Brakes are the most essential system in your race car. The consequences of being unable to stop can take a fun day at the track and turn it expensive or even deadly in the blink of an eye. In this article, I will explore the basics of how disc brakes work. I’ll use my time attack/wheel-to-wheel BMW E90 as an example of how you can use fundamentals to make your braking system worthy of competition abuse. Warning: this will be nerd-heavy, so save it as a bookmark.
Kinetic Energy Into Thermal Energy
When you step on the brake pedal of your average road car, the force your leg produces is multiplied by 6:1 to 7:1 for brake systems without a brake booster and 4.5:1 to 5:1 for systems with a booster. That force is then changed into a hydraulic pressure via the master cylinder. The hydraulic pressure reaches the brake calipers at the wheels through a series of hard and soft lines. The hard lines are typically made of steel and/or stainless steel. The hard lines connect to soft lines, which are made out of rubber or a combination of rubber, synthetic materials, and stainless steel. This allows for articulation, as the calipers are attached to the suspension, so the freedom to move up, down, in, and out is necessary. That force pushes a piston, or pistons, inside the caliper onto a brake pad made of friction material to clamp the brake rotor. The rotors are attached to the wheel via wheel studs; thus, this is how the car slows down. The brake pads convert a vehicle’s kinetic (motion) energy to thermal (heat) energy through friction on the rotor. This heat is dissipated into the surrounding air through convection (heat transfer between masses at different temperatures). Generally, the front brakes absorb 70% of the vehicle’s kinetic energy, and the rear brakes absorb the remainder. The front brake rotors are typically oversized and vented better than the rear. Now that we know the basics, we can work on improvement. If you have drum brakes, I advise you to change them to disc brakes for better thermal management.
Line Em’ Up, Brake Hoses
The first modification you should make is ensuring you have PTFE (polytetrafluoroethylene) brake hoses (often called brake lines), usually wrapped in stainless steel. Be sure to purchase from a company registered with the United States Department of Transportation (more on that later). I recently had a revelation: rubber brake hoses are a wear item. The Department of Transportation (DOT) regulates all brake hoses on cars sold in the United States. They must conform to the FMVSS 106 and SAE J1401 tests. One specification that an average brake hose assembly must achieve is the ability to withstand pressure of 4,000 psi for two minutes without rupture. They also must pass the whip test, which continuously bends soft lines on a flexing machine for 35 hours at pressure. The last big one would be the slow pull and fast pull tensile test. In the slow test, the hose must withstand a pull force of 325 lbs without separating from its end fittings. During the fast pull test, that 325 lbs is upped to 370 lbs with the same stipulations of not separating from its end fittings. To have a brake hose on an OEM car in North America, they also have specific markings, and must meet temperature resistance, ozone resistance (for cracking), and corrosion resistance standards.
I know what you are thinking, “Then why the hell am I replacing the ones that came on the car, Devin?” Well, sit back down, junior. Your brake hoses live in a harsh environment. They are peppered with salt, dirt, and brake dust, among other things. These all cause corrosion on fittings. Then, there are multiple heat cycles with dramatic cooling. I could go on and on!
Brake lines don’t get the inspections they deserve and are often forgotten if they are working and have no signs of leakage. You should inspect your brake hoses for blisters or bubbles. If you rapidly push down on the brake pedal and see the hose deform, replace them. You should also look for cracks by trying to flex the hose. If there are any, replace the hose. Check the brackets that hold the brake hoses to a suspension components. If they are damaged or rusty, they may pinch the hose, meaning you should replace them.
Fittings are plated to prevent corrosion. If this plating wears off, it can accelerate wear. Brake hoses also tend to swell with age, which causes restricted flow. Brake hoses can also deteriorate from the inside with no warning signs. Moisture in brake fluid (brake fluid is hygroscopic and absorbs moisture from the air) also damages the reinforced fabrics. Lastly, you should run your finger along the length of the hose, and if there are any irregularities, you fucking guessed it, replace the hose.
DOT Compliant
You want to ensure you get your hoses from a company registered with the United States DOT to sell brake hoses. This means their products have been tested to the above standards and passed. I see this often in the grassroots motorsports world where a consumer assumes that when it says “stainless steel lines,” they are all the same. It’s certainly possible to buy a worse product than your OEM rubber hoses.
