In the world of automotive engineering, original equipment designs need to represent the best available compromises between performance, durability, cost, weight and ease of production. “Jack of all trades, master of none” is a pretty good description of OE designs.
When it comes to our clan of racers and hardcore enthusiasts, however, we can modify vehicles to better suit our purposes. We aren’t constrained by the many compromises that manufacturers must face. Fortunately, the automotive aftermarket is brimming with options to satisfy our need for speed—and our need to slow down.
In the braking sector of the aftermarket, there are dozens of companies supplying hundreds of products claiming to make our cars stop shorter. Sifting through marketing hype to get to solid engineering can be a daunting task. Add to this the nearly unlimited possibilities of salvage yard hop-ups discussed daily on the various online message boards, and you quickly have too many possibilities to count.
Some of these options will help lower lap times. Others will make you curse the day you stumbled upon them. We’re here to help.
When it comes to brakes, a component upgrade can sometimes be a system downgrade. It all comes down to how the new part affects the system in three areas: gain, balance and compliance.
No Pain, No Gain
In team sports, the success of the team is typically expressed in terms of its win-loss record. Within the team, each player is analyzed in terms of how his personal effort helps the team achieve its goals.
The specific metrics are almost unlimited—goals per game, plus/minus goal differentials, shooting percentage, yards per carry, earned-run average, and on and on—but the point of these metrics is always the same: to quantify the contribution of the individual to the success or failure of the team.
Brake systems can be described in a similar manner: Each component contributes to the performance of the system. While the specific units can be different for each component, in general we will refer to the “gain” of a component as the output of that component divided by the input it receives. Likewise, the system gain is the product of all component gains.
A Balanced Diet
Simply stated, a balanced brake system is one in which the front and rear brakes will achieve wheel lockup at the same time. If the front brakes tend to lock first, the system is said to be front-biased, and if the rear brakes tend to lock first, the system is said to be, you guessed it, rear-biased.
An important note here: Because of dynamic weight transfer, it’s impossible to achieve balance at all decelerations on all surfaces without a closed-loop control system, but that’s a discussion for another day.
Strength Under Pressure
Compliance is defined as “extension or displacement of a loaded structure per unit load.” This definition is actually appropriate for the discussion of brake systems because every component within the system exhibits some compliance. This is because the brake pedal creates a force input to the system.
Because the function of the brake system relies on the conversion of mechanical force to hydraulic pressure and then back to mechanical force, we describe the compliance of the system in terms of the volume of fluid required to achieve the target pressure.
Since we know the bore of the master cylinder that generates this pressure, we can relate the volume to master cylinder piston travel. We can take this one step further and, through a knowledge of the pedal ratio, relate master cylinder piston travel to brake pedal travel.
Now that we have laid out some basics, we can start looking at some problem areas.
Big Front Calipers
Nothing makes people say “ZOMG!” quite like big front calipers, especially when they’re painted bright red. But big calipers do more than just look hot—they provide more clamp force on the rotor. More clamp force means more brake torque, which shifts the system balance toward front-bias. Big front calipers also require more fluid volume, which means longer pedal travel and less fluid reserve for when pad fade strikes.
So, unless your system is rear-biased, big front calipers are going to hurt, not help, your system balance. And if your master cylinder was marginally sized before the swap, you could wind up putting all your pedal force into the floorboard rather than into generating higher line pressures.
Real Needs
Big calipers usually save weight because they’re often made from aluminum instead of iron. It’s appropriate to use them in two main situations: when you need less unsprung weight, and when you need more clamp force due to increased piston area. Because of the variety of aftermarket caliper sizes, not all aftermarket calipers will give an increase in piston area compared to the existing caliper.
Big Front Rotors
There are as many sources of big front rotors as there are sources of natural male enhancement. At least the big front rotors give a real increase in diameter.
Assuming all other system parameters are held constant, increasing rotor diameter increases front brake thermal mass and front brake torque output. Think carefully—does your car really need that?
If you don’t see heat-checking or other thermal distress in your existing rotors, then you don’t need more thermal mass. If you do see evidence of overheated brakes, there are a couple of things you can try—assuming they’re not prohibited by racing rules—before dropping the long green on a set of big front rotors.
