Modern braking systems tend to have disc brakes on the front and drum brakes on the rear. More-expensive models have four-wheel disc brakes. Disc brakes do a great job stopping a car and are simplistic in design and maintenance.
Another method to increase the braking in vintage muscle cars is to add disc brakes to the front wheels with parts intended for a similar-model car or from a kit designed by a manufacturer to work with the model of car you are performing the upgrade on. Since the front brakes do 75 to 90 percent of the braking, changing from drum brakes to disc brakes on the front is one of the most effective braking upgrades.
Stability, Steering, and Stopping Distance
Tires are literally where the rubber meets the road. Tires are the link between the vehicle and the road surface, and they are the final piece of the braking system. Tires actually stop the vehicle and play an important role in the change of speed and direction. Because these circular devices are involved in transmitting braking, motion, and lateral forces, any one of these forces can and will affect the others. The Motorcycle Safety Foundation (MSF) teaches its riders about this concept in what it calls “the traction pie.”
The MSF has a traction pie graph that represents the total amount of traction that a tire can have. The pie-like segments define areas for acceleration force, braking force, turning, and a reserve. The four segments of the traction pie are ever changing, shrinking, or growing, depending on the action happening at the time. For example, under strong acceleration, that segment of the pie will be larger. The braking segment will shrink to nearly nothing, and turning will probably be somewhere in size between the acceleration and the braking segments. The reverse would be true if the condition was hard braking instead of hard acceleration.
The MSF goes on to explain that the total traction can be consumed by those three segments when they consume all the reserve. After that point, the tires will lose traction. In this explanation of traction, brakes play a key role in stability, steering, and stopping.
In addition to the various brake components, aftermarket manufacturers often produce their own lines of spindles and steering arms too. Spindles can be purchased that raise or lower the ride height of a vehicle but keep the steering geometry correct.
In a traction pie concept from the Motorcycle Safety Foundation (MSF), a circle that represents the total amount of traction available is divided by forces that consume that traction. Acceleration and braking take a large part of the available traction. Turning consumes some of the traction, and whatever is left over is held in reserve. According to the MSF, a reserve should always be maintained. If the reserve is fully consumed, a loss of traction will result in a skid or a spin.
Engineering Details of Braking Performance
The engineering behind brake performance is much deeper than most motorists realize. A great number of factors need to be considered when designing a brake system for a specific vehicle. The vehicle itself figures into these equations.
“The effectiveness of any brake system depends on factors like the weight of the car, braking force, and total braking surface area,” said Mark Chichester of Master Power Brakes. “You have to factor in how efficiently the system converts wheel motion into heat and how efficiently the built-up heat is removed from the brake system. The buildup and dissipation of heat go a long way toward explaining the major differences between drum and disc brakes.”
Drum brakes are at a clear disadvantage when it comes to dissipating heat. As drum brakes get used hard, the brakes fade because of excessive heat buildup in the drum. The drum absorbs heat until it reaches a saturation point and is unable to absorb additional heat.
With disc brake systems, the rotors are not confined in a tight space; they are exposed to the outside air that provides a cooling effect and helps combat brake fade. Most basic disc brake conversion kits have rotors that are made of cast iron.
Cast iron is inexpensive and has great wear properties. Cast iron is also heavy, so those enthusiasts looking to gain performance by losing weight may want to consider rotors made from other materials, such as ceramic composites. Kits with these ceramic composite brakes are engineered to be heat resistant and able to handle higher compressive loads at higher temperatures.
Force Conversions
To understand how the force from stepping on the brake pedal is converted into pressure at the brake’s friction pad or shoe, some elements of the common components in the braking system must be explained.
The force of a driver stepping on a brake pedal to stop a car traveling at a high rate of speed would need to be tremendous if the force wasn’t multiplied. For instance, a young soccer mom in a 4,000-pound sport utility vehicle (SUV) running down the highway at 70 mph would need to use all 120 pounds of her body weight along with both feet standing on the brake pedal to even start slowing down the vehicle. Using that illustration, it is obvious that multiplying the force applied to the brake pedal is critical when engineering a braking system.
Heat is the enemy of brakes, and one of the key advantages to using disc brakes is that they are more effective in the heat due to their ability to shed heat better.
The vacuum-operated brake booster works today much as it did 50 years ago when muscle cars ruled the road. Drawing vacuum on the front of the diaphragm removes atmospheric pressure. The rear chamber is vented to the atmosphere and the pressure multiplies the force a driver applies with the brake pedal. This cutaway shows where the diaphragm placement in the brake booster is and how it works in relationship to the brake pedal actuator rod.
How the force is amplified from the driver’s input into braking force is referred to as brake system gain. This gain is done mechanically and through vacuum assistance. It all starts with the driver stepping on the brake pedal. Without extra exertion, an average-size person delivers about 70 pounds of force on the brake pedal pad. The brake pedal is really a mechanical lever, and the positioning of the pedal pad in relationship to the mounting point (where the pedal pivots) and the point where the pushrod is attached to the master cylinder is how the force of the driver’s action is multiplied.
How Upgrades Affect Vehicle Performance
There are several things to consider when planning a brake upgrade. These include gain, modulation, heat capacity, cooling rate, and weight.
Gain
Gain is a fancy term for multiplying the mechanical advantage. These gains can come from changing the pedal ratio, adding a brake booster, upgrading to a larger caliper piston size, or changing the size of the rotor. Larger discs allow for more brake torque because the brake pad will apply pressure at a larger radius, while larger caliper pistons (or more pistons) result in more area of applying a specific pressure.
One of the quickest and easiest ways to improve gain in the braking system is to change the brake pedal. The braking ratio can be changed with an increase or decrease of distance between the pedal’s hinge point and where the master cylinder piston connects to the pedal.
