Braking Theory & Construction (updated 7-23)

[ZfrkS62]

This is my 9,000th post. I was going to wait until it was finished, but i realized i can't stay off JW for that long I am going on vacation tomorrow afternoon and will be off JW entirely for a week (you spamming bastards are going to kill me with unread posts when i get back )

If you feel lost in visualizing what is being discussed, fear not, i will have illustrations added at some point. Until then, you can look for illustrations at www.howstuffworks.com
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DISCLAIMER: I AM NOT GOING TO BE HELD RESPONSIBLE FOR ANYTHING YOU SCREW UP BY WORKING ON YOUR CAR YOURSELF. IF YOU DON'T THINK YOU CAN HANDLE A JOB, DON'T DO IT! THIS IS SUPPOSED TO BE AN INFORMATIVE GUIDE, NOT A LICENSE TO CALL YOURSELF A MECHANIC!!

Braking system theory and maintenance



In this section we will be looking at how the most important system on your car. The Brakes. This simple system is the most important feature of any given car, yet the most commonly ignored, overlooked and generally forgotten system in terms of maintenance. That is until you hear the tell tale sign of ear shattering squealing as your brake pads waste away the final few miles of friction material.

Hydraulic Theory

Your brake system is a hydraulic circuit and operates under Pascal's Law, which says "Pressure within a closed system is equal and undiminished in all directions." This means that that the fluid is not moving. For you math nuts, it follows this formula:

Pressure = force/area
Force = Pressure x Area



Components

As mentioned, the braking system is quite simple. It is a closed hydraulic circuit which uses hydraulic pressure to force friction linings against either a metal rotor, or drum, depending on the set up. We will start running down the system components from the very beginning of the braking process.

Brake Fluid

This is the life blood of your braking system. In the US, there are 3 classifications of brake fluid that fall under Department of Transportation (DOT) and Society of Automotive Engineers (SAE) standards. Because brakes use friction to stop the vehicle, large amounts of heat are generated. These heat is naturally absorbed into the brake system and subjects the fluid to heat that exceeds the 212*F boiling point of water. There are 2 boiling points used to classify fbrake fluid. Dry, and Wet. The dry boiling point is pure brake fluid, with no moisture contamination. The wet boiling point is the boiling point of the fluid with 2% moisture contamination.

DOT 3 and 4 are polyglycol based, similar to engine coolant. Polyglycol based fluids are hygroscopic, which means they absorb water. This is important because if moisture in the system was not absorbed, it could form water droplets that could freeze, or boil and cause localized corrosion of the metal lines. DOT 3 and 4 are mixable. Because brake fluid can absorb so much moisture so quickly, it is important that fluid containers do not be left open for more than 1 hour. Less in high humidity areas. 18 month old fluid can contain between 2-3% water. 3% contamination in DOT 3 fluid will drop it's boiling point by 25%. DOT 4 is affected more adversely as a trade off for absorbing moisture much slower. It will lose 50% of it's boiling point with only 3% moisture.

DOT 5 fluid is a synthetic fluid used mainly in racing applications and very high performance vehicles. This fluid is a silicon based fluid with a boiling point of 500*F, dry; and 356*F wet. However, this fluid is not hygroscopic, so it does not absorb water. Therefore, the wet boiling point will never come into play.

Boiling points: DOT 3 DOT4 DOT5
DRY: 401F 446F 500F
WET: 284F 401F 356F

Dot 5 fluid contains 3 times the dissolved air that glycol based fluids do. Because of the high air content, the silicon fluid has a spongy feeling to the pedal and reguires more pedal travel before the brakes are fully applied. Silicon fluid contains 15% dissolved air. Glycol 5%. Also because of the air content, rapid cycling will cause the fluid to froth, so it should NOT be used in ABS applications.

It is important to note that even though DOT 3 and 4 are interchangable, they are not generic, meaning no two companies make it the same way. Different brands will perform differently. These fluids are still interchangable.

Brake Pedal

This is the input from the driver to start the braking process. It is a 5:1 mechanical lever, meaning that for every 1lb of force imparted on the pedal, 5lbs are imparted on the Master Cylinder.

