Brake FAQ

What is coefficient of friction?

One of the important criteria when evaluating a brake pad is the ‘coefficient of friction’. The coefficient of friction is the ratio of the force of friction between two surfaces and the force pressing them together. The smaller the coefficient of friction, the smaller the force that is required for the two surfaces to slip. The higher the coefficient of friction, the stronger the force that is necessary for the two surfaces to slip. The coefficient of friction is 1 when 100 kg of force (parallel to the ground) is required to move an object that weighs 100kg. If the same object can be moved with 50 kg of force. The coefficient of friction is 0.5. When calculating the coefficient of friction for the braking of a car, the braking torque that occurs when braking, and the fluid pressure that was required to apply the brake pad is used.

In general, OEM brake pads have a coefficient of friction of 0.3～0.4, and performance brake pads have a coefficient of friction of 0.4～0.5. The higher the coefficient of friction, the less fluid pressure (lighter push of the brake pedal) is required to create a high braking force. If the coefficient of friction is too high, there is too much friction and makes it very difficult to brake. The most important factor for the coefficient of friction is for the brake pads to reach its maximum friction level immediately after the brake pedal is stepped on. If the initial braking power is bad, it is commonly reffered to as brake pads that do not work well (Having bad initial bite). The second important factor is the stability of the coefficient of friction at various temperatures. It is common for the coefficient of friction to be lower at low and extremely high temperatures. This is a problem because there will not be enough braking power for street-use if the temperature is too low. Another problem is the decrease of the coefficient of friction at extremely high temperatures. Users who drive on race circuits need stable braking power even in those conditions. A quality racing brake pad will have stable performance from start to finish. The third important factor is the stability of the coefficient of friction at different vehicle speeds. Brake pads will be dangerous if a stable coefficient of friction was achieved during braking at 60km/h but unstable at 180km/h, selling the product would not be possible. At DIXCEL, we are constantly doing research and development to come up with a brake compound that has stable yet high coefficient of friction. To increase the coefficient of friction and stabilize the brake compound, materials like bronze, steel fibre, fibreglass, Kevlar, ceramic, titanium, carbon, etc are very popular. It is the ongoing challenge at every brake manufacturer to make the mixture of the materials to make the best brake pad.

What is fade resistance?

Fade resistance is evaluated by (1) the temperature at which brake fade occurs, and (2) how little the coefficient of friction changes after brake fade occurs.
Brake fade is the decrease in braking power. Brake fade occurs when high temperatures causes a part of the resin material to vaporize. This creates a film between the disc and pad, which significant decrease the coefficient of friction. In general, OEM pads have a fade point (the temperature at which brake fade occurs) of around 300℃～350℃. Performance brake pads usually have a fade point around 400℃～700℃. (The fade point varies by material and the type of use.) Performance brake pads are made to have a higher fade resistance rate to minimizes the decrease of the coefficient of friction, even after brake fade occurs. The fade resistance rate is based on ratio of the coefficient of friction before and after the brake fade, so 100% would mean no change in the coefficient of friction. In general, OEM brake pads have a fade resistance rate of 40～50% where as performance brake pads have 60～80%. At DIXCEL, our goal is to increase the temperature at which our brakes fade and minimize the decrease of the coefficient of friction after brake fade. The research and development is ongoing to come up with better mixture of resins and materials to sell the best brake pads.

About brake pad wear

A common thinking is that ‘performance pads have faster wear rate compared to OEM pads’.
This is neither false or true, since the wear rate can vary. This is because pad wear rate is different depending on the temperature of the brake pads. The graph on the right shows the wear differences of the OEM brake pads to our performance pads; the M and Z type pads. The values are the measurement of pad wear after 0.4G braking from 80km/h, which was repeated 1000 times for each type of pad and each temperature. By looking at the graph, it is easy to see how the pad wear rate increases significantly as the pad temperature rises. OEM pads have a low wear rate when pad temperature is below 150℃, but have a much higher wear rate when the pad temperatures are above 300℃. When pad temperatures reach above 300℃, brake fade occurs for the OEM pads, and become almost useless in terms of braking power. Performance pads are designed for use on race circuits and for street performance, so they are developed to have optimal pad wear rate at temperatures around 250℃～600℃. At lower temperatures, the performance pads will have higher wear rate at lower temperatures when compared to the wear rate of OEM pads at low temperatures. If you are going to use your brake pads for ordinary street use, the statement ‘performance pads have faster wear rate compared to OEM pads’ will be true. Performance pads are for users who need pads for use on race circuit and for street performance, that last longer, safer, and superior performance compared to the OEM pads.

