WC Engineering > Technical Articles > Brakes: Bigger is Better?
Brakes: Bigger is Better?

Most people think first about making their car faster, second to handle better, and last is to slow down faster. Then, soon after someone has had their first or second track event, they realize that the brakes are an area that can have a significant effect on lap times. No matter how fast you can accelerate, you still need to burn off some speed at some point. The more brake power available, the higher the deceleration rate, the less time that it takes to drop the speed. Hence, you can carry your speed down the straight before you brake.

The amount of braking force that is available is dependent upon a few things:
  • Tire traction
  • Brake disc sizes
  • Pad coefficient of friction
  • Caliper clamping force
  • Weight shift
  • Front/rear braking bias
    Tire Traction

    Tires are a significant item in many areas. Choose tires for your own driving needs and be honest with yourself about your needs. Don't buy a set of shaved tires that are just barely street legal as your only tires if you only go to one track event a year. On the other hand, don't buy a tire with a tread wear rating of 240 as a set of "track day only" tires. Your choice should reflect your true needs and if you get serious about doing track events, consider getting a dedicated set of tires. I run Yokohama A-032 tires on the street and track. They have a tread wear rating of 60, last about 3000-4000 miles, and have the best wet and dry traction that I have ever used, but they do have a sound issue. I'm willing to accept the drawbacks of the sound and tread life. Remember to be honest about your choices. For me, I don't believe that it is possible to have too much traction.

    Brake Disk Sizes

    The disk size has two items to address: diameter and thickness. The diameter is easier to understand and easier to see. As a simple rule, the larger diameter of the rotor, the more force that is available to stop a wheel, just like using a longer wrench makes it easier to break a frozen bolt loose. If you kept the same caliper and same pads, but installed a larger diameter rotor, you would get greater stopping power. This power can be referenced in Newton-meters of torque. Factory rotors are sized according to many factors: wheel sizes, calipers used, unsprung weight, tires, price, etc. Initially, it would seem that the largest diameter rotor that can be obtained should be used, but those other factors must be kept in mind. Even, a reasonably larger diameter rotor will provide increased stopping power.

    The brake rotor thickness is more difficult to determine. The thickness of a rotor is proportional to the amount of heat we want to retain and dissipate, two very different properties. A properly designed brake system will take into account the optimal operating temperature of the rotors and the pads. A thin solid rotor will retain more heat than a thicker vented rotor. The key is to choose a rotor that will dissipate the excess heat, but still remain within the working temperature range of the brake pads. Some brake pads operate well while cold, others need some heat before they have any bite. The brake disk thickness must be chosen to match the pads and to match the driving that will be done. A short, tight road course (Las Vegas Speedway, 1.8 mile) may need different pads or possibly rotors. Due to the high demand for braking, more heat will need to be dissipated. We may need to use thicker rotors or higher temperature pads. A long open track (Road America, 4.0 miles) may need lower temperature pads or thinner rotors to order to keep some heat in the system. Different pads again may be needed for street driving. Another approach may to be to use a thick, vented rotor and adjust the amount of cooling air that is available to the rotor.

    Brake rotor material affects its thermal characteristics and friction and wear properties. I'm often asked about the prices of rotors and why some are more expensive than others. It is a simple matter of quality. Good rotors run truer and last longer. For your everyday car, you can go to the dealer and buy front rotors for $120 each, or you can go to your local discount auto parts store and get "the same part" for $20 each. Simple question: does your life depend upon a $20 rotor? Mine doesn't. Good rotors cost good money and my life is worth it.

    Brake Pads

    We may need to carry several pads depending on the use of the car. One type for street driving, another for short tracks, and perhaps another for long tracks. And different drivers may prefer a different "bite" depending on their driving style. As mentioned above, the pads and the rotor thickness can be related. Take a good look at the proper operating temperature for the pads and choose accordingly. Hawk and Performance Friction make excellent pads.

    The coefficient of friction of a pad can change with the temperature. Some pads have excellent cold bite (good for the street) and others have almost no bite until they have some heat in them. The amount of friction available combined with the mechanical advantage of the larger diameter rotor must be matched to the traction ability of the tires. Some other areas to consider are brake dust, noise, and rotor life. You may find a pad the fits your driving style only to find out that they tend to chew up the rotors. Then again you may find street pads that leave an excessive amount of brake dust.

