All About Braking

What is Friction Co-efficiency?


One of the important elements in the evaluation of brake pads is “friction co-efficiency”. Friction co-efficiency represents the degree of friction between two objects, note the smaller the figure is, the smaller the friction between the objects are, i.e. the objects are easier to slip, while the larger the figure is, the bigger the friction is, i.e. more difficult to slip. Simply friction co-efficiency can be explained like this, to move an object of 100kg horizontally, it takes a force of 100kg, while if the object is moved by a force of 50kg, the friction coefficient between the object and the road surface is 0.5. In the case of brakes for vehicles, friction coefficient is calculated using braking torque generated during braking (G deceleration) and fluid pressure (pedal pressure) required for generating the torque, the formula for the calculation is as follows,

Normally, normal pad friction coefficient is around 0.3～0.4 and that of sports pads is 0.4～0.5. If the friction coefficient is high, that means light fluid pressure (i.e. light pedal pressure) can produce a lot of braking force, however if the friction co-efficiency is too high, it causes jerks making it difficult to handle. The most important thing about friction coefficient is how fast the friction co-efficiency of the brake pad builds up to its maximum after the brake is applied. The Pads are deemed inefficient if this process takes a long time, this is generally described as having low initial bite (braking force).

The second most important thing is it does not get affected by temperature. Normally, friction coefficient tends to be low when the brake disc temperature is low or extremely high. If the friction coefficient is low at low temperatures, the brake pad will not generate enough braking force in street driving conditions; if it is low at high temperatures, it will cause problems on racing circuits. Efficiency at high temperatures for so called racing pads and ones with stable braking force from the start line to the goal is regarded as of good quality.

The third important thing is the stability against change of speed. It is deemed too dangerous and thus cannot be put on the market if the friction co-efficiency is unstable at the speed of 180km/h even if it braking is stable at the speed of 60km/h. We, brake pad makers, are doing our best everyday to develop materials which retain a stable and high level of friction coefficient in any circumstance. We always aim to present the best performance compound by combining a variety of materials including well-known elements which are used to enhance or stabilize the friction coefficient such as steel fiber, copper, glass fiber, Kevlar, carbon, ceramic and titan.

What is fade resistance? (Anti-fade stability)

“Fade resistance” (Anti-fade stability) is measured by (1) how high the fading point (temperature) is, (2) the difference (drop) between friction co-efficiency before and after fading. Fading is the phenomenon that friction co-efficiency of brake pads sharply drops when resins used in friction materials discharge gas (vapor) beyond a certain temperature which makes a thin layer between the pad and the rotor and causes them to separate. Normally the fading point (the temperature where fading starts) is set at around 300℃～350℃ at an OEM pads, while that of sports pad is around 400℃～700℃ （Fading points vary depending on each material or the way they are used）. For sports pads, fading rate (**) is set as high as possible to keep a stable friction coefficient even after fading starts. Normal pads’ fading rates are around 40 ～50%, whereas those of sports pads are set around 60～80% to maintain friction coefficient at the same level as that of normal pads before fading. We, sports pad makers, are working hard everyday on research and development of composition and content of resins used for the pads and other fibrous materials. ** It represents difference in friction coefficients before and at fading with friction coefficient at fading presented in % against friction coefficient before fading as 100%,

Abrasion of pads (Pads wear)
It is often said that sports pads wear away faster than normal pads but this is correct and wrong at the same time. That is because the degree of abrasion (wear rate) of brake pads varies much depending on the temperature they are used at. The graph below compares degrees of abrasion (wear rate) of normal pads and two types of sports pads (our material M and material Z) when they are applied with 0.4G braking at 80Km/h a thousand times at different temperatures. As shown in the graph, abrasion of pads increases parabolically against temperature, and the higher the temperature is the higher wear rate is. Abrasion of the normal pad is very small when used at 150℃and under but, at 300℃ and over, it could go up more than twice the abrasion of the sports pad. However, as normal pads start fading at 300℃ and decay to a practically unusable degree, there would be few users who might experience the intense abrasion at such a temperature range for OEM pads. For sports pad, while a high temperature range, 250℃～600℃, which they are often exposed to in winding roads and/ or circuits is paid more attention, the wear rate at lower temperatures is higher than that of normal pads. To conclude, for normal road use only, sports pads wear faster than normal pads do, while they last longer when used for
winding roads and circuit and excellent with safe braking ability.

