The evolution of Pro Touring is an interesting one. It's gone from thinly disguised race cars that only young guys with healthy backs could endure on the street, to mega-dollar show cars that log most of their mileage driving up and down trailer ramps. However, in an interesting and somewhat unexpected turn of events, the most recent crop of six-figure Pro Touring machines have proven themselves on the track in ways no one could have imagined just a few years ago. Thanks to events like the Optima Ultimate Street Car Invitational and the American Street Car Series, these high-profile street machines have earned the respect of the staunchest of naysayers while smoking Corvettes and Porsches along the way. Playing in this arena requires some serious suspension hardware, and that's right up the Roadster Shop's (RS) alley. While the suburban Chicago-based shop may be best known for their ultra high-end, award-winning muscle car builds, it has been manufacturing complete chassis and cutting-edge suspension hardware since day one. To get a better understanding of how factors such as suspension pickup points, spring location, camber curves, bushing design, and sway bar stiffness affect ride and handling, we had Jeremy Gerber explain the technical elements of design that go into all of the Roadster Shop's suspension components.
There are many shortcomings of a factory muscle car suspension design, and perhaps the most significant is the location of the suspension pickup points. It is easy to correct factors like floppy bushings and weak hardware by simply upgrading to a bolt-on aftermarket control arm. What you cannot achieve is the relocation of the suspension pivot points or changing the geometry of how it functions. When your average enthusiast looks at a double wishbone suspension, they oftentimes see no more than a pair of spindles attached to some control arms with a set of coilover shocks providing the motion control. The majority of people see the control arm pivots as a simple means of attachment that have no purpose beyond connecting the control arms to the spindle. In reality, the location of these pivot points are the core and most important feature determining the performance of your front suspension. On the RS Fast Track suspension, there are countless hours of engineering and design time invested in simply locating the control arm pivot points in order to yield the best possible performance in the industry. In a computerized 3-D suspension model, we are able to plot the location of the roll center and instant center of the suspension system and optimize its location and manipulate ride and handling characteristics. The pivot points are more than just mounting locations. They form an invisible grid in the suspension design software that allows mapping the swing arm lengths of the control arms, antidive percentages, caster and camber gains, and a variety of other performance factors that ultimately contribute to designing a high-performance suspension system.
Ride and Handling Balance
A hallmark of running an aftermarket front clip or frame is having an outstanding balance of ride quality and handling. There are many clear and simple advantages of utilizing a complete aftermarket chassis, such as added strength, rigidity, and ride height. While these key items are very straightforward and easily identifiable to the customer, what cannot be shown in an advertisement is the ride quality, feel, and performance of the chassis. In order to obtain this, we have found that beyond our in-depth engineering of suspension components there is a lot of knowledge to be gained from building, testing, and tuning vehicles. We go to great lengths to design a complete package where the rear suspension works together well with the front suspension, making the car perform great as a complete package. Many overlooked items, such as exhaust routing, crossmember locations, and body mount locations lend themselves to the ride quality of a car. It is very important to make sure that no moving or vibrating parts come in contact with the body of the car in an effort to eliminate interior vibrations. The springs and coilover shocks lay a massive roll in the ride quality of the car. In order to use them to the best of their abilities, however, we have optimized the motion ratios of our front and rear suspension packages to allow the use of lighter spring rates for a smoother ride while not sacrificing performance. Simply put, where some suspension systems require the use of a 600-lb/in spring to hold your vehicle up to its ride height, a Roadster Shop chassis may only use a 450-lb/in spring. We do not rely on high spring rates for roll resistance through cornering, and instead we have designed a system that lets light springs provide great ride quality while utilizing splined sway bars to keep the roll angle of a car to a minimum.
