows how phasing angles are set up on most production passenger cars. Notice the differential pinion angle is as many degrees up as the transmission angle is down. The angles are parallel. Without load on the suspension, the angles total 6 degrees. These angles should add up to a maximum of 7 degrees.
This picture illustrates the problems with running parallel phasing angles. The differential is shown with a torsional load from acceleration. The pinion tries to push upward, causing the springs to wrap up. At this point, the maximum tolerable angle of 7 degrees has been exceeded. This is bad for U-joints.
In this picture, you can see the pinion angle is set fewer degrees down than the leaf-spring set-up. This is how short-track coilspring suspensions should be set up. Unlike leaf-spring suspensions, the trailing arms used in a coil-spring suspension typically minimize upward movement of the pinion.
The angle on the transmission is typically measured off the back of the transmission on the driveshaft yoke’s seal surface. It can also be measured off the engine block, since the oil pan’s gasket sealing surface is parallel to the crankshaft (just take into account that there is a 90-degree angle difference from the transmission angle). The pinion angle on the rear end is taken from the face of the pinion yoke. Finding the flat surface on the transmission and the pinion yoke is easy, but there may not be space to place your gauge. If you have a metal straight edge, you can rest it against the flat surface and attach your magnetic gauge to the straight edge.
For a car that is set up for handling around corners, the optimum pinion angle is different than if you were setting your car up for serious drag racing. Incorrect pinion angles can cause chassis vibration and premature U-joint wear and failure. Without the correct angles, the needle bearings in the U-joint caps do not rotate (as shown in the U-joint section). Those needle bearings need to rotate in order for the U-joint to operate reliably and smoothly.
Optimal Pinion Angle
Now that you know how to measure pinion angle, it’s time to find the optimum pinion angle. There are many different schools of thought in this area of suspension tuning. I am going to go over pinion and transmission angles. Both angles are equally important when it comes to optimum suspension tuning. When referring to both angles combined and their relation to each other, they are referred to as phasing angles.
Pinion angle depends on your application. Production passenger cars and basic street cars operate fine with parallel pinion and transmission shaft angles. Many shops still use this old-school design for building cars, and it’s fine for street use. However, if you are going to build a car that will be pushed to its limits on a road course, then forget that school of thought. During acceleration, torque causes the pinion to tilt upward. If you set your pinion angle a few degrees upward, the pinion will want to travel even further upward during acceleration. This is explained in more detail in the leaf spring section. The most common transmission shaft angle is 2 to 3 degrees down. Leaf-spring suspensions allow the pinion angle to rotate upward when the springs wrap up under acceleration. Angling the pinion downward compensates for this upward travel. Serious short-course racecars run a pinion angle of as much as 4.5 degrees down. A downward pinion angle of 2.5 to 3 degrees is a good place to start for Restomod (high-performance street and road course) applications.
This is the standard U-joint. It is strong enough for everyday driving. If you are going to apply excessive torque and/or sticky tires, stepping up to an off-roadstyle U-joint is a good idea.
This is an off-road U-joint. It has a grease canal and is made of stronger material than a standard U-joint. Off-road U-joints are a good compromise between standard and racing U-joints because they’re stronger than standard units, but still serviceable.
This is the racing U-joint. It has no provisions for servicing. It does not have grease canals, which increase its strength. The only lubrication comes from small grease reservoirs in the end of the U-joint legs.
The combined pinion and transmission angle should not exceed more than 7 degrees. A combined pinion angle of 3 degrees and transmission angle at 2.5 degrees add up to 5.5 degrees, which does not exceed the maximum. If you run parallel phasing angles like some old-school street rodders have been using, you can easily run into problems. For example, think of the transmission angle set at 3 degrees down and the pinion angle set up 3 degrees to run parallel with the transmission. When the leaf spring wraps up, the pinion angle can rotate upward 2 or more degrees. If you add the transmission angle at 3 degrees to the pinion angle of 3 degrees, it would add up to 6 degrees combined. During wrap-up, the 6 degrees can become 8 or more degrees, which exceeds the maximum allowable range totaling 7 degrees. This will dramatically shorten the life of your U-joints.
Restomods equipped with coilspring rear suspensions can run with less downward pinion angle. The trailing arms on a coil-spring suspension typically limit the amount of pinion lift (where the front of the differential tilts upward), so pinion angle can be set at 1.5 degrees down.