Power Brake Booster

There are 2 types of power boosters for the braking system. The vacuum booster; which is the most commonly used. And the Hydro-boost system, which is a hydraulic system powered off the power steering system. These systems are mainly used in diesel powered vehicles since diesels do not produce sufficient vacuum to operate a vacuum booster. The brake booster is located in the engine compartment between the firewall and master cylinder.

Vacuum Power Booster uses the difference in atmospheric pressure (about 15psi) and vacuum to apply force. Inside the unit is a diaphragm that seperates the vacuum and atmpospheric pressure. When the brakes are at rest, the unit is pulled into a vacuum. When the brakes apply, atmospheric pressure is pulled into the unit on the other side of the diaphragm and assits in the application of the brakes. The unit is NOT serviceable and should not be taken apart as there is a high tension spring compressed inside of it and will cause severe injury if it unloads!!!


Master Cylinder

The brake master cylinder is comprised of the Reservoir which holds the brake fluid, one or two pistons, depending on the application, cup seals which seal the fluid pressure and pushes the fluid through the brake lines. A secondary cup which is an o-ring that seals the back of the master cylinder from leaks. It doesn't seal any fluid pressure. A spring inside the cylinder helps return the piston assembly to it's rest position. The unit is connected to the brake power booster through a pushrod.

Fluid fills the pressure chamber through a compensating port. When the pushrod moves against the piston, the cup seal moves past the compensating port and builds pressure in the pressure chamber, forcing pressure to build in the lines.


In 1967, Tandem Master Cylinders were required in all hydraulic braking applications. The reason for this is that if hydraulic pressure is lost in one part of the system, total braking failure is not experienced. In a tandem master cylinder, two pistons work within one cylinder bore. The rear piston is the secondary piston and pushes fluid to the brakes it serves, while pushing fluid against the rearward facing cup seal of the front (primary) piston. The hydraulic pressure forces the primary piston to push fluid to the brakes it serves.

During hydraulic braking failure in the system served by the secondary piston, the lack of hydraulic pressure will cause the secondary piston to bottom out against the primary piston, forcing it to build pressure in it's half of the system.

There will always be enough braking power to stop the vehicle in the event of one half of the system failing, though it will take more effort to stop due to the reduced braking ability.

Tandem systems are either split longitudinally, or diagonally. In a Longitudinal split, one piston will operate the front brakes, while the other operates the rear. In the Diagonal split system, the brakes on opposite corners are operated. The diagonal split is generally used on front wheel drive vehicles since weight transfer can shift 80% to the front under a hard stop, rendering the rears all but useless.

Some master cylinders have what is referred to as a step bore. This moves a large amount of fluid quickly when the pedal is first pressed to quickly apply the brakes. The smaller bore provides a pressure boost during stopping. This all results in less pedal travel.

Master Cylinder check valve

On systems with drum brakes, there is a check vlave the retains 6-25 psi in the system when the brakes are not applied. This keeps the rubber seals of the wheel cylinders sealed against the bore. Not all systems have this check valve however as cup expanders on the ends of the center spring which expand the seals against the cylinder bore.

The check valve is a simple flapper valve, aka duckbill valve. As the fluid pushes on the valve, it pushes open, allowing the fluid through. When your foot is released from the pedal, pressure is released from the system and the valve closes. But fluid pressure pushes the valve off it's seat against a calibrated spring until the spring tension overcomes hydraulic tension and completely closes the valve.

[graywolf624]

Way to completely diss me by not including a link to the article I wrote and posted here long ago

[ZfrkS62]

If it was before 9/2004, i'd have had no way of knowing

Don't worry, one of my plans is to compile an index of helpful links and articles (Mopsdrops' articles not included)

[graywolf624]

I think you were here.
I was just kidding but here you go:
The Truth About Brakes


The Working of Brakes

Over the past several years I have seen many myths perpetrated by the main stream. The purpose of this article is to dispel some of those myths while explaining basic concepts. Through the course of this article you will learn about how brakes work. You will also learn the advantages and disadvantages of cross-drilled, slotted, and vented rotors. Lastly, you will learn about brake bias.

