As mentioned in earlier chapters, the stiffness and strength of the components play a large role in making a car handle and perform well. The same can be said for the vehicle’s chassis and body structure, which is the backbone of the entire system. If the chassis isn’t strong enough to support the abuse, you may be leaving a lot of performance on the table, even with high-dollar suspension components. That’s why it’s important to prepare your car’s frame or unibody structure for hard cornering, hard braking, and hard acceleration.
This Tech Tip is From the Full Book, DETROIT SPEED’S HOW TO BUILD A PRO TOURING CAR. For a comprehensive guide on this entire subject you can visit this link:
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First things first: You must always start with a rust-free car. A little rust in the front fenders or, maybe, in the quarter panels isn’t an area of concern, but any time you see a vehicle with rusted floorpans, trunkpan, or rocker panels it’s best to stay away. If there is significant rust in these areas, it likely means that the chassis structure has been affected by rust also. Regardless of the chassis configuration (full-frame or unibody), rust weakens the metal significantly and creates all sorts of problems down the road. For West Coast guys this isn’t usually an issue but any car guy east of the Mississippi River has dealt with rust at some point in his life.
It’s never a good idea to start with a total rust bucket, unless you plan on gutting the body and sitting it over a custom tube chassis. At that point, the body is just a shell and the chassis is brand-new so rust is no longer a concern. Obviously, it takes a special breed of car guy to go all out on a tube chassis Pro Touring build. But trust me it’s been done.
If you’re building a truck, a Corvette, or a GM A-, B-, or G-Body, you’re dealing with a vehicle with a body-on-frame configuration. That means the chassis is a separate structure from the body, which is an ideal setup in terms of strength. This full-frame can also be affected by rust but it isn’t nearly as fragile as the chassis structure of a unibody design. Full-frame cars are generally heavier than unibody cars but provide a great platform to build upon.
The strength of the body-on-frame design is undoubtedly the highlight of this design but many applications still require additional bracing to keep the structure solid and stiff. Early Chevy Impalas (1958–1964), for instance, feature an X-frame design, as opposed to the more generic perimeter design of many other makes and models. Don’t ask why General Motors thought this was a good idea but the X-frame certainly doesn’t lend itself well to high-performance driving.
Moving on through the years, General Motors stepped up its game with the A-Body platform but it took the perimeter design a bit too far by building a strong structure around the perimeter of the car. This leaves a lot to be desired in the crossmember and bracing department, making for a frame that flexes during harsh driving conditions. For most full-frame cars (especially the GM A-Bodies) additional chassis braces are necessary. Luckily, they are widely available to fit the popular 1964–1972 GM midsize models.
In most cases, you can spot a full-frame car just by the size of it. Ford and General Motors placed full-size models (Impala, Bonneville, Galaxie, and Monterey, among others) on a full-frame, based on the weight of the vehicle. They felt the heavier car needed a full-frame to support it, and they were right.
Chrysler Corporation didn’t play into this mindset, choosing unibody construction for most of its models, starting in 1960. The heavy weight, along with metal deterioration caused by rust, spelled disaster for many Mopars from the 1960s and 1970s, which is part of the reason they’re worth so much money. There aren’t many of them left!
Despite the original flaws and deterioration after years of use, full-frame vehicles provide the necessary strength for a bulletproof Pro Touring build. You may need to strengthen a few points on the frame, especially the areas around the suspension mounting points, but you’ll save quite a bit of time and money on things such as subframe connectors, shock tower braces, etc., on a full-frame design.
The weakest points in a full-frame are usually located around the “kick up” where the frame rails go over the rear-end housing. These joints can sometimes flex, and they are susceptible to rust, which causes the joints to weaken even further. For the suspension to do its job, the mounting points must be capable of taking the stress of harsh driving without flexing, as even 1/16 inch of flex can cause changes in the geometry. Unfortunately, full-frame cars are not the norm in the Pro Touring world, so most folks have to spend a lot of time and money to make their unibody vehicle as sturdy as a body-on-frame vehicle.
With this in mind, a number of enthusiasts have built custom frames for cars that originally used unibody construction. This is a major task, as it usually requires the removal of all floorpans and trunk pans to even get started on a custom chassis. Then new pans must be fabricated to work with the chassis, and this usually leads to the original result of full-frame cars: extra weight. It certainly isn’t a beginner’s project. The end result is usually a full-on race car, instead of a practical Pro Touring vehicle that you can legitimately drive on the street.
Until the 1960s almost all American auto manufacturers used the body-on-frame configuration, even though Nash introduced the first successful unibody car in 1941 after a somewhat flawed attempt by Chrysler almost a decade earlier. The term ”unibody” is actually short for unit body, which actually means unitary construction. This type of construction means that the body and chassis are essentially the same unit.
