Brakes: A Tech Article

Brakes are the first line of safety and control in your car. You never think about them until they are not there. Trust me, its not a good feeling. And there is no moment more eye opening than feeling the brake pedal fall to the floorboard with no pressure. Today I’m going to explain what the function of each part is and wipe out some of the myths associated with them.

The brake works by pressurized fluid pushing in the piston forcing the pads against the rotor. The added friction slows the car by turning the kinetic energy of the moving vehicle into heat. Simple enough. But facing the limitations and tuning the system are not simple enough. So I’ll break down and analyze each part. And hopefully wipe away some of the myths placed on it.

I’ll start with the biggest part, the rotor. Basically its a round piece of cast iron. Heat treated to reduce warpage during its life cycle. Usually there are venting ports through the middle with vanes that push air through to keep them cool. Most people think that a bigger rotor will give you shorter braking distances. This is false. With the same tire, braking will be the same. Even little rotors will easily lock up the tire. A larger rotor does provide more leverage, but the main thing it buys is more heat tolerance. It’s ironic that the one thing that brakes are supposed to due can be its downfall. Overheating the rotor will eventually cause the rotor to warp. You can feel a vibration in the brake pedal when they are warped. It is important to keep the rotor as true as possible because a warped rotor does not give smooth braking force. In a hard braking instance, the panic stop, the spike in clamping force will play havoc on the car’s anti-lock systems causing them to cut on and off. Which leads to longer stopping distances. With all the other inattentive drivers out there, using cell phones, texting, as well as the good old fashioned drunks, its best to keep the brakes as good as possible to keep you and your vehicle safe.

Now I’ll move on to the caliper. This is where the pads are housed and where the clamping force is applied. Inside the caliper is a piston, that when the pedal is pushed the fluid pushes the piston out and the pads contact the rotor. Now most of you have heard of multi-piston calipers before. Two piston, four piston, six piston, etc. The larger numbers are used on high performance and racing cars. So more pistons will make your car stop shorter right? The quick answer is no. More pistons give more even pressure across the pad face and more importantly for a non ABS car, better driver feedback. In some racing series anti-lock brakes are illegal. So the driver, in order to get the shortest braking distances possible for that unbelievable lap time, use a technique called threshold braking. Basically the driver is bring the brakes to the point of locking up the tire and releasing the pedal, just slightly, to stop the lock up. This is how a modern cars anti-lock system works also. Lock, release, lock, release. For a driver that has tripped the anti-lock brakes, you will feel a pedal vibration as the ABS system actuates. Don’t freak out. Just keep pushing. This is the shortest possible braking distance your car is capable off without Michael Schumacher driving it.

Sorry for the tangent, but I felt I should get the ABS stuff in before you got bored and quit reading. Back to calipers. The caliper sees most of the heat in a performance application. So maintenance is required. It is the sturdiest part of the braking system, but it is not uncommon to have to get the seals replaced. I would recommend about once a season for the race car, once every two or three fluid changes for the track beast, and about once every ten years for the pedestrian road car. The brakes are your #1 safety equipment. It’s easy to forget about them, until they go out.

The liquid used to push the piston in is the brake fluid. It is a very specific hydraulic fluid made to resist expansion due to temperatures. The fluid is the weak link in the braking system. When your brakes tell you that you have pushed past their thermal limit, its the fluid that will fail. Us performance junkies say we boiled it. And that is not far from the truth. Once the fluid hits its heat maximum it boils, creating air in the brake line. And air can compress unlike a fluid. So you end up with no pedal pressure and no brakes. I have been in that situation before and there is no worse feeling that charging down a mountain road and feeling the brake pedal go to the floorboard.

Now about 99.9% of road cars use what is called in the United States as DOT 3 brake fluid. DOT 3 is cheap, fairly resistant to temperatures and humidity. For a road car, there is no better alternative. But for a track or high performance car, it has its limitations. You can use the full braking power a few times, but you need to allow the brakes and fluid to cool off before diving deep into them again. My failure was downhill in a fairly heavy ’86 Mustang, with stock 10.5” front rotors and not much time to cool off from braking zone to zone. It didn’t help I was using the gas in between the corners either. Now if I drove semi-fast and lifted earlier or lengthened out the brake distances I would have had no problems. So don’t think that it was the road that caused the failure. It was the driver. And other than getting a later model system with larger rotors and calipers and investing in DOT 4 brake fluid, there’s not much I could do. So now I get to the alternative. DOT 4 is what all the race cars and ultra high performance cars come filled with. It has a significantly higher boiling point than DOT 3. But there are some downsides. DOT 4 sucks up water like a ShamWow. Any water. Especially the humidity in the air. The more water it picks up, the lower its boiling point becomes. Most race teams vacuum the fluid directly out of the can and into the brake system. 99% of it never sees air so its higher boiling point is preserved. And the vacuum system isn’t cheap either. But its worth it to them because of the extra heat it buys the race car. Because of how quickly the fluid picks up water, it needs to be changed much more frequently. With a race car all fluids are changed between races. Its simple insurance for the team. Why would you not spend the $50 worth of fluid that could fail and cost you the race. The one thing about DOT 4 is that the fluid is much thicker than DOT 3. This can cause an ABS system to not work. So if your upgrading, check the viscosity rating. A heavier fluid could damage the ABS system. You should also check the each brand’s heat ratings as well. There are some DOT 3 fluids that outperform lower end DOT 4’s thermally with the lower viscosity.

