These days, GM has the unibody thing all ironed out. The result is lightweight chassis that are substantially stiffer and stronger than yesteryear’s full-frame cars, and for resounding evidence, look no further than the fifth-gen Camaro. Not only does it handle the tremendous loads imparted by 426 hp, 4,100 pounds of mass, and over 0.90 g of lateral grip, it does the deed without main ’rails running along its perimeter to help stiffen things up. Unfortunately, even full-frame cars of the ’50s and ’60s aren’t nearly as stiff as modern unibody cars, and twist up like the storyline of Lost once some modern horsepower and tire grip are thrown into the mix. Recognizing this problem long ago, companies have been churning out chassis stiffening components for quite some time. Unlike unibody cars like Camaros and Novas that can be substantially stiffened up with subframe connectors and aftermarket front and rear clips, there is no easy fix with full-frame Tri-Five Chevys. The solution is engineering a completely new frame from the ground up, and innovative aftermarket chassis builders have done just that. To find out what’s involved in pulling off such a conversion, and how much a full-frame swap can improve ride and handling, we sought out the expertise of the top chassis builders in the business. Thanks to Craig Morrison and Matt Jones of Art Morrison Enterprises, and Steve McClenon of Hotrods to Hell, we got thoroughly schooled on the inner workings of an aftermarket full-frame chassis. Here’s the scoop.
Matt Jones: The original frames used in the ’50s and ’60s were merely a way for the engineers to mount suspension components to some sort of structure as cheaply as possible. Most of the structural integrity was handled by the body, which was deemed enough for the vehicle’s intended usage without much additional support from the frame. By today’s standards, vintage cars with full frames or unibody construction are very flexible. Open-channel cross-sections were very popular during the ’50s and well into the ’80s because they were cheap to produce and provided increased cabin space. Materials were designed to be thin, which reduced material cost and tooling wear during production. Today, much more focus is placed on overall structural strength as a way to improve the overall quality of a vehicle. Modern cars can have more than double the torsional stiffness than vehicles made in the ’50s and ’60s. Chassis stiffness is very important to ride and handling, and factory muscle car bodies need all the help they can get. With increases in spring and roll rates, more stress is placed on the body and frame. Stiffer structures will allow the suspension to work more efficiently, which makes transitional handling more crisp, and also reduces interior rattles by a significant amount. They’re also safer and provide improved ride quality.
While it is difficult to increase the body’s strength, we can focus our attention on the chassis to make sure it is as stiff as possible while maintaining packaging constraints. Designing an entirely new frame allows much more freedom of design than simply designing around an existing frame. Suspension design can be revised for more aggressive handling and to accept modern lightweight components, such as tubular control arms and lighter steering racks. Framerails can be narrowed or shaped to allow wider tires while still maintaining an acceptable turning radius. Finally, material shape and thickness can be optimized for increased strength without gaining too much weight.
Steve McClenon: When designing a new frame, you don’t have to do much to make it stronger than a stock unit from the ’50s and ’60s. Factory full-frames like those used in Tri-Fives and A-bodies are like wet noodles. They use open-channel ’rails and have very shoddy welds. It makes you wonder how they held together at all, but most people never even realize that since the frame is covered up by the body. In comparison, the frames we build use fully boxed tubing. Our frames are lighter and stronger than stock, but they also take up less space and offer more ground and exhaust clearance. Plus, the rear framerails have been narrowed to allow fitment of 345mm rear tires. One thing to keep in mind is that original Tri-Five frames are nearly 60 years old, so frames that are still in good shape aren’t common, which makes an aftermarket frame more appealing. Our frames are designed to retain all the factory suspension pickup points, and we offer a full line of control arms, springs, shocks, brakes, rearends, steering boxes, and sway bars to go along with them. They’re designed so that you won’t have to hack up the car to install one. Just drop your body onto one of our frames using the stock mounts, and you’re good to go. The improvement in stiffness will make a car feel like it has full ’cage in it.
GT Sport Chassis
Craig Morrison: Art Morrison’s GT Sport frames have several innovative features that can make a Tri-Five Chevy handle like an exotic sports car. We start with an all-new frame built from 2x4-inch rectangular steel tubing. Combined with crossbracing that’s strategically placed throughout the chassis, the result is a frame that’s significantly stiffer than stock. Revised suspension pickup points allow lowering the car 3 to 4 inches while retaining full suspension travel. The body can be lowered even more with our optional drop spindles. The control arms, spindles, coilovers, and sway bars included with our Tri-Five chassis have been designed to take full advantage of the all-new frame design. The control arm angles have been optimized to create an ideal camber curve, while minimizing the side scrub movement of the tires. Furthermore, the control arm angle increases antidive characteristics for improved stability under braking and a smoother ride.
