A properly configured braking system has the same impact on your lap times as adding more horsepower.
However, the development of a balanced and effective racing brake system for track use must take into consideration an overall design of the complete brake system. The word “system” is no understatement for your racing brakes’ effectiveness. This system is only achieved through the proper grouping of the components starting with the brake pedal and going all the way to the tires on the ground.
The most significant difference between developing a racing brake system for road courses and oval tracks is that you need to consider not just stopping power and balance but also the much more complex application of side to side and recovery balances for those racing among the curves.
“Basically when it comes to creating the proper pressures between brakes and calipers, we do quite a bit of sizing of proper master cylinders for customers,” says Jason Wahl, CEO at Tilton Engineering Company. “To determine the master cylinder sizing, we have to go all of the way back and do calculations. it’s an equation with 34 variables in it, so it’s pretty complex.”
The Starting Point For System Design
The driver, suspension and track can be determining factors at the starting point of your brake system. Surprisingly, most racers and suppliers will state that the best overall system starts at the tires, which may seem odd, though it’s the most effective strategy.
A common saying that still rings true is, “It’s not the brakes that stop a car, but the tires.” James Borner, Design Engineer at Performance Friction Corporation (PFC Brakes), assists in shedding light on an effective overall brake system.
“What you need to focus on is the brake system as a whole,” Borner says. “You must look at the pedal assembly, brake lines, and wheel components. The pressure generated is the central operation of these component specs. For us, the brake pad itself is one of the more important pieces of the brake system. When the pad itself is working as intended, road race brakes have three phases.”
“You have your initial application of the brakes and then the applied deceleration of the race car over a given distance,” Borner continues. “The third phase is the characteristics of the brake release as you accelerate. That is the most important in my view. This third phase determines how quickly you go from braking and generating friction, to being drag free so that now your power can go back into accelerating the vehicle forward.
Transition From Braking To Acceleration
The transition from the braking phase to accelerating is extremely important, especially in road racing where you’re trying to gain an advantage when accelerating out of a curve. Borner stresses that this acceleration point is where passing occurs, more often than not. The PFC zero drag calipers that Borner talks about is in relationship to brake recovery. This illustrates one of his key validations.”
A lot of the passing you see in the braking zone does require all three phases of the brake event to be successful, with the initial brake applied and deceleration being the most critical. – James Borner, PFC Brakes
“The first advantage is the ability to brake later than the competitor,” Borner explains. “This requires a system that is quick to react and sustains the higher declaration rate. Again, tires are extremely critical here as well. If successful, then phase three comes into play; that essentially is where the pass is made with better late braking and then accelerating out of the corner quicker allows the pass to stick.”
Border adds, “Sometimes this passing maneuver does not stick but not necessarily due to any issues from the braking system. If you brake too late and do not have the correct line your speed slows too much and you cannot recover, and the other driver will just pass you right back.”
“Our zero drag calipers are more of a reference to what PFC thinks is a better braking system for circle track and road racing,” Borner claims. “We designed a very stiff brake caliper, so you don’t waste energy with deflection and deformation as you initially apply the brakes. If you have a caliper that flexes even slightly while you are releasing brake pressure, the caliper is moving back down into its rested position. Then, the actual release of the brake pads takes longer.”
As two companies that complement one another, both share the same philosophy when it comes to designing an overall racing brake system. With Tilton Engineering offering a wealth of master cylinders, racing pedal assemblies, bias bar components and more, Wahl describes their extensive phone services.
Extensive Technical Design Assistance
Wahl describes the complex worksheet including 34 variables that Tilton Engineering uses to calculate master cylinder sizing. “It goes all the way back to looking at the vehicle dynamics and calculating the amount of weight transfer they get under braking,” he says. “Our calculations are not made on a static condition but a dynamic condition.”
We do a large number of master cylinder sizing for racers. We don’t make calipers or rotors, but we do a lot of sizing for calipers and master cylinders. That is 95-percent of our incoming tech phone calls. – Jason Wahl, Tilton Engineering
“Imagine a tipping point or pivot point of the contact patch of the front tires and determine the amount of grip in the tire,” Wahl continues. “That is why tire compound is factored into our worksheet. You need to know the weight of the vehicle, the wheelbase of the vehicle, the size of the tires, and obviously the size of the other braking components.”
“Our specialists will input vehicle numbers on weight, balance, center of gravity; all of the technical details from the vehicle,” Borner describes. “We then calculate what we think is the optimum starting point for an effective brake system whether it is six-piston front, four-piston rear or two-piston rear calipers and the related pads.”
Wahl explains that typically with most cars running on pavement, whether circle or road race car, it’s going to work out with the front brakes doing about 70 percent of the work. So, all components upstream from the calipers including the racing pedal assemblies, master cylinders, and bias adjusters must be sized around that basic theory.
“One of the worst examples of not proportioning front and rear brakes are those people, out of simplicity sake, who put the same size caliper and rotor on both the front and back of the car,” Wahl says. “Yes, in theory, one can gauge front/rear master cylinder sizing enough to balance the forces, but thermally that system is never going to be very balanced.”