The characteristics of aggression to the brake rotor

Most brake pad makers put the braking performance first and characteristics of attack to rotor second or third. As we have released a numerous rotors and offered support to races in a number of categories, we have a good understanding how much rotors affect not only braking force but also anti-abrasion capacity, anti-fade stability and anti-judder capability. Therefore, at our company, when we are carry out research and development we also take into account rotor attack characteristics. Why, then, are brake pads with strong rotor attack characteristics a problem? If brake pad with a strong rotor attack characteristics is used, it scrapes the rotor significantly and the rotor could be turned into something like a LP record. In such a
state, the sliding surface is so rough it causes a dull response of the brake pedal. In addition, as it becomes impossible to keep a normal balance, judder is also caused. Additionally, as the rotor surface is now like a file, it cuts into the pad and removes the friction material thus the pad and rotor attacks each other, which in turn shortens the lifespan of the pad. Due to the thinning friction material and the rotor, the brake system itself becomes heated up easily, and so eventually become prone to fade.

Heat conduction

This may not be unfamiliar to most of you but it is a very important aspect in comparing the efficiency of brake pads. Friction materials consist of a variety of materials and steal fiber is one of them. Steel fiber is suitable for enhancing the pads’ performance and stiffness but too much of it causes not only stronger rotor attack characteristics but also higher heat conductivity which puts the surrounding parts under excessive heat. Some of the typical heat harms are -

●widening of calibers　
●distortion of rotors
●fluid leakage from bleed screw
●cracks　
●jagged surface rotor　
●too much wear of pads
●peeling phenomenon due to curving of the backing plate　
●brake fluid vapor lock

These are all very dangerous phenomenon which could immediately cause crash on circuits. High braking performance is particularly expected from racing pads and so contains a large quantity of steel fiber. However, that causes the pads to be heat conductive and also to heat up easily. We have succeeded in developing a race pad with low heat conductivity by using as little steel fiber as possible. The name of the pad is R01. Please experience it with your car this excellent performance cannot possibly be shown in data.

The life of the Rotor

If you think you do not need to change the rotors, then you have danger right in front of you. Rotors have a lifespan and in the case of the front rotors, about 1.0mm wear on one side. * i.e. 1.0mm wear on either side of a rotor, inside or outside. In the case of the rear rotors, it’s 0.5～0.75mm wear on one side.　The minimum thickness is indicated on each rotor （Please be careful as the minimum thickness is different depending on part number）. Use beyond minimum thickness is very dangerous because of the lower heat resistance. In the worst case, rotors could be distorted or cracked easily. We do recommend you to change rotors before such things happen.

What is the Heat treatment process?

At our company, a strict temperature control is being implemented. Detailed information on duration and temperature cannot be made public due to corporate confidentiality; however, we are going to use some imaginary figures for explanation.　First, raise temperature by 5℃ every 10 minutes and,　when the temperature reaches 300℃, keep heating for 8hours to maintain the same temperature. After that, cool it down lower the temperature by 5℃every 10 minutes. On a graph, it is easy to see the temperature control is ideally done. Heat treatment basically takes 24 hours for the whole process. By taking as long as 24 hours, and by carrying out heat treatment slowly keeps perfect control on the temperature, distortion is avoided, the binding of metal particles is reinforced and precision in heat resistance is improved. * The cited figures in the temperature changes in the heat treatment are imaginary and so different from the time and temperatures in our actual heat treatment.


What is the heat resistance temperature of disc rotors?

Unlike brake pads, it is very difficult to indicate the exact temperature i.e. “ ～℃”. Original rotors and basic materials for all types, SD type or HD type, are at the same risks of cracks or distortion when temperatures becomes beyond 600℃ or higher, however, cracks or distortion do not always happen if temperature goes over 600℃ depending on each condition of use. The affecting elements depend on as how fast it was cooled down, how often the temperature was repeated over 600℃ or how the state of the brake system including caliber was, are often more important.