With a unibody design, the floorpans, rocker panels, and inner fenders are part of the vehicle’s chassis structure. With most muscle car and pony car applications tipping the scales at 3,000 pounds or more, that is a lot of weight to be supported by sheet metal. The advantage of unibody construction for auto manufacturers was weight savings and ease of construction at the assembly plant. Generally, the car would technically have frame rails on the front and rear of the vehicle but these rails would be welded directly to the floorpan and cowl area. General Motors was the exception with its bolt-on front subframes.
It wasn’t until the late 1950s that Ford adopted this chassis design for its larger cars, such as the Lincoln and Ford Thunderbird. From there the trend grew, and Ford used a unibody construction on other models, including the Falcon, Mustang, and Torino (which was changed to body-on-frame in 1972). Ford passenger cars have always been plagued with minimal engine bay real estate, thanks to huge shock towers. It is a major chore to stuff a large engine into any early Mustang, Falcon, or Fairlane without doing major work to the front suspension.
Detroit Speed’s new Aluma-Frame for 1964–1970 Mustangs is a bolt-in front suspension module that replaces the original front suspension and removes the shock towers altogether. The Aluma-Frame retains the original front subframe rails and provides lots of great advantages, including improved suspension geometry, rack-and-pinion steering, and adequate engine bay clearance for many modern power plants. (Read more about this front suspension system and how it helps Mustang guys enter the Pro Touring world in Chapter 4.)
General Motors jumped onto the unibody bandwagon when it introduced the Chevrolet Corvair in 1960. Although the Corvair was a totally different animal with its four-wheel independent suspension and rear-mounted engine, it served as the first of many unibody vehicles built by the General.
Two years later, the Chevy II was introduced; it was a much more conventional compact car for the time. It, unlike any other vehicles at the time, had a bolt-on front subframe that was obviously a manufacturing decision designed to save the company time and money. But, hey, it turned out to be a great move for Pro Touring guys because it allows easy subframe removal and installation. A few bolts here and there, and the entire front end of the car can be removed. Brilliant.
When General Motors joined the “pony car” market in 1967 with the Camaro and Firebird, it introduced its F-Body chassis design, which has turned into the most popular Pro-Touring platform in the industry. The F-Body continued the bolt-on front subframe approach, and later passed it on to the 1968–1974 Chevy Nova, and BOP (Buick, Oldsmobile, Pontiac) equivalents.
As mentioned earlier, Mopar used unibody chassis construction for many years, even on its full-size cars. When the muscle car and pony car craze hit in the mid-to-late 1960s, Mopar had a number of high-performance models, all of which featured a unibody construction with torsion-bar front suspension and leaf-spring rear suspension, a common design element in most of its models in the Chrysler, Dodge, and Plymouth lines.
The Mopar crowd doesn’t get much love in the Pro Touring world but many of the Mopars from the 1960s and early 1970s are very capable performers. It’s just a matter of strengthening the unibody structure with subframe connectors and adding a mild roll cage to have a great platform to build upon. Aftermarket support for Mopars isn’t quite up to par with the GM brands but if you’re willing to shell out some dough for custom components, you can make a Mopar handle with the best of them.
Despite the difference between the Big Three’s attempts at unibody construction, they all share the same problem: strength and rigidity in harsh driving conditions. Drag racers and road racers deal with the same struggles when dealing with unibody cars, so the idea is to remove the tendency for the chassis to flex under hard loads. The answer, in most cases, involves fabricating or installing a set of subframe connectors. The connectors are self-explanatory, connecting the front portion of the frame rails to the rear portion of the frame rails. In other words, they provide support in the middle floorpan area, where manufacturers relied solely on the floorpan and rocker panel structure to take the abuse.
Luckily, subframe connectors are usually an easy install. Some are bolt-in style, suitable for a guy building a car in his driveway, but they tend to hang below the rocker panel pinch weld to avoid the dips and pockets in the stock floorpan. This obviously isn’t the most attractive look, so most hardcore Pro Touring enthusiasts go with a weld-in design that is recessed into the floorpan.
Weld-in connectors are a bit more work but it’s totally worth the effort because they look a lot better and they provide much more strength. Weld-in subframe connectors essentially make the unibody as strong as a full-frame car, as long as all of the suspension mounting-points are rust-free. Weld-in subframe connectors provide definite advantages but also make GM’s bolt-on front subframe a bit more permanent.
Subframe connectors are very important although they will add a few pounds to your car. With heavy-duty components comes extra weight. The additional pounds are offset by the strength of the unibody structure, and you’ll still come in lighter than a full-frame car from the same era.
There was a time when a roll bar or roll cage was used only in a serious race car. Times have certainly changed; many Pro Touring builds feature a mild roll bar, which provides safety, as well as additional strength for the unibody structure. Roll bars also offer a great mounting point for five-point safety harnesses. They have therefore become commonplace on Pro Touring cars and trucks.