About pad wear sensors

The pad wear sensors, also known as pad wear indicators, let the users know when the pads need replacement. There are two different types of pad wear sensors, mechanical and electric. The mechanical type generates a screeching noise when the metal clip attached to the pad comes in contact with the disc, to indicate the need for pad replacement. The electric type turns on a warning lamp on the instrumental panel of the car when the electric wire built into the pad becomes disconnected, to indicate need for pad replacement. Most Japanese and American cars use the mechanical type. Some Japanese luxury cars and most European cars use the electric type. There are two types of electric pad sensors. BMW, Mercedes, etc, use detachable wear lead wires. The other type is the built-in type, which is used by VW, Audi, etc. The wear lead wire is connected to the brake pad during the manufacturing process, therefore not detachable. In addition to the Premium series which have the built-in sensors as original, we also carry built-in sensors on select models on other pad types. (Please check the application table for details)

What is residual stress?

Residual stress is a structural weakness that occurs in the casting process of iron, the main material used in brake discs.

During normal street-use, residual stress does not cause any problems with the discs. However, when the discs are used at race circuits at high temperature conditions for an extensive amount of time, the residual stresses within the disc can lead to thermal cracking and deformation. Dixcel offers certain brake discs with a heat-treated surface, which relieves residual stress and prevents thermal cracking and deformation from occurring.

Comparison chart of the performance difference with/without a heat treatment

• The comparison test above was done using a non-heat treated disc by another manufacturer and our heat-treated HD type disc. The R01 brake pads were used for both tests.
• All of the testing above was done on the Twin Ring Motegi race circuit.
• The data values above can vary depending on the conditions (weather, pads used, vehicle set-up, driver, lap time).
• The data values should only be used as a reference to better understand the performance differences.

What is Heat-Treatment?

At DIXCEL, a strict temperature control is implicated at each step of the heat treatment process.
The details of the time and temperatures of the heat treatment process cannot be disclosed, so hypothetical values will be used to explain the heat treatment process.

First, the temperature is increased by 5 degrees centigrade every 10 minutes. When the temperature reaches 300 degrees centigrade, the temperature is kept the same for 8 hours.
Next, the discs are cooled by temperature being lowered by 5 degrees centigrade every 10 minutes. The graph on the left shows that the temperature control is very ideal.

The whole heat treatment process is completed over a period of 24 hours. This allows for slow and gradual process under perfect humidity control.
This helps prevent deformation, strengthens the bonds between the molecules, and allows for an increase in heat resistance.
※The temperatures and times listed in this explanation are hypothetical values. The actual temperatures and times that DIXCEL uses are different.

Benefits of slots and a heat-treatment on a race circuit

The advantage of a slotted disc is an increase in stopping power. The advantage of a heat treated disc is an increase in durability.
For the slotted discs, our testing results showed an average braking power increase of 15-20 percent. The heat treated discs have better protection against thermal cracking, vibration, and distortion.
They also increase the life of both the pad and disc.
For users who want best of both worlds, DIXCEL recommends the FS or HS series discs.

Brake Disc Material

Cast iron and carbon are the two main materials used to make a brake disc. The benefit of Carbon is that it has a high thermal resistance and is lightweight. The downside is that Carbon is expensive, so it is used mainly by high budget racing teams. Cast iron is the more commonly used base material. There are three types of cast iron, each type has different graphite composition; grey cast iron, CV cast iron and ductile cast iron. Grey cast iron (flakes graphite cast iron) has excellent processibility and anti-abrasion capability.

