Brake System Design Theory

    This section covers the basic concept of what the brakes do, what things influence how the brakes work, and the basic concepts of building a better braking system.

    System Design:

      There are not many adjustment knobs to turn, levers to slide, or things that can be fiddled with, in the braking system of an automobile. This means that the performance is really determined by the large, often expensive, non-adjustable parts in the system, and how well those parts have been chosen and selected to work with the other parts in the system. This really becomes an issue of choosing the right parts to begin with, or having to go back and replace large and expensive pieces, to correct issues and problems after they emerge.

      What the Brakes Do:
      The brakes slow the car as quickly as possible while maintaining directional control. That last detail is an important one. Without directional control, the car may spin or veer out of control as it slows, and hit something. If the brakes are locked up and the wheels and tires are not rotating, the result is a 2,000-3,000 pound box of metal, plastic, and glass, resting on four giant pencil erasers, skidding out of control, while the driver prays he does not hit anything before the car comes to a stop, when wild ride ends. Keeping the tires rotating in a circle, just a little bit, allows the driver to control the vehicle’s direction, while decelerating as quickly as possible. This is called “Threshold braking” ( Applying the brakes just barely short of the point that they lock up, and controlling the brake pedal to keep the tires just barely rotating. The rear tires act like rudders to keep the front pointed in the direction of travel, and the front tires allow for some control to change direction while slowing down. This becomes an important aspect of not finding yourself looking out the windshield in the direction you came from, while the back end of the car rapidly closes on large, heavy objects. Antilock braking systems have been designed to control braking for even the least talented and least skilled driver. They typically do a rather ham-fisted job that is better than the least competent among us, but nowhere near the level of anyone who cares enough to practice and put forth the effort to learn to control how to operate a brake pedal.

      Brake Bias or Balance
      Brake bias is a measurement of the balance between the front and rear brakes as related to the required force to safely stop the vehicle in a controlled manner, and the actual force being provided by the brake components installed on the car.
      The majority of cars are made with different sized brake calipers and different sized rotors on the front and the back. In most cases, the calipers and rotors on the front are quite a bit larger than those on the back. This is because that, with few exceptions, the front brakes do most of the job of braking and slowing down the car, and the rear brakes essentially help as much as possible without overpowering the fronts. Oversimplified, the slowest wheel tries to get in front. The shopping basket with the one front wheel that refuses to turn and wiggles left and right, will push over to the middle of the front while all of the other less damaged wheels trail behind. Or take your car out on a piece of deserted pavement and pull the emergency brake, the rear wheels lock up and the car spins around and slides backwards.
      So the goal is to get all of the brakes working together, with the front brakes providing most of the braking work, and the rear brakes working as much as possible to help the front brakes, without overpowering the front brakes, and messing things up.

      What if the front brakes lock up way before the rears?
      When the front brakes severely overpower the rear brakes, the front brakes will lock while the rear brakes are not providing much effort to slow the rear wheels. The car will continue in a straight line, sliding forward in a straight line, the rear tires rolling and acting as rudders to keep the nose of the car in front. This is called brake understeer. If the driver is trying to turn and brake at the same time, the car will continue in a straight line, through the outside of the turn, off the road surface and into the trees, the driver staring forward in horror until the car finally comes to a stop.

      What if the rear brakes lock up first?
      When the rear brakes severely overpower the front brakes, the rear brakes will lock up, the rubber on down facing part of the rear tires will turn to smoke, and those tires will flat spot. This is called brake oversteer. If the driver is driving in a straight line while attempting to decelerate, the rear end will lift, become unstable, and crab from left to right repeatedly as the car slows down. If the driver is unfortunate enough to lock the rear brakes while decelerating and turning at the same time, the rear end will snap and swing in the direction of the turn, and the driver will quickly be looking at the direction that he came from, staring in horror through the windshield, as the car continues to slide backwards until it finally comes to a stop, also, usually off of the paved surface, in the trees.

    What Affects Biasing?

      Static Load; Axle weight.
      Weight distribution is the percentage of weight on the front axle, versus the percentage of weight on the rear axle. Most cars have a front engine layout and have more weight over the front axle than the rear. Front engine front wheel drive cars have the additional weight of the transmission moved even more forward, and no drivetrain weight on the rear axle, leading to even more weight on the front axle. The Geo Storm and Isuzu sister cars (Impulse and Stylus) are front engine cars and front wheel drive (or all wheel drive derived from a FWD layout), and have a weight distribution of 64/36. That’s almost 2/3 of the weight on the front tires, and one third of the weight on the rear tires. Take the curb weight as 2410 pounds, and that is 1542 pounds on the front tires and 868 pounds on the rear tires. Nearly double the weight on the front tires, making it very easy to understand why it takes more braking force at the front to slow the car, and why the front brakes have been designed to deliver a braking force more than double that of the rear brakes.

