Brake means to stop. What should be stopped? Obviously, something that is in motion. Braking is an action that is used to bring a moving object to rest.

How an automobile especially a motorcycle is brought under the performance bike category is defined on the basis of its speed, power, torque, handling, comfort and lastly by braking power. What is the point of riding a bike which has a good top speed of over 300km/h but cannot be stopped in time? If there is a turn approaching in 50 metres and you need to bring your bike travelling over 250km/h to 80km/h and you have only 8 seconds to do that. The bike would slide into gravel trap and would fall. A bike even after having the best engine, ergonomics, handling and best in everything but has poor braking is just like riding a Spanish fighting bull. No matter how strong it is, ultimately it is going to make you fell off itself. The dial on the tachometer in a motorcycle should reach 300km/h mark from 0km/h as early as possible to make the bike the best in class. But it should also perform the same action in reverse order that means to bring the dial from 300km/h to 0km/h in short period of time (the reverse mechanism). “What goes around comes around”. How a simple life teaching could be followed in everything to make it safe. This article is not on the working or what disc brake is but on the types of rotor discs used in a disc brake.

Age?

The first disc brake system was developed in England (1890). The first automobile to have a disc brake was a car developed by Frederick William Lanchester (Lanchester motor company) in Birmingham factory in 1902. The design did not succeed because it used a copper disc which was fine but not good enough in useful braking at high speed.

The first motorcycle to roll out with a disc brake system was MV AGUSTA 600 in 1965 which was a touring cafe racer style motorcycle. Honda was already an emerging brand in all classes of motorcycle. The same brand achieved mastery in the disc brake and rolled out its first production bike with the name HONDA CB750.

There are many types of braking systems but here I would be focusing only on disc brake. When a bike is in motion, it has kinetic energy stored. Kinetic energy has to be removed from the bike in order to brake or reduce its speed. This is done by introducing another force to the system which is friction. Friction is developed when two objects rub against each other. This is basically the science involved in a disc brake system (mechanical brakes). In motorcycle, the profile of disc is different on front and rear wheel. Front wheel has always larger disc fitted than the rear wheel. Weight of the rider with the weight of the bike shifts to the front section of the bike. It mainly gets concentrated on the front wheel at the time of braking. To stop this incredible force, a larger sized disc is required to stop it. This is the reason why a bike has greater disc on the front wheel and smaller disc on the rear wheel. As the size of the disc increases, the efficiency of braking follows along with it. The other reason for increased size is the weight of a larger disc which is more on front wheel. This weight increases some amount of traction which is nothing but friction( a kind of stopping force). Friction in the brake along with the traction assists in braking. A disc brake consists of a calliper, master cylinder, friction pads and a rotating steel disc. I am not getting into a detailed working of a disc brake as there are many articles on that. But still to give a rough idea behind braking, the friction pads on the calliper start to rub against the rotating disc fixed on the wheel as soon as the brake lever is pulled by the rider. The whole unit is connected with each other by a set of cables that controls the system hydraulically (DOT 3 oil).  

The two types of disc brake system are:-

 Opposed piston/fixed calliper

This type of disc brake has pistons on both side of the rotor disc. Pistons on either side move back and forth towards the disc and rub against it to stop the disc fixed on the wheel while braking.  

Floating or sliding calliper disc brake

This type has set of pistons only on one side of the disc. Calliper is designed and fixed in such a way that the whole calliper slides or floats back and forth towards the disc to rub against it when the brake is applied.

This was just an overview of disc brake which would now help you to understand the main topic of this article.

Flat disc

It is the most basic type of disc made of cast iron which is flat as well as smooth and rotates on the axle shaft. The disc is uniformly made up of iron with larger surface area for increased contact patch for friction pads that improves braking power. Due to ease of producing the disc, it is cheap in cost and easy in installation.

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There is a significant increase in temperature when the brake is applied due to rubbing action of the friction pads against the disc. This may lead to a dangerous wear and tear problem and may be as serious as brake failure in some case.