Pinion Angle Adjustment For Leaf-Spring Suspensions
On leaf-spring suspensions, there are a couple of ways to adjust the pinion angle. The more common way is to install shim-style wedges, which are available in different degrees from several manufacturers. The other method is to install a pair of adjustable leaf-spring pads. These are great for project cars that will not be sitting at fully loaded ride height for quite some time. They can be installed early in the process of a project, then adjusted and welded later when you’re close to finishing the car. They’re a nice alternative to welding a stationary spring perch to the differential housing early on in a project, and having to use shims to adjust the pinion angle later.
There is an alternative to welding the adjustable perch, too. Once they’re installed and the pinion angle is adjusted, carefully drill a hole through each perch and axle tube. Then thread the hole in the axle tube to accept a bolt. Disassemble the rear end to clean all metal debris from inside the axle housing, and then reassemble the rear end. This is not the easy or preferred method, but it is an alternative for people without welders.
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Pinion Angle Adjustment For Coil-Spring Suspensions
Adjusting pinion angle on coilspring suspension systems is completely different than on leaf-spring suspensions. There are three ways to adjust coil-spring suspension pinion angle. The third is the easiest and most common.
You can see the needle bearings in the cap. When the joint turns, the needle bearings spin, keeping the U-joint from causing vibrations in the drivetrain and causing a shortened life.
A 2-degree wedge has been placed between the spring perch and the leaf spring. Shims typically come in 2- and 4- degree increments.
- Cut the spring perches off the rear differential housing and weld them back on at a different angle.
- Modify the length of the stock trailing arms.
- Purchase fully adjustable aftermarket control or trailing arms. These are readily available and are the most convenient way to get full adjustability from your coil spring rear suspension at any ride height.
Many different aftermarket companies offer adjustable upper control arms. These are easy to install, stronger than stock arms, and give you adjustability. More information on these arms is given later in this chapter.
Universal Joints
There are three types of universal joints available: Standard-Duty U-Joint – This Ujoint is great for everyday driving. It’s completely serviceable, with a zerk fitting located on the body between two of the legs. However, the hole for the zerk fitting can be a weak area when a lot of torque is applied.
Off-Road U-Joint – This U-joint is for off-road/road-race applications. It’s completely serviceable, but the zerk fitting is located at the tip of one of the U-joint caps. The body is much stronger since the fitting hole is not located near the center. This U-joint is also made of a stronger steel than the standard-duty U-joint. Race-Only U-Joint – This U-joint is for racing uses only. It isn’t serviceable and has no zerk fittings. The only way to service this U-joint is to completely remove it, lube it by hand, and then reinstall it. It’s stronger because the U-joint body is solid steel, with no internal passages for grease.
Driveshaft angles of the driveshaft are very important to the life of the Ujoint. Inside of the U-joint caps are needle bearings, which have to rotate inside of the cap when the U-joint is spinning. The needle bearings will not spin if the U-joint is installed incorrectly or the driveshaft angles are incorrect. If the needle bearings do not spin, they wear out. This will eventually cause strange drivetrain vibrations that are hard to pinpoint.
You can usually tell if your U-joints are worn excessively if you hear a clunking noise from the drivetrain when shifting from reverse to a drive gear. A worn U-joint can cause excessive vibrations in the drivetrain. The most sure-fire way to check the U-joints for wear is to safely put your car up on jack stands or a hoist. With the engine turned off, but in gear, physically attempt to rotate the driveshaft clockwise and counterclockwise. If you can feel play or see the pinion or transmission yoke not move in conjunction with the driveshaft, it’s probably time to replace the U-joints. There should not be any visible play between the U-joint and its end caps.
Leaf Springs
Sometimes people overlook leaf springs as an important part of a car’s performance. Leaf springs are not only an easy way to locate the differential housing, but they can also mean the difference between a comfortable or a harsh, uncontrollable ride. They also set the ride height for your car. Leaf springs are a single unit, but should be thought of as two separate units that do distinct jobs. The front half of the leaf spring locates the rear-end housing in the chassis. The rear half of the leaf spring is responsible for the ride quality. The spring effectiveness is affected by bias. Front leaf-spring bias means the front half of the spring has higher strength and spring rate than the rear. A spring with too little front bias will tend to promote leaf spring wrap-up (explained later in this chapter) and possibly induce wheel-hop. More front bias will help cure those problems, yet less rear bias will help the ride of the car be softer. More rear bias will also help make the rear of the car more stable around corners and firm up the ride a little.
