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Tkart magazine

Tech Focus | Carbon: wheel rims, floor trays and pistons

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09 June 2016
Carbon fibre is still a relatively recent material, with characteristics of lightness and impressive resistance, such as to replace, especially in motor sport, including karts, materials such as steel, aluminium, magnesium and titanium. At least for certain applications.

Carbon fibre is composed of carbon filaments of infinitesimal thickness (5-10 micrometres). Equipped naturally of great tensile strength, this increases further when the filaments are intertwined, as is the case for the wires of the top of a boat.
The resistance of the filaments, however, only regards torsion. To also make the carbon resistant to bending and compression, the fibre must be combined with a resin, creating what is called a composite material.

Carbon is widely used in motorsport, particularly in F1, but with strict limits. In karting, for example, it is forbidden in the construction of chassis. It may, instead, be used for seats, floor trays and chain covers, with a good weight saving and an increase in rigidity. In fact, carbon has elastic properties, and it is therefore resistant to bending and torsion, significantly higher than either steel, used to manufacture standards floor trays, and fibreglass, used for the seats. It is interesting how some companies have even dared to try and make wheel trims and pistons out of carbon composite, but were unsuccessful.
Carbon composite is made by combining braided carbon fibre and epoxy resin, and its final specifications depend on the characteristics of these elements. The production process is fundamental. The simplest is the hand-coating of the resin on the fibre sheets, which, however, can lead to air bubbles and imperfections that make the carbon less resistant. The alternative is vacuum lamination by means of pumps which suck the air from a bag containing the component, eliminating bubbles between fibres and resin and distributing it more evenly. The process strong>ensures that quality is greater than that achieved in an autoclave, where very high pressures make the parts extremely compact and practically free of imperfections. This is how key elements are made, such as the bodies of the cars, helmets, but also floor trays and seats. However, non-structural items such as chain guards are also produced by other processes. The temperature is also important and must always be adjusted. Autoclaves have their own regulation system. It goes without saying that the production process affects the final cost of components.

Components (bearing housings, wheels, pedals, hubs, small parts, etc.) made of aluminium and magnesium are used for karts. The difference is related both to weight and performance, given the different levels of rigidity and the different coefficients of heat dispersion of the two materials. In recent years, carbon has been introduced to elements such as seats, floor trays and chain guards. The resultant weight gain is important, especially this year, in the OK category. The minimum weight has been significantly reduced from 158 kg to 145 kg. Looking at the data, according to the manufacturer’s Nek, floor trays made of composite carbon weighs less than half that of steel: 590 g against 1270 g. Tillet, instead, has indicated a weight of 1100 g for aluminium floor trays, compared with the 330 g of the carbon composite. Tillet also produces a seat made of composite carbon mixed and Kevlar. Its weight is about 0.8 kg, compared with 1.1 kg of the fibreglass model (Tillett data).
In addition to reducing the weight, the carbon allows an increase the rigidity of a particular component, thus allowing to affect, for example, and the structure. In particular, seats and floor trays cause a general stiffening of the frame and, thus, greater difficulty of the same to lift the inner rear wheel while going round bends, remaining closer to the ground. A situation useful for high power and high traction vehicles, such as the KZ, while for vehicles with lower power, a stiffening of the frame may affect the kart too much which may be counterproductive. It must be said that a carbon component is not necessarily more rigid: first of all, the fibre can be mixed with Kevlar or fibreglass, and it all depends on the total thickness of the sheets of carbon with respect to those of the fibreglass. The orientation of the fibres is also important. Also, the indications on the effect of floor trays and more or less rigid seats are only indicative, so what mainly determines the grip is the balance between loads on the front and rear wheels. Both carbon seats and floor trays are made specifically with different levels of rigidity for the needs of each chassis and driver.
Carbon has resistance (also superior to steel) and lightness (best aluminium and fibreglass) properties that make potential use enormous. Not surprisingly, an attempt has also been made to manufacture pistons and carbon rings. A super light piston would have great potential in the improvement of engine performance, precisely because of the reciprocating motion, and therefore the continued acceleration and deceleration of the piston. That’s why a top motorsport company like Lame has produced a prototype. The problem is the low thermal expansion of the carbon piston, which does not “follow” the expansion of the cylinder when the engine warms up.
Carbon has indisputable advantages in terms of lightness. It is also very durable, although a bit fragile: in case of major shocks, it breaks rather than bends, like metals. Rigidity may be another element for improving the performance of karts since, in theory, it allows an increase in grip. The disadvantage is that carbon fibre is expensive, as is as the production of components which are currently still necessarily artisan and cannot be automated, like steel and aluminium components.
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