Very Informative Brake article

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The Renegade
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Very Informative Brake article

Post by The Renegade » Wed Aug 30, 2006 8:00 pm

**** A big thanx to Callumgw for bringing us this article, and to Steve Gavin for writing it. ****


Pedal Feel Contributors:................

First some quick definitions: Brake Pedal ratio is the measurement of how much mechanical assistance you are getting from the pedal. Example: a ratio of 4.1 will give you 41 pounds at the booster input rod for a 10 pound load at the pedal itself.

Dead or lost travel is how much pedal stoke is required before you actually start stopping.

Dead Travel or Lost travel and overall poor pedal feel is made up of the following:
[(Travel as measured at the brake service pedal assembly pin (where the booster connects)]
(Imagine traveling from the pedal through the brake system to the rotor, all lost travel must be multiplied by whatever your brake pedal ratio is.)

1. Tolerance between brake pedal pin and booster input rod. This can be quite a bit for systems that use a pin mounted brake light switch. If you do have such a switch do not remove it or take up the slack as your brake lights will be on all the time. If you don't have a pin-mounted switch just get a tighter bushing. If you do, you're s**t out of luck.
* will be felt during first few mm of travel

2. Slack in the brake pedal assembly itself. To see how good or bad yours is, with the car off pump the brakes until hard (2-3 pumps) and then grab the pedal with your hands and see how much it moves around.
* will be felt through first few mm of travel (I hope)

3. Dash flex. This can range dramatically from vehicle model to vehicle model. Not much you can do about this.
*felt during medium and high decelerations stops on most cars, on Fords dashes flex with the breeze

4. Lost travel in booster. This is designed to be there to allow for booster expansion due to climate and use over time. Only adds half a mm (multiplied by the pedal ratio).
*felt only in the first few mm of travel

5. Flex of booster shell. Can be a real problem on some designs. All you can do is try and brace the booster or replace with a better product.
*felt on medium and high deceleration stops

6. Design tolerances in the Master Cylinder. Varies greatly from one to another. Simply, if you want less lost travel in the TMC (tandem master cylinder), you have to pay for a more expensive one. A minimum lost travel of about 1-1.5mm is required for proper and safe operation. However, I've seen some with double that. (Again multiplied by the pedal ratio)
*felt during first 10mm or so of pedal travel

7. The brake tubes and ABS unit. Maybe .0000001mm here. Don't worry about it.

8. Brake hoses. Get steel braided ones, there worth it. Rubber hoses flex quite a bit even under low pressure.
*felt almost all the time

9. Brake Caliper Piston Roll Back. This is usually the worst offender. Only way to get rid of this is to get better calipers. Roll back is how far the piston moves away from the rotor when pressure is released. The bigger the gap, the more you have to push on the pedal to get contact.

10. Caliper Deflection. The caliper actually flexing under pressure, like 9 you can only improve this with better calipers.
*felt during medium and high deceleration stops

11. Brake pad backing plate. If this is flimsy it will flex and not allow a good contact between the rotor and pad forcing you to apply more pressure and therefore more pedal travel. Fixed by replacing pads with higher quality ones.
*felt most of the time

12. Brake pad material itself. If the material is low density it will compress like a sponge. And if it's a low mu (friction) pad it will require more pressure and therefore more pedal travel.
*felt all the time

13. Rare, but a problem on really cheap brake pads: The bonding process used to bond the brake pad material to the backing plate. A poor process will cause the material to "squirm" around.

Well those are the biggies, but not the biggest. The biggest is AIR in the system. Before you do anything else do a really good and thorough brake bleed. And only use the fluid it says to use on the cap, DOT 3 or DOT 4 or whatever.

Also, changing the brake pedal assembly to one with a lower ratio. Remember all lost travel is multiplied by the pedal ratio, the lower the ratio the less dead travel at the pedal pad. This also firms up the pedal as you're getting less mechanical assistance. Just be careful, because if you're brake booster fails it will take more force on the pedal to come to a stop.

Pedal feel can also be "tuned" with a different booster.

