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TKART magazine Tech Talk | The secrets of the brake disc
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05 February 2018
The main part responsible for braking on a moving kart, the brake disc is a simple component in its basic concept, albeit nowadays analysed in minute detail and made using the characteristics of different technologies and concepts
Nowadays, you say "brake disc" and think of a concentrate of technology, made using cutting-edge materials according to projects and simulations on a computer. All true, yet this very modern component hides ancient origins, which legend dates back a few thousand years. It is said that the first use of the brake disc was on a vertical lathe used to model vases and consisted of a large disc at the bottom that was put into motion and stopped by an operator’s feet. Just a legend, presumably, but whatever its actual origin, to this day the brake disc remains the simplest and most effective way to dissipate the kinetic energy of a moving vehicle (certainly
more efficient than the "drum brake" system). Therefore, karts cannot do without this component, mounted on the rear axle and, on vehicles in the shifter classes, on the two front wheels, also. Naturally, in order to brake, the disc has to have pads and calipers, which in turn is controlled by a hydraulic pump operated by the pressure applied by the driver’s foot on the brake pedal.
The kinetic energy of the moving kart, calculated according to the formula showed above, needs to be dissipated in the form of contact friction between the pads and the disc (and, therefore, being transformed into heat), in order to stop the kart
Ecin= ½ mv<SUP>2</SUP>
For example: m=180 Kg; v=120 km/h (33.33 m/sec). Ecin = ½*180*33.33*33.33 = 100000 (Joules) approx.
Drilled disc: you need to pay attention to holes in cast iron because cracks can form between one hole and the other over time
Slotted disc: this type of machining of the braking surface has the task of revitalising the pads and discharging the gas produced during hard braking (with "Fanding", which decreases the coefficient of friction at high temperatures)
Milled disc
"Petal" disc: this type of machining of the brake disc also has the advantage of reducing its overall weight
In addition to ventilating the rotating disc, internal fins make it lighter still (anything that rotates has to be as light as possible!)
Straight, inclined, "helical" or "peg" shaped fins (sectional views)
Straight, inclined, "helical" or "peg" shaped fins (sectional views)
The "Finite Element Method" technique creates a "mesh" of the model of a disc, simulating the type of material. Constraints, associated conditions and the stresses on the disc and pad caused by braking are then applied virtually
The same procedure is applied to the simulation of the conditions of a pad placed under stress
The friction coefficient (the force applied to the disc and the disc-pad materials) varies according to the sliding speed and decreases as pressure is applied. Heat and friction stimulate the system and lead to wearing on both the pads and the disc itself, sometimes even provoking the permanent deformation of the two components. Maintenance is essential in order to avoid having problems. Starting by checking that the disc is straight, which is performed by positioning a dial gauge on the braking tracks: when rotating the disc, the oscillation must be less than 0.1 mm, otherwise it must be replaced (grinding is not recommended as there is a risk of excessively reducing the thickness, the minimum size of which is usually specified on the disc itself).
In order to counteract stresses, the geometry, thicknesses and materials must be carefully chosen when designing discs and pads. With regard to materials, cast iron (malleable grey or spheroidal cast iron, with a hardness of around 250 HB, or white cast iron, which is much harder, albeit more brittle) is most commonly used for discs. In some cases, special cast iron is used, for example "GJS600" spheroidal cast iron, whose tensile strength is typical of steel (about 600 N/mm2). Another material used in kart discs is stainless steel of the "AISI 400" family, which can be subjected to hardening in order to obtain a hardness of around 50 HRC. Stainless steel works well with sintered pads (copper, etc.), while pads made of organic material (traditional, black) are more suitable with cast iron.
Another decisive parameter in the choice of disc is the braking surface, considered both in terms of shape and width. The braking surface can be drilled, slotted, milled in various ways or even have the shape of a “petal”.
Ventilated discs are used to increase the dissipation of heat generated by braking. They are made using casting, through a process that involves the interposition of a suitably designed core in sand and resin (to make it more solid to handle during the process). This discharges inside the disc, creating fins of various shapes. The air sucked in coming from the sides is moved and expelled thanks to the radial "fan" effect created by the internal fins during rotation. It should be emphasised that the braking surface can exceed a temperature of 600° C. A carbon disc in F1 can exceed 1000° C.
In order to limit costs, karting regulations prohibit the use of these materials (consider that the manufacturing process of a carbon-finished disc lasts months). The regulations also prohibit special materials such as carbon-ceramics.
In the past, aluminium discs with ceramic, molybdenum or other castings made using "flame spraying" or "plasma spraying" processes have also been tested and used. Thanks to its remarkable lightness, this kind of disc was popular for maximising the reduction of the gyroscopic effect.
A disc is subjected to a lot of stress, both by heat and the pressure of its caliper that may not be perfectly balanced. This can lead to permanent deformation. The phenomenon begins with the appearance of a "blue" colour on the tracks. Another characteristic is that almost all discs used in karting are floating.
The force applied to the disc and the materials used for the disc and pads determine the coefficient of friction. Indicatively, the value is around 0.4 (that of the tyre with respect to the ground is about 0.8)
The most common flotation method uses circular centring and connecting pins, which allow slight axial play
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