      Dynamic Load; Weight Transfer
      But there is another variable in this that tilts the scales even more. When a vehicle is decelerating, the weight of the vehicle shifts to the front axle. It is easy to see this in action. Looking at a car that is rapidly slowing down, the nose goes down and the tail lifts. Sitting inside the same car, anything that is loose slides, falls, or flies forward. The passengers find themselves pressed forward against their seat belts. That big gulp in the drink holder splashes against the dash and firewall. Stuff that used to be in the backseat ends up under the front passenger’s feet.
      For brake system design, it is accepted that half of the rear axle weight will be shifted to the front axle during heavy braking. Back to our example, the 2410 pound Geo Storm, with 64/36 weight distribution and a 1542 pound static front axle weight and a 868 pound static rear axle weight, and we come up with a 1976 pound dynamic front axle weight and a 434 pound dynamic rear axle weight.. That’s an 82/18 ratio.
      So we’re at a 4 ½ : 1 ratio of dynamic load at the maximum braking potential of the vehicle.
      These numbers do not translate directly into required braking force numbers, but provide a very good illustration of why the front brakes are so much more powerful than the rear brakes. It is important to pay attention to the OEM braking force numbers, work to maintain the front/rear brake force ratio if it is working, and adjust from that starting point only in small steps and only as needed to correct problems with the vehicle’s braking characteristics.

      Road Grip. Tires and Pavement
      Any combination of tires and pavement that results in increased friction, will normally increase front brake bite. Stickier tires and stickier pavement provide more bite for the front tires and front brakes for slowing the car.
      Any combination of less sticky tires, overheated tires, less sticky pavement, gravel, dirt, grit, low temperature, weather, water, rain, or ice, that results in decreased friction, will normally increase rear brake bite.
      This becomes an important consideration when making decisions to fine tune the car for the track conditions on race day. Pavement temperature, rain or dry conditions, and the type of pavement (concrete, asphalt, blacktop, grit covered or broken pavement) will all affect the braking characteristics of the vehicle.
      These are equally important considerations for building a dual use car for street and track, when less grippy street tires are used for daily driving and extremely grippy race tires are used at the track. The brake balance will change with the tires being used, and a good balance with one end of the tire grip range can be a bias problem with the other end of the tire grip range.
      A recipe for disaster is to take a car that has been tweaked to the limit for dry, hot, summer track use, and then attempt to drive it on cold, snow and ice covered roads. Without addressing the brake bias and adjusting for less rear axle braking, chances are the car will be too dangerous to operate.

      When the Tail Lifts…
      Another important idiosyncrasy of nose heavy cars, especially FWD cars, is the tendency of the tail to lift off the ground during heavy braking. During low and moderate braking, less weight shifts forward, and a tail light car will have enough rear axle weight to keep the rear tires on the ground and the rear braking force transfers to the ground without locking up the rear brakes. But raise the speed the vehicle is being slowed from, and push the braking force to the threshold, and the rear of the vehicle will become light enough to lift the rear tires off of the ground, and the rear brakes will lock. As the vehicle slows, the weight returns to the rear axle, but the rear brakes remain locked, and the tail will try to slide sideways.
      There are several ways to deal with this situation, including using a brake proportioning valve to reduce the rate of rear brake fluid pressure increase, effectively re-curving the rear brake action related to the front braking action.
      There is a discussion of this very phenomenon on one of the Fiat 500 forums:

      Tread Patch
      Alignment, specifically camber, can also affect brake biasing. A high amount of negative camber will rotate the tread face of the tire away from the pavement that the car is resting on, thereby reducing the amount of the tire that is actually able to apply stopping force (or acceleration force for that matter) to the pavement. This must be considered when adjusting the suspension alignment for race (or appearance) purposes. A heavily tilted tire may provide optimum cornering ability at high speed, but may also sacrifice all of the available braking force to tire smoke.

      Changing Vehicle Weight and Weight Distribution.
      The normal process of stripping the interior out of a car to reduce vehicle weight can be very counter productive to weight distribution and brake bias for a front engine, front-wheel-drive or transverse-engine-all-wheel-drive vehicle. As mentioned, the Geo Storm and Isuzu sister cars start out with almost 2/3 of their weight on the front axle. The majority of weight removed from a passenger vehicle by stripping out the interior results in weight taken off of the rear axle. Aside from the front half of the carpet and the front door panels, all of the interior is behind the center line of the wheelbase. So almost all of the couple hundred pounds of possible weight reduction from stripping the interior comes off of the rear axle, resulting in a very nose heavy car and increased front brake bias. It is actually possible to get close to a 80/20 weight distribution on some FWD cars. The required braking force differential between the front brakes and rear brakes with such a split is almost comical, requiring giant brakes on the front, and tiny brakes, as might be found on a shifter cart, on the back.

