February 1, 2009

Last month we kicked off our Camaro-Mustang Challenge road race project with an introduction to the class and an overview of the selection and preparation of the donor '95 Mustang GT chassis. This month, we start building it. If you missed Part 1, you should know that the CMC is a bucks-down series designed as the ultimate grass-roots level of road racing. And even if you don't plan to build an all-out racer, most of the build will transfer to any Fox Stang. With that said, we'll get started.

Last month we did the deed of stripping and repainting the interior and front clip from the firewall forward, so now we can begin the fun part of bolting on the suspension and brakes.

The CMC rules make selection of chassis and braking components relatively simple, as the modifications allowed are minimal and require retaining mostly stock parts and geometry. Major components that can be upgraded are shocks, springs, and bushings, but suspension geometry and major control points and components must remain stock. With a few years of experience in the class with our previous car, we had a solid baseline setup already figured out and knew just where to turn for the needed parts-Maximum Motorsports.

Most readers are familiar with Maximum's full-on road and drag race suspension components consisting of tubular K-members, coilovers, and torque arms-but none of that stuff is legal in CMC. If you dig a little deeper into Maximum's catalog, though, you'll see that the company has a starter suspension kit called the Road & Track Box that is ideally suited to CMC. Consisting of upgraded shocks, springs, caster-camber plates, aluminum rack bushings, a bumpsteer kit, K-member braces, upgraded control arm bushings, a solid steering shaft, and rear lower control arms, these parts form a solid suspension foundation and are essentially a turnkey suspension system for a CMC Mustang.

You can tailor the spring rates and shock selections to the intended use of the car, and for our race-only setup, we selected MM's race-valved Bilstein front struts and Bilstein heavy-duty rear shocks to match our 1,000-pound front and 250-pound rear spring rates. Installing the parts is pretty straightforward, and the photos and captions hit the highlights. We'll get into suspension setup and tuning in future installments.

Next to handling, braking is high on the list of items that must be dialed-in to achieve success in road racing, and there are several good options for brakes on a CMC Mustang. The rules limit choices to 12-inch rotors and twin-piston front calipers with a maximum of 40mm pistons. The most common parts used are '94-'04 Cobra/Bullitt/Mach 1 calipers combined with 12-inch rotors in the front and 11.65-inch rear rotors, combined with single-piston rear calipers from those same applications. Other options include the stock '94-'98 single-piston front and rear calipers, or the '99-'04 GT twin-piston front calipers. Fox chassis Mustangs have an even wider selection of available braking components, but the most common solution is to convert to SN-95 front spindles and five-lug axles to allow using the late-model brakes and increase the selection of wheel options. This swap has been well-documented over the years in print and on the web, so we won't get into it here.

Next month we'll wrap up the construction portion of the project with the engine, drivetrain, wiring, and safety components. After that, we'll be ready to go racing!

All About Spring Rates
Aftermarket suspension springs are generally described with either a linear or progressive spring rate. This indicates the pounds of force required to compress the spring one full inch of travel. For example, a 1,000 lb/in linear spring requires 1,000 pounds of force for every inch it is compressed. A progressive-rate spring such as a 750-850 lb/in spring has a variable rating to increase rate as the suspension cycles the spring. Typically, the progressive spring acts in its lower rate range when it is close to the car's static ride height, and increases in stiffness as the suspension is compressed. Progressive rate springs are popular in aftermarket suspension tuning to give a firm but comfortable ride while cruising, with the higher rate coming into play when the car is driven harder to reduce body roll in cornering. Linear rate springs are commonly used in racing applications, either with coilovers or in the stock location, because it is easier to tune and predict their performance than with a variable-rate spring.

In addition to spring rate, another important spring term to understand is wheel rate. This is the effective spring rate the wheels and tires see as the suspension applies leverage against the spring. In basic terms, the farther the spring is located from the centerline of the wheel, the more leverage the wheel applies against it, and the lower the effective spring rate of the combination. Friction from the control arm bushings, ball joints, and dampers also contribute to wheel rate, but for general comparisons, the wheel rate of the stock Mustang spring location close to the inner control arm pivot point is about 25 percent of the spring rate. With a coilover spring mounted concentric to the front strut, the effective wheel rate increases to approximately 90 percent of the spring rate, hence the much lower spring rates used in coilover applications that still result in higher effective wheel rates. For example, a 1,000-pound spring in the stock location yields a wheel rate of approximately 250 pounds compared to a 450-pound coilover spring that generates a wheel rate of approximately 400 pounds.

Spring Rating Tech Tip
The rate of a coil spring is determined by the diameter of the coiled wire, the inside and outside diameter of the coil, and the number of coils in the spring. The only way to change the spring's rate is to increase or decrease one of those dimensional variables. Cutting full or partial coils from a spring will increase the rate, but a spring that has sagged or lost height with age will only change the static load borne by that coil, not its rate.

Rating A Rollcage
When buying or building a race car, one of the most important things to consider is the rollcage. The 'cage is akin to the foundation of a house-don't buy either one with problems. When assessing a used race car, don't get distracted by long lists of trick goodies and speed parts, that stuff all bolts on and off-a rollcage is forever, good or bad. If you are building a new car, you've got pretty much one shot to get it right. The first thing to consider about a 'cage design is whether it meets the basic rules of the sanctioning body you intend to race it in. If it has all the minimum required tubes, and is made from the correct material (some groups require at least DOM mild steel), next check to make sure it is legal for the intended class. Many classes restrict the points where the rollcage can be attached to the chassis, so make sure you won't have to cut any bars out to meet the rules. It can be costly to adapt a car built for drag or circle track racing to road racing, as there are often big differences in rollcage standards that make crossing over difficult. For example, drag racing rules typically differ greatly in the design of side impact door bars and diagonal supports in the main hoop, as well as allowed material. Finally, try to assess the overall quality of the construction. Are the bends well made, do the tubes fit tightly to the interior, and do the welds look like they were done by a pro or a guy with a tube of bathtub caulk?

Avoiding Common Rollcage Problems

  • Wrong material: SCCA and NASA require all newly constructed rollcages to be built with drawn-over-mandrel (DOM) mild steel, and will not certify rollcages made from electrostatically-welded (ERW) tubing, which is common in street and drag cars.
  • Wrong size tubing: Most sanctioning bodies specify a minimum outside diameter and tubing wall thickness based on the minimum weight of the car. Make sure the rollcage meets these specs.
  • Missing or illegal bars: Some classes restrict rollcage designs to a maximum number of chassis attachment points, and they may have specific rules regarding the construction of main hoop braces, door bars, diagonals, roof and dash bars, and other features. Make sure you have a rulebook with you when inspecting an existing rollcage or getting a quote from a builder.
  • Poor weld quality: Not only is bad welding as unsafe as it is unsightly, if it's really bad, your car may not pass tech. A word of advice: Building rollcages is not a good way to learn how to weld.
  • Incomplete welds: This is one that trips up a lot of people. NASA requires all welds to be completed 360 degrees around the circumference of the tube; SCCA only requires 270 degrees.
  • Poor design or construction: Even if a rollcage meets all the basic rules and specs for its intended class, there is a big difference between one that is really good and one that is just adequate. In addition to safety, chassis rigidity is a beneficial side effect of a good rollcage.
  • Poor condition: Beware of rust or heavy corrosion, or signs of previous impacts.