Aftermarket companies offer many different spring configurations. Each company has a different idea about what design is best for each application. One company may believe that a 1971 Mustang would work better with a higher rate (stiffer) spring, which produces more oversteer. Another company may offer less front bias (less front spring strength), causing the leaves to wrap up easier under hard acceleration. Some companies offer only leaf springs, while others offer leaf springs with rates that are balanced as a system with their other suspension components for a better overall package. Typical spring rates for leaf springs range from 85 to 250 lbs. The rates are determined by the vehicle’s weight, model, year, and performance level. The best source for information on having your own springs built is a company that has been in business of producing springs for manufacturers and some of the top custom car builders since 1937. Eaton Detroit Spring can build the perfect spring for your custom or stock application.
This has a late-model Thunderbird four-bar suspension with stock trailing arms. These are not adjustable in any way except to cut them to the length you desire, which is more hassle than it is worth.
On the right, you can see a stationary spring perch. On the left, you see an adjustable perch. They are both offered by Detroit Speed & Engineering. There are many benefits to using adjustable spring perches.
Pro Suspension Components offers this set-upthatreplacestheuppercontrolarms on your 1979 to 2004 8.8-inch-equipped Fox-body. The system is called Pro-Link, and it allows easy pinion angle adjustment and change of instant center. (Photo courtesy Real Speed Parts)
If you throw a set of 153-lb springs on your Mustang and install mismatched front and rear shocks and sway bars, you may get lucky and get a car that handles great. In most cases, mismatched components can leave much to be desired. This is a good reason to consult a knowledgeable suspension engineer or use a complete suspension package from a reputable company. At the very least, do some homework to increase your own knowledge before spending your hard-earned money on parts that might not work well together. Asking other enthusiasts what worked for them is a great way to get answers.
Wrap-Up
Leaf springs have a major enemy: wrap-up. Massive amounts of horsepower and torque under your hood will put a smile on your face. But if you can’t get your power to the ground, it’s less fun and sometimes it’s downright frustrating.
When you hit the throttle, the torque of the engine is transferred to the rear end via the driveshaft. With the forces of the ring and pinion rotating against each other, the pinion is pushed upward. Since the rear end is mounted to the leaf springs, the springs try to change from their typical arc shape to the shape of an “S” as the rear-end housing rotates upward. This phenomenon is referred to as wrap-up. Wrap-up can be more or less extreme, depending on the leaf springs you have. With any hint of traction, the wrap-up can turn ugly and produce wheel-hop. The way to combat wrap-up is to install a set of leaf springs designed with more front bias.
Steel Multi-Leaves
Most Ford Motor Company vehicles are equipped with rear suspensions with steel multi-leaf springs. Not all aftermarket multi-leaves are created equal. I will point out the different features to help you decide what the best spring will be for your application.
Leaf Spring Rates
Spring rates are determined by placing the spring on a test table under a spring press. Mono-leaf springs are laid on the table with the arc facing down, opposite of the direction they’re mounted on a car. If it takes 150 lbs of force to compress the spring 1 inch, it’s a 150-lb spring. The higher the rate, the stiffer the spring. Mono-leaves are pretty basic in this aspect.
Multi-leafs are also rated on a testtable, but they have a progressive rate. The progressive rate comes from the secondary leafs that are stacked in succession with the main leaf. The extra leafs change the spring rate significantly. Before the main leaf can be compressed 1 inch, the secondary leafs begin to resist compression, and typically increase the amount of weight it takes to compress the spring.
Here you can see the basic parts of a multi-leaf spring. A mono-leaf spring would only have one leaf. The center bolt holds the leafs together. The round portions at the ends of the spring are called spring eyes.
If you use sticky tires with a leaf-spring suspension, the leaf spring can be overpowered if the suspension is not set up correctly. The torque can turn the leaf spring into an “S” shape.