      The Factory “Safe” Setup; Idiot Proof and Set For Slippery Conditions
      Automobile manufacturers dumb down cars to keep people from being able to hurt themselves.
      With the suspension, they back the handling away from oversteer to mild understeer, because sliding in a straight line is considered safer than spinning.
      Automobile manufacturers do the same thing with the brakes. A little bit of brake understeer is considered safe, because sliding in a straight line with the brakes locked up is considered safer than spinning. This allows the driver to apply the brakes, temporarily lock up the front brakes, and then slightly back out of the brake pedal to get the car back to its braking threshold.
      But automobile manufacturers must also provide a product that is safe and predictable in all possible driving conditions and make sure that the vehicle is safe, even in adverse conditions. As mentioned earlier, as grip is reduced, brake bias moves to the back of the car. The car manufacturer designs and adjusts the car so the brakes are balanced to provide safe operation on snow and ice. So with the factory brake setup, the rear brakes are often noticeably underpowered on good, dry pavement, in order to be safe and predictable on snow and ice.

    Driving Style Influence On Desired Brake Balance

      Right Foot Braking.
      The performance driving school instructor will drill students on accelerating and decelerating in a straight line, and maintaining a steady speed while turning. The claim being that cars, especially FWD cars, can only do one thing at a time well. So the racer accelerates in a straight line, and then brakes in a straight line before turning, and keeps the vehicle speed at the adhesion limit while turning. Once the turn is completed, the driver allows the steering wheel to unwind and begins accelerating toward the next turn. The brakes and gas pedal are never used simultaneously and the driver never makes a heavy deceleration panic stop while turning.
      Tuning a car to work well with this driving style typically involves doing things to encourage “rotation”. The rear spring rate is increased (heavier springs, larger rear sway bar, stiffer rear dampers) so that the tail will kick slightly loose, inducing a slight but controllable “lift off” oversteer. Slightly increasing rear brake bias can also work well with this driving style. Many drivers, especially Miata owners, will concentrate on increasing rear brake bias to improve turning and “rotation”.

      Left Foot Braking.
      Left foot braking violates a lot of the foundational ideas of right foot braking. The basic theory is that in various specific driving situations, stability is enhanced with the use of the brake pedal while also using the accelerator pedal, and the car can be forced to do things that would not normally be possible with a right foot braking driving style.
      Specific to turning and rotation with a FWD car, the driver can maintain speed under engine power while enhancing cornering traction by slightly applying the brake. The driven front wheels are allowed to overpower the brakes and continue pulling the car, while the rear, non-driven wheels decelerate and resist forward movement. The result is basically a very subtle emergency brake drifting maneuver, that counteracts the FWD tendency to understeer and takes the cornering to neutral and slight controllable oversteer. The action can be controlled on a finite level with the right foot on the gas pedal and the left foot on the brake pedal. The driver holds the wheel at a steady point and uses the gas and brake to increase or decrease the radius of the turn, all the time continuing to apply power and preventing the car from slowing through the turn as it would with right foot braking. The car goes through the turn quicker and exits with a higher speed.
      This driving style works well with a brake system designed to have a slight braking understeer bias or front brake bias, as is typical with the stock “safe” brake setting that most manufacturers try to achieve. This is also why a driver with a left foot brakeing technique can usually squeeze a little quicker time out of a stock or unmodified car.

    Looking At Braking Forces

      We are going to refer frequently to the brake force and bias calculator tool on the TCEPerformanceProducts.Com website:

      This tool illustrates a lot about how changing one piece affects the overall effectiveness of the system and changes the balance of the system. It also illustrates which component changes make very large changes in braking force and balance, and which component changes make very small changes in braking force and balance.

      This tool is not exact and is intended only for illustrating how changing specific variables affects the resulting braking force. The tool also can not calculate drum brake force. Because of this we are going to use the Isuzu Impulse and Stylus sister cars with four wheel disk as the baseline to work from. The Storm GSi uses the same front brakes as the Impulse and Stylus XS, and we can estimate the rear drum braking force as about 10% less than the Impulse/Stylus disc brake results.

