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Brake FAQ

Planning brake upgrades for your NA/NB Miata

copyright 2021 SUPERMIATA LLC

 Why big brakes?

  • Greater thermal capacity to increase fade resistance
  • Longer pad life
  • More brake torque
  • Improved modulation and brake feel
  • Weight reduction
  • Bling!

 Benefits of a balanced brake upgrade

  • Quicker stops
  • Firmer brake pedal
  • Better feel and modulation
  • Reduced pedal travel

 Weight transfer
  When a car decelerates, weight shifts forward. Anything which allows the car to decelerate quicker increases this forward transfer of weight. Driving on a warm, smooth race track with a racing line full of rubber will generate more friction than a cold bumpy street covered in a film of diesel, for example. This traction improvement has the same effect as a softer compound tire. Planning a performance brake upgrade should take into account any factors that can change forward weight transfer such as tire size/compound, road surface, aerodynamics, suspension tuning, vehicle weight.

 What is brake bias?
  Brake Bias refers to the distribution of braking force front to rear. The amount of brake bias required is determined by the amount of forward weight transfer under braking. Ideal bias allows the fronts to lock slightly before the rears. For this reason, all automotive brake systems are designed to generate more peak front brake torque than rear. OEM engineers consider the tire, typical vehicle load and public road surface in determining OEM brake bias. Any changes to the hydraulic system, rotors, pads, tires, suspension or road surface can require a change in brake bias. As forward weight transfer increases, brake bias needs to be shifted forward to maximize braking potential.

 Pedal effort vs pedal travel

The NA/NB's all are designed to lock up the OEM tires with less than 100lbs of force on the pedal.  This is done by balancing the various system attributes: 

  • Master cylinder piston diameter
  • Caliper piston number and diameters
  • Fixed or floating caliper
  • Pad mu (friction coefficient)
  • Rotor diameter
  • Pressure proportioning front to rear
  • Changing to higher ratio vacuum booster
  • Pedal geometry (leverage)
  • Overall combined deflection of all the components

 Changing any component of the brake system can affect the amount of brake torque for a given pedal effort. A mismatched system can result in either a very firm pedal that requires too much force to lock the wheels or a mushy pedal that locks the wheels with too little effort. The aim is the ability to lock the wheels with about the same pedal force as OEM but with less mush than OEM. This is one reason why we offer a range of pad compounds with mu listed (friction coefficient).

 Thermal capacity
  One Miata owner might upgrade the front brakes simply because it looks cool, never planning to run it on track or make other brake mods. For that driver, just using a street friendly, quiet, long lasting, low dust pad compound front and rear on an 11" front rotor with a compact Wilwood Dynalite caliper leaves things pretty well balanced. Just a slight increase in front torque bias and thermal capacity, but lots of bling. Our BX11 with 2 piece rotors and BP-10 pads fits the bill.

   A higher performance Miata with 15x9" wheels wrapped in gummy 225 tires planning for hard track driving will need more thermal capacity and brake torque. Our BX11 with optional 3" brake ducts and race pads is a great upgrade.

   In Q3, 2021 we will add a full race spec BX1175 kit with a thicker 11.75" diameter rotor, much larger caliper and almost double the pad volume of our BX11 kits. More pad volume increases pad life.

   A more powerful car will reach the braking zone sooner, allowing less time for brakes to cool. When it does arrive in the braking zone it will also be going faster. More kinetic energy to convert to heat and less time to do it. A more experienced driver will carry more speed into the braking zone, get there sooner and brake harder than a less experienced driver. Everything else being equal, a more experienced driver requires a system with greater thermal capacity. Always err on the side of too much brakes, particularly if you do HPDE and have more power than OEM.

 Brake ducts
Everything else being equal, a track car will just about double its pad life by adding functional brake ducts. Overheated brake pads tend to wear unevenly. If the outer pads get less airflow they wear faster than inner pads.  If the rotor runs too hot the top edge of the pad wears faster than the bottom edge. If the pads fade from being overheated and rotors are also too hot, the driver must increase pedal force to stop the car. This can result in a wedge shaped wear pattern know as tapering.