Muscle cars are by no means lightweight machines, and the stress on the chassis is compounded by today's modern tire compounds and massive horsepower output. Consequently, all RS framerails are built from 10-gauge mild steel, providing a super-solid structure for the rest of the chassis. In addition to this, we integrate a variety of 0.250-inch plate steel gussets and 0.125-inch wall tube crossmembers to make our chassis as rigid as possible. There is a huge difference between a dedicated race vehicle and a muscle car you intend to drive on the street, and strength in the suspension components is one of the key differences. We sacrifice nothing to provide the strongest control arms and suspension components possible. Our lower control arms are built from 1.625-inch tubing with 0.125-inch wall thickness, which is more than twice the diameter and strength of other systems on the market. Our upper control arms are built from the same material, only in a 1.25-inch diameter. We use 4130 chromoly, pre-hardened upper control arm shafts that can withstand any abuse the road or track can throw at them. Heavy-duty machined components are utilized all throughout the suspension to make sure that no pothole or imperfection in the road will cause harm to your suspension. Years ago, a 500hp big-block Chevy was considered a high-performance street motor whereas today it isn't uncommon to see 850-plus horsepower LS motors that are lighter and more efficient. When you factor in the effects of wider and stickier performance tires, it is crucial that a suspension system and chassis be up to par. We cut no corners when it comes to strength and materials. We do not use commonly available components that may fit the bill. All components have been designed and built specifically for our application. By doing this, RS suspension components can handle in excess of 1,500 hp. With built CV axles utilizing chromoly CV joints and 300M axleshafts, even our independent rear suspension units are able to accommodate these massive horsepower demands.
Spindles and Camber Gain
Spindles play an important role in overall suspension design, and the C6 Corvette spindle is an outstanding foundation to build a performance muscle car suspension off of. They are a very high strength and lightweight design, and offer a nice, tall ball joint that allows for great camber curves to be built into the suspension. When raising the location of the upper ball joint on a spindle in relation to the lower ball joint location, you are now given a larger window to raise and lower your upper control arm pivot points, thus increasing or decreasing the camber curves. When set up properly, you can achieve some pretty fantastic camber curves that are an improvement even over the stock Vette geometry. The taller design limits you to a minimum of a 17-inch wheel diameter, but provides a good sturdy base with its height that resists the massive twisting forces that are transferred through them during hard braking. Another key advantage is that there are countless aftermarket and OEM performance brake packages available as a bolt-on for the stock C6 spindle. This spindle design utilizes a bolt-in sealed bearing hub that is available in stock form as well as upgraded units designed to withstand the abuse of road racing and autocrossing.
Dropped Steering Arm
While the C6 spindle has many great features, the one downside to it is the stock steering arm location. The Corvette is set up with the crank pulley of the motor behind the steering rack so the height of the steering rack is not a factor. However, when retrofitting the C6 spindle into a muscle car, the engine's oil pan is now directly above the steering rack, causing a conflict and a massive packaging issue. The solution is our billet aluminum dropped steering arm. It lowers the mounting location of the steering rack and tie rod considerably while still retaining the best steering geometry and Ackerman possible. As a result of this, we are able to mount the steering rack low in the chassis and accommodate most any engine package.”
Splined Sway Bars
Splined sway bars have been used in racing for many years and have recently been adapted into Pro Touring cars. They offer many strength and versatility advantages over a traditional sway bar. More specifically, splined bars are available in a variety of wall thicknesses and diameters to provide different levels of roll stiffness for different applications. We have a very solid baseline for what bars work the best for muscle cars, but for the more serious driver it is nice to be able to fine-tune the car with the use of different wall thickness sway bars. In addition, we utilize a broached sway bar arm that can be cut to any length and made adjustable to provide infinite tuning possibilities for the roll stiffness of your car. Many suspension systems simply use mild steel DOM tubing for a sway bar that really provides no roll resistance. Since they are not sprung steel, they can fatigue very easily.
