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.
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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.
The automobile was originally designed with a body-on-frame configuration. By the 1960s, many manufacturers were making the switch to unibody designs, maintaining the body-on-frame design only with larger cars. For General Motors, the B-Body (Impala and other similar models), A-Body (Chevelle), and G-Body (Monte Carlo) featured a full frame.
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.
Body-On-Frame
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.
This is an example of a very clean, rust-free, second-generation Camaro. The floorpans, trunk pans, and quarter panels are common rust-prone areas, and they are important parts of the car’s structure because the Camaro features a unibody construction.
It’s always important to do a “cross out” measurement to make sure the unibody is square. This involves choosing four equal reference points (body mount bolts suffice if you cannot locate the original “gauge holes”). Using a tram gauge or plumb bobs, measure diagonally, and compare the two measurements. As long as the reference points are equal, the measurements should be very close. Factory tolerance is 1/8 inch, so if the numbers are within 1/4 inch, consider yourself lucky!
If rust or chassis damage is an issue for your project, you can always opt for aftermarket components to repair the damaged metal. Of course, this requires some intense fabrication and welding skills, depending on the severity of the damage. For chassis and floor repairs, a MIG welder is preferred for its ease of use. Starting with a rust-free car is ideal.
When General Motors introduced the A-Body platform in 1964, it was the backbone of the first string of muscle cars. The Chevelle, GTO, 442, and GS shared the same chassis, which featured a multi-link rear suspension and a perimeter-style frame. This design leaves a lot to be desired because of a lack of bracing around the suspension mounting points but it’s a strong platform to build upon.
Corvettes can sometimes be confusing to the novice because they were not technically a muscle car or a pony car. They were considered a sports car, and they featured a body-on-frame design from day one. Corvettes also featured a fiberglass body from day one, and an independent rear suspension starting in 1963.
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.
Unibody
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.
This unfinished Camaro provides a great visual of a unibody chassis construction. The satin parts are essentially just sheet metal, used for the floorpans, trunk pans, etc.; the glossy portions are structural components. This example has been modified with subframe connectors and other structural bracing.
General Motors is the only manufacturer that used a bolt-in front subframe. The frame unbolts from the chassis, unlike the Ford and Chrysler unibodies that feature a welded front frame section. This removable subframe makes for easy assembly but suffers from a bit more flex due to the movement of the bolts and bushings.
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.
Chrysler Corporation relied heavily on the unibody platform, but its chassis designs featured an integrated front and rear frame section. Large cars, such as this Dodge Charger, are a little too heavy for high-performance handling without first installing additional bracing and subframe connectors.
Ford also used an integrated front and rear subframe. This Ford Mustang has the floorpan removed for rust repair, which shows exactly how much this chassis relies on the structure of the sheet-metal floorpan. This chassis is greatly strengthened by extending these front subframe rails to meet the rear rails.
Subframe connectors are pretty simple pieces of metal but they’re very important chassis components. If you’re serious about building a Pro Touring car with a unibody chassis, a pair of weld-in subframe connectors is a must-do modification to take advantage of the new suspension.
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.
Subframe Connectors
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.
You hardly notice the subframe connectors when they are installed but they offer a great advantage over the stock unibody layout. Bolt-in connectors are available but they generally hang below the floor, which looks a bit tacky, and lack the strength of weld-in connectors. These through-floor subframe connectors require a bit more work but they work nicely and look great.
A full roll cage may not be necessary in your Pro-Touring machine but a simple four-point roll bar can offer many advantages. It provides additional chassis stiffening, and it provides a safe mounting location for the five-point safety harnesses.
Roll Cage
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.
As with any aspect of a Pro Touring build, it’s easy to get carried away with the roll cage, such as on this Chevy II. Door bars are necessary in some racing organizations but they are overkill on a standard Pro Touring machine. Unless you’re a serious racer, you’ll hate climbing in and out of a jungle gym.
On a body-on-frame platform, the roll cage must go through the floorpans and weld directly to the frame rails. For unibody designs, the roll cage tubing must be attached to a 6 x 6– inch steel plate, which is first welded to the floorpan.
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.
Mini-Tubs
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:
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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.
Although Chevrolet continues to be the make of choice for Pro Touring builders, they are cursed with small wheel tubs. An F-Body (Camaro or Firebird) only holds a 275/40R17, and early Chevy II/Nova platforms are even more limited. Mini-tubs are a must if you want wide tires!