There is a common fallacy out there that increasing your brake pad size in terms of swept area will increase the stopping power of your car through greater friction. From a standpoint ignoring operating temperatures this is in fact false. The force of friction is determined by physics as the force down on the object times the coefficient of friction. As such there is no surface area in the friction equation. However, the temperature of the pad varies throughout its use changing the coefficient of friction at each point along its temperature slope in a non-linear/non-progressive manner. As such it is possible that a larger pad will change the friction force favorably given pad makeup. It certainly will change the amount of time before the brakes enter the proper range and when they leave the range. It will also influence when and how long it is at the peak performance point. Meanwhile, modifying the pad material can change this operating range. As such the affect of increase in pad size on braking friction would depend on the makeup of the pad. Also note that the only way to modify the force down is to change the brake piston force (by size changes or number for example).

This does not mean that a larger brake pad does not help braking! The benefit of a large brake pad comes into effect when you consider thermal dissipation. The larger the pad the more this thermal temperature (created by the interaction between the pad and rotor) is spread amongst a pad. This means less temperature is concentrated at one point on the pad and the rotor absorbs more heat. This decreases the likelihood that the pad itself will heat beyond operating temperature. If the pad were to go beyond operating temperature it would glaze over resulting in brake fade. Furthermore, a larger pad results in a longer service life of the pad since there is more pad material to consume.

**Note: This is not to say that a huge pad is the way to go. I am simply telling you the benefits of a bigger pad. Do not. I repeat do not buy a huge pad thinking that will be the end all. However, consider a pad with a better material makeup for a large difference.








Cross-Drilled /Slotted Rotors

The second thing you can do to improve your brake performance is often to go to a larger rotor. We all know that this gives the rotor further ability to dissipate heat away from the pads through itself and through the air (conductive and convective heat transfer). So obviously a larger pad, a larger rotor, or both result in better brake performance by avoiding brake fade.

But what about cross drilled or slotted rotors? Well the common belief in the main stream is that somehow slotted or cross-drilled rotors allow for better performance by handling heat. This is 100 percent false. The individuals involved in such fallacies mention that air through the holes or slots work to cool the rotor (convective heat transfer into the air from the rotor). The issue is that from physics we know that metal transfers heat better then air by a significant amount. As such the larger mass of the rotor becomes more important then the larger surface area of the rotor in any situation other then the optimal. Cross drilling and slotting rotors are not optimal manners of creating metal to air transfer through larger surface areas. There is not much airflow through the holes or slots. Furthermore for cross drilling the holes will fill with brake dust in effect lowering the cooling ability of the rotors vanes they pass through.



Rigidity

From the information above we can glean that the rotor begins to work as a heat sink. Now by cross drilling or slotting we are decreasing the overall amount of metal to transfer this heat to. Clearly we are decreasing performance of the rotor to dissipate heat amongst itself. Furthermore, the holes of a cross-drilled or slotted rotor decrease the area of the pad that contacts the rotor. This concentrates the heat more on certain areas of the pad, which has similar effects to that of using a smaller pad. As such the pad heats up more quickly.

We are also damaging the brakes structural rigidity. The iron in a brake rotor is made of a crystalline structure. By drilling holes in said surface we cut the end grains creating a situation that breeds cracks. Furthermore, even if we were to cut the rotors correctly to avoid cutting the end grains structural rigidity is still decreased. The temperature around the holes will be slightly less then that of the entire rotor leading to temperature stress. Moreover, the decreased mass will result in lowered rigidity.




Advantages

So what do cross drilled and slotted rotors accomplish? The main original purpose of slotted and cross-drilled rotors was to vent gases that buildup between the pads and the rotors. However, this reasoning is no longer valid. As the years have gone by pads have been designed that produce very little gas. Furthermore many pads come with groves in themselves that allow for the removal of any minor gas that is created. A slotted or drilled rotor always decreases the rotors capability to dissipate heat amongst itself. A slotted or drilled rotor will also clean off the brake pad as it passes the slots at the expense of faster pad wear. As such there are benefits for rally and dirt tracks. Furthermore, the slots or holes themselves can serve to wipe off the top layer of glaze that tends to appear on your brake pads. Some racers say this last part is beneficial while others question whether the slots will fill before the deglaze affect is ever helpful. I have yet to determine the answer to this question.