        Baseline Numbers, Impulse XS / Stylus XS (Storm GSi with rear drum brakes):
          Leg input in pounds: 60
          Master cylinder diameter: .875
          Front caliper piston diameter: 2.0
          Front pad coefficient of friction (cf): .30
          Front rotor diameter: 9.7 (pad is set in from edge of rotor by .1 inch)
          Front pad radial height: 1.63
          Calculated Front Rotor Torque: 2530 In.Lb.
          Rear caliper piston diameter: 1.25
          Rear pad coefficient of friction (cf): .30
          Rear rotor diameter: 10.1
          Rear pad radial height: 1.4
          Calculated Rear Rotor Torque: 1065 In.Lb.s (Drum Brakes ~950 In.Lb.)
        Baseline for Storm 12V/SOHC Base Model Front Brakes:
          Leg input in pounds: 60
          Master cylinder diameter: .875
          Front caliper piston diameter: 2.0
          Front pad coefficient of friction (cf): .30
          Front rotor diameter: 9.0
          Front pad radial height: 1.63
          Calculated Front Rotor Torque: 2233 In.Lb.
          Calculated Rear Drum Torque: ~950 In.Lb.

      What Increases Braking Force?
      Enter the variables for the Impulse XS / Stylus XS brake system into the TCE Performance Products Brake Force Calculator, and play around with some of the numbers to see what changes the calculated output numbers for the braking force.

      Start with the specs for the Impulse XS / Stylus XS brake setup as provided above. Try changing one variable at a time and see how that affects the braking force.

      Starting with the baseline numbers, tinker with the brake caliper piston size and number: Change the front caliper piston diameter from 2 inches to 2.125 inches. The front braking force changes from 2530 In.Lb. to 2856 In.Lb. A single piston caliper with a 2.25 inch piston changes that to 3202 In.Lb.
      Change the front caliper piston diameter from a single piston of 2 inches to a two piston caliper of 1.5 inches diameter. The front braking force changes from 2530 In.Lb. to 2846 In.Lb. A four piston caliper with 1.62 inch pistons changes that to 3319 In.Lb.

      Go back to the baseline numbers and tinker with the brake rotor diameter:
      Change the front rotor diameter from 9.7 inches to 11 inches. The front braking force changes from 2530 In.Lb to 2937 In.Lb. And with a 12 inch rotor, that changes to 3251 In.Lb.

      Go back to the baseline numbers and tinker with the brake pad coefficient:
      Change the front brake pad Coefficient of Friction from .3 to .35. The front braking force changes from 2530 In.Lb. to 2951 In.Lb. And with a .40 CF pad, that changes to 3373 In.Lb.

      Three ways to increase braking force:
      • Increase brake caliper piston area.
      • Increase rotor diameter.
      • Increase break pad friction coefficient.

      What Decreases Braking Force?
      Change the front brake pad radial height from 1.63 inches to 2.63 inches. The front braking force changes from 2530 In.Lb. to 2216 In.Lb.
      Change the brake master cylinder diameter from .875 inch diameter to 1.0 inch diameter. The front braking force changes from 2530 In.Lb. to 1937 In. Lb.

      These two variables are counterintuitive. One would expect that a larger master cylinder would increase the braking force, and that a larger pad would increase the braking force. But the result is just the opposite.

      The brake pad area is not a variable on the calculator. Only the radial height. The radial height as in the width of the surface that the pad sweeps on the face of the rotor measured from the very edge of the rotor to the other edge of the pad. This measurement determines the center point of the braking force and its distance from the axis of rotation. Take the diameter of the rotor, deduct half the width of the pad. That is the center point where the brake pad is being applied to the rotor. So a wider pad moves the center of that braking force closer to the axis of rotation, and decreases the braking force. Larger pads do not increase braking force. Increasing the pad radial height decreases braking force. Independent of the pad radial height measurement, the pad area will only affect how long the pad lasts. This is why performance calipers tend to be made so that the brake pads are in a crescent moon shape, narrow and wrapping around the edge of the rotor.

      A larger master cylinder will require more leg input to achieve the same pressure through the brake line. That is the variable just above the master cylinder diameter. This means that if the master cylinder diameter is increased from .875 inch diameter, to 1.0 inch diameter, then the driver will have to press on the brake pedal harder to achieve the same braking force. How much harder? 31% harder. That is an increase from 60 Lb. to 78.5 Lb. to apply the same braking force. This will give the illusion of increasing the vehicle braking, because the pedal will feel harder. The problem is that the driver has to press the pedal 31% harder to apply the same force. This will reduce the driver’s ability to control and keep the car at the threshold braking point, because the driver must apply significantly more pressure to the pedal to reach threshold braking. Making the brake pedal more difficult to use, will also increase stress and fatigue on the driver.


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