Roadster Shop utilizes jounce bumpers on our front and rear suspension systems to limit the suspension travel during compression. This prevents the shock from absorbing the force of heavy impacts on the road. By doing so, it increases the life and performance of a coilover shock. If a suspension system does not have a jounce bumper, it is relying on the shock to take the impact of road obstacles like potholes and large dips, which can severely damage your coilovers and even cause shock mounts and other components to shear off during impact. Aside from the protection it provides to a coilover, a jounce bumper also offers performance gains since they work as additional spring rate when the suspension begins to fully compress. The RS Fast Track suspension also integrates an upper bumper that limits the extension of the suspension travel. This aids in the assembly process and allows us to bring the coilover shock outboard as close to the ball joint as possible. This enables orienting the shock as vertical as possible while preventing the upper control arm from contacting it. In contrast, many suspension systems make sacrifices in this area and resort to mounting shocks at unacceptable angles.
A lot of research and testing has gone into the bushings that we use in our Fast Track suspension. We have chosen a combination of a bronze Oilite sleeve and an Acetyl bushing, which we feel is the best possible design for a vehicle that will be both street and performance driven. The Acetyl bushing provides almost no deflection while still providing a barrier from road feel and vibrations. It is a much better solution to using Heim joints, which can be noisy and often wear out quickly. Rubber bushings can provide a nice ride, but will deflect heavily under cornering loads and drastically inhibit the performance of your suspension. Over the years, we have seen mostly negative results with the use of urethane bushings. Not only are they noisy, but they still deflect very easily. Unless they are properly designed and have supporting washers, they are prone to wearing our rather quickly and deforming under normal driving conditions.
Maximizing Tire Clearance
Fitting massive front tires is the norm these days in Pro Touring machines. Most Pro Touring cars compete and drive with street compound tires, and as competition gets more and more intense so does the need for more traction. Going to larger front tires is a natural fit when looking to achieve more bite without using a softer compound tire. In order to package these larger tires, we have specifically designed our upper and lower control arms to avoid any interference when moving through their steering motion. A swept-back lower control arm can accommodate front wheel backspacing up to 8 inches while still retaining greater steering angles than most stock and aftermarket suspension systems.
The solid axle four-link rear suspension is probably the most common rear suspension seen on muscle cars, street rods, and Pro Touring cars because of its ease of packaging and relatively good performance. It has been proven over the years that, when set up properly, it can work quite well. However, simply put it cannot match the performance advantages of an independent rear suspension setup. If you look at modern cars, the only thing on the road with a solid rear axle is a pickup truck. For the diehard drag racer, a parallel four-link solid axle will always be the best performing option, but for the Pro Touring and cruising enthusiast there are some significant advantages to be had with our IRS. For general driving and cruising on roadways, the ride quality will be a huge advantage, as the IRS can absorb uneven surfaces and bumps by allowing the wheels to move independently. This allows the tires to do a better job of dampening and maintaining traction. This advantage is even amplified in racing applications where dropping a tire off track or putting one tire on the rumble strip can really upset a solid axle car. With an IRS setup, the car will maintain traction and control while allowing one wheel to absorb the obstruction.
Furthermore, an IRS allows precisely controlling the camber of the rear tires. We have built-in camber curves that match well with our front suspension, creating a very predictable and great-feeling total package. Close attention has been paid to the balance of roll center height and sway bar rate in order to optimize traction and handling of a vehicle on street compound tires. Heavy muscle cars with street tires require a substantial amount of weight transfer during cornering. This allows loading the rear tires sufficiently to make them stick to their maximum potential without causing a car to understeer. A 1.25-inch splined rear sway bar has been integrated into our IRS cradle and can be easily swapped for lighter or heavier bars to fine-tune your car's performance.
