February 1, 2009

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.