Leaf Spring Features
Think of the leaf spring as a front section and a rear section that are separated at the axle-locating pin. A spring with more front bias will require more leverage to bow the front half of the leaf spring than the leverage required to bow the rear. The secondary leafs change the bias of the leaf spring as a complete package. A good spring for Restomod applications will have more front bias. This means that the secondary leafs are more prominent in the front half of the leaf spring unit. However, don’t forget about the rear half of the leaf spring unit. It’s important too. Minimizing the number of leafs or their lengths in the rear half of the spring saves weight, but it can hamper your handling characteristics. Without the proper number of leafs in the rear half, the leaf spring may be too soft and better suited for drag racing.
Anti-Friction Pads
Multi-leaf springs are constructed of multiple steel leaves, and the friction between the leaves contributes to the spring’s stiffness. To reduce the friction and noise, some manufacturers include anti-friction pads between the leaves to improve ride quality for street use. This is something to look for in a spring, since Restomods are meant to be driven as much as possible.
Lowered Springs
There are quite a few aftermarket companies producing lowered rear leaf springs. Lowering springs typically lowers the ride height of the car by changing the rate, arc, and overall length of the leaf spring. Some springs are available with the spring eye configured differently than the original spring. For instance, if the front spring eye is curled upward, the front spring eye can be built with the spring eye curled downward. This effectively lowers the car approximately 1.5 inches without using a lowering block. The benefits are not only the lower stance; you also get lower roll steer (steering changes induced by body roll), decreased torsional spring twist, decreased lateral movement, and a lower roll center (an imaginary line the body and frame pivot on during body roll). Beware: Installing lowered springs can change the pinion angle of your rear end. You might want to re-check your pinion angle after you lower your car to make sure everything lines up correctly.
To eliminate bind and friction, pads are placed between leafs. Here you can see the anti-friction pad between the end of a secondary leaf and the main plate (primary leaf).
Leaf springs can be customized to lower or raise the ride height of your car. You may notice the leaf spring eye is reversed. See how the main leaf is on top and then curls down. This custom spring was made to lower the car.
The most common way to lower the rear of a car with leaf-spring rear suspension is to install lowering blocks. Here you see the different sizes of lowering blocks. BellTech offers lowering-block kits, which include longer U-bolts, nuts, and blocks.
When the housing is separated from the leaf spring with more than a 2-inch block, the housing gets too much leverage. With power applied to the drivetrain, the rearend housing can wrap-up the leaf springs into an “S,” which can lead to wheel hop
This Global West solid bushing does not bind in radial motion. The center and sides are not attached. The center is allowed to turn while the sides are bound. These bushings are equipped with grease fittings (not shown) so they can be serviced.
Lowering Blocks
The most common way to lower the rear of a leaf spring car is to install lowering blocks – basically a spacer that goes between the spring and the rear-end housing. Most Ford leaf spring cars have the rear-end housing located on top of the leaf springs. This makes lowering a car with blocks very easy. Trucks typically have the rear-end housing located under the leaf spring. Some early Ford cars are also designed this way. In this case, you can move the housing and relocate the rear axle mount pads to locate it on top of the leaf spring. After you do this, you can use the lowering block to further lower your vehicle.
Running more than a 2-inch block moves the axle housing too far from the leaf spring. This gives the axle housing more leverage when torque is applied, and will promote leaf spring wrap-up and wheel hop. If you have to run more than a 2-inch block, you should check into getting some lowered leaf springs.
Leaf-Spring Bushings
Leaf-spring bushings come in three types: rubber, urethane, and solid. Your choice of leaf-spring bushings should depend on what kind of driving you plan on doing. One builder may prefer using rubber in the front spring eye and urethane in the rear spring eye to keep road shock to a minimum. Since the front eye of the leaf spring takes most of the road shock, using rubber there makes sense. The urethane bushing in the rear eye minimizes lateral (side to side) leaf-spring movement. On the other hand, another builder may prefer to use solid Delrin and aluminum leaf-spring bushings front and rear for better handling performance, at the expense of ride quality.
Rubber
Rubber leaf-spring bushings are great for stock grocery getters. They minimize the transfer of road feel to the chassis because they absorb the shock from uneven road surfaces. On the other hand, rubber bushings allow the rear suspension to travel in ways you may not want it to, which can make the car unpredictable in high-performance driving.