Cut-in is what sets the initial point of boost (when it kicks in)
2-stage (knee height) is what sets how much initial force
Boost ratio is just as it sounds, it sets how much assist you get
Run-out is the maximum assist

By lowering the cut-in and increasing the 2-stage you get a better initial bite sooner. Just have to be careful you don't go overboard and have the driver eat the steering wheel at every stoplight (like an '80s Audi).

Difference between Feel and Performance:............................

I want to make it clear there is a difference between making a brake system feel better and actually perform better.

The stopping distance of a car is not necessarily directly related to the feel. A poor feeling brake system can have very good performance, i.e. Jaguar. While a great feeling system can have lousy performance, i.e. Ford Focus

The only way to decrease braking distances is to create more heat through friction and conduct that heat more efficiently.

Basically, you need a bigger contact area between the rotor and pad (bigger pads and rotors) with the best contact patch possible between the two. Also higher friction levels with higher mu brake pads.

And most importantly.........REALLY GOOD TIRES!

And you need a way to get rid of that heat at a faster rate. More rotor mass and/or better conduction (vented, etc.).

How a car stops is simple, it takes Kinetic Energy (energy created by motion) and transfers it to Heat (Thermal) Energy. It does this by the mechanism of friction.

KE=1/2 the mass of the vehicle multiplied by the square of it's velocity

Stopping distance is determined by the rate of KE to TE transfer, or also know as WORK.

Simply, to stop sooner you need to transfer Kinetic Energy into Heat Energy faster.

There are no other tricks; you need higher levels of friction and ways to dissipate the heat quicker.

The big things to do to get shorter stopping distance:

1. Best Tires for the conditions (use common sense here, no Pilots in Feb in Alaska)
2. Higher Friction Brake Pads and/or rotors
3. Bigger diameter rotors
4. Calipers with more piston area

The other things you can do that give you that extra advantage:

1. LOSE WEIGHT (the car I mean), less mass, less KE

2. Minimize rotational inertia of wheels/tires. Get lighter wheels and tires (all else being equal). Note: the farther away from the center of rotation the mass is the great the inertia, so a 17" wheel will have greater inertia than a 16" all else being equal.

3. Try and balance out the braking, if the rears can do more work it spreads out the work. Less weight transfer to the front BY MOVING THE BATTERY TO THE TRUNK, stuff like that.

4. Practice! Get to know how your system works so you can best utilize it.

5. Don't drive to fast, remember it the square of the velocity. The amount of KE increase from 40KPH to 80KPH is not 200%, but 400%

Also, venting built up gases under the pad was a big problem, but pad tech has come so far that it's not something I would worry about (assuming your using race pads for racing). Again it's one of those things that are there but only makes a very, very, very minor difference (if any) most of the time. Same thing with slotting, which was developed to vent gases and clean pads.

Things like 2-piece rotors, X-drilling, slotting, cryogenics, heat-treating are all "at limit" technologies. In other words they only make a noticeable difference (if any) at the very limits. If you drive on the street in a manner that actually utilizes these technologies regularly you're probably dead and not reading this!

Brakes are as much art as science, there are just so many different variables involving not only performance but also feel and consumer wants it becomes a real pain!

Again, you have to know what you want to get out of your system and where you're going to use it.

In the end if you got the cash it won't hurt, but you get to a point of diminishing returns and you have to wonder!

Brake Performance Vs. Brake Feel:......................

Brake feel: pedal effort and pedal travel for any given desired deceleration. Ease of brake pressure modulation, accuracy and precision of modulation. Feedback through the pedal "describing" pad rotor contact dynamics and pressure fluctuations.

Brake Performance: Level of brake torque produced and the resistance to brake torque loss, better known as fade.

For performance I do not use the term stopping distance because that involves more than just the brakes.

Benefits of "better" feel:

Brake feel is just like steering feel. The better the feel/feedback is the more of the inherent performance you will actually be able to use.

Imagine a car that has 1.0g of lateral grip, but has lousy steering. Too light, completely dead, nothing happens just off-center, poor linear response and too much or too little ratio. Like a Mercedes AMG

How much of that great 1.0g are you ever going to be actually using with poor steering?

Same with brakes. Improving feel will allow you to better use what you have.

However, it will not objectively increase brake performance. If you are already getting the max out of you system then better feel will not get you more.

The formula below is a decent way to estimate what, if any, increase you have in brake torque after an upgrade:........................................

