Just saw a TV commercial talking about a performance vehicle and how the large rotors installed help it to stop faster. I think there is some confusion on big brakes, big brake kits, caliper size, and number of pistons. Don’t get me wrong, I think big brakes are good, but let’s look at the whys and how’s. First, if you're cruising along at 50MPH, and you slam on the brakes to the point of lock up, it doesn't matter how large your brakes are. At that point, it's the tires responsibility to stop your car. The brakes have done all they could; they stopped the wheels from rotating, what else can they do? Nearly every car I’ve ever driven is able to lock up the wheels under street driving conditions.
First, let’s make sure we know the basics on how brakes work. We’ll only look at disc brakes here, because, well, drum brakes are lame. Here is a picture of a rotor, caliper and pads assembly.
Figure 1: Disc Brake Parts
When you push on the brake pedal, you push on a piston (the master cylinder), which pushes on hydraulic fluid (brake fluid). The fluid is pushed through the brake line to the brake caliper. The caliper contains one or more pistons in it. These pistons push on the brake pads. The pads push on the rotor, which is physically linked, to the wheel. The harder you push on the brake the more the caliper squeezes the pads against the rotor to slow the wheel down.
All That Force
To achieve the large force required to stop a 4000-pound vehicle with your foot requires some serious force multiplication! The master cylinder’s piston is much larger than the pistons contained within each of the brake calipers. Not only is it larger in diameter, but it also moves further in and out compared with the pistons in the calipers. The pads within the calipers are very close to the rotors at all times, therefore the pistons within the calipers only have to move a very small distance to cause the pads to contact the rotor. This is how the brakes generate all their force. The large movement of your foot and leg pushing against a large diameter piston to push small diameter pistons a very small distance causes those small pistons to move with a much larger amount of force. This is all based on Pascal’s Principle. Here’s how it works:
Figure 2: Pascal’s Principle
Forgive my crude drawing, but this will make this easy to understand. F1 is the force you apply to the brake pedal and F2 is the force that will be applied to the brake pad by the piston inside the brake caliper. Say the area of that caliper’s piston (A2) is 4 square centimeters, and the master cylinder’s piston (A1) has an area of 50 square centimeters. Pascal figured out that F1 times A2 has to be equal to F2 times A1. Or said another way A1 divided by A2 times F1 equals F2. So, the area of the master cylinder’s piston divided by the area of the brake caliper’s piston times the force you push on the pedal equals the force pushed on the brake calipers piston. Let’s try some numbers. Say you push on the pedal with 100 pounds of force. In that case, the brake caliper’s piston will see 1,250 pounds of force! That poor brake pad!
As mentioned above, the caliper contains one or more pistons, the brake line connections, and holds the brake pads in place.
Figure 3: Caliper Components
Figure 2 shows a typical single piston caliper. When the brakes are applied, fluid pushes the piston outward against the pad, which squeezes it against the rotor. In this caliper the single piston pushes against one pad, which also causes the outside pad to contact the rotor. Every caliper contains two pads, one for each side of the rotor. In some cars multiple piston calipers are used, such as 2, 6, or 8. In general larger calipers with more pistons, and possibly larger rotors, are placed on the front. This is because when you brake, the center of gravity of the car shifts towards the front, causing the front wheels to be loaded up with more weight. This makes those wheels more difficult to stop rotating, and consequently you’ll need more braking force and potential for the front brakes.
So why do we want big brakes? The main answer is heat, and how it is dissipated. Your car could have very small rotors installed, and still be able to lock up the wheels at 60MPH. Try to do it repeatedly, however, and you’ll notice that on the fifth or sixth hard stop from 60MPH, those same brakes no longer lock up the wheels. Keep at it, or use higher speeds, and you’ll have trouble slowing down at all, no matter how hard you push on the pedal. Keep at it still more, and the pedal will start to become soft and possibly drop to the floor and you’ll loose braking altogether! As the temperature of the pads and rotor heat up, the friction generating capability between the pads and rotor go down. It’s a bummer, but it’s true. Small rotors heat up faster because there is less mass, or material to heat up. Larger rotors take longer to heat up, and will therefore withstand more repeated stops, higher speeds, and larger loads. The second reason for larger rotors is a longer torque arm. The larger rotor allows the pads to be placed further away from the center of the wheel/hub. By increasing this lever-arm distance you can increase the braking force. Using multiple pistons on calipers allows a more even force to be applied to the pad, and can also give the pedal a more firm feel. With a larger the number of pistons, you can apply force to more parts of the pad. It’s the old how many circles can you fit in a rectangle problem. In addition, on calipers with 8-12 pistons, you’ll notice that the manufacturer places more of the pistons toward the outside of the pad, which again increases that torque arm length.
Figure 4: Eight-Piston Caliper
Figure 5: Twelve-Piston Caliper
Clearly from Figure 4, and 5 you can see that adding pistons increases the pad surface area being pressed upon by pistons.
So what happened when we lost braking altogether? In that case the brake fluid became so hot that it boiled and turned into a gas. As we all know a liquid cannot be compressed, but a gas sure can! Now when you press on the brake pedal you’re just compressing the boiled brake fluid gas. This doesn’t provide any push against the piston inside the brake caliper, and no braking takes place. Once the fluid cools down, it returns to its’ liquid state and your brakes now work again. When purchasing brake fluid, you will notice that the manufacturer lists a wet and dry boiling point. Brake fluid has a strong tendency to absorb moisture; the stuff literally sucks the moisture out of the air. The most common way moisture gets into the brake system is from leaving the container of brake fluid open. The dry boiling point given is the temperature at which the fluid will boil when no moisture is present. The wet boiling point is when 3% of the fluid by volume is water. Unless you change your brake fluid very frequently, the wet boiling point is the more important number to look at. As you can see, it’s important to have a high boiling point for your brake fluid – as soon as the fluid boils you have no brakes!
Do you need big brakes? I would upgrade the brakes on a stock car or truck if you have experienced any sort of fade problems on the street. In my own experience, this has never happened – unless you are really exceeding the posted limit! If you road race the car, you will probably end up wanting larger brakes, or if you frequently tow heavy loads with your truck. You don’t want to wait to find out that your brakes will fade on the street or track. So, go out and try some things, and see what happens with the brakes you have in a safe environment. This is generally true; please try things out with your vehicles, you bought them, go play! Try some panic stops from 70MPH; make sure the wheels lock and the ABS activates. If you are towing a heavy load for the first time, try some stops from highway speeds before you encounter a situation where you do need to stop, only to find out you needed triple the stopping distance. Test those things out and make sure they are good, don’t just assume you need something you may not, and certainly don’t assume you do not need it, if you might. Happy braking!
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