Friction caused by rotating and sliding elements; losses generated by tyres; aerodynamic resistance ... there are different friction factors that considerably reduce the power generated by an engine which is transmitted to the axle. Let’s analyse their influence and understand how to minimise them
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WHAT IS IT?
HOW DOES IT WORK?
IN PRACTICE
Each engine has a net power, expressed in Horsepower (30-40 hp for engines with or without gears), which generates a certain thrust (power at the crankshaft). However, in practical terms, it clashes with a series of forces that limit and block the movement of the kart. The best known is the weight-based opposition according to the first law of dynamics: a=F/M, where a is the acceleration, F is the thrust generated by the engine torque and M is the kart plus pilot mass. But there are other elements that act against the thrust of the propulsion. First, friction, the resistance between the various mechanical components in relative movement: such as the bearings, axle and hubs of the rear rims,
rather than friction of the chain on the wheel and sprocket. Even more important is friction caused by the rolling of tyres, resulting from tyre contact with the asphalt and, above all, the deformation of the body due to the rolling motion, which creates an important force blocking the advancement of the kart. Lastly, aerodynamic resistance, an element strongly linked to the speed of the vehicle, but also to its aerodynamic shape, must be considered. Thus, in short, friction losses (mechanical and aerodynamic) can be divided into frictional power losses of the chassis organs (bearings and transmission chains), tyres, and aerodynamics. So they can be analysed better.
WEIGHT, FRICTION AND AERODYNAMIC RESISTANCE OPPOSE THE GENERAL FORCE GENERATED BY THE ENGINE AND REDUCE KART PERFORMANCE
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FOOTPRINT ON THE GROUND
The difference between tyre - asphalt contact surfaces with the vehicle still and the rear still moving
WHAT IS IT?
HOW DOES IT WORK?
IN PRACTICE
VEHICLE SPEED
The relationship between the rolling coefficient f and velocity of kart V has a constant value of f up to about 60 km/h for a conventional tyre and 110 km/h for a radial one, which therefore maintains a lower f value at all speeds. Subsequently, the relationship becomes exponential to the square, with a sudden similar increase of f for both tyres.Read more
TYRE STRUCTURE AND MATERIAL
The rolling coefficient f, depending on the speed V, for tyres made for different uses. The difference on these tyres are the materials, body structure and type of treadRead more
This chart shows the rolling power lost, depending on the speed, depending on the material with which the tyre is made.Read more
VERTICAL LOAD AND INFLATORY PRESSURE
The rolling coefficient f generally decreases as the inflation pressure increases (lower footprint and tyre deformation), and increases with an increase based on the vertical load on the tyreRead more
The rolling coefficient f generally decreases as the inflation pressure increases (lower footprint and tyre deformation), and increases with an increase based on the vertical load on the tyreRead more
Among the above-mentioned friction elements, those relative to the bearings and chains are difficult to calculate. In general, it can be said that, for bearings, it depends on the type of the bearings, their diameter and the bearing load. The latter parameter is approximate to the load factor on the individual rear wheels (for the axle bearings) and the front. It is more difficult to assess the losses caused by rolling resistance due to the deformability of the wheel, but in this case the calculation can be improved by experimental data from the automotive world. Tyre losses can be deriving from two factors: rolling resistance and sliding between the tread and asphalt.
WHAT IS IT?
HOW DOES IT WORK?
IN PRACTICE
Below 120 km/h, in other words in most of the conditions under which a competitive kart races, 90% of the resistance is derived from the first component and only 10% from the second. Therefore, it is clear that it is the rolling resistance that needs to be analysed more thoroughly for a kart.
Due to the weight on tyres when they are in contact with asphalt, they deform and creates a tyre-asphalt contact surface of a certain thickness. This footprint is moved towards the direction of motion with respect to the vertical axis passing through the centre of the wheel (fuselage). The vertical pressure between a tyre and asphalt is greater in the direction of motion. In this way, the pressure can be
seen as a vertical upward force shifted in the direction of motion with respect to the previously indicated axis, which creates a torque that opposes the rolling of the tyre. The total force opposed to rolling can therefore be calculated experimentally considering the vertical force N and the rolling coefficient f. In turn, the latter can be experimentally calculated based on a number of parameters such as kart speed V, tyre inflating pressure, vertical wheel load, tyre size, radial or conventional structure of the tyre , tread material, temperature and condition of the track’s asphalt, as well as forces on the sides, on bends, and longitudinal forces, during acceleration and braking. All these parameters are analysed in the graphs of the previous slide.
ROLLING RESISTANCE
The vertical force N pushes the wheel in the opposite direction to the rolling motion.
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