Photos and Illustrations Courtesy of The Brakeman, Inc.
The braking systems in various forms of racing differ significantly. Dirt cars, such as sprint and late models, vary greatly from asphalt vehicles, such as road race and NASCAR. Drag racing is especially unique and has its own special requirements.
In most forms of motorsports the repetitive use of the brakes creates a significant heat buildup. The longer the event, the greater the buildup, hence rotor and caliper size, pad compound and other factors take on significant importance. The type of racing also impacts which elements take on added importance.
In drag racing brakes have been an overlooked element for far too long. Recently, several factors have become apparent, forcing this oversight to be addressed seriously. Most drag racing tracks were built in the ‘90s, ‘80s or even earlier when speeds were dramatically lower. Shutdown areas, as you can imagine, were much shorter. Combine that with the tech requirement for parachutes over a given speed, and brakes were considered important for staging and little else. Much has changed in the past few years, and there have been serious consequences as a result, forcing an examination of the brake requirements to return maximum safety to the sport.
Let’s examine brake system and see how various components affect the overall performance of the system as it relates to drag racing. First, we begin with the input force. This relates to the driver either stepping on a pedal or pulling the brake lever to actuate the master cylinder to create line pressure. This line pressure is then moved through the brake lines to the calipers to create clamping force. The clamping force acts on the brake pad to clamp the rotor, which, in turn, slows the tire from turning through friction with the track surface.
That all seems pretty simple and should provide a steady, predictable stop, but let’s take a look at what happens when various components are either left out or fail to do their job.
Input force is generated through a pedal or lever, all of which have a ratio. This ratio is the distance from the pivot to the center of the pedal/handle, compared to the distance from the pivot to the master cylinder pivot. This ratio acts in unison with the master cylinder bore size to create line pressure. Line pressure is what makes the caliper work. The higher the line pressure to the caliper, the harder the caliper works. Well, you might be wondering if it’s that simple, why don’t we just dump more pressure into the caliper to make it work harder? The answer to that question is simple physics. We are limited by the travel of the pedal/lever and the size of the master cylinder. In the formula, line pressure equals input pressure multiplied by pedal ratio divided by the surface area of the master cylinder. We can only go as small on the master cylinder as the caliper requirement for fluid.
To clarify, when you step on the pedal with 100 pounds of force (which is quite easy to do when you are sitting down) multiplied by the 6:1 pedal ratio, you have 6 multiplied by 100 pounds or 600 pounds of force on the master. If the master cylinder bore size is 1 inch, the surface area of the master is .785 square inch. Dividing 600 pounds by.785 square inch theoretically yields 764 pounds of line pressure. If you have a particularly rigid caliper, then you have the ability to reduce the master cylinder bore size since the caliper will not require as large a volume of fluid as a more flexible one. In this example, a 7/8-inch master cylinder, which has a surface area of .601 square inch, with all other factors remaining the same, will up the line pressure to 998 pounds from the 764 pounds previously noted. The final element of the equation for torque is the coefficient of friction, or Mu of the pad. Pad coefficient of cold friction can vary from .3 to .6 or more, depending on the materials and operating temperature. More on this later.
Normally, the resultant torque from even the lower line pressure would be sufficient. However, other factors in the system reduce the actual pressure due to various conditions, such as frictional binding in the pedals, growth of the brake lines, bending or flex of the calipers, etc. Hence, there is a limit to how small the master cylinder size can be, which limits line pressure. This is why your choice of components is so critical.
For example, plumbing your car entirely in braided line will increase the fluid requirement proportionally for every inch of braided line in the car, as it will swell far more than solid line. This also causes a “hysteresis” effect, meaning there is a delay in the very important release characteristic of the caliper.
When you combine this delay with the fact that most calipers flex, the resultant drag caused by this delay discourages anyone from running an extremely critical valve. This valve is called a residual valve. Its purpose is to maintain residual line pressure so that the calipers are ready to react on the next application. Because the master cylinders on drag cars are often mounted below the height of the caliper position, brake fluid will roll back downhill to the caliper while the car is traveling down the track. Combined with the severe vibration found on the higher horsepower vehicles, there is a disastrous loss of fluid in the proper area of the system at a critical time (at the finish line and shutdown).
Racers often try to overcome this problem by simply mounting the reservoir above the height of the caliper to stop the fluid rollback. This alone will not stop the serious vibratory effects on high horsepower cars. The fluid will still move away from the caliper itself. A properly functioning residual valve is a key component in the brake system.
Consider that the rotor does not stop the car. The rotor is a heat dissipater and a lever only. If the caliper is unable to clamp the rotor, then it makes little difference what material or what diameter it is. If you are converting to carbon, please note there is drawback to carbon rotor/carbon pad systems that merits consideration. Carbon parts have very poor cold friction and require warming. This could cause low torque and a resultant line creep, when attempting to stage the car. It also means that there will be a delay in the deceleration rate at the finish line due to the time it takes for the pads to come up to temperature to increase the torque. A good comparison of this principal is the torque output of an engine getting better as it approaches its optimum rpm.
A finish line speed of just 200 mph is 293 feet per second. At this speed a vehicle can travel more than 1,000 feet through shutdown in four seconds. At 300 mph, feet per second increases to 440. This means that in three seconds, if you apply the brakes exactly at the finish line, you will travel ¼-mile or the equivalent of the race in less time than it took you to get there! On tracks where every foot of shutdown area is critical, this clearly could create a disastrous condition.
Since the key component of the brake system is really the caliper, it is important that this component be capable of the demands placed upon it. As a guideline, if you can see the caliper flex during bleeding, your caliper is too weak. A simple way to verify is to use a pair of vernier calipers over the center of the brake caliper and have someone step on the brake; flex of more than .020 is an indication of an inadequate caliper. The aforementioned combined with the loss of clamping force from weak calipers flexing (thereby wedging the brake pad into the rotor) results in a major reduction in deceleration rate. This wedging of the pad due to flex also creates inconsistent application, causing bounce or shake, which reduces the contact patch and time and increases stopping distance. This actual rate of deceleration is far different from what the basic line pressure math in the beginning of the article would indicate.
Simply inserting an exotic brake pad with a high Mu does not fix the larger problem; it is only a single factor in the equation. In many cases, this high torque level is only achieved at high temperatures, and low temperature performance degrades accordingly. There is no single magic bullet that can cure the ills of a weak or improperly designed brake system. All facets of the system must be designed to work in concert.
Obviously, all components of the system must be in proper operating order to provide a safe system. The factors discussed in this article are offered as a guideline to help ensure that the potential pitfalls are dealt with so that when you need the brakes, they will be there.
For years, racers have either used components based on price and weight alone, or failed to monitor the condition of the components of the system and gradually allowed the points listed above to reduce the overall effectiveness of their brake system. Most drag racers running high-speed cars place the bulk of the requirement of stopping the car on the parachute. Do not be misled! Every car, even over 300 mph, should be able to make a safe and complete stop with no parachute at all, on every pass. Parachutes often deploy incompletely or fail to open at all. This alone should alert us to the importance of the previous statement. A good brake system can make you a better racer, improve your 60-foot times, save your equipment from potential disaster at high speeds and most importantly, save your life. Don’t settle for less.