While factory Tri-Fives came with 2 degrees of positive caster, we set the angle at 5 degrees in our GT Sport chassis. This not only enhances straight-line stability, but it also increases the tire contact patch and optimizes weight distribution during cornering. Likewise, we positioned the steering rack so that the bumpsteer curve closely matches the camber and caster curves. This means that the car will track straightly even on uneven surfaces. Significant time was invested in dialing in the roll center as well. For the first 3 degrees of body roll, the roll center is maintained almost perfectly, and the rate of vertical movement is at a 1:1 ratio with the suspension movement. That’s just another way of saying that the car will remain very stable and predictable during acceleration, braking, and cornering.
When a customer orders up one of our Tri-Five chassis, they get a brand-new frame, tubular upper and lower control arms, lightweight spindles, Strange coilovers, and front and rear sway bars. In the rear, the stock leaf springs are replaced with a triangulated four-link suspension. This offers vast improvements in rearend control under acceleration and cornering. Moreover, eliminating the leaf springs in conjunction with narrowing the framerails frees up enough space for 345mm tires. A power steering rack and 9-inch rearend housing are included with our frames as well.
For the ultimate in flexibility, our Tri-Five frames are available with engine mounts for Mouse motors, big-blocks, and LS series small-blocks. The frame will also accommodate most GM automatic transmissions as well as Tremec and Richmond manuals. Another nice feature are passages built into the crossmembers that allow routing the exhaust through the frame to further improve ground clearance. This frame has been proven to produce 0.94 g in lateral acceleration on the skidpad with street tires.
Matt Jones: Engineering a brand-new frame is a tremendous technical challenge. The very first step is to conduct some market research to figure out what most customers will want and what they may expect. For instance, are they willing to trim some sheetmetal for aftermarket transmissions or larger tires? Will they want the largest tires possible, front and back? Will the vehicle be raced at autocross events or just street driven? The next step is closely studying the vehicle platform for which we plan to design a frame. While most vehicles are said to use the same frame design throughout a generation of model years, the truth is they are usually different. Bumper mounts typically change, as well as rear ’rail design and body mount locations. All changes must be found before the design process begins. Next, we examine how the OEM frame is packaged in the vehicle. Most vehicles do not allow for a 335mm-wide tire without moving the framerails inboard, but care must be taken to see how much the ’rails can move without significant changes to the body or other components. The floorpan will dictate what suspension can be used, and how low the vehicle can sit. From there, the process gets much more high tech as we digitize the OEM frame and body. Using a Faro arm-measuring device, every important detail is digitized in 3-D to determine where to place the engine, bumper, and body mounts. At the same time, gauge or tooling holes can be mapped to later check our sample frame for possible damage to ensure we record valid data. Also, body mount location is compared between the frame and body to see where possible differences lie. Lastly, the floorpan shape will determine how narrow the framerails will be, how low the vehicle can sit, and which suspension will package the best.
Craig Morrison: The amount of time and effort it takes to install an Art Morrison full-frame kit depends on the applications. Some cars, like ’58-64 Impalas, required significant modification to the floor because of how low the floor hangs down. Depending on the skill level of the individual building the car and how intricately the new floor is designed, the installation process can take between 40-80 hours to install. Fortunately, that’s not the case with ’55-57 Chevys, as the body design lends itself very well to an aftermarket frame, the installation process is very simple. There are a few tabs for routing the OEM emergency brake cables that need to be trimmed off, and the original pinion snubber needs to be removed. Other than that it is a bolt-on project. With Art and I working together the first time, we were able to make the chassis swap in about five hours. With the body off of the frame, installing the suspension components, brake lines, fuel system, and engine and trans is much easier. For the dramatic improvements in ride and handling that our Tri-Five aftermarket frame offers, the time required to install one is a small price to pay.
