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TKART magazine Tech Talk | The forces generated by tyres
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Gianluca Covini
25 September 2022 • 14 min. read
What are the basic physical laws governing the contact area between the tyres and the asphalt (an area about the size of a pocket diary)? Find out in this article to better understand the dynamics behind the kart you drive
How does the kart move, brake, accelerate and turn? The absolute protagonists of these actions are the tyres, which transform the inputs from the engine, brakes and steering, into acceleration (and deceleration) on the track. How? By exchanging, through contact with the asphalt, so-called "tangential" forces, i.e. applied at the point of contact between the wheel and the asphalt and with a "horizontal" direction, therefore parallel to the asphalt. To understand how these forces develop, it is necessary to divide them into two types: longitudinal forces, which develop along the longitudinal axis of the wheel, and transverse forces, also called lateral forces, along the transverse axis of the wheel. The former are those that allow the kart to accelerate and brake, the latter allow you to turn left and right.
The accelerations can be derived from the forces applied to the contact between tyre and asphalt. in fact, the force is equal to the mass for the acceleration (F = m x a). The longitudinal and lateral forces respectively depend on two very important parameters, which must be kept in mind and which we will describe in the paragraphs below: the longitudinal rolling and the drift angle. Furthermore, as we will see, the longitudinal and lateral forces can also combine, and in this case we are talking about combined grip, which is the most critical situation for the tyres, the one that the best drivers know how to manage best. Combined grip occurs, for example, at the entry to a bend, where the tyres have to both brake and turn, or when exiting bends, where the lateral component is added to the longitudinal component given by the thrust of the rear tyres.
The longitudinal forces allow the kart to accelerate and brake, the transverse forces are those that allow you to turn left and right. The contact area, given the high deformability of the tyres, develops within a small area, called, precisely, the contact area, which varies according to many factors, including inflation pressure and vertical dynamic crushing of the tyre. Dynamic crushing derives from rolling: the faster the tyre rolls, the more it "inflates", decreasing the contact area. To give an idea, by adding all four contact areas gives an overall surface more or less like that of a pocket notepad.
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