 In our testing, we have found that 3" is the minimum hose diameter to flow enough air to matter. 2.5" or smaller ducts flow so little air that their cooling benefit can nearly be matched by simply removing the OEM backing plates. Corrugated hose has a much thicker boundary layer which is the mass of air on the inner walls of the hose that doesn't actually move but is instead turbulent and chokes total airflow. The ony flow that actualy reached the other end of the hose is the laminar flow in the center of the hose not affected by this boundary layer. Total airflow drops as the hose gets longer. The hose that feeds the brake duct must be as straight as possible. Every bend reduces airflow. So the goal is the shortest distance from intake to caliper with the fewest bends and a 3" hose diameter. While purpose built race cars will have as many as three separate hoses feed to each hub, caliper and rotor face, that sort of tech is beyond the reach of most Miata owners. The hottest part of the brake is the pad/rotor interface. The best way to cool that interface is forcing air through the internal vanes in the rotor as rapidly as possible. So a single feed is ideally aimed at the center of the hub to feed the vanes inside the brake rotor. The outer face of the wheel is a low pressure zone, the inner face a higher pressure zone. This natural pressure differential draws air from the inside of the rotor, up through the vanes and out the face of the wheel.  The patented Supermiata Boxmount technology was designed to allow more airflow under the caliper while simultaneously increasing mount stiffness without adding weight. 

 Here is an example of a 3" duct on race prepped NB2

 

Brake Torque
  On a budget or to meet competition class regulations, one can substantially increase available brake torque without adding much thermal capacity by just swapping high mu race pads into an otherwise stock system. Our Stage 1 brake kits offer this option. This example with small rotors, small OEM-size pads and no brake ducts will have limited thermal capacity compared to a setup using larger, thicker directional-vane rotors, higher volume pads and comprehensive ducting however. With limited thermal capacity, expect much shorter pad life and the potential for significant fade in repeated hard decelerations when compared to a big caliper, higher volume pads and larger rotors.

 Brake torque vs hydraulic advantage
  In the context of a 90-05 Miata, pedal geometry remains constant so the leverage your foot has on the M/C (master cylinder) isn't adjustable or tunable. Some race builds have aftermarket billet aluminum pedal assemblies with separate front & rear M/C's and adjustable pedal geometry. For the OEM brake pedal the amount of line pressure generated for a given pedal force can only be tuned by changing caliper and M/C bore diameters. You may need to decrease hydraulic advantage if brake torque potential increases but you will never need to increase hydraulic advantage. This means smaller M/C or more caliper piston area than OEM will never be needed.

   We do not recommend swapping to a larger 1" master when using OEM style low mu street pads. The larger M/C creates a firmer pedal but may not generate enough psi (brake line pressure) to lock the brakes at an acceptable pedal force. Taking advantage of a larger M/C requires higher mu pads.  Reducing hydraulic advantage can be done when available brake torque increases.

 Mixing and matching caliper and M/C piston sizes
  Hydraulic advantage refers to the system multiplying the force put into the pedal as psi. Understanding the relationship of hydraulic and booster ratios can get a bit confusing. The OEM system is designed to generate a certain amount of line pressure which in turn generates a specific amount of brake torque.

 

Changes that increase brake torque for a given pedal effort

  • Increasing caliper piston area (diameter)
  • Decreasing master cylinder bore diameter
  • Increasing pad mu (friction coefficient)
  • Increasing rotor diameter
  • Increasing pedal leverage ratio
  • Increasing brake booster assist ratio

 Reversing any of these parameters has the effect of reducing brake torque.

 Booster ratios

A higher boost ratio will reduce pedal afford compared to a lower ratio.

NA/NB1 90-00  ~4.74:1

NB2 01-05  Sport with ABS 9.7:1

NB2 01-05 Sport without ABS & MSM (w/ABS) 6.4:1

 
Caliper piston area

90-02 Front  2.01" area

01-05 Sport Front 2.125" area

90-02 Rear 1.25" area

01-05 Sport Rear 1.375" area

 

Master cylinder bore

90-02 7/8"

01-05 Sport & MSM 15/16"