Urethane
The next step up in performance is to replace the rubber spring-eye bushings with urethane bushings. Urethane bushings don’t distort like rubber bushings, so they keep the springs in proper position. They help reduce body roll and torsional movement of the leaf spring. Urethane leaf-spring bushings should be installed with the manufacturer’s suggested lubricant. Under harsh conditions, they should be serviced on a regular basis to reduce squeaks. Since urethane is denser than rubber, some people feel the ride becomes too harsh, especially when using them in the rear spring eye. Using a urethane bushing in the front and rear spring eyes increases the transfer of road shock to the car. When pulling in and out of driveways at an angle, you will get some binding in the front spring eye. This binding can periodically cause noises under the rear seat. It sounds like someone dropped a water balloon on a car from a second story window (not that I would know). Even the twisting motion of the leaf springs under hard braking can produce this sound. Most of the road shock and binding come from the front spring eye, which is why some people stick with a rubber bushing up front.
This is a parallel four-link rear suspension set up for cornering purposes. A panhard bar in front of the rear-end housing keeps it from moving from side to side. (Photo courtesy John Parsons)
These are leaf-spring bushings. They come in different forms: rubber, urethane, and solid. Each has different characteristics, so do your research and choose the best option for your car and the type of driving you’re doing.
Three-link rear suspension systems have two lower links connected to the housing (like a four-link), but the upper links are replaced with a single link on the top of the differential housing. This set-up is not a typical three-link, because it has a decoupled torque arm just below the third link. This set-up works well, but it is not the best for regular street driving.
Solid
If you don’t care about an extremely comfortable ride, you can completely do away with flexible bushings and install solid bushings made of aluminum and Delrin. Unlike urethane and rubber bushings, solid bushings have a solid center and separate solid sides. This allows the center to turn in a radial motion without binding on the sides. Solid bushings resist chassis roll, promote more predictable handling, and they do not bind. It is frustrating to drive a car on a road course and have it turn into a corner differently, even though you entered it exactly as you had every other time. Solid bushings are a great fit for Restomods built for frequent outings at the road course. However, solid bushings create a harsh ride because they do not absorb as much energy from uneven road surfaces.
Coil Springs
In the U.S., Ford Motor Company has not used parallel leaf springs in any car line since 1980, but they are still used in trucks. The following is a list of a few of the cars equipped with coil spring rear suspensions: 1979 to present Mustangs; 1965 to 1978 LTDs and Galaxies; 1972 to 1979 Torinos and Rancheros; 1967 to 1988 Thunderbirds; 1965 to 1978 Marquis and Monterey; 1979 to 1986 Capris; 1974 to 1988 Cougars; 1972 to 1976 Montegos; 1978 to 1983 Zephyrs; 1968 to 1979 Lincolns; and more models and years. Typical Ford coil systems have two coils and four locating arms. The coil springs simply locate between two spring pockets. The length of the springs, the control arm articulation, and the weight of the vehicle all work to keep the springs in place.
Aftermarket coil springs are available from many different manufacturers. They come in stock and lowered heights. Different spring rates are also available. As with leaf springs, coil springs with higher rates are stiffer than lower rate springs and the rate is measured the same way.
As with leaf spring manufacturers, each coil maker has its own ideas about which rate will work best with your car. Some manufacturers design their systems to produce more or less oversteer than the others. They can accomplish these differences by changing spring rates or sway bar diameters front and rear. A good aftermarket suspension manufacturer carrying full front and rear packages can offer technical advice to help you with your application.
Trailing Arms for Coil-Spring Rear Suspension
Coil-spring and coil-over rear suspensions locate the rear-end housing with links commonly known as trailing arms or control arms. Many aftermarket companies offer upper and lower control arms with different bushing ends for different types of driving. Not all control arms are created equal. Some are better for drag racing than they are for running on a road course. Some arms are great for both. Restomods are meant to be driven as much as possible and would benefit from getting power to the ground in a straight line and around a corner.
Most aftermarket trailing arms have urethane bushings in each end. Urethane bushings have “stiction” (or static friction) and will wear out over time (as will any moving part). When the bushing wears out it distorts and allows fore and aft movement of the arms, which will change suspension geometry. These are better than stock, but are not the only designs available.