Ventilated disc

When an automobile is assembled with a high performance engine i.e. increased power, torque and acceleration which demands for greater force to stop the bike. Flat disc brake cannot tolerate the heat produced in a bike travelling at a high speed while it brakes. Ventilated disc becomes a better option in such a vehicle. The main problem with braking is the heat generated in the area of the disc that gets sandwiched in between the friction pads. This heat does not get any way to escape out of the disc thereby increasing the temperature as well as danger. In a ventilated disc brake, the size of disc brake is bigger than a flat disc. But the increase in the size also increases the amount of heat generated in the disc while braking and raises the temperature. Another disc is fixed to the first disc/rotor. There is small gap in between the two discs and the gap is provided with slots to dissipate the heat generated in it. The pattern of the slots may be straight or little tilted or curved. Curved slots in the disc work like a turbo which improves dissipation of the heat. This design of the disc is for four wheelers. Ventilated disc becomes a standard disc in the upcoming types of disc brakes for this reason.

In a motorcycle, a disc similar to the above one cannot be used due to lack of space. However, the same concept could be adapted by fitting another disc on other side of the wheel.

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Ventilated disc brake is little heavier than a flat disc due to the increase in size and number of discs used.   

Drilled/Cross-drilled disc

The account of its usage states way back in 1960 when it was first used in racing cars. It is the most widely used disc brake around the world and the name itself tells the whole story. An improved version of a ventilated disc that has several small holes or air holes drilled in it on the surface of disc for a better heat dissipation. Holes are drilled in either cross pattern or wave pattern to cover whole area of the disc and to experience equal amount of heat getting dissipated from every section of it. These holes actually dissipate heat through the drilled holes. These also serve as an escape route for water, gases and other things that may come in contact with the disc when the bike is travelling on different road conditions. It reduces the problem of brake fade under repetitive and hard braking experienced in the previous type of disc. Drilled holes reduce the weight of the disc to make it lighter and reduce stress on the wheels. It also enhances the overall look of the wheel.

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Drilling holes means removing excess amount of material which in turn reduces the surface area of the disc. The disc is not made to be fitted in high performance vehicles as it is prone to crack under high temperature due to the presence of the holes. The holes along with the surrounding solid region of the disc have temperature difference which means that the holes would cool faster than the other portion of the disc. This gives rise to temperature difference making the disc prone to cracks. Although dust and debris are not a big problem in drilled but sometimes if the property of the dust is kind of stubborn that it faces dust trapping issue.

Slotted/Cross-slotted disc

This is an improved version of drilled disc where instead of small holes, long curved slots are machined in a cross pattern. The slots are curved in outward direction to throw the dust or water out and away from the centre of the disc. The longer slots reduce wear off problem which ultimately improves the overall life of the disc. Dust trapping is not a problem in slotted disc due to greater escape area. The disc works well in rain as it has better capability to throw off the water getting splashed on it in wet conditions. The main purpose of using this disc is the improved heat dissipating characteristic than the former. Also, the friction pads are made of a material that rubs against a metal that develops a specific gas which then gets sandwiched in between the disc and the pad. This is known as out gassing and does not allow the pads to make direct contact with the disc. Therefore slots also provide an escape window for such a gas.

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Even after all these qualities, a slotted disc is not used in commuter bike much due to the increase cost of producing one especially because of the complex m/cs used to cut slots. And we know, commuter should be a cheap and well affordable for the crowd. There is higher rate of friction pad getting worn out. It is also noisier when the pads rub against the slots.

Slotted and Cross-drilled disc

In slotted disc, there are several slots cut in the disc covering every section of the disc. The same is observed in a cross-drilled disc. There is enough space in between two adjacent slots or holes even after this. Designers thought of using the advantages of both the patterns to form a new type of disc. Slotted and cross-drilled disc has slots as well as holes systematically covering every portion of the disc to overcome disadvantages of the two.

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The amalgamation of the two processes involved in building one makes puts this disc on the higher side of the cost.

Dimple disc

This type of disc has no drilled holes but partially drilled dimple on the face of the disc. This retains the strength of the disc due to no removal of any material which was not observed in the type of discs mentioned earlier. This improves the lifespan of the disc. This also provides more surface area for the friction pads to scrub hence improving braking power. Out-gassing is also done in the disc with no compromise of the material.

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It has more weight making the front wheel heavy. It has less dust and water throwing capability.

Petal/Step Disc

In all the earlier type of discs, the shape of the disc was perfectly circular/round. In petal disc, the shape of the disc is changed by machining into a wave, petal like edges to make the periphery of the disc to look like a flower petals or steps. The testing revealed that a petal disc could brake more effectively and efficiently than a conventional disc with circular shape. Friction pads get a larger area to rub against due to the petal shape of the disc improving the braking power. The design of the disc avoids overheating problem while constantly dissipating heat to keep the disc cool. Petal disc is no doubt lighter than the conventional circular discs which make the front wheel lighter. Lighter front wheel makes the bike quick to accelerate and also makes the front end nimble for easy cornering.

