07 Oct Education Station: Brakes
Welcome to the first installment in a new weekly feature of Fluid MotorUnion’s blog, Education Station (name subject to change). The aim of this project is to make simple the many systems that occupy space in and help to power your vehicle. Not only will this help to educate our customers so they can better understand why we do what we do, but this will hopefully educate everybody that doesn’t already know what’s contained in these features. After all, the more you know, the better prepared you’ll be if you encounter a problem on the road, or a potentially shady mechanic (hint: there is no blinker fluid). Our first installment breaks down the braking system. Enjoy!
When broken down to its simplest ideas, understanding the braking system is relatively simple. Essentially, when you press down on the brake pedal, the system uses pistons and incompressible fluid (brake fluid) to multiply and transmit pressure to each of the four brakes on the car. Maintaining that braking power relies upon three major components: The brake fluid, the master cylinder and the brake booster. The brake booster assists the master cylinder, which in turn creates pressure in the brake fluid, transferring stopping power from your foot to the brake calipers, pads and rotors.
The Brake Booster
The brake booster, otherwise known as the vacuum booster, uses air pressure to multiply the braking force applied with your foot on the pedal. Have you ever driven an old car without power brakes, or have you attempted to stop a vehicle that’s rolling but turned off? You’ll find that you must apply significantly more pressure on the pedal than you’d expect. Not surprisingly, vehicles require hundreds of pounds of pressure to actually stop the car, and if it weren’t for the booster, your right leg would tire very easily on the ride home from work.
The booster assists the braking process by using air pressure differences. When the pedal is depressed, it pushes a rod through the booster into the master cylinder. The booster is filled with a partial vacuum, which means there is very little air pressure. As the pedal is pushed, a diaphragm opens, which lets in outside air to one side of the booster. When this happens, the unequal air pressure between the sides creates additional pushing power.
The Brake Master Cylinder and the Brake Fluid
For safety reasons, the brakes on your wheels are broken into two circuits – front and rear. If one circuit springs a leak or suffers another issue, the other will still be able to supply braking power and slow the vehicle down. The brake master cylinder supplies pressure to both circuits, allowing you to slow your vehicle. Understanding this portion of the braking system requires a little knowledge on how hydraulic pressure works.
Hydraulic pressure means pressure applied through a fluid medium. The atoms that comprise a liquid are already pretty tightly packed, so when pressure is applied to a fluid contained within, say, a cylinder, the liquid will not compress but instead attempt to expand. Let’s use a syringe as an example; by pushing on the syringe’s cylinder, the pressure sends the liquid through the needle, since it cannot compress and needs somewhere to go. If there were no needle, then the syringe couldn’t be pushed past a certain limit.
This is essentially how the brake system functions; the action of pushing on the gas pedal pushes pistons within the brake master cylinder, which in turn puts pressure on the brake fluid, transferring the pressure from your foot to the brake calipers and pads. Using pistons of different sizes, braking power can be multiplied many times; the ten pounds of pressure your foot exerts on the pedal can end up exerting over a hundred pounds of pressure on the brake pads. If this weren’t the case, and all the pistons in the brake system were the same size, it would require superhuman efforts to bring 3000 pounds of steel to a standstill.
The following is a picture of the brake master cylinder, brake booster and brake fluid reservoir on a 2008 Honda Civic Si. Although it’s a Honda, this setup still looks very similar to others by BMW, Mercedes, Lexus, etc:
Brake Calipers, Pads and Rotors
After the braking power is multiplied and sent through the brake lines to the calipers (assuming you have disc brakes and not drum brakes, which most modern cars do), the physics of friction take over, which in turn stop your vehicle.
Friction is a measure of how hard it is to slide one thing over another. What may look smooth to the human eye will, on the microscopic scale, contain hundreds (if not thousands) of peaks and valleys, all of which interact and bump up against the peaks and valleys of other objects. This creates friction.
Brake pads consist of steel plates with a friction material bound to the surface. The calipers use the hydraulic pressure from the brake lines to push a piston behind the pads, putting the pads against the brake disc (or rotor), which creates friction, slowing down the discs and therefore the car.
Pads are built with a failsafe system to detect when they need to be replaced. As the friction material wears down through heat transfer and production of brake dust, there will be a soft metal piece on the pad that will eventually make contact with the rotor, causing a loud squealing noise. This noise, when noticed during normal driving conditions, lets you know that it’s almost time for a brake job.
The following is a picture of the brake pads, caliper and rotor on a Mercedes ML-class SUV. Disc brakes look very similar to one another:
Issues That Can Arise with Braking Systems
-Loss of Vacuum Pressure in the Booster
Were the booster to fail, there would not be any difference in air pressure between the sides, and the brakes would require a greater applied force to transmit the same power. When this happens, you will feel your brake pedal become much more difficult to depress, similar to the feeling of pushing the pedal when the car is off. While the car will still be drivable and capable of stopping, the work required will increase to a great degree. This is what cars without power brakes feel like normally. Isn’t technology wonderful?
-Increasing Heat within the System
Heat is a problem with brake systems, to the point where the majority of brake problems can be explained with heat in some way.