If you’ve seen the interior of an all-out race car, you’ve seen the maze of round tubing that makes the roll cage. This jungle gym isn’t practical, even for occasional street driving, because it’s a hassle to get in and out of the car, and it also creates a serious hazard. (A roll cage creates a hazard; that’s sort of ironic, right?) When you consider the extensive tubing, and the fact that it is designed to protect a driver who is wearing a helmet, the danger might become a little clearer. Even a small collision in the parking lot could slam your head against one of the bars, which is never a good situation. Always take great precautions when driving a caged vehicle on the street. Even though it’s ugly, roll-bar padding may be your best bet to protect your noggin.
The simpler four-point roll bars that are generally used in mild Pro Touring cars are less dangerous on the street. They’re also fairly easy to install because they don’t require nearly as much fabrication and installation time. Prefab kits are pretty common and easy to install, thanks to the bends, notches, and cuts being in all the right places from the get-go.
Most prefab four-point roll bars do not pass tech at NHRA- or IHRA-sanctioned drag strips. These drag racing organizations require a minimum of five roll-bar mounting points for cars that run quicker than 11.50 seconds in the quarter-mile. If you’re building a multipurpose car, this is a very important detail to consider.
The NHRA rulebook also provides some great guidelines that translate to Pro Touring cars, while relating to the average car guy. It designates that roll bars on all body-on-frame cars must be connected to the frame (not to the floorpan). It also states that all roll bars in unibody cars must be attached to the car using 6 x 6–inch steel plates welded to the floorpan. These plates strengthen the mounting point for the roll bar and provide additional rigidity, allowing you to get the most out of the aftermarket suspension components.
If you’re serious about road racing, refer to the Sports Car Club of America (SCCA) rulebook for exact rules on tubing sizes, appropriate mounting configurations, and other regulations. The SCCA is very strict on all of its rules but if you can pass SCCA tech, you’re good to go with pretty much any racetrack or racing organization.
Chassis modifications are plentiful in a typical Pro Touring build, and one of the most popular is to widen the original rear wheel tubs or install new tubs. The wheel tub (also known as the wheelhouse or wheelwell) is usually the limiting factor of tire and wheel fitment on muscle cars and pony cars from the 1960s and 1970s.
This Tech Tip is From the Full Book, DETROIT SPEED’S HOW TO BUILD A PRO TOURING CAR. For a comprehensive guide on this entire subject you can visit this link:
SHARE THIS ARTICLE: Please feel free to share this post on Facebook Groups or Forums/Blogs you read. You can use the social sharing buttons to the left, or copy and paste the website link: https:// musclecardiy.com//uncategorized/ how-to-build-a-pro-touring-chassis/
The largest tire you can fit on a stock first-generation GM F-Body (Camaro or Firebird), for instance, is a 275/40R17 on a 17 x 9.5–inch wheel with 5.0- to 5.5-inch backspacing. Although this is a much larger tire and wheel than the original setup, the desire for larger rear tires is a common thread in the Pro Touring scene. By widening the tubs or installing a prefab mini-tub kit, you gain lots of real estate, allowing you to install larger rear tires for increased traction.
Detroit Speed offers wide tubs for most common GM applications, as well as complete mini-tub kits for leaf-spring cars. The installation process for wheel tubs requires a great deal of work, as the original tubs must be removed by drilling out the spot-welds. Next, the necessary cuts must be made to fit the new tubs into place, and then they can be plug-welded to the car.
Although a pair of widened tubs gains you some necessary room, you also have to consider the location of the original leaf springs if you’re planning to utilize them on your car.
If you’re swapping to a custom rear suspension system, such as Detroit Speed’s QUADRALink, leaf springs are the least of your worries. If you’re keeping the springs, you need offset shackles to move the springs inboard and free up the valuable space for rear tires and wheels. Moving the leaf springs inboard interferes with the original fuel tank location on a GM F-Body, but Detroit Speed offers narrowed fuel tanks for this purpose.
According to Detroit Speed, its mini-tubs and mini-tub kits provide enough room for a 335/30R18 tire, mounted to an 18 x 12–inch wheel on a 1969 F-Body. For the Chevy II crowd, Detroit Speed’s mini-tubs allow for 10-inch-wide wheels and 295 tires on 1962–1965 models and an 11-inch wheel and 315 tire on the 1966–1967 models. If you have dealt with tire and wheel fitment on a Chevy II, this is a major improvement! Ford guys can also rejoice because mini-tubs for a Mustang can give it enough room for 10.5- to 12-inch wheels and 295 to 335 tires, depending on the year of the car
This increased footprint certainly helps in the traction department, and it’s just plain cool looking, regardless of the application.
Project: Deep Tubs Installation
Detroit Speed forms its mini-tubs in-house for all popular applications. Installing them is a pretty involved process that includes a substantial amount of cutting, grinding, and welding. As long as you’re willing to put forth the effort, a mini-tub kit is the only way to fi t big tires under the rear of a muscle car.
written by Tommy Lee Byrd and Kyle Tucker
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