This Tech Tip is from the full book HOW TO BUILD FORD RESTOMOD STREET MACHINES. For a comprehensive guide on this entire subject you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
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The most effective control arms available have spherical bearings on one or both ends. The spherical bearings offer full range of motion without bind or the possibility of distortion. Some applications will not benefit from having these spherical bearings at both ends, but require a more lateral “fixed” position of one end of the trailing arm. In these cases, one end may require a solid bushing and a spherical bearing in the other end. Each application is different.
This is the Hot Rods to Hell Inc. Cassette Truck-Arm Suspension. It uses longer suspension arms like those used on Winston Cup series and other big-buck racing cars. This system has proven itself on the track, and now on the street. The set-up can be installed in leaf- and coil-springequipped Restomods.
This Fox-body is equipped with a Griggs Racing rear suspension kit that replaces the upper links with a lower center-mounted torque arm that mounts to the differential and a special cross-member/subframe connector set-up. The lower links are replaced with a stronger set and an adjustable panhardbar set-up has been installed.
Total Control Product’s rear suspension system has a right and left lower link, a short lower torque arm, and a Watt’s link mounted under differential housing. It also has lay-down coil-over shocks operated by rocker arms. The whole system bolts to a subframe, which bolts to four brackets that can easily be welded to the original frame.
Some rear trailing arms are designed to keep the rear axle housing in a certain position for proper geometry and articulation with a certain degree of bind. In these cases, one end may require a solid bushing and a spherical bearing in the other end. The spherical bearings offer full range of motion without bind or the possibility of distortion. Some aftermarket suspension companies offer upper control arms with adjustable length. After installing these on your car, you can adjust them to get optimum pinion angle for U-joint life and improved suspension geometry. Except for racing applications, aftermarket companies only offer adjustable upper control arms for adjusting pinion angle. The lowers are typically non-adjustable and rigid, since they need to take the brunt of most of the road shock.
Instead of replacing the upper control arms on your Fox-body with the typical boxed or adjustable arms, a company named Real Speed Parts offers a product called Pro Link. Pro Link allows you to change the instant center by relocating the upper control arm points on the rear axle housing with a bolt-in bracket and link system that allows for multiple adjustments. It also allows for easy pinion-angle changes. This system has been proven to improve performance on road courses and drag strips. It has been tested to withstand up to 2,000-hp engines at the drag strip. Real Speed Parts offers adjustable heavy-duty lower control arms to round out a completely adjustable Fox-body rear suspension.
Link/Bar Suspensions
The most common link system is the four-link. This is popular in drag racing, but unless it’s modified slightly, it won’t make for a very good candidate in a Restomod. Either the top or bottom links need to be angled front to rear so that they converge as close as possible. This allows for more axle articulation than is possible with parallel links, which helps keep the tires planted in the turns. Angling two of the links approximately 20 to 30 degrees to the centerline of the car also eliminates the need for a panhard bar, which locates the axle side to side in the car.
Another common type of link system gaining popularity in the Restomod and Pro-Touring crowd is the 3-link. The 3-link uses two lower and one upper link to eliminate the inherent binding of the four-link design, and allows for complete axle articulation. Using only one upper link means that the link and its attaching mount must be extremely strong. Consider using at least 11 ⁄2-inch diameter thick-wall tubing for this link, and no less than 3 ⁄4-inch diameter bolts. For the lower links, use at least 13 ⁄8-inch tube and 5 ⁄8-inch bolts. On the subject of attachment bolts, consider using airframe (also called AN bolts) bolts because they are very high-quality. The 3-link design, as well as the four-link, needs some type of side-to-side axle locating mechanism, such as a panhard bar or a Watt’s link. The panhard bar attaches to the axle on one side and the frame rail on the other.
Three-link rear suspensions consist of two lower links and a third link centered above the differential housing. The system in this image shows a torque absorber to take out some of the shock caused by hard acceleration and deceleration. Three-link systems require a panhard rod or the equivalent (not shown) to keep the rear end centered in the chassis.
Martz Chassis has come out with a rear suspension kit for your early unibody Restomod. It features a non-parallel fourlink, weld-in crossmembers, coil-over shocks, and a rearmounted, stock-car-style sway bar. (Photo courtesy Martz Chassis, Inc.)