First, try removing the dust shields found behind each rotor. (Be sure to protect your tie rod and ball joint dust boots from the heat.) Second, add cooling ducts. Cheap and effective cooling ducts can be fabricated from stuff sourced from your local big-box home improvement center.
If you can lock your front brakes, you don’t need more torque. If you really do need more brake torque but not more thermal mass, perhaps a more aggressive friction material is what the doctor ordered. Keep in mind, however, that an increase in front torque without a similar increase in rear torque will make your car more front-biased, and this will hurt stopping distance if the brake system was balanced before the change.
Also keep in mind that if the oversized rotor is heavier than the OE part, the increased unsprung mass and increased rotational inertia will make the car less responsive to throttle, steering and brake inputs.
Real Needs
Big rotors are okay when evidence supports their use—for example, when the existing rotors show heat damage even after ducting has been added. As explained by the formula for kinetic energy (KE=1/2mv2) a 10-percent increase in vehicle speed equals a 21-percent rise in kinetic energy since we’re dealing with the square of the speed.
If you’ve souped up your car to reach higher speeds than originally designed, its stock brakes often won’t cut it anymore. Big rotors might be required.
The key is to keep the system balanced. If larger front rotors are installed with no other changes, the system will become more front-biased. This is because the caliper will need to be moved radially outboard to accommodate the increased rotor diameter. Doing so increases the effective radius, which means more torque is generated by the same clamp force. In order to keep the system balanced, an adjustable proportioning valve might be needed to increase the pressure of the rear calipers.
Cross-Drilled Rotors
Cross-drilled rotors are another brake component that generally falls into the natural male enhancement category. The popularity of cross-drilled rotors is a carryover from the bygone days when outgassing vapors from the pad material reduced the friction level between pad and rotor.
The holes gave this vapor layer an escape path, allowing the friction material to remain in contact with the rotor. Modern friction materials don’t outgas like that anymore, and drilled rotors don’t provide the performance improvement they once did.
In addition, a drilled rotor has less thermal mass than an undrilled rotor of equal dimensions. Each hole is a prime location for cracks to form due to thermal and mechanical overload.
There are degrees of severity when it comes to stress concentrations, depending on both the type of concentration and the type of loading that the part sees. When a load is applied axially to the hole (as when the caliper squeezes the rotor), it’s not as bad as when the load is applied radially to the hole (trying to turn the round hole into an oval hole). On a related note, a chamfered hole has a lower stress concentration factor than a straight hole.
Again, ask the key question: Are these holes—and their potential problems—what the car needs?
If some protection against outgassing is required, or you want something sexier than plain rotors, go with slots instead of holes. With this route, compromise in rotor strength is negligible compared to holes.
Real Needs
Even though cross-drilled rotors have some downsides, we still see them on both race and street cars. If the rotor has been designed with these holes included—in other words, the manufacturer didn’t just take a hole punch to a standard rotor—then generally it’s safe. We have to trust that firms like Porsche and GM do their homework before speccing cross-drilled rotors for their latest supercars.
In a top-tier racing environment, professional race teams know exactly how far they can push a set of rotors, whether drilled or not. For example, the rotors seen glowing at Daytona are probably not the same ones used at the next race. When frequent replacement is part of the plan, the risk of part failure is lowered. Plus, the reduced mass offers performance gains that are worth the cost of frequent replacement.
Race Pads
In a track environment, a braking system is subjected to multiple high-g decelerations from fast speeds. Plus, there’s very little time in between decels for cooling. The resulting high temperatures will destroy OE pads in short order.
As a result, friction material suppliers have developed pads that maintain a high output at elevated temperatures. Because these pads reach their peak effectiveness at temperatures much higher than what is seen in typical commuting, they are a poor choice for a street car. Since these pads are so abrasive, they are sure to disappoint commuters with decreased rotor life and embarrassing brake noise. They also create dust that can destroy the finish on that prized set of wheels.
Real Needs
High-performance brake pads suited for autocross and street use are available from many manufacturers. These pads won’t survive an endurance race, but they are designed to handle the temperatures seen during an autocross run while providing good feedback. Many are also rotor-friendly and not too dusty.
Brake Booster
Here’s an idea commonly discussed online: Want better brakes? Then toss the booster. We’d rethink that move.
In our brake math sidebar, the brake booster provides a 6:1 gain in a system with an overall gain of 96:1. Take away that 6:1 multiplier, and the system gain is reduced to 16:1.