Craig Morrison: When building a chassis, round tubing and square tubing each have their own sets of pros and cons. Race cars are built from round tubing, while production cars are built from square tubing. The primary benefits of building a round-tube chassis are strength and reduced weight. Since a rollcage is integrated into the chassis design, the overall chassis is extremely stiff and provides a very rigid platform for mounting the suspension. While lightweight, this space frame approach requires massive amounts of fabrication and usually requires a builder to hand fabricate every panel in the car except for the exterior body panels. A more traditional rectangular chassis that fits under the floorpan is generally a lot easier to install into a car, but because it isn’t welded to a rollcage they usually aren’t as rigid as a full round-tube chassis. From a street car standpoint, the fabrication is straightforward, you can keep a stock interior, and the car is easier to build.
Strength and Durability Testing
Matt Jones: Strength, durability, and performance are measured in two ways at Art Morrison Enterprises. The first is computer simulation, and the second is real-world testing. Before prototype production begins, designs are validated via Finite Element Analysis (FEA) and computer suspension simulation. Each part is checked for strength and durability, and we remove as much excess bulk as possible. Prototype samples can then be checked for stiffness using real-world methods, and our in-house vehicles tell us what the vehicle will do on public roads and perhaps what may need to be adjusted. The benefit of building chassis for over 35 years is the wealth of knowledge that accumulates in chassis design. Every design has been proven hundreds if not thousands of times in the past, and knowing which designs do and don’t work allow us to confidently work toward the right direction.
Craig Morrison: All the chassis built at Art Morrison utilize framerails that are manufactured in-house. For our Tri-Five frames, we start with 2x4x0.012-inch wall tubing, and then cut the ’rail sections to length. From there they are bent, utilizing one of our two mandrel benders. A mandrel bender inserts a die into the center of the tube to support it on the inside while bending it to prevent the tubing from wrinkling. When the ’rails are bent, they are trimmed to a final length and then loaded onto the jig table. The brackets are laser-cut and formed by one of our suppliers, and bolted onto a jig table. Once the chassis is completely loaded, the welding begins. A typical chassis requires 8-10 hours of welding to join everything together. From there, the chassis is pulled out of the table and all the welds are inspected for completeness. Afterward, any weld spatter is removed.
Matt Jones: One of the biggest drawbacks of building a suspension around a stock chassis is that significant changes to the camber curves, caster, and roll center can’t be made without moving the stock suspension pickup points. This is particularly true with cars like the ’55-57 Chevy, since suspension technology has advanced quite a bit since they were originally designed. The luxury of a ground-up redesign when using a new frame design eliminates many of these restrictions. Generally, the overall aggressiveness of the suspension will be based on customer preference, and every aspect of the chassis design is a balancing act. The steering knuckle design is determined by what size wheels the customer will typically run. For example, Tri-Five owners don’t usually run 20-inch wheels like the typical Pro Touring muscle car, so C6 Corvette knuckles aren’t a good idea in these applications. Control arm shapes are contoured for tire clearance, and brackets and crossmembers are shaped to ensure adequate engine clearance. Likewise, the availability of different antiroll bar materials will somewhat determine the roll center location. Higher roll centers will effectively increase roll stiffness and vice versa.
Furthermore, spring availability will have an affect on motion ratio requirements and roll gradientsor body rolland wheel rates are determined by the preferences of the typical customer. Lastly, the steering rack will be placed in the optimum location and checked for clearance with all engines that may be used in the chassis. Multiple iterations of a chassis are usually required for each new suspension design, as one change creates a domino effect that leads to other changes. For example, if the engine requires a U-shaped antiroll bar for harmonic balancer clearance, the resulting roll rate change must be accounted for to see if the roll center height must be adjusted again. At that point, the steering rack will be relocated for the optimum bump curve and then rechecked for clearance. The steering rack relocation will also require rechecking tie rod to antiroll bar link interference. In the end, various parts of the suspension will be redesigned several times until everything is satisfactory.
Matt Jones: Reducing unsprung weight is vital in almost all aspects of performance, especially in vehicles that use live-rear axles. Higher unsprung weight will have a negative affect in both handling and ride quality, the two most important aspects in chassis performance. To put it into perspective, modern independent rear suspensions using aluminum components will typically have about 295 pounds of unsprung weight, while live-axle systems weigh in at about 385 pounds. That represents a 30 percent increase in unsprung weight. Consequently, it’s very important to make the suspension as light as possible in both the front and rear. Lighter suspension components can get away with a lower spring rate, which then leads to lighter damper valving, both of which give a significant increase in ride quality. CHP