Brake Torque Increase %=

[(caliper piston area new/old) * (effective radius new/old)* (brake pad friction coefficient new/old)]

*effective radius is the distance from the center of the hub to the center of the brake pad

*For sliding calipers multiply the areas by 2

Any answer equal to 1 means no increase. Any answer less than 1 means you've gone backwards. Any answer greater than 1 means a torque increase.

This will NOT give you the actual brake torque, just the difference.

Now things like steel lines, stiffer calipers affect feel, not performance. Only force applied, torque arm and friction do that. You can change things like lines, calipers, pedals and boosters to change the characteristics. Such as, initial bite point and initial deceleration level, pedal travel and effort, pedal force multiplication. However, these do not actually produce more brake torque.

Stopping Force (estimated):.........................................

(for ease and consistency try and use meters and kg)

1. stopping force = weight of car * longitudal coefficient of friction of tires

2. Stopping Force = pedal force * brake pad coefficient of friction * mechanical force ratio * (1/radius of the tire) * brake rotor effective radius
.....solve for mechanical force ratio

3. Front Force = weight front + total weight * tire friction * height of CG * (1/wheel base)

4. Rear Force = weight rear - total weight * tire friction * height of CG * (1/wheel base)

5. % front = Front Force/Stopping Force

6. % Rear = Rear Force/Stopping Force

7. mechanical force ratio front = mechanical force ratio * %front
mechanical force ratio rear = mechanical force ratio * %rear

8. Take that ratio and divide it by the pedal ratio and booster boost ratio.

This is the ratio of master cylinder bore area and caliper piston bore area.

(MC-master cylinder)
A larger MC results in...
> less pedal travel
> a firmer pedal
> less hydraulic advantage

A smaller MC results in...
> more pedal travel
> a softer pedal
> more hydraulic advantage

Larger caliper piston area results in...
> more pedal travel
> a softer pedal
> more hydraulic advantage

Smaller caliper piston area results in...
> less pedal travel
> a firmer pedal
> less hydraulic advantage

2-Piece rotors:.....................

I have seen a lot on 2-piece rotors. Some of the information contained in them is correct some is mythical.

Some definitions.

2-Piece rotor: A brake disc rotor that has a separate hat (cap) usually made from a lightweight metal. There are two types of common 2-piece street rotors. One uses a bolted hat and the other a pinned hat (also known as a "floating rotor" design).

The bolted type is just what it sounds like. Usually an aluminum hat bolted to a cast iron rotor. The only real benefit of this design is weight savings.

The pinned type has usually stainless steel pins that attach the aluminum hat to the rotors. This allows the rotor to "float" on the pins. The great advantage of this design is that it allows the rotor to move freely. When the rotor expands and contracts there is much less chance of binding or distortion. As you can imagine this cuts down on warping and uneven wear (DTV). The disadvantage of this design is really high costs and increased NVH.

As far as better heat conduction, not really. It does help a bit, buts it's not enough to make it worth the extra cost. The nice think about the weight savings is you can get larger rotor with out taking a weight penalty.

- It may help keep your wheel bearings cooler.

Lug Nut Torque:........................

Over-torque of wheel lug nuts is one of the prime causes of brake rotor distortion. This can lead to permanent warping of the rotors, uneven wear of the rotors and pads and lots of brake chatter (NVH).

With today's very stiff alloy wheels, like from BBS, SSR, Volk, etc., when you torque down the lug nuts the wheel-mounting surface will force what ever it contacts to take its shape. Which means whatever that surface looks like will be what the rotor looks like.

Get a torque wrench and check the torques on every lug nut and make sure they are within the specs (which you should be able to find in your owner's manual). And make sure that every lug nut is torqued down exactly the same. Even if all 5 on a wheel are within specs, not having all 5 be equal will introduce distortion.

And if you think that your light alloy wheel can't possibly be that stiff, you wrong they are MUCH stiffer than the brake rotor or even the hub.

Don't assume that torque the lugs to the lowest range in the spec is the best. Try and keep it nominal not at the extremes of the range.

Much of the problems with rotor warp, brake chatter, disk thickness variation can be traced back to over-torqued and uneven torqued lug nuts.

After coming back from the shop, get that torque wrench and check the lug nuts yourself.