Even if it were practical to increase all other components by 25 percent (hint: it’s not), the result is a system gain of only 50:1, which means Joe Driver will be required to exert twice the original pedal force to get back to the original boosted deceleration. That might be okay for a few laps, but Joe is bound to get worn out in an enduro.
Real Needs
If the high overlap of radical cams or the low manifold vacuum of wide-open throttle operation are limiting the helping force of a vacuum booster, consider adding an electric vacuum pump to supplement (or replace) the engine as a vacuum source.
Boosting Your Knowledge of Boosters
Brake boosters come in two types, each named for the source of its helping force. First, there’s the common vacuum booster, which uses engine vacuum on one side of a large diaphragm and a controlled vent to the atmosphere on the other side to generate helping force (force = pressure x area). Then there’s the less common Hydro-Boost—that’s actually a Robert Bosch trademarked name—which uses pressurized fluid provided by the power steering pump to generate helping force.
- Vacuum Pros: No parasitic loss on drivetrain; familiar pedal feel.
- Vacuum Cons: Packaging; low manifold vacuum equals low helping force.
- Hydro-Boost Pros: Easier to package; consistent helping force.
- Hydro-Boost Cons: Parasitic loss; added hoses equals added potential failure modes; pedal feel is nothing like a vacuum booster; one thrown belt equals no helping force.
Changing Master Cylinder Bore
When installing larger calipers, or when swapping from drum brakes to discs, the new components will require additional fluid volume from the master cylinder. There are two ways to get more fluid from the master cylinder, and each has its risks.
The first is to keep the original MC and hope that the pedal doesn’t hit the floor before maximum deceleration is reached. The second is to increase the master cylinder bore and hope that the driver can provide the higher input force this requires.
Aside from installing larger calipers or swapping from drum brakes to discs, it’s unlikely that you’ll ever need to run a different master cylinder. This modification is often discussed online, however.
Mastering Master Cylinders
The master cylinder’s job is simply to convert mechanical force to hydraulic pressure. In order to choose the right master cylinder, it’s important to understand the relationship between bore, stroke, input force, output pressure, and output volume.
Pressure equals force divided by area, so an increase in bore diameter will cause a decrease in output pressure for the same input force. In other words, an increase in bore diameter will require an increase in pedal force to achieve the same pressure. Volume equals piston area times the piston stroke, so a larger MC bore will require less pedal travel to move a given volume of fluid. In short, a larger master cylinder will yield a firmer pedal, but it’s not going to make the car stop any quicker.
Removing Proportioning Valve
The proportioning valve’s job is to decrease the pressure rise rate on the rear brake circuit relative to that on the front. This keeps the system from going rear-biased under high-decel stops, where there’s a lot of dynamic weight transfer from the rear axle to the front. There is a certain pressure level where the proportioning valve moves into action—it’s called the “knee” or “split” point. Removing the proportioning valve increases rear gain at all pressures above the knee point.
If your car really needs more rear brake gain, first look to a more aggressive friction material. Then, if class rules allow, look at an adjustable proportioning valve. One of these will reduce rear pressure rise relative to front, but it will also allow for adjustment of the knee point.
If your vehicle is equipped with ABS or ESC, it’s possible that the brake control system is responsible for rear proportioning. If you disable or remove your ABS (to avoid the dreaded “ice mode” activation mid-corner), you might also remove your proportioning valve without knowing it.
If you don’t know exactly what you’re doing, or if the vehicle will be used anywhere other than a closed course, please don’t permanently disable or remove your ABS. The results can be disastrous.
What Have We Learned?
When changing brake components, you must consider how each component affects the gain, balance and compliance of the system. If marketing hype—or Interweb wisdom—is your only guide, braking system performance can suffer.
However, if you do your homework, make your selections based on what the vehicle tells you, and consider how each component affects the system, then you might find that the improvement in system performance results in lower lap times and helps you get to the pointy end of the field.
Just remember: Despite what the media would have you believe, bigger is not always better.
Patrick Caherty is a principal engineer for TRW Automotive Brake Systems in Livonia, Mich. He’s a graduate of the University of Maryland’s College of Engineering, a four-time GRM $200X Challenge competitor, and a lifetime gearhead. He would like to thank his lovely wife Victoria for allowing him the time to write this article.
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