Bret Voelkel’s ‘69 Mustang is shown tearing up the road course with its Air Ride Technologies AirBar four-link rear suspension and ShockWave front suspension kit. New technology has taken airassisted suspension systems to the next level. (Photo courtesy Air Ride Technologies via Scott Killeen)
Martz Four-Bar
For your Restomod, you might decide you want to replace your rear leaf-spring suspension with a completely new custom four-bar rear suspension. Martz Chassis builds a complete rear suspension system to replace the traditional leaf-spring set-up. This system will allow you more adjustability over leaf spring rear suspension and can be finely tuned in the pursuit of better handling without the weight of leaf springs. This four-bar set-up has a weld-in support brace (with integral driveshaft safety loop) for the front of the links and a weld-in support brace for the integrated coil-overs; adjustable panhard bar; and optional sway bar. It includes all the proper brackets to attach the suspension parts to the rear axle housing. The kit options are solid rod ends or urethane bushing ends and a 7 ⁄8-inch stock-car style sway bar. This system has been tested at speeds over 150 mph on Martz’s Mustang shop car. The kit allows you to keep your back seat, and it is perfect for cars with mini-tubs. Martz claims it can be completely installed in a weekend by a seasoned car guy with welding skills, so if you aren’t up for some serious surgery, this might not be for you.
Air Ride Technologies AirBar
When you think of air ride suspension, you may think of boat-like cushyhandling characteristics. Bret Voelkel, the president of Air Ride Technologies, is changing those preconceived notions with new technology. Air Ride has been pumping new technology into its products for years. As of 2005, they are offering AirBar rear suspension systems for 1964-1⁄2 to 1970 Mustangs. It replaces the rear leaf springs with four-link rear suspension and the ShockWave 7000 units. A rigid cradle is bolted to the frame and underbody structure using existing factory bolt locations. AirBar lowers the car approximately 2 inches, with a deflated height typically 5 to 6 inches lower than stock. Bret installed the AirBar rear suspension with a ShockWave system in the front of his 1969 Mustang, which has proven itself street and road course worthy.
Independent Rear Suspension
If you want to switch to something completely different from the average Restomod’s solid axle and leaf-spring suspension, an independent rear suspension (IRS) may be for you. With IRS, the left and right tires can move independently of the differential housing, which is solidly mounted to the frame. Two half-shafts (also known as drive axles) extend outward from the differential to rotate the hub assemblies and wheels. This design allows one tire to travel and move independently without affecting the other tire. A big benefit of the IRS is the possibility to have camber changes during suspension travel to increase the contact patch of the tire to the pavement when driving hard around corners.
Factory IRS
The most popular IRS for street rods is the early Jaguar rear end. The Jaguar IRS lacks forward trailing arms and upper locating links. It allows torque steer when high horsepower is supplied to sticky tires. Toe and camber problems are also common. For these reasons, consider one of the following set-ups instead.
Cobra and Thunderbird IRS
Your Restomod can be converted from the factory live-axle rear suspension to an IRS system for 1989 through 1997 Thunderbirds and Cougars or 1999 through 2004 Mustang Cobras. The Thunderbird and Cougar IRS subframes are not the same as the Cobra units. The Cobra IRS subframes and parts are stronger and have been found to adapt to older cars easier. The Thunderbird and Cougar have a wheel bolt pattern of 5 on 4.25 inches and the Cobra bolt pattern is 5 on 4.5 inches, which is more common with muscle car-era Restomods. The T-bird and Cougar IRS hubs can be upgraded with Cobra hubs to convert the bolt pattern, but you will still have other inferior parts. The Cobra IRS units are the systems of choice.
These IRS systems are attached to a subframe unit. The coil springs and shocks do not attach to the subframe, so you need to fabricate mounts in the receiving chassis. An easy way to eliminate the need to make coil spring mounts is to convert the IRS to coil-over mounts. If you are going to mount the coil-over in the stock shock absorber position, be careful. The lower shock mount on the IRS control arm was originally designed to take the road shock. It was never intended to bear the load and weight of the vehicle, so the lower shock mount should be beefed up a bit.