• Grey cast iron has the advantage of being easily mass-produced, making it the most commonly used by discs manufacturers. The drawback is that it could be deformed or cracked under repeated sharp changes in temperature in the high temperature range (about 800℃).
• Ductile cast iron is an excellent material. The tensile strength of ductile cast iron is equal to that of steel. Ductile cast iron also has an high anti-heat capacity (stability against expansion and contraction). Unfortunately, it has low surface hardness, which can cause abnormal wear and/or abnormal heating due to its high exothermicity, if the material is used for brake discs.
• CV cast iron (Compact Vermicular cast iron) has an intermediate character between grey cast iron and ductile cast iron. The quality control of CV cast iron during the manufacturing process is extremely difficult, so the quality varies. Sometimes its closer to grey cast iron but other times is closer to ductile cast iron. After extensive testing, grey cast iron with special additives are being used in DIXCEL brake discs. OEM Products often use grey cast iron with FC150～200 (FC is numerical representation of the strength of cast iron). DIXCEL uses grey cast iron with FC200～250 for higher durability. After extensive research and development, DIXCEL has developed a disc which has special additives to strengthen the disc’s vulnerability to sharp temperature changes in the high temperature range. The superior precision and balance of the disc goes without saying.

The structure and the shape of a brake disc

The two most popular types of brake discs are solid and ventilated discs. Ventilated discs have cooling vanes between the braking surfaces which allows air to flow through, and has a cooling effect on the disc. More cars are becoming equipped with ventilated disc on the front brakes, and high-performance cars have ventilated discs on the front & rear.

■Solid disc

■Ventilated disc

Straight type: Number of fins is generally 24-48. Fewer fins → light weight More fins → higher rigidity	Curved vane: Spiral shape allows cooling air to be vented out.	Pillar type: Smoother due to fewer obstructions to air flow

The precision machining of brake discs

Brake disc engineering involves the precision procedures Disc Thickness Variation (DTV), run-out, Mounting Surface Flatness (MSF), friction surface parallelism, and balance.

DTV･･･Disc Thickness Variation
Disc Thickness Variation is a measure to see if there are any variation in brake disc thickness along the entire braking surface. DIXCEL tolerance is 1/100mm

Run-out
Run-out is a test to see if the disc will spin without any vibration. The parallelism of the mounting surface and the outer friction surface is measured. DIXCEL tolerance is 5/100mm.

MSF (Mounting Surface Flatness)
This is a measurement to make sure the disc will not vibrate after installation on the car. The flatness of the disc mounting surface is measured, and DIXCEL tolerance is 5/100mm.

Friction Surface Parallelism
Friction surface parallelism is a check to see if the two friction surfaces are parallel. The parallelism is check on the entire friction surface. DIXCEL tolerance is 2/100mm

Balance
The balance is to check if the weight balance of the disc is evenly distributed. If there is an uneven distribution of weight, it can cause unwanted vibrations. The uneven balanced is fixed by adding balance weight or shaving off excess weight.

If any of the precision machining standards are not met, the risk of a disc developing vibrations will be high. On top of the five precision machining standards, the friction surface of the disc is machined to improve the bedding process of the new brake pads, and provides more stable braking from initial use. The groove between the mounting surface and the friction surface of the disc is designed to optimize the cooling effect, which will prevent thermal cracking and distortion. At DIXCEL, we put all discs through a thorough final product inspection. Be rest assured our products are of the highest quality.

Bedding-in brake discs

■Street use only

It depends on a combination of pads/discs and conditions of the road you drive on, but bedding-in takes roughly 300～1,000km to complete. During this period, please avoid fast or abrupt driving or do not drive in a way as to force the temperature up. Bedding-in will be completed simply via normal driving.
It is recommended that the mechanics complete the very beginning phase of the bedding-in for customers. The beginning phase of the bedding-in is the period when a brake pedal feels spongy, or when the friction does not pick up instantly after the brake pedal is pressed.
Do NOT drag brakes for bedding-in. Just repeat normal braking from the medium speed with medium foot pressure until the car comes to a low speed of 10kph or so (to achieve full contact between pads and discs. Short braking only promotes the contact on the outer part of brake discs). Repeat the braking until the pedal is rigid and the friction feels linear to the foot pressure.
* The number of braking required varies depending on 1) the condition of the disc surface, 2) the pad friction surface and 3) the types of friction material.
If the disc surface is uneven, machine them or replace them if the thickness is near the wear limit which is engraved on the edge of the discs. With new or resurface discs, it is also advisable to sandpaper the surface of friction material if the brake pads are used for better results.

■Circuit use

When using a new disc for the first time on a circuit, start with 50% braking for about 5 minutes and then go back to the pit once and take at least 5-minute intervals. Then, repeat 70～80% braking for about 10 minutes. Pit in again and take an interval of about 10 minutes. Finally, gradually increase from 80% to 100% braking, and the bedding-in of discs on a circuit is completed.