Independent rear suspension systems have gotten a bit of a bad rap in the aftermarket industry mainly due to the outdated and poorly engineered Jaguar-style IRS design. The Jaguar-style suspension does not utilize an upper control arm, and retains the spindle and wheel location by relying on a half-shaft–style axle. Should a half-shaft or U-joint break, the wheel of the car is free to move inward and outward uncontrollably, causing a very dangerous situation. Without using an upper control arm to retain the spindle and hub assembly you are very limited to the horsepower you can apply. Imagine the massive twisting force a rear tire exerts when it is under had acceleration. If there is no upper link to counteract this force, you are relying on the lower suspension pivot for stability, which is now seeing this twisting force amplified by the leverage effects of the suspension. On top of the lack of strength in these systems, there are no performance gains to be had with them. The camber gains are very poor and often produce positive camber, the bushing and suspension hardware have massive amounts of deflection, and there is no antisquat available to assist in traction. There are many companies that build very high-end, Jaguar-based suspensions that are well suited to lightweight street rods and are very aesthetically pleasing for show cars and cruisers. For the Pro Touring enthusiast, however, it is mandatory that an IRS system use a full upper A-arm assembly. The strength and performance benefits that come with this design simply cannot be matched with a Jaguar-based unit.
Many traditionalists fear an IRS because they feel they're too weak to handle high-horsepower levels, and this is mainly due to the poor strength and performance of Jaguar-based IRS systems. We have gone to great lengths to make sure that the RS Fast Track IRS can withstand massive amounts of horsepower. For instance, we have utilized the best materials in our CV axles that start with an unbreakable 300M axleshaft using specially machined chromoly, high-performance CV joints. These axles have been tested in drag race applications with more than 1,400 hp. We offer upgraded shafts and higher spline outer bearing assemblies to support horsepower numbers in excess of 2,500. Likewise, our control arms are made out of 0.125-inch wall DOM, 1.625-inch diameter tubing with Acetyl bushings that will not deflect under hard acceleration. Also, our IRS cradle has been CAD designed to withstand the forces of high-horsepower motors.
Our Fast Track IRS is available on any Roadster Shop chassis, as a stand-alone cradle, or as a custom-built back-half kit. It can be a very easy installation when it is integrated into a complete RS chassis. On some applications, rear floor modifications are required to accept the IRS system. While many RS chassis are designed to be direct bolt-ons to original bodies, most original bodies were not designed to accommodate an IRS. We simply could not let the factory floor limit the capabilities of the suspension from a strength, performance, and handling standpoint. In most cases, simple sheetmetal work is required for the installation. With the ragged condition of the average muscle car project and the rust and panel repair they require, this would not be outside the realm of anyone doing a restoration. We also offer our IRS as a stand-alone suspension cradle that can be installed by any shop or homebuilder. Fabrication skills are required, but the installation is simplified since the suspension is completely assembled in a stand-alone cradle. When weighing out the performance versus ease of installation compromise, we simply could not come to terms with sacrificing strength, performance, or ride quality simply so it could be a bolt-on part. As a result, it is not offered as a direct bolt-in application. The floors and subrails of muscle cars were never designed to accept an IRS, and the decision was made that like the front suspension, this could not be a limiting factor. So much would have to be changed, in fact, that the design compromises would have negated the benefits of building an IRS in the first place.
To maintain the quality and performance of our suspension products, we utilize a vast amount of modern equipment and technology. Every component is individually designed and engineered in 3-D modeling and tested for strength and fitment. The suspension geometry is extensively designed and mapped out in a complex suspension design and analysis program. All framerail and crossmember components are designed in either 2-D or 3-D software and then cut on a combination of high-definition CNC plasma machines and lasers. Components are bent using heavy tonnage CNC press brakes. To make sure our control arms maintain exact tolerances, we use CNC mandrel benders and have the tubes cut and notched by CNC pipe lasers. We have over 40 application-specific machined components that are CNC cut on four-axis milling machines. All of our components and assemblies are designed and tested in Solidworks 3-D software to make sure everything is up to par prior to building any prototypes. Additionally, heavy-duty fixtures are used to put all the components together once the sub-assemblies are complete. All of this technology allows us to produce high-quality, high-performance products over and over again.