Brake Air Ducts:.........................

This is only for auto-x and racing.

Getting all that extra air on the brakes going down a straight gets the brakes nice and cool for the next corner.

Cheap and effective way to keep rotor temperatures down and therefore reduce fade.

Rear Brakes:.......................

Just want to add a little warning about the rear brakes. Be VERY careful when it comes to making changes back there. Upgrading the rears to increase braking power, not just feel, can be a double-edged knife.

More braking power to the rears will only do you good if
1. the balance is woefully off and the rears are under-utilized, if you can easily lock-up the rears your already at full potential
2. you have increased the weight the rear tires carry, the grip of the rear tires and/or minimized the weight transfer to the front.

Putting more powerful brakes on the rear alone do nothing more than cause you to lock up or go into an ABS event sooner. You must increase the rear tires load to benefit from an increase in the rear brakes power. Otherwise, the rears should be looked at only from a "feel" standpoint.

"Mini brake test"................................

2 friends and me performed a little brake test this morning.

I have a friend that works at Teterboro Airport and was able to gain access to a nice flat and large area. Not a runway.

To make it interesting we brought my 2000 BMW 328i with sport package.

Atmosphere Conditions at the time (benefit of being at an airport):

Temp: 54 F, Humidity 55%, Barometer 30.28in and rising, Wind 6 MPH East, partly cloudy

Surface was perfectly dry and smooth. Surface was concrete.

The vehicles:

2002 Impreza WRX Sportwagon with manual transmission, SSR Competition wheels, 225/45ZR17 Bridgestone Potenza S-03 PP tires, Tein H-Tech springs, rear 20mm anti-roll bar. All else stock, including brakes.

2000 BMW 328i with manual transmission and the sport package, all stock. The tires: 225/45ZR17 Michelin Pilot Sports.

The WRX tires have 650 miles on them; the BMW tires have 7000 miles on them. All pressures where set at the recommended settings as posted on the driver's doorjamb.

WRX has 720 miles on the pads; the BMW has 7000 miles on the pads.

Test procedure:

10 consecutive stops from 60MPH (using the car's speedometer, so chances are it wasn't exactly 60MPH). That's all I had the time to do.
Consecutive means the time it took to turn around get back to the start and then 30 second wait.

3 drivers. Myself, worked as a brake engineer and have experience on well over 100 different vehicles. Driver 2, Ralph, is a "car guy" and knows how to drive. Driver 3, Carmine, good driver but not a "car guy."

All numbers are rounded to nearest whole number, so no 120.5 feet, that would be listed as 121 feet. I do this because of the nature of the test and the measuring equipment. Which consists of 10 cones spaced 15 feet apart and a tape measure.

The BMW's DSC (ESP system) was turned off, so both cars had ABS only working.

For time reasons I was the only driver to do all 10 runs. Ralph and Carmine just did one each in each vehicle 15 minutes after I was done to add subjective data.

The results (all in feet):

Run 1.......................121............................117
Run 2.......................121............................116
Run 3.......................121............................116
Run 4.......................123............................117
Run 5.......................125............................117
Run 6.......................125............................119
Run 7.......................128............................121
Run 8.......................133............................121
Run 9.......................133............................121
Run 10......................133............................121

Take these numbers for what their worth. Please keep in mind the difference in tires and miles on them. And the "test equipment" used. Disclaimer: This is for entertainment purposes only.

Common Brake Fluid Boiling Points (from 8complex as posted on

|| Wet Boiling Point || Dry Boiling Point
Castrol SRF || 518°F || 590°F
Earl's HyperTemp 421 || 421°F || 585°F
Motul 600 || 420°F || 593°F
AP-600 || 410°F || 572°F
Neosynthetic 610 || 421°F || 610°F
ATE-Super Blue || 392°F || 536°F
Valvoline || 333°F || 513°F
Castrol LMA || 311°F || 446°F
Earl's HyperTemp 300 || 300°F || 568°F
Ford HD || 290°F || 550°F
Wilwood 570 || 284°F || 570°F
PFC-Z rated || 284°F || 550°F
AP-550 || 284°F || 550°F

All brake fluids absorb moisture, some faster than others (except silicone which is not recommended for anti-lock brake systems). Castrol SRF resists moisture contamination (non-hygroscopic) more than any other fluid we tested; therefore change intervals can be greatly extended. This reduces the effective cost over a season of racing. Many drivers say that they can run the same fluid all year long with only bleeding off the fluid in the calipers for each event. This way a can or two will last all year. Other fluids (hygroscopic type) require additional flushing of the system for each track event to maintain the lowest percentage of moisture and the highest boiling point.