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Petal disc make the friction pads to wear out faster.

Perimeter disc

We all know that the disc of a disc brake is assembled on the wheel hub. This is done in order to apply an overall constant stopping force on the centre of the wheel to make the bike stable under braking phenomena. Perimeter disc is a very unique type of disc in terms of its shape and assembly. Mostly seen in Buell motorcycles, the disc is assembled on the edge of wheel rim instead of been assembled on the wheel hub. This improves the surface area and improves braking. Also, as the disc is located in a less packed region of the wheel i.e. at the periphery of the rim, it dissipates heat better from both inner and outer portions of the disc. Buell Motorcycle Company is such a talented brand which you would come to know if you go through the models in detail. I have already discussed it in one of my earlier articles comprising vast information on frames. In my earlier article where I had explained about torque, there I had mentioned that if the distance from where the force is been exerted or applied is increased then it would increase the overall torque. Assembling the disc on the edge of the rim increases the distance of the disc from the wheel hub which increases torque ultimately increasing the stopping power of the brake. It also improves the overall look of the bike.

 

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Due to the design and the location of the assembly, special type of rim has to be fitted to the bike which is very expensive to increase the overall cost of the bike.  

Carbon disc

It is a conventional circular disc which is made up of a very contrasting material called carbon ceramic. Silicon resin with small grains of carbon fiber is moulded into a circular disc which is later heated and pressurized. A cycle of repetitive cooling and heating over a period of time forms the final product i.e.silicon carbide. The disc is drilled with holes to help dissipate the heat generated while braking. This is ultra light with enormous amount of strength and this is the reason why they do not have any impact even after reaching tremendous high temperatures and is less prone to brake fading which impressively increases the life of the disc. Carbon disc straight away improves the stopping power due to the property of not getting deformed under harsh braking making the bike brake from 300km/h to 80km/h or even less if required in less time and distance. Carbon ceramic is less reactive than steel and iron and hence it is immune to corrosion. The sticky black dust produced by steel disc and rust often makes the surface of the disc rough which inherits scraping sound when the disc is rotating. Carbon disc runs quieter than a steel disc as it neither rusts nor produces any such dust. Carbon disc is not used in road legal vehicles as the operating temperature of the disc is extremely high. This extreme high temperature is reached only in high performance racing automobiles like MotoGP bikes, NASCARS etc. Carbon brake was used in aircraft for the first time somewhere in the 1970 era. Later F1 cars adopted the use of carbon brakes due the impressive property of carbon ceramic. In 2001, the first production car rolled out of the factory which was Porsche 911 GT2.

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The brake is very expensive due to the material and the method of producing one. It starts to function only at extreme temperature so it cannot be used in road legal bikes where every street is covered under a speed limit for public safety.

The set up of disc brake is different in two-wheelers and three/four or many wheeled automobile. Wheel of a car is smaller with big axle and is designed in such a way that it could permit setting everything on the axle or the region in between the axle and wheel but the last part that locks on the wheel hub is the wheel itself. If there is a choice of dual disc/rotor on one wheel then it could be done only by a special design that is a single unit with slots. These slots give an impression of two separate discs that are integrated to become one with those horizontal slots due to its design. This is followed for each wheel. A motorcycle on the other hand has a different geometry of wheel with larger diameter. It has a swingarm and fork instead of a long axle. The front wheel is assembled in between the two forks. There is enough room to fix a disc in between the wheel hug and fork. This is the reason a slimmer disc is designed for a bike than a car which has thicker disc brakes for effective braking. This is done to stop a bigger mass in a car than a bike and also a car has bigger room for a broader disc to be fixed on the axle hub. You might have observed that when the wheel is removed from a car; the disc does not come along with it which is basically what happens in a bike where the wheel when removed also brings out the disc with it as the disc is fixed on the wheel itself. A separate disc is added on the other side of the wheel to improve braking power without compromising the even geometry on both the sides. Rear end is fitted with a single and smaller disc or drum brake in some commuters as there is chain sprocket on the other side of the wheel. However, a bike with single side swingarm has sprocket as well disc brake on the same side as it has enough room on that side due to the absence of swingarm on the other side. The same trend is followed in a car also where both the rear wheels either have discs or drum brakes. The braking could be taken to various levels of performance by increasing the number of piston, calliper and size of the disc. The material chosen to build the part could also enhance brake effectiveness.    

                by AutoVogue