Now I shouldn’t have to say it because I covered it in the boiling segment. But air in the lines is bad. It gives the spongy pedal feeling because the air can compress and fluid won’t. If you feel that spongy pedal, get the car serviced as soon as possible. It is a very hazardous condition. And when you check your oil, which should be at least every fill up or week, look at the rest of your fluids as well. This includes the brake fluid and coolant. Because ending up on the side of the road because of overheating or in an accident because you have no brakes is not the car person way.

The master cylinder is the connection from the pedal to making the fluid press on the pistons in the calipers. Usually it is cast in one piece with a reservoir on top of it. It normally has the brake proportioning cast or built in. The proportioning gives the front more pressure than the back. Which is what you want. Despite what Forza says, 60 to 65 percent of the braking happens on the front of the car. When you brake, the nose dives down putting more weight on the front tires. So to balance it out the manufacturers balance more brake pressure to the front. Because locking up the front tires first is much easier to handle than the back end locking up and trying to swap ends with you on corner entry. Most aftermarket brake manufacturers make their products for a specific car. Most of the time they know the amount of proportion in the stock master cylinder and balance a small amount more to the rear. Always to be safer, the car maker usually over balances toward the front and that can increase braking in a dry scenario. But you add weather elements to the equation and its better to be safe than have your product try to kill your consumer. Now if you are upgrading to an aftermarket system as stated it balances back toward the rear decreasing brake distances. The reason for that is that most of the people who upgrade to the aftermarket system are putting it on a track car or show car. Something that is used sparingly or only in good weather. As well as being a performance minded individual who would understand that you would give more distance to the car in front of you when it rains. Now if you are upgrading the brakes on an older car I suggest you buy the whole system at once. This is the safest way to make sure that the entire system works as intended and as safely as possible. If you have no options or are certifiably crazy and cheap as your author, and are swapping in another car’s system, take the whole thing. If you plan on using C6 Corvette brakes on your 240SX, you had better use the Corvette’s master cylinder to go along with the calipers. And make sure the hard brake lines are close to the same diameter as well. Any deviation can cause the front/rear bias to be off and reduce braking effectiveness.

The pads are the where the pressure hits the rotor. They are the wear item in the braking system. When you hear the screeching sound when you use the brakes, its time to get new ones. There were many myths associated with the pads so I’ll head them off first. At one time you had to bed the pads in. Which is you would need to treat them through multiple heat cycles rapidly. This was also true of the rotors. There was a condition when the pads heat up they would produce a gas. That would seep out of them. Under heavy usage the gas would get strong enough to push the pad off the rotor. Which if you look at the pad it has a line cut in the middle for just this purpose. It also applies to cross drilled, slotted and dimpled rotors as well. These are ways to reduce the gas under heavy and repeatable braking loads. While the pedestrian daily driver would never reach the temperatures associated with gassing, I would still bed them in for a car that is going to be tracked or used hard.

Now there are many types of brake pads. There are the compounds that leave no brake dust to those that aggressively wear the rotor. Make sure you are using the right kind of pad for your application. You don’t want a severe duty pad that causes unnecessary wear to the rotor for your street car or screeching noise every time you press the pedal. But you might want that in a track only car. When you get to performance pads there is a three icon rating system. The three variables are temperature, initial friction, and extended friction. Most road pads fall in a lower class in temps, and have a strong initial bite with a lessening extended friction. Which means it comes on hard and then loses friction the longer its held. The common idea is that it is more natural for a non performance driver to push harder the longer the pedal is pushed. And for most drivers it is true. But for racing applications it might be reversed. Because the racing driver normally has such pedal feel, because of the multi-piston system, and ability to threshold brake that they can adapt better to a rising friction compound. As well as the fact that the slower a wheel gets the more clamping force can be applied before lockup.

So that is brakes in a nutshell. Hopefully, you have gained a little insight into the function and know the warning signs to look for when it comes to the maintenance of your braking system. As always leave any feedback or questions in the comments as well as any tech requests you want a clearer explaination for.