FYI - The Castrol SRF is around $77/container versus $10-15/container for the rest.

Silicone Brake Fluids

Fluids containing Silicone are generally used in military type vehicles and because Silicone based fluids will not damage painted surfaces they are also somewhat common in show cars.

Silicone-based fluids are regarded as DOT 5 fluids. They are highly compressible and can give the driver a feeling of a spongy pedal. The higher the brake system temperature the more the compressibility of the fluid and this increases the feeling of a spongy pedal.

Silicone based fluids are non-hygroscopic meaning that they will not absorb or mix with water. When water is present in the brake system it will create a water/fluid/water/fluid situation. Because water boils at approximately 212º F, the ability of the brake system to operate correctly decreases, and the steam created from boiling water adds air to the system. It is important to remember that water may be present in any brake system. Therefore silicone brake fluid lacks the ability to deal with moisture and will dramatically decrease a brake systems performance.

MINIMAL boiling points for these specifications are as follows:

|| Dry Boiling Point || Wet Boiling Point
DOT 3 || 401ºF || 284ºF
DOT 4 || 446ºF || 311ºF
DOT 5 || 500ºF || 356ºF
DOT 5.1 || 518ºF || 375ºF

Poly Glycol Ether Based Brake Fluids

Fluids containing Poly glycol ethers are regarded as DOT 3, 4, and DOT 5.1. These type fluids are hygroscopic meaning they have an ability to mix with water and still perform adequately. However, water will drastically reduce the boiling point of fluid. In a passenger car this is not an issue. In a racecar it is a major issue because as the boiling point decreases the performance ability of the fluid also decreases.

Poly glycol type fluids are 2 times less compressible than silicone type fluids, even when heated. Less compressibility of brake fluid will increase pedal feel. Changing fluid on a regular basis will greatly increase the performance of the brake system.

FLUID SPECIFICATIONS All brake fluids must meet federal standard #116. Under this standard are three Department of Transportation (DOT) minimal specifications for brake fluid. They are DOT 3, DOT 4, and DOT 5.1 (for fluids based with Polyalkylene Glycol Ether) and DOT 5 (for Silicone based fluids).

Wet vs. Dry Boiling Point

WET BOILING POINT - The minimum temperatures that brake fluids will begin to boil when the brake system contains 3% water by volume of the system.

DRY BOILING POINT - The temperatures that brake fluid will boil with no water present in the system.

How does water get in there?

Water/moisture can be found in nearly all brake systems. Moisture enters the brake system in several ways. One of the more common ways is from using old or pre-opened fluid. Keep in mind, that brake fluid draws in moisture from the surrounding air. Tightly sealing brake fluid bottles and not storing them for long periods of time will help keep moisture out. When changing or bleeding brake fluid always replace master cylinder caps as soon as possible to prevent moisture from entering into the master cylinder. Condensation, (small moisture droplets) can form in lines and calipers. As caliper and line temperatures heat up and then cool repeatedly, condensation occurs, leaving behind an increase in moisture/water. Over time the moisture becomes trapped in the internal sections of calipers, lines, master cylinders, etc. When this water reaches 212º F the water turns to steam. Many times air in the brake system is a result of water that has turned to steam. The build up of steam will create air pressure in the system, sometimes to the point that enough pressure is created to push caliper pistons into the brake pad. This will create brake drag as the rotor and pads make contact and can also create more heat in the system. Diffusion is another way in that water/moisture may enter the system.

Diffusion occurs when over time moisture enters through rubber brake hoses. The use of hoses made from EPDM materials (Ethlene-Propylene-Diene-Materials) will reduce the amount of diffusion OR use steel braided brake hose with a non-rubber sleeve (usually Teflon) to greatly reduce the diffusion process.

DOT what?