“The front calipers and rotors in this kind of incorrectly designed system will always heat up the front more than the rear,” Wahl continues. “Since the coefficient of friction of a brake pad changes with temperature, they might be balanced in the scenario of brake pressure and force, but they won’t operate properly due to heat fade.”
Determining Brake Line Pressure
Many brake product specialists recommend your brake pedal/master cylinder assemblies generate pressure in the 1,200 psi range as the ultimate line pressure under severe braking conditions. Such variables including the strength of the driver’s legs, the brake pedal ratio and the volume of the pistons in your brake calipers play into your choice of your master cylinder piston diameters. Again, each of our companies interviewed recommend using their help for your master cylinder choice.
Since nearly all circle track and road race applications utilize independent master cylinders between the front and rear brakes, the individual master cylinder bore sizes vary between each. With the overall design of your components in place, you pretty much have that fundamental 70-percent forward balance of braking power in place.
How leading racers fine-tune that balance is where another critical component comes into play. The definition for brake bias is the “The front and rear distribution of braking power. The brake bias should match the traction of the vehicle while braking.” The adjustable bias brake pedal assembly is your mechanical application for adjusting that bias.
With individual master cylinders and an adjustable brake pedal assembly, a mechanical balance bar is located between the brake pedal and each front and rear master cylinders. Wahl recollects, “Back in the 1950s, the British Cooper formula cars had balance bars and two master cylinders. That fundamental mechanical linkage is still used today.”
“Tilton’s 600-series pedal assembly uses a very conventional bulkhead mounted master cylinder with two bolts that bolt the body of the master cylinder to the pedal assembly,” Wahl describes. “Then a threaded balance bar will move side to side with adjustment to change the force from the pedal to each master cylinder.”
“As you move the balance bar bearing closer to one master cylinder, you’re putting more load on it and less load on the opposing master cylinder,” Wahl finishes. “That’s how you’re getting front/rear bias through the balance bar.”
The Tilton 900-series takes the bias adjustor to a much higher level. With a gimbal style balance bar connected to pivot-mount master cylinders, this design eliminates brake migration through the braking zone.
“In an extreme road racing or oval track application, the geometry and the friction in extreme braking create what we call brake bias migration,” Wahl says. “When a driver gets hard on the brakes, you just initially slow the car down. As the car slows, you modulate your foot force off the brake pedal to complete your cornering. With that, the brake bias shifts more and more to the rear of the vehicle.”
Adjusting Brake Bias On The Fly
“With a traditional bias adjuster, as much as seven- to nine-percent change in brake bias takes place. It depends on how far off-center you have the balance bar adjusted and the sizing of the rest of the system”, Wahl finishes. “With the new system, everything pivots off a fixed point, and everything is on bearings, so it eliminates a lot of the friction and the geometry issues you have with that changing bias.”
The adjustable pedal assembly is a terrific application, but what about when race day arrives? Ever changing variables like track conditions, tire wear and brake heat may affect your braking in mid-race. Almost as quickly as the adjustable pedal assembly came about, the advent of a remote bias adjuster was invented.
The Tilton remote bias adjuster allows the driver to “tweak” brake bias from the dashboard. Simply described, it is a cable connecting between a knob within driver’s reach and the threaded bias adjuster on the brake pedal assembly.
Another addition focusing on brake pressures is the addition of brake pressure gauges. All our brake pressure, volume, and related theories rely on utilizing a brake pressure gauge. Think of it this way: You would never have a racing engine without an oil pressure gauge.
Though the bare minimum would be a pressure gauge connecting from the caliper bleeding orifice to the gauge, that inexpensive option is a must for the toolbox to measure and diagnose any brake problems. Race cars running in professional series will utilize active brake pressure gauges as a full-time component in their system.
Using a high-pressure brake gauge on each front/rear system is an absolute method to measure brake bias and monitor any changes ranging from a single race day to an entire season. Most high-quality gauges are liquid-filled and measure up to 2,000 psi.
One instance pointed out by Borner is the application of monitoring brake pressure to diagnose possible problems. “We stress specifying the proper brake pad for your particular type of racing; even your driving style can change the brake pad you use.”
“One thing you’ll chase with any kind of flex in your rotating brake assemblies is brake pressure,” Borner continues. “If your pads are soft or if your caliper flexes, this could be due either to a design problem or wear. After two to three seasons of use, a heat cycled aluminum caliper and bracketry get softer.“
“If you’re chasing increased brake pressures to do the same job as previous races, it is an indicator of flex or fluid problems,” Borner expands. “You could have a fluid compressibility problem, air in the system, component flex or a pad issue.”
Diagnosing brake performance can be determined by working from the brake rotor backward. “It’s really a function of your brake pressure and your pad friction that generate your deceleration,” Borner says. “Stable brake pads and disc rotors can hit a certain deceleration friction rate that is stable; then the next step back is how is my caliper doing? Brake pad chamfering for caliper flex is your next sign.”
The function of race braking has one common denominator: heat. Heat cycles can slowly impact even the best system. It could be the simple effect of brake fluid that has seen too many heat cycles or additional force from worn components causing heat from higher pressures needed to get the braking job done.
Power and traction may be high on the list for winning races, but a well-tuned brake “system” can get you out of the corner quickly. I’m referring to that same corner where most good passing of your competitors is made, so the best brake is as good as horsepower.