A few quality companies that come to mind are Stoptech and Goodridge. Stoptech, which is owned by Centric, is an OEM level manufacturer. Their aftermarket lines are pressure tested to 4,500 psi instead of the 4,000 psi minimum. Goodridge is another company that takes things a step further and tests their hoses at 12,750-13,500 psi. Goodridge also manufactures for OEMs as well. Both of these companies adhere to FMVSS 106 and SAE J1401 tests.
Many older cars you see on the track could benefit from new lines. I noticed the hoses on the E90 were cracking and had clear signs of corrosion on the fittings, so I replaced them with Goodridge hoses. These lines will give a firmer pedal feel compared to the old hoses that could have deformed under hard use. There is a caveat. Many new sports cars come from the factory with pretty awesome hoses, so going with quality aftermarket lines might just be a lateral move—cars like your Porsche GT2s, GTRs, ZL1s, GT350s, etc. What this simplifies down is that going OEM or with aftermarket companies that meet or exceed the DOT standards is the safe bet. Next we can go to the brake fluid step.
Overlooked, Brake Fluid
I’ll go on a limb and say that brake fluid is likely the most overlooked fluid in a street car. Brake fluid is supposed to be changed every 2-3 years or 50-100k miles, depending on the manufacturer. Brake fluid is a hydraulic oil that is the medium that transfers your leg strength from the brake pedal to the calipers. When it’s new and fresh, it’s basically incompressible. However, brake fluid is hygroscopic, so it absorbs water. Water is absorbed through the caliper piston seals, via condensation from calipers heating up and cooling repeatedly, and via diffusion through rubber brake hoses over time ( PTFE hoses eliminate this). You may be saying, “Mr. Devin, sir, isn’t water also incompressible?” Damnit! I’m getting there, hold on.
Two important things to remember is that you cannot eliminate water from your brake system. Still, you want to keep contamination from water as low as possible because a small amount of water can drastically reduce the boiling point of the brake fluid. Water has a boiling point of about 212° Fahrenheit. Once it reaches its boiling point, it becomes steam (gas). Gas is compressible, which is the last thing you want in between your brake pedal and your calipers. In most cases, water is known to reduce brake fluid’s boiling point by over 33%. This leads to overheating the brake fluid, the boiling fluid then converting to gas and causing air bubbles.
When driving on track, this phenomenon is felt as a “soft” brake pedal. If you keep driving through this situation, you will need to pump the brakes to get the car to slow down. If the brake fluid boiling is really bad, you can’t generate enough brake pressure and slowing down will be caused by things outside of the car. No bueno.
There are some solutions that you can do to avoid overheating your braking system.
- Use less brakes, duh
- Find a way to cool down the brakes, brake cooling kits, etc.
- Install titanium, stainless steel, or ceramic caliper pistons.
- Titanium backing plates on your pads.
- Brake fluid with a higher boiling point.
There are plenty of kits available to help most popular vehicle applications with brake cooling. In the last ten years, many sports cars come with brake cooling aids from the factory. Titanium caliper pistons are becoming readily available. You can expect to pay $100-200 USD for a pair of pistons. Titanium is a very poor conductor of heat and reduces this heat transfer by a significant amount. A GTR with 20 pistons would be around $2000 to $4000, so it’s not a cheap option. Motorsports-grade big brake kits typically come with stainless steel or titanium pistons. Stainless steel can be about half the cost of titanium. Still, you must be careful when using aftermarket caliper pistons as dissimilar metals have different rates of expansion which can cause issues of the pistons being too loose or too tight depending on the temperature of your brake system. Titanium backing plates will transfer less heat from the friction material on the pads to the pistons, which are pushed by the brake fluid, thus lowering the heat the fluid sees. This leads us to arguably the easiest and most effective way to keep brake fluid temperatures in check: better brake fluid.
The DOT classifies brake fluid as DOT 3, DOT 4, DOT 5, and DOT 5.1. DOT 3, DOT 4, and DOT 5.1 brake fluids are compatible and ether-based (which is why they’re hygroscopic). DOT 5 brake fluid is silicone and it doesn’t absorb water. It’s incompatible with the other brake fluids, ABS systems and therefore, most modern cars. It is used mainly for classic cars and the U.S. Military. Brake fluid is also classified into “dry boiling point” and “wet boiling point,” which we will focus on.