DOT: Acronym for "Department of Transportation" -- an American federal agency or "Department of Transport" -- a British agency

DOT 3: This brake fluid has a glycol base. It is clear or light amber in color. Its dry boiling point is 401° minimum and wet boiling point of 284° minimum. It will absorb 1 to 2 percent of water per year depending on climate and operating conditions. It is used in most domestic cars and light trucks in normal driving. It does not require cleaning the system and it can be mixed with DOT 4 and DOT 5.1 without damage to the system. The problem with it is that it absorbs moisture out of the air and thereby reduces its boiling point. It can also damage the paint on a vehicle.

DOT 4: This brake fluid has a borate ester base. It is clear or light amber in color. Its dry boiling point is 446° minimum and wet boiling point of 311° minimum. It is used in many European cars; also for vehicles in high-altitude, towing, or high-speed braking situations, or ABS systems. It does not require cleaning the system and it can be mixed with DOT 3 without damage to the system. The problem with it is that it absorbs moisture out of the air and thereby reduces its boiling point. It can also damage the paint on a vehicle.

DOT 5: This brake fluid generally has a silicone base. It is violet in color. Its dry boiling point is 500° minimum and has no wet boiling point in federal DOT 5 specifications. It is used in heavy brake applications, and good for weekend, antique, or collector cars that sit for long periods and are never driven far. It does not mix with DOT 3, DOT 4, or DOT 5.1. It will not absorb water and will not damage the paint on a vehicle. It is also compatible with most rubber formulations. The problem with it is that it may easily get air bubbles into the system that are nearly impossible to remove, giving poor pedal feel. It is unsuitable for racing due to compressibility under high temperatures. If as little as one drop of water enters the fluid, severe localized corrosion, freezing, or gassing may occur. This can happen because water is heavier and not mixable with silicone fluids. It is unsuitable for ABS.

DOT 5.1: This brake fluid has a borate ester base. It is clear or light amber in color. Its dry boiling point is 500° minimum and wet boiling point of 356° minimum. It is used in severe-duty vehicles such as fleets and delivery trucks; towing vehicles, and racecars. It can be mixed with DOT 3 or DOT 4 without damage to the system. It maintains higher boiling point than DOT 3 or DOT 4 fluids due to its higher borate ester content. It is excellent for severe duty applications. The problem with it is that it costs more than other fluids and there is limited availability. It also absorbs moisture out of the air and thereby reduces its boiling point. It can also damage the paint on a vehicle.

What causes a mushy pedal?

DOT 5 fluid is not hygroscopic, so as moisture enters the system, it is not absorbed by the fluid, and results in beads of moisture moving through the brake line, collecting in the calipers. It is not uncommon to have caliper temperatures exceed 200° F, and at 212° F, this collected moisture will boil causing vapor lock and system failure. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel.

Brake Fluid and Cold Temps:......................................
Kinematic viscosities: All brake fluids (DOT 3, DOT 4 and DOT 5) must meet a minimum viscosity test of not less than 1.5 centistokes at 100° C (212° F) and must not be more than the following to meet their various classifications (the larger numbers indicate higher kinematic viscosities just like with motor oils).

DOT 3 1500 Centistokes at minus 40° C
DOT 4 1800 Centistokes at minus 40° C
DOT 5 900 Centistokes at minus 40° C

*-40° C = -40° F*

A centistokes is is 1 mm^2/s


It's no big secret brakes are a critical component in high performance road or track cars. Hopefully some of the info I post here will answer a few questions and help you the consumer divide the marketing fluff from the facts and make a more educated decision when spending your hard earned dollars.

This is going to take a while so please be patient...... Still going.............

Disc Brake rotors,
Disc brake rotors are made from various materials such as Cast Iron, Steel, Ceramic, carbon, Aluminium, and Titanium to name a few.