In the hydraulic portion of the brake system, three numbers are always interdependent on each other – Pressure, Temperature, and Volume. They’re part of the ideal gas law, which states that PV = nRT, which for the sake of simplicity can be broken down to just PV = T. In the brake lines, the volume cannot change (as the lines and cylinders don’t expand in response to additional pressure), so when the pressure rises, so does the temperature. If you track your car, you’ve likely noticed that your braking power decreases after several runs. The constant additional pressure in the lines brings up the temperature of your brake fluid, which will eventually begin to boil, leaving your braking power at zero. You can tell this is happening when your pedal stops responding and sinks to the floor.
Increasing heat also affects the brake pads and rotors. Just like how fire is created by rubbing sticks together, friction creates heat. Riding the brakes or constant heavy use on a track can greatly increase the heat between the pads and rotors. This heat decreases the efficiency of the pads and rotors, contributing to what is commonly known as brake fade. When pads and rotors are hot, leaving them together (such as using your emergency brake) can cause the pads and rotors to fuse together, rendering your car immobile and requiring replacement.
-Leaks within the System
Just as how the loss of vacuum pressure can create difficulty in braking, a loss of brake fluid through a leaky line may cause serious problems. Just like in the syringe example from earlier, if a liquid under pressure has somewhere to escape to, then it will escape. In this case, applying the brakes would force the fluid through the leak, as opposed to applying force to the pistons in the brake caliper, eventually rendering the brakes useless.
-Warping and Brake Noise
If braking is done inconsistently, or an excessive amount of pad material manages to transfer to the rotor unevenly, problems can arise. Rotors can become warped, which will cause unsteady braking, typically accompanied with wobbles and rumbling under braking conditions. Similarly, these problems can affect brake pads as well, creating an inconsistent pad surface and therefore reduced braking ability and increased noise and wheel/pedal rumbling.
How To Address Some of These Issues
-Issues with Brake Boosters or Master Cylinders
Replace them. Some aftermarket master cylinders are out there, which change a variety of components (piston sizes, etc.) to change pedal feel and overall braking power. What you choose all depends on how you use your car (race-only, daily driver, etc.), as some parts are much better suited for race-only applications where braking is quick and heavy.
-Issues with Brake Lines and Fluid
If the lines are leaking, they’ll need to be plugged or replaced, with the latter consisting of a much more thorough solution to the problem. Some cars come with rubber brake lines, which may unexpectedly expand in response to pressure, especially as the rubber ages and becomes weaker, decreasing braking ability and increasing likelihood of brake fade. Upgrading to braided stainless steel brake lines is a logical step to fixing this problem, even if the car is only driven on the street.
If you track your car often, or if you’re very brake-happy, and your brake fluid constantly reaches high temperatures, it’s best to swap out your brake fluid with one that has a higher boiling point. Typically, these kinds of fluids tend to be more expensive, but they can withstand high pressure for much longer periods of time, keeping the braking system performing optimally under a variety of conditions.
-Issues with Calipers, Pads and Rotors
We at Fluid MotorUnion typically suggest replacing both pads and rotors at the same time. While it is a more expensive solution than just replacing one or the other, it carries good reasons with it. One way to fix a warped or overused rotor is to machine it down to a smooth surface. While one possible way to fix things, German manufacturers have come out against this action, ad we’ve followed suit. Only under certain (and very, very limited) circumstances do we perform rotor machining.
Replacing either the pads or the rotors without the other can create problems, just as easily as it can fix them. When pads and rotors are both installed and new, it allows time for the pad to evenly distribute some of its material on the rotor, creating an even surface with additional friction for stopping power. New pads on old rotors mean that the new friction surface of the pad is interacting with the grooves and material of the old rotor, which is one of the major reasons for unappealing brake noises. Similarly, new rotors and old pads don’t allow for a solid transfer of pad material, leaving the braking less than optimal. As a result, replacing both at once will extend the life and usefulness of your new brakes, requiring replacement far less often.
There are literally dozens of different kinds of pads and rotors on the market. While the car was engineered to use OEM (original equipment manufacturer, the company that made your vehicle) pad and rotors, moving to an aftermarket setup may provide for a much better braking system.
Aftermarket brake pads can contain friction materials better suited for a variety of situations, from street driving to racing. Racing pads typically contain a high friction material, which on the street can cause a high amount of squealing and brake dust (matter ejected during the friction process, the reason your wheels get darker and dirtier than the rest of your car). Many street pads are built with a higher coefficient of friction than OEM pads, but are also engineered to minimize dust and noise.
Aftermarket rotors tend to utilize different methods for maintaining low temperatures than their OEM counterparts. Some are cross-drilled (they look like they have little holes drilled on the surface) or slotted (they look like they have lines going across them). These drills and slots allow the gases created during heat-heavy braking to escape, keeping the brakes cooler and performing better.
Upgraded calipers typically bring additional pistons to the braking system. Brake calipers are usually measured in terms of how many pistons they contain (called pots). Therefore, 4-pot brakes have two less pistons than 6-pot brakes. Additional pistons create additional pressure and therefore additional braking power. Larger calipers are typically best suited for racing applications. Moving to a larger caliper can be more expensive than just upgrading pads and rotors, but it all depends on how much braking power you want or need in your vehicle.
…We hope you enjoyed this article on breaking down the braking system. We are planning on doing an Education Station each week for 12 weeks, focusing on several different systems, which might include anything from HVAC to Engine Internals to Steering/Suspension and more!