    Caliper Selection and Clamping Force

    Many factory calipers are a single-piston piece. This is good for a simple caliper and they keep the piston inboard and allow good wheel clearance and fewer moving parts for lower service issues. They do have their drawbacks, however. Typically they are made of cast iron so they are heavy and they have some type of "slider" so clamping force isn't always equal from side to side.

    Multiple piston (or "pot") calipers offer more even clamping force than a single piston. The force is distributed across the pads better, yielding a better contact area. Often times when using a four-pot caliper, the total brake fluid volume is less than a single piston piece, so for a given brake pedal effort we can get more clamping force. (P1×V1 = P2×V2) For this reason, master cylinder capacities must be referenced when designing a brake system or changing calipers. On most good 4-, 6-, or 8-pot calipers, the pistons are of staggered sizes which gives more even pad wear. And when selecting a caliper for an application, it is critical that the proper leading or trailing calipers be used. The calipers will be marked with the proper disk rotation. Failure to follow this simple step will result in uneven pad wear, noise, and possible piston damage.

    Weight Transfer

    We have all felt the transfer of weight as a car slows down. I'm not going to go into all the details because there are entire books on the subject. All I'll mention here is that the majority of braking is done by the front brakes. Front brakes are always larger than rear brakes, but they must be properly sized to match the weight transfer and the rear braking capacity. Our cars have a large proportion of their weight to the rear which gives us a good front to rear weight bias, excellent rear traction, and due the mid-engine configuration, we have a lower polar moment of rotation which is better for handling.

    A serious drawback to the rear weight of our cars has to do with our center of mass. As with any projectile (our cars are just a projectile that we are trying to control), the center of mass (our engine/transaxle assembly) will always try to be "in the lead" with reference to the rest of the car. During acceleration, this isn't really an issue because the weight transfer puts more force on the rear tires. The center of mass is just in front of the rear wheel centerline so dealing in purely relative terms, the center of mass is in the lead. On the other hand, under braking or deceleration, the center of mass still tries to be in the lead and in this case it is at the wrong end of the car. This has the effect of putting more downward force on the front tires and reducing the force on the rears. The brake force bias must be such that the rear brakes never lock up before the fronts. Remember, we have a center of mass that is in the rear and it will take the lead if given the chance. If the rear tires lock up before the fronts we have just given the center of mass the freedom to go wherever it would like, typically instigating a serious "ground loop."

    Front/Rear Brake Bias

    Significant weight shift occurs during deceleration, so the front brakes need to do more stopping than the rear brakes. Larger rotors and more powerful calipers are used in the front to help maintain the proper balance. Also, a differential bore master cylinder is sometimes used to vary the amount of hydraulic pressure that is fed to each end of the system. The front-to-rear bias is critical when modifying or upgrading any braking system.

    Other Points of Interest:

    Single-Piece Rotors vs. Two-Piece Rotors

    When we want to use bigger rotors we have to sacrifice something. Larger rotors have more weight than stock rotors. Stock rotors are also cheaper because they're made in mass quantities. For after-market rotors, the investment involved in making a one-off casting is high, therefore most manufacturers offer brake rotors that are just the outer disk. It is then up to the brake system designer to engineer a brake rotor "hat" or "bell." The hats are typically an aluminum alloy that can be made on CNC equipment in small quantities and to the exact specifications for the application. The combination of the larger rotor with an alloy hat can be the same or only slightly more weight than the stock brake disk. When attaching a hat to a disk, only AN-type aircraft bolts and jet nuts should be used. Use bolts that have only shank through the disk and hat, not any threads, even if you have to use a washer. Good quality bolt kits are available from many sources.

    Master Cylinders

    Most production cars use a single master cylinder, typically with two pistons running in two bores. Each of the pistons are sized for the stock front and rear brakes. When upgrading brake systems, the master cylinder's volume and pressure must be known and accounted for.

    Many race cars have gone to the use of two separate master cylinders: one for the front and one for the rear, with a "bias bar" between them. This allows different master cylinders for different brake calipers. The balance bar is connected to the brake pedal and the cylinders and can allow the driver to adjust the braking bias while driving the car. It can be tuned for different track conditions, fuel loads, or just driver preference.

    ABS Systems

    I personally do not own any cars with ABS systems. I even removed the ABS system from my 1991 Esprit SE. ABS can be helpful if you need ballast, an excessive electrical load, or a discount on your insurance.

    In all seriousness, a properly designed ABS system may have advantages in adverse weather conditions and may give a less experienced driver an advantage in emergency situations. But if the system is not used correctly or the driver expects it to correct for their mistakes, then ABS is not the solution.

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