Cast Iron would account for the majority of road/track cars used today. Why? It's inexpensive to produce and its thermal v's strength properties are ideal for most applications. The key difference between cast iron and steel is the carbon content. As a basic rule cast iron has greater than 2% carbon where steel is less than 2%. The type of iron primarily used in disc rotors is grey cast iron which has a carbon content greater than 3%. Grey cast iron has a flake type carbon formation which when produced correctly acts like small veins or capillaries drawing the thermal energy from the braking surface into the mass of the disc rotor. High performance disc rotors like what I used to design had a carbon content of 3.7% which is huge compared to standard discs on the market which are usually around 3 to 3.3%.
The key mechanical elements to a well designed disc rotor is diameter and mass. Diameter is controlled by wheel size and the disc mass is controlled by the active corner weight of the vehicle. What about cooling vanes? Cooling vane design plays a greater role in marketing than it does in disc rotor function is most cases. Think about it! Once a vehicle is released what marketable features are left to differentiate one disc rotor to another. You can slot it, drill it, or change the vane design.
In regards to replacement disc rotors the diameter and thickness of the disc is fixed. A disc that is lower in mass will heat up quicker than a heavier disc. The trade off is increased rotating mass which comes up in discusssion quite often when armchair engineers are involved. Rotating inertia does have an effect on braking dynamics but it's not quite as influential as it's often portrayed. When a disc rotor wears, the mass naturally decreases. Lower mass equals higher temperatures and higher temperature increases wear. The same deal is applicable for disc pads which will come up later.

I haven't done a lot of work with Aluminium disc rotors but have studied the viability. A few brave vehicle manufacturers have tried to use this light weight material but few have been successful due to either functional or cost reasons. Aluminium is approximately 1/3 the weight of cast iron and has approximately three times the thermal conductivity. Once you study the figures you find that the 3 times gain in heat transfer is cancelled out by the 1/3 equivallent mass on the same size disc rotor. A basic understanding of heat transfer will make this clear.

Carbon disc rotors are used on F1 vehicles but rarely anything else. Cost is prohibitive and suitability for production car use is non existant. Carbon discs can handle temperatures up around 2500 deg C and generally only last one race due to wear. Carbon brakes dont work well at low temps.

Ceramic brakes are popping up on Porsches, and high end Mercs these days for a modest $15k to $20k a set. Excellent friction and almost non existant wear but I'd hate to drop one....

Heat Transfer
Joules/sec = 0.5 x mass x vel² Note: Velocity squared. Speed has a greater influence than vehicle mass. The faster you go the more energy transfered. So in theory the faster you go the more mass in the disc you need to cope with the increase in kinetic energy. More mass spinning at higher speeds is also a bad thing when you consider rotating inertia. Now you've got a problem. There are ways around this when designing a disc for a particular purpose.

Vane Design
Straight - This is the most common design found in production cars. Straight vane discs are fine for normal street applications and are the least expensive to produce. Most straight vane discs are non directional. In performance and track applications straight vane disc can suffer from ballooning between the vanes resulting in disc vibration.
Curved - Curved vane discs are the popular choice for dedicated race applications. A correctly designed curved vane disc has very efficient air flow (cooling) characteristics and strength due to the longer vanes. The longer vanes offer better support of the braking surfaces. Curved vane discs are more expansive to manufacture due to additional tooling required to produce left and right handed discs.
Pillar - The pillar style vane design was originally patented approx 20 years ago but little was done to develop and introduce the design into mainstream disc products until the late 90's. The advantages of pillar design is the dispersal of supporting columns between the two braking surfaces (rings). The pillar design solved the problem of unsupported areas between the vanes that is typical of straight vane discs and some poorly designed curved vane rotors. A pillar vane disc will never be as effective as a well designed curved vane rotor but it's not far off the mark. To their advantage pillar vane discs are much less expensive to manufacture as the same disc can be fitted as left or right hand.

Slotted & Drilled discs
If you have a close look at the brake torque calculations (When complete??) you will notice there is no place in the equation for the effect of slotted or drilled rotors. Why is that? If you were to fit three cars with each of the three styles of disc rotor I.E Plain, slotted, and drilled and conducted a simple stopping distance test based on the same initial speed and pedal pressure (brake torque) you would be dissapointed to find that they will all stop at the same distance. Ooops! As a young naïve brake designer I used to beleive the marketing too until I started testing.
Where a drilled or slotted disc does make a measurable difference is when you introduce a third party to the equation like brake dust, water, sand, ECT.
Slots and holes evacuate these third party materials and maintain more consistant braking effort over a longer period of time. The key is more consistant braking.
Drilled disc rotors don't belong on the track with a few (vey few exceptions) Where the braking potential of a vehicle is well and truely above what is required like on some Porsches, Bikes, BMW's then they can be used without any issues and potentially better dust evacuation.
Slotted disc rotors are the popular choice for production car track work. The ideal slotted disc has the least amount of slots required to maintain adequate dust removal and no more. In most cases 4 or 6 slots is adequate. Too many slots can have a negative effect on braking. Excessive pad wear, material deposits, lower friction co-efficient can be the result of a poorly designed slotted disc. Don't waste too much time analysing slot designs most of them are marketing tools. If you remember two basic rulles you'll be fine. 1. Less is more 2. Simple straight or curved sweeping (long slots) that cover the pad area are best.