In circuit driving, which Rotor is more beneficial: With slots Versus Without Slots or Heat versus non heat treated?

Firstly about slots, rotors with slot perform better than ones without. (According to the data from our tests, performance was improved by 15～20% by using slots. Also the gas discharge effect of the slot can lower the temperature of the whole brake system, which makes stable driving possible.

About running in of rotors

First, here is an instruction for street use only. It depends on combination of pads and conditions of road you drive on, it takes roughly 300km～1,000km of running-in on ordinary roads. During that time, please refrain from fast or abrupt driving or do not drive in a way as to force the temperature up. Running in of rotors will be completed simply via normal driving.

Circuit use

Distortion or cracks in rotors happen easier if optimum driving on a circuit is conducted straight from the beginning this in turn causes judders. When using a new rotor 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 a 5 minutes interval. After that, repeat 70～80% braking for about 10 minutes. Pit in again and take an interval of about 10 minutes. After that, gradually increase from 80% to 100% braking and the running in of rotors on circuit is completed.

Materials for disc rotors

Materials used for rotors are roughly categorized into carbon and cast iron. Carbon has advantages such as high heat-resistance and light weight but as it is expensive its’ use is limited to some racing purposes such as F-1. More commonly used is cast iron, there are three types depending on the shape of graphite contained; grey cast iron, CV cast iron and ductile cast iron. Grey cast iron (flakes graphite cast iron) has excellent processibility and anti-abrasion capacity and so has advantage that it is easily mass-produced thus it is most commonly used by rotor makers. Its drawback is that it could be deformed or cracked under repeated ups and downs of temperature at high temperatures (about 800℃). Ductile cast iron is excellent as a material and has a tensile strength which equals that of steel. Its anti-heat capacity (stability against expansion and contraction) is also high. On the other hand its surface hardness is low, and so it sometimes causes abnormal wear (jagged surface rotor) or abnormal heating due to its high exothermicity if used for rotors. CV cast iron (Compact Vermicular cast iron) has an intermediate character between grey cast iron and ductile cast iron but as its quality changes some times to that of grey cast iron but other times to that of ductile cast iron, it is extremely difficult to control quality in the manufacturing process. At our company, after extensive tests with actual cars , we have been using grey cast iron with some special additives.　Other Products, ones with FC150～２００ are often used but Dixcel use ones with FC200～250 for higher durability we have managed to improve on the additives to offer even more heat-resistant rotors. Needless to say, they are at a high level in terms of precision and balance.


The structure and the shape of the disc rotors

Solid discs and ventilated discs are the main types of disc rotors. Ventilated discs have a shape of two iron discs put together and when wind goes through the rotor, it can be effectively cooled down. Some cars at present have brakes of this type for front brakes, this includes high performance cars such as the Skyline GT-R, and ventilated discs are used for the rear brakes as well. Ventilated discs are further divided into subgroups depending
on the shape of the fins. Straight fins are most commonly used in many cars, but some high performance cars such as the Supra and Lancer Evolution adopt curve vane or pillar type. Disc rotors are used in a variety of forms depending on their sizes or cars they are used for.


Merit and defect of the number of slots

Generally speaking, the more slots there are, and the higher the friction coefficient becomes, but at the same time, there will be roaring noise and the pads will be worn out faster.

Merits of more slots

● Higher friction coefficient
● Control on judder
● More efficient cooling

Defect of more slots

● Faster abrasion of pads
● More roaring noise
● High cost

Differences in effect between aggressive rotation and conservative rotation on slotted rotor

Both ways present slot effects, however, friction co-efficiency in aggressive rotation is higher compared to conservative rotation. On the other hand, abrasion of pads is faster in aggressive rotation than conservative rotation.

The data above is a test data on an assumption that our Z type is used on the streets and on a circuit. The figures changes depending on the materials of the pads. Also abrasion figures change depending on the purposes. Taking all those characteristics into consideration, our six-slot SD type and HS type adopt aggressive rotation.

* Please remember six-slots rotor is for aggressive rotation while twelve-slots rotor is for conservative rotation. Please also be careful not to fit them in wrong ways.