 Brake Bias and proportioning valves
  It's a common misconception that an adjustable proportioning valve can "fix" a brake system that is setup with poorly matched parts. An adjustable proportioning valve should only be used to fine tune for small variations in tire to road friction such as different tire compounds or changing track conditions. You can't fix mismatched brake components with a prop valve any more than you can fix understeer by using a bigger steering wheel. More on that subject in the next section about trail braking. OEM master cylinders are "tandem" style meaning they use one piston behind the other in a common bore diameter that feeds line pressure equally to front and rear brakes.  OEM and adjustable proportioning valves have little to no effect on line pressure at low pedal force / low system pressure. At low pedal force, front and rear line pressure is about the same. The actual bias being accomplished with rotor diameters and caliper piston sizes generating more front torque. As pedal force and line pressure increases with harder braking, the valve will have a "knee point" where rear pressure begins to be reduced relative to the front. This compensates for reduced rear traction as weight is transferred to the front tires in maximum braking. Modern cars, for the most part, no longer use a mechanical proportioning valve. Instead they use a variety of sensors around the car and often use active electro-hydraulic proportioning controlled by the chassis computer.  A mechanical proportioning valve is used in the non-ABS NA/NB Miatas. The OEM Miata valves have different knee points depending on the year and are non-adjustable. Aftermarket valves have adjustable knee points. Purpose built race cars have separate and different bore size front and rear master cylinders as well as a mechanical bias bar that allocates line pressure according to driver adjustments.



  

The Holy Grail: Trail braking
  Trail braking is a technique learned by intermediate and advanced level drivers where brake line pressure is smoothly and gradually released at the end of the braking zone during corner turn in phase. This technique allows the driver to precisely balance slip angle at all four tires, yaw (rotation) rate, car position on track and speed. It can only be used when the entry speed is high enough for the tires to generate slip angle during brake release. This technique requires lots of entry speed and minimal steering input, the important weight transfer being controlled with the brake pedal. Get it wrong and you either understeer off the corner or spin. Less experienced drivers tend to release the brakes more abruptly and rely on extra steering corrections to adjust the cars attitude, tossing it in and managing, which is less effective than trail braking.  Trail braking begins at very high line pressure in a straight line then transitions to low line pressure as the driver begins to turn in. What this means is that if your basic brake bias is way off and you are either locking fronts or have sudden oversteer as you turn in, you can't really fix it by maxing out the prop valve. An example problem might be a car that has too much rear brake bias. The driver cranks in lots of prop valve to choke off rear line pressure which fixes straight line braking. But as the driver eases off the brake and the prop valve drops below its knee point, the bias change, rears lock and around she goes! Common problem actually. Once the braking system is set up correctly for your particular tire & suspension you should almost never need to chop a lot of rear line pressure. This means a good setup has the valve nearly full open most of the time. An advanced driver might fully open the valve to increase rear brake torque a bit in the rain to aid turn in on a slippery track.

   Good trail braking technique is a heck of lot easier with finely tuned brake bias. The driver that has not yet learned to trail brake to rotate the car will tend to want a car that is "looser" so that it rotates without trail braking. The less experienced driver may initially be less concerned over brake balance but will eventually find the limitations of the brake setup as their skill increases. The more experienced driver will want a more neutral setup and be very concerned over getting brake balance just right as it’s vital to initiating precise rotation and car placement on corner entry. Even a moderately skilled HPDE or race driver can hit apexes and generate good exit speed. Lower lap times come with additional entry speed translated into higher minimum corner speeds. The myth that exit speed lowers lap times is what your HPDE instructor may tell you. They want you to enter turns slower to ensure you both get home in one piece, not because it’s faster :)

 

A few last notes

  • Stopping power for one stop is ultimately limited by tire traction, not by brake size, pad compound, or caliper type. The trick is to balance brake torque at each end for the pedal effort you can comfortably apply, for the traction your car has.
  • Adding too much front brake torque without balancing the system can actually result in longer stopping distances. This results from the fronts now having too much torque for a given line pressure so the driver presses the pedal less to keep the fronts from locking. That lower total line pressure reduces rear brake torque which keeps them from delivering the maximum deceleration the rear tires are capable of generating. In this example, bias has shifted too far forward and the car takes longer to stop. A common problem resulting from not making a pad change in the rear to balance out front upgrades.

 Brake balance matters!



 

copyright 2021 SUPERMIATA LLC

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