The answer of slotted and cross-drilled rotor usefulness seems to lie with whether the benefit of cleaning the pads outstrips the loss in heat dissipation. In terms of cross drilling there are so many costs that nothing is accomplished beyond perhaps giving you a certain bling look. In a motorcycle or other extremely light vehicle the decrease in rotational inertia and unsprung mass might perhaps be useful (once other more efficient avenues are exhausted). However, in a street car or race car the speeds and weight of such vehicles will make the relatively miniscule decrease be outweighed by the need for more heat dissipation. Slotted rotors, meanwhile, share the positives of cross drilling but notably are slightly less subject to the costs. They do not impede airflow through the rotors vanes, nor do they have as large an affect on structural rigidity. Therefore, the need for slotting depends on your application.








Vented or Vaned

So what do ventilated rotors accomplish? Well, the concept is that they will help cool the rotors. We discussed earlier that giving up mass for surface area to gain cooling of the rotors should only be done when optimal. Vanes are the optimal method of achieving these goals. The rotors are designed to increase surface area and to flow air in the middle of the rotors. The increased surface area to the air clearly provides for more cooling from the air at the cost of mass. So why does this method work while the others fail? The first reason is that a ventilated design flows a lot of air through a rotor. A ventilated rotor acts as a centrifugal pump sucking air into the rotors. This is why rotors with curved vanes provide better braking.

A slotted or cross-drilled design will flow very little air under heavy braking. As such the vanes of the ventilated system are far more efficient. Moreover, air moves through the center of the rotor cooling the rotor more evenly and efficiently. Furthermore, the ventilated design does not decrease the contact patch of the pad on the rotor. Finally, the design has different structural rigidity qualities then that of a cross-drilled or slotted design.




Brake Bias

So now you know that increasing your pad size and rotor size will help to stop your brake fade. You also know that swapping the pad, increasing the rotor size, or increasing the force of the pistons on the pad can increase your stopping force at the tires. Finally, you have learned to stay away from cross-drilled and look very closely at whether to use a slotted rotor.

So does that mean it is time to go get that fancy front brake kit for your car? Well, potentially no again. The first thing to consider is that in any braking setup the tires are the ultimate limiting piece. You cannot stop faster then your tires allow you to stop, ever. As such, if your car can lock it’s tires under braking consistently then better brakes will not improve your braking performance. (I stress the consistent part, as brake fade must also be combated.)

Furthermore, most people understand the idea of brake bias, but fail to understand its application. A typical car is setup with the front brakes being far more effective then the rear. Now the first thing we must realize is that from a dynamic stand point your car should have stronger front brakes. When you brake physics transfers more weight to the front axle that must be accounted for. However, in this dynamic state we also have brake bias. Your typical street car is slightly dynamically biased towards the front. This leads to the front tires locking up before the rear tires under heavy conditions. Such a situation is obviously not optimal for a car stopping quickly.

You want the stopping bias to be roughly equal given the acceleration you are traveling at (please note that the bias depends on the acceleration of the vehicle). When you have a front bias you get a more stable stop (as opposed to a rear bias where a lock can cause spins), but you also get further forward weight transfer and longer stopping distances. Most cars stock come with a minor front bias for the layman. So it is clearly discernable that by going with a bigger front brake kit you are possibly increasing your stopping distance if you do not equally modify the rear brakes, change your pads, change your tires to ones that do not lockup, or set the clamping forces lower on the front brake. Without making such changes the larger effective radius can lead to an earlier lockup of the front wheels.

[ZfrkS62]

Ah yes, i remember this popping up somewhere now. i think it was an argument that brought it up.

I'll slot it in with the braking modification thread once i get it going.

[TopGearNL]

The guy from The Gumball Rally that had the Brake problem on his SLR should read this topic really

Thanks, I haven't got a car but still very interesting stuff!

[ZfrkS62]

Originally Posted by TopGearNL The guy from The Gumball Rally that had the Brake problem on his SLR should read this topic really Thanks, I haven't got a car but still very interesting stuff!

The SLR is a different animal in terms of braking because of the carbo-ceramic brakes. Since they are a fairly new concept in terms of road use, i don't know anything about them really. That and i didn't follow the gumball at all so i don't know what he experienced.

[ZfrkS62]

info on tandem master cylinders has been added.

[ZfrkS62]

i think that was in an article graywolf posted. and yes, they are more for the look. Even looking at full race cars, you don't see crossdrilling.