The dry boiling point is the temperature at which the brake fluid boils when fresh out of the can. The wet boiling point is arguably more important than the dry boiling point. The wet boiling point is the rating for when the fluid boils after absorbing 3.7% water in the brake system. DOT 3 fluids must have a minimum dry boiling point of 401F and a minimum wet boiling point of 284F, determined by the DOT. DOT 4 fluids dry minimum is 446F and wet minimum is 311F. DOT 5.1 has the highest boiling points, with 509F for dry and 356F for wet.
DOT 4 fluid will have a higher boiling point during the early portion of its life, but once it begins to absorb water, its boiling point will decrease faster than a DOT 3. DOT 5.1 fluids are simply DOT 4-type fluids that meet DOT 5 performance requirements. Because of this, they typically can be mixed with DOT 3 or DOT 4. They are often referred to as ”DOT 4 Plus” or ”Super DOT 4” because they are more similar to DOT 4 fluid by chemistry than a conventional DOT 5 fluid. The downside to 5.1 brake fluid is the cost, which can be three to four times the amount of DOT 4 fluid.
You should be aware that if you run a DOT 4 / 5.1 fluid, you should change the fluid more often than if you use a DOT 3. The hygroscopic nature of these fluids means they should be replaced at least annually in your race car. Saving the leftover fluid isn’t an option, so buy your brake fluid in small containers. Once a new bottle is opened, the fluid will absorb moisture in the atmosphere no matter how many ziplock bags you put it in. Castrol SRF is the best brake fluid on the market as of late 2023. It has the best boiling points by a decent margin. Personally, I also use HAWK 660 to save a few dollars because Castrol SRF is so expensive it can make it challenging to make my mortgage payment.
Rotors
While talking about temps, let’s focus on the pieces of metal that have to deal with a lot of heat—the brake rotors. When you step on your brake pedal, the pads are compressed against the rotor, creating friction. This enables your vehicle to stop while also producing tremendous heat. The most common rotors consist of two iron discs connected by vanes. The two essential factors for rotors used in motorsports are heat capacity and thermal stress management. A good design for heat capacity involves finding a balance between incorporating sufficient metal mass in the friction ring to absorb and manage the kinetic energy transferred into the brake system, while simultaneously minimizing the rotating mass of the wheel assembly.
Managing thermal stress reduces fatigue and extends the disc rotor’s lifespan. Regardless of the manufacturer or disc design, grey cast iron is prone to fatigue in motorsport applications. Metal fatigue is the stress and strain cycle induced by thermal expansion and contraction during heating and cooling. Without question, slotted rotors are the way to go. Slots in the disc rotor serve to remove waste material (dust) from the friction surfaces. They contribute to maintaining consistent friction performance over the lifespan of the disc and pad. In contrast, when using plain discs, careful attention to friction material selection is essential to prevent surface glazing and a subsequent decline in friction performance. Drilled rotors are dogshit and shouldn’t be used because the holes are a stressor and add fatigue that eventually will end with cracked rotors. Note, for an endurance application for most grassroots teams, you may want to go to blank rotors that have an anti-glazing feature. Slotted rotors excelrate wear so pads will need to be changed more often.
Generally speaking, a two-piece floating rotor is the way to go for a motorsports application. Prominent benefits include decreased weight and improved heat dissipation, facilitated by the aluminum hat functioning as a heat sink. The cast iron disc can expand radially (outward) relative to the aluminum bell. The cast iron disc will heat up while the aluminum bell remains cool. The rotors are less likely to warp or crack from excessive heat. Another significant advantage is the maintenance cost savings of replacing just the friction ring while retaining the center bell for reuse.
I’m going to stop here, this article is running long. In the following write-up, I’ll cover brake pads and identifying the operating temperature.
TLDR
- Buying parts from OEM suppliers is always preferred.
- Castrol SRF is the best brake fluid.
- Slotted rotors are the way to go, and getting them in a 2-piece design is best if you can afford them. Unless endurance racing, which anti-glazing blank rotors are best?