The Cobra IRS is mounted to a bolt-in subframe. This set-up is waiting for engineers to fit it to the FR100 project. FR100 is a ’53 Ford pickup, built by McLaren Technologies for Ford Racing Technology group. (Photo courtesy Ford Racing Performance Parts)
This is the same McLarenbuilt ’53 Ford. Most of the original parts of the Cobra IRS have been replaced with fabricated parts. The strength of an IRS unit depends on the specific year, and some are stronger than others. This truck really gets flogged, and the IRS has worked exceptionally well on the road course and drag strip. (Photo courtesy Ford Racing Performance Parts)
DVS Restorations
The following information about the differences between Cobra IRS systems is from David Stribling of DVS Restorations. The 1999 Cobra unit has 28-spline axles, a 3.27:1 gear ratio, Traction-Loc, and soft rubber frame mounts. The 2000 Cobra “R” unit (considered to be the best of the best) has larger 31-spline axles, 3.55:1 gear ratio, Gerodisc hydromechanical limited-slip unit (stronger and designed to handle the more powerful Cobra R DOHC 5.4-liter engine), improved CV Joints, improved upper camber slot location, and harder-durometer rubber frame mounts. Cobra units for 2001 had slightly improved axles (31 on the insides and 28-spline on the outsides), a 3.27:1 gear ratio, and TractionLoc. Similar to the 2000 Cobra R, the 2003-2004 Cobra unit has 31-spline axles (redesigned again), 3.55:1 gears, TractionLoc, redesigned camber, and redesigned tie-rod location for improved bumpsteer.
DVS Restorations has done all the homework on adapting Cobra IRS units into 1965 through 1970 Mustangs, including all the custom brackets and modifications necessary for each Mustang year. The 1965 and 1966 Mustangs are narrower than the 1967 and later cars. This proved to be a problem for the Cobra IRS, since the width of a 2000 Cobra R IRS is approximately 65 inches from hub to hub. The 1965 and 66 Mustangs’ inner quarter panel lip to inner lip is approximately 66.25 inches. That only leaves 1.25 inches, or room to play with wheel backspace/offset and tire sizes— and that’s not much! DVS offers a kit to bolt the IRS into the rear of the 1964-1⁄2, 1965, and 1966, but it works best with flared quarter panels. The 1967 and newer Mustangs are wide enough to accept the IRS without flaring the quarter panels. The standard Cobra IRS subframe is a good fit with the DVS kit, but it leaves the car sitting a little too high for some builders, so DVS offers a modified IRS subframe for the builders who want their Restomod a little closer to the ground.
It may raise your eyebrows a little, but the C4 Chevy Corvette IRS is lightweight, a completely self-contained unit, and has proven its great potential for street and track use. For various reasons, an IRS unit from one specific year may be more desirable than another from a different year.
Duane Carling at CTM Engineering did a ton of unbelievable research to put together the original IRS kit designed for the 1964-1/2 Mustang. If you want IRS for your Restomod Mustang or Falcon and don’t want to do any welding or cutting, this may be the kit for you. (Photo Courtesy CTM Engineering)
The Original T-5 IRS (for early Mustangs)
If you’re looking for IRS for your early Mustang, Falcon, Comet, Cyclone, or Torino, and you want something rare without making any frame modifications, there is an independent rear suspension system is for you. A gentleman named Duane Carling from CTM Engineering has an incredible story about the IRS built for the Mustang back in 1964. The following information gives a little bit of the whole story. For more, get a hold of CTM Engineering. The company information is listed in the Source Guide in the back of this book.
Not a lot of people are aware of this IRS, but Ford had a team of designers building an independent rear suspension for the original Ford Mustang. In fact, the IRS made it onto a few test Falcons back in 1964 because the Mustang wasn’t even available yet. Duane was looking at some pictures while doing research and found a picture of a 1964 Falcon with IRS that had been taken back in 1964. Soon Duane was on a quest to attain more information on this rare suspension system. One dead end led to another, until he eventually found the man who was the head of the IRS project. From there, the story goes beyond your wildest dreams. After many years, Duane found actual prints, original one-off parts, and other info that led him to piece together the original IRS built for Ford and tested by Shelby. The rumor is that it made an improvement in the handling of the Mustang, but the improvement didn’t outweigh the cost of building it.
CTM Engineering now builds these systems just like they were back then. It uses custom parts, original parts from other cars, and a few Jaguar parts. The system bolts into the original factory locations on the 1964 and newer leafspring equipped chassis.
Written by Tony Huntimer and Posted with Permission of CarTechBooks
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