《 Comparisons of the method of recommended bedding-in 》

Advantages and disadvantages of slotted rotors

Generally, the more slots on a rotor, the higher operating friction level. The downside is they increase noise (noise from the discs rotating) and cause faster pad wear.

What is the heat resistance temperature of brake discs?

Unlike brake pads, it is not precise to indicate disc heat resistance temperatures in the form of ‘up to ℃’. All brake discs are made of generally the same material, so they all run the risk of possible thermal cracking and distortion when temperatures reach 600℃ or higher.

Many different factors cause these problems, so we do not specify the heat resistance temperatures of brake discs.

Brake squeal

In general after replacing brake pads, brake squeals can occur frequently.
While it is a well known problem throughout the industry, the cause is often not well understood.
Or information regarding the squeal are seldom available to study.
At Dixcel, we offer a free and easily available troubleshooting guide for you to understand the mechanisms, causes, and prevention measures of brake-squeals.

How brake squeals are generated

When braking, the pad and the disc rub and cause a contact vibration. (vibration = frequency = noise) The sound produced by this contact is then amplified by the disc. In a way, the disc itself functions as a speaker and amplifies the noise, which can sound like a squeal. See illustration below (fig. 1).

When the brakes are applied, the pads are pushed by the piston and pressed against the rotor. There would be no issue if a strong and uniform pressure could be applied, but in reality this often isn’t the case and vibrations occur. The vibrations are then amplified by the rotor, causing a squeal. In severe cases, these vibrations are transmitted to the car’s calliper, suspension, and body, creating a variety of noises.

This issue is kept in mind, since the vibrations are expected to be absorbed by:

1. the brake pad’s softness (attenuating characteristics)
2. the brake disc’s softness (attenuating characteristics)
3. the shim between the pad’s back plate and the piston.

One might think:
If brake’s absorption is not effective → vibrations grow and create an unpleasant noise

Brake squeal only occurs when the pads and rotors come into contact with each other, so we tend to think that the pads and rotors are the source of the problem, but that’s often not the full picture. The pads and rotors may be generating the squeal, but the underlying issue is still present.

Case study on brake squeal (2) – the calliper piston

It’s important to understand the function of the calliper piston, which is intrinsically linked to brake squeal. Although it can cause brake issues, it’s overlooked in most cases without suspicion. 

As shown in the figure (Fig. 1), the calliper piston is held in the cylinder body by a piston seal. The elasticity of this piston seal pulls the pad back when the brake is released, but when the elasticity of this seal disappears, the separation between the pad and rotor is reduced, causing drag. Additionally, the vibration absorption is also reduced, making it easier to transmit pad vibration to the calliper, which causes squeal. 
Piston seals and dust boots are vital parts, and that their deterioration can lead to various problems, but people tend to forget to replace them. 

In general, it’s necessary to replace the tire once every 100,000 km, but it will vary depending on the area you live in and your driving style. If your driving is demanding, replacement will be needed sooner, and on the circuit, it’s best to replace it once every 5 drives. If you’ve never replaced the seal (boot) and have increased mileage, and are experiencing problems such as brake squeal, we recommend replacing it. It is very likely that the effects will subside.

Some people may be surprised to hear the cause of brake squeal isn’t directly around the brakes, but the deterioration of these small parts (see Fig. 2) can lead to big issues. It’s not wrong to suspect the replaced part (in this case the brake) as the cause of the squeal, which is often true, but it’s not the full picture. 

We recommend the best way to solve the brake squeal is to check the replaced parts and at the same time check if there are any abnormalities in the peripheral parts that have not been replaced, and if there are any that have deteriorated.

My brakes are squealing.

In many cases, brake squeal is caused by a combination of various factors, and have to all be considered, so please collect as much information as possible in the following manner.

◆Vehicle model
◆Pad/rotor part number
◆Date of installation, mileage after installation, total mileage
◆Type of sound (squeal, croak, creak, gurgling, rattle etc)
◆Type of occurrence (low speed or high speed, just before stopping / warm temp or cold temp / rainy day, winter, early morning, etc.
◆Rotor (new or used)
◆Rotor condition (friction material adhesion, step reduction, etc.)
◆Pad condition ( Remaining amount, uneven wear, surface deterioration, etc.)
◆ Calliper piston condition (whether the piston seal and dust seal have been replaced)
◆ Countermeasures for squealing (if countermeasures have already been taken, the time, method, effect or not, etc.)