Brake calculations

Pedal force = 30kg
Pedal ratio = 4:1 - The bake pedal is a lever who's fulcrum (pivot point) is design to multiply the pedal force. Generally 4 to 6 times.
Force on master cylinder = 30 x 4 = 120kg

Master cylinder diameter = 1"1/8 (2.86cm) Master cylinder diameter is governed by required output pressure and fluid volume required to extend caliper pistons.
Master cylinder area = 6.413 cm²
Boost ratio = 1.82 - vacuum booster
line pressure - 120kg/6.413x1.82 = 40.87 kg / cm² (581 PSI )

Caliper piston diameter
To be continued………………………

Heat treatment
Heat Treatment of metals doesn't necessarily mean hardening. Performance disc rotors can be heat treated for the purpose of stress releiving the product before use. Disc rotor distortion (Warpage) is often the result of accumulated stresses in the disc. When you stress releive a disc rotor you are returning the internal stress to the base point of zero. The Disc rotor manufacturing process often generates stress in the disc before it even reaches the car.

Deep freezing or Cryogenics is a process used to refine the microstructure of various metals. I have used this process to alter disc rotor microstructure for specific applications. For general motorsport use, Cryogenic treatment isn't going to make dramatic changes in braking. Crygenically treated cheap Chinese rotors are popular in the USA where the process can take a low quality product to being a low quality product that wears a little better. Marketing does the rest of the convincing. Where Cryogenics proves most useful is in the treatment of wearing engine components like cranks, cams, pistons, rods, and even complete blocks.


Brake fluids

Two piece rotors
Two piece rotors were once only seen on high end race cars due to exorbitant prices and limited range. Eventually designs were changed to lower the costs and they became an affordable option for weekend racers and the fashion conscious car owner. The two piece disc has quite a few credible advantages, weight reduction around the non braking surfaces, improved heat dispersion protecting bearings from overheating, and the ability to allow the disc to expand and contract more freely.
All very important features for high speed driving.
Two piece rotors can be divided into two different categories, floating and fixed mounting system. The fixed mounting system is very popular these days as they are inexpensive compared to floating discs and are more suitable for combined street and track use. The fixed mounting design consists of an aluminium mounting bell, a flat disc and high tensile bolts which join the two together. The joint is rigid due to close fitting tolerances. Fixed mounted discs are also low maintenance due to the lack of moving parts and tend to last longer. The disadvantages are the restriction of free movement of the disc when it expands. The expanding disc must also to some extent stetch the aluminium as it grows which can lead to distortion in extreme cases. Generally fixed mounted two piece discs under 330mm in diameter wont have an issue due to the smaller amount of expansion. Floating two piece rotors are highly recommended over fixed when the diameter exceeds 330mm or in extreme racing applications where freedom of movement is critical. In addition to free movement of the expanding disc the side or axial float also helps reduce piston knock back as the disc can move axially and remain centred in the caliper. This axial float is usually only 0.2mm but it is enough to reduce knock back and a soft pedal. The floating disc mounting system comes in different styles, floating round bobbins encapsulated at either end which travel along an elongated hole in the disc or hardened steel keys which are fixed to the disc and the slotted aluminium bell floats axially while the retaining keys allow free expansion. The only disadvantage of the floating style other than cost is noise. If the car isn't racing then the discs are clanging as they flop around the fixings. Most manufacturers fit springs of various types to eliminate noise these days and some work but this offen defeats the purpose of freedom of movement.
Trust no-one but yourself.

The beast:
Mid North Coast Member. :twisted:


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