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TKART magazine Tech Talk | What you need to know about kart chassis tubes
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12 November 2018
The basis of any successful kart is its chassis: a frame made of steel tubes welded together according to a precise design. Seemingly the epitome of simplicity, in truth it hides a multitude of technology and knowledge regarding materials, shapes, mechanical characteristics, treatments and welding
Of the many different types of vehicles in the world of motorsport, the kart is perhaps the one with the most “racing” of souls: it is simple, light, has an excellent weight/power ratio and is capable of lateral acceleration on bends that is greater than that of the most expensive supercars. Moreover, thanks to high performance combined with low costs, it has the peculiar characteristic of attracting both professionals and those who dream of a career behind the wheel, as well as mere enthusiasts who want to have fun. The basis of this particular vehicle is the chassis, made of steel tubes welded together and without a suspension. Seemingly a very simple object, from a construction point of view, in truth building a chassis requires considerable technical knowledge (the choice of shapes, geometries, thicknesses, etc.) and materials (elasticity, strength, treatments, etc.). In order to create a successful chassis, the main manufacturers use "FEM" (finite element method) computerised simulations and specific software; above all, they dedicate
an incredible number of hours to testing out on the track, collecting data and analyses, using the results to modify their final projects.
Precisely because it lacks a suspension, a kart chassis must be designed to absorb the "defects" of a track. Moreover, given the absence of a differential, which is not allowed in the regulations, and the presence of a rigid axle, it has to be able to "free itself" and perform optimally, especially on bends, when it is necessary to prevent the engine from dropping to low levels of torque. Therefore, a kart’s chassis acts as a shock absorber, working both in torsion and flexion. In recent decades, the geometric shape of kart chassis has remained virtually unchanged, a sign of having more or less achieved optimisation. The materials, the thicknesses and the treatments to which the tubes of a chassis can be subjected, as well as the precision and quality of the welds, are the aspects that have been continuously changing and evolving. All of which occurs in relation to the type of tyres used.
Given the lack of a suspension, on karts it is the chassis that must be able to absorb the "defects" of a track
The design of a chassis from the '90s (the Birel R series) compared to a chassis with the latest approved "standard" design
A kart’s chassis is mainly stressed by flexing and torsion. Hollow section round tubes offer the best compromise between resistance to torsion and weight. In the early ‘90s, CRG created an oval tube chassis, known as the "Epoch", but the project was later abandoned
The mathematical formula for calculating and/or verifying the torsion stress to which a hollow section round tube may be subjected
The mathematical formula for calculating and/or verifying the flexion stress to which a hollow section round tube may be subjected
The tubes (made by specialist companies, sometimes according to specific requests by manufacturers) are formed using a drawing process, which consists in passing a rough piece of ductile material through a die, or several dies, until it is the desired shape and calibration. During the drawing, the material undergoes work hardening, which benefits mechanical resistance. After the drawing, the tubes undergo annealing in a controlled atmosphere, then straightening, finishing, checking and storage
The tubes, of course, can have sections of different types: round, square, rectangular, oval, etc. However, kart frames only use hollow section round tubes, whose moment of inertia is the same on both horizontal and vertical axes. This facilitates and simplifies the various adjustments for the different conditions and characteristics of circuits. In fact, the goal is not merely to design a chassis with good performance, but one that is also simple to understand and responds faithfully to any adjustments. A chassis with oval section tubes, for example, would behave in a totally different manner depending on the direction in which the section is welded, in addition to which managing the joints between the various tubes and the welds would be more complicated during construction.
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