Brake Squeal Troubleshooting

*This troubleshoot guide is based on the assumption that you are driving in cities, on highways, and on mountain passes at normal speeds.
In the case of driving on a circuit or a similar situation (repeated braking from high speed on a highway, etc.), please note that this guide may not apply.

What is most important when choosing a brake fluid?

Most drivers today are concerned with the boiling temperature of their brake fluid, and not too concerned if it meets DOT specifications or not.
This is acceptable for drivers who only use their cars at the race circuit.

Everyday drivers who use their cars for street use should be more concerned if their fluid meets DOT specification. Brake fluid that do not meet DOT specifications can speed up the deterioration of brake components over a extended period of time. It can also lead to the malfunctioning of ABS during cold weather. Most people do not know much about brake fluid, and it can be hard to find relevant information.
That is why we recommend reading the following section, ‘Understanding Brake Fluids’, a guide explaining brake fluid and its functions in greater detail.

What is DOT specification?

DOT is the abbreviation for the ‘Department of Transportation’, which is an American government transportation department. The DOT set standards such as FMVSS (Federal Motor Vehicle Safety Standard) very similar to the JIS in Japan or DIN in Germany. The following table shows the DOT brake fluid specifications:

□ Dry Boiling Temp. : Boiling point when the fluid is brand new, no moisture absorption.
□ Wet Boiling Temp. : Boiling point with fluid that has 3.7% water by volume. After 1-2 years of fluid use.
□ Viscosity : a measure to represent the brake fluid flow property. The higher the value, the more difficult for the fluid to flow. If the value is high when the air temperature is low, the fluid can have a negative effect on ABS performance.
□ pH value : value to show acidity / basicity of a solution. If the pH value is lower than 7.0 (high acidity), the fluid can accelerate corrosion of other brake components.

The higher the DOT number, the higher the brake fluid performance ?

This is not exactly correct. The DOT numbers categorize the fluids by various uses.

DOT 5.1 has strict viscosity standards at lower temperature in addition to having a high boiling point temperature. Therefore, in cold climate areas, the DOT 5.1 brake fluid is very commonly used on most cars. The most widely distributed brake fluid is the DOT 4, which has a dry boiling point temperature around 270℃ and a wet boiling point temperature around 170℃. The boiling point temperatures of DOT4 is very similar to those of DOT5.1. The major difference is the viscosity at low temperatures.
Today, cars are commonly equipped with ABS, and DOT5.1 fluid is used since its viscosity helps the ABS work consistently even in cold climates.

Which DOT specification is best for race circuit use?

Most racing brake fluids are developed for circuit-use only. The boiling point temperatures easily exceeds DOT5.1 specification but the viscosity and pH levels do not pass DOT5.1 specification. This is why racing brake fluid do not pass DOT 5.1 specification.

Is it safe to use racing brake fluid for street-use?

The use of racing brake fluid that passes DOT specification is safe for street-use. Racing brake fluid that does not meet DOT specification can speed up the deterioration of brake components over an extended period time. It can also lead to the malfunctioning of ABS during cold weather.

What is Super DOT4?

A combination of the high boiling temperature of DOT5.1, and the low temperature viscosity characteristics of DOT4.

Does viscosity at low temp (DOT5.1) create a smoother pedal feel on circuit ?

No, low temperature viscosity may appear smooth, but does not affect the feel of the pedal on the circuit. The fluid temperature in circuit runs can reach over 150℃ and viscosity characteristics in that temperature range are not much different from other grades.

There is actually no significant correlation between pedal touch and fluid viscosity. Instead, the feel of the pedal changes with the amount of water vapor absorbed in the fluid. If the amount of water vapor absorbed in the fluid is small, the pedal push will be hard, and if it is large, the pedal will feel ‘spongy’.

Is it safe to mix old and new fluid ? Or mix different grades of fluid ?

In short, it is not recommended in either case.
If both are glycol-based brake fluids that pass DOT standards, there will be no major functional problems, but the performance will not be average, and the lower grade will retain the lower performance characteristic.

Therefore, we recommend avoiding mixing two fluids, and instead replacing the brake fluid entirely.

How much fluid is needed for replacement?

For passenger cars, 800ml to 1L is generally required when replacing the full amount. (This does not apply to trucks, etc.). For partial replacement of front calipers only, 300-400ml is sufficient.