This is a 1968 Camaro with stock tubs. The rear tires are 235/45R17s, mounted to 17 x 8–inch wheels. It offers a great look for a mild-mannered Pro Touring build but most folks want a much wider tire and wheel combination out back.
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.
On the other side of the coin, this 1969 Camaro features a Detroit Speed mini-tub kit to squeeze a set of 335/30R20 tires into the rear tubs. A massive combination like this also requires a narrowed rear end to achieve the “deep dish” wheel look.
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.
1 Before doing any cutting or grinding, you should start by marking the desired cuts. Detroit Speed suggests making a mark 23 ⁄4 inches inboard of the tub. Some trimming is necessary to get a perfect fit around the curved transition areas.
2 The rear seat panel must be cut using the marks seen here. Then, the spot-welds are drilled out, and the piece can be placed to the side until the new tubs are installed. The rear seat panel will be sectioned and re-installed when the new tubs are welded into place.
3 It’s tough to cut out the wheel tub in one piece, so cut it into sections if necessary. Then you can trim the floorpan and trunk pan on the marks determined by the templates.
4 Most of the cutting can be performed with a pneumatic cut-off wheel. It doesn’t offer a super-fast cut but you can be very precise, compared to the high speed of an electric grinder with a cut-off disc. Use a drill with a spot–weld cutting bit to make easy work of the original spot-welds on the rear seat panel and truck hinge bracket.
5 Now the underside of the wheel tub can be marked and prepared for cutting. The marks extend down into the rear frame rail. Detroit Speed provides templates for these panels and suggests using 1/8-inch steel plate.
6 Remove the wheel tub, leaving only the rear frame rail notch to be cut and removed. A pneumatic cut-off wheel or plasma cutter can be used on the frame rail, which is slightly thicker steel than the wheel tub.
7 The new, deep tubs can now be test-fit to check for any clearance issues. This generally requires several tries to get the perfect fit. Additional trimming around the corners may be necessary for a flush fit all the way around.
8 Here are the rear frame rails with the new 1/8-inch plates (not included in the kit) tack-welded into place. These notches help gain more real estate, and the plates require moderate fabrication skills. The rear portion of the frame rail notch features a slight curve, which can be made without any special metalworking tools.
9 Depending on the clearances, Detroit Speed suggests leaving a 1/2- to 3/4-inch strip of metal on the floorpan and trunk pan. These strips can be hammered over (using a dolly) to act as a mounting flange for the new tubs.
10 With all clearances checked and the frame rails notched, you can clamp the wheel tub into place for the final time. Locking pliers (lots of them) are suggested, but you can also use sheet-metal screws or Cleco fasteners to secure the tub before welding.
11 Drill holes every 2 inches in the wheel-tub flanges, where they attach to the frame rail, floorpan, and trunk pan, but only drill through the wheel tub. Then, use a MIG welder to spot-weld the wheel tub to the car.
12 If you’re building a leaf-spring car the following crossmember steps can be skipped, but if you’re using a Detroit Speed QUADRALink rear suspension kit, you must install a new rear crossmember. After the necessary portion of the floor is removed, the “blank” crossmember is fitted and marked.
13 The crossmember can then be cut on the marks, which follow the bumps and contours of the floorpan. Use a cut-off wheel to make the cuts, and use a grinder to fine-tune the panel.
14 After some tedious fitting, the rear crossmember can be welded into place. Welding this type of L-joint can be tough from the bottom side, so welding it from inside the car may be easier. Either way, sand the welds smooth with a grinder.
15 With the rear crossmember in place, the wheel tubs can be finish-welded. The rear crossmember serves two purposes: It strengthens the rear frame rails and raises the rear upper shock mounting points, leaving plenty of shock/spring travel when the car has a lowered ride height.
16 Take your time when grinding the welds and you’ll end up with a beautiful finished product like this. The plug welds are easy to grind without digging too deeply into the metal but any joint that is butt-welded (such as the frame rail filler plates) requires special attention, to prevent weakening the joint.
17 Because the trunk hinge bracket usually mounts in the curve of the wheel tub, it must be sectioned to fit the new, wider tub properly. When you get it test-fitted, and everything checks out, the trunk hinge bracket can be spot-welded to the tub. The finished product isn’t alarmingly different from the original but it certainly provides a lot more real estate for wide tires and wheels. The rear crossmember allows for the QUADRALink but it also helps stiffen the rear portion of the chassis.
18 Four spot-welds are now unnoticeable, thanks to a few minutes of careful grinding. Always be careful when assembling or disassembling trunk hinges because of the extreme tension on the spring.
written by Tommy Lee Byrd and Kyle Tucker
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