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24/04/2014
WHERE THE TYRE MEETS THE ROAD

A matter of touch


If the total grip available is partly used to brake or accelerate, that reduces the amount used to steer the vehicle. Therefore, braking or accelerating when cornering is reduced due to less longitudinal grip.

Massimo Clarke

The tires connect the vehicle to the ground, supporting the load elastically and are, therefore, able to ensure adequate comfort. Moreover, the forces transmitted to the road surface, both longitudinal and transversal, enable a vehicle to accelerate, brake and change direction. These forces work through the area of contact with the ground or '"footprint", whose extension and geometry  plays a fundamental role, together with the characteristics of the compound, on how the tires and consequently the car will behave.Due to the load, the tire is deformed elastically, " being compressed " on contact, the process constantly repeating as the wheel turns. Therefore, what takes place is a series of compressions and extensions, whereby the area of the tread that touches the ground constantly changes.

 

A new portion of rubber replaces the previous one and the deformation affects not only the structure of the tire (particularly the side walls that flex), but also, on a different scale, the tread compound where it touches the ground. A noteworthy consequence of this elastic deformation of the tire is constituted by the fact that , due to the above mentioned compression, the rolling radius of the wheel is lesser than the geometric one.

Therefore a complete revolution of the wheel causes a shorter forward movement than in theory (theory being, the circumference of the tire in static conditions and without any load impressed on it).The dimension and shape of the footprint are influenced by such factors as the size of the tire, its profile, the rigidity of the carcass, the inflation pressure and the load exercised on the wheel. A larger contact area with the ground guarantees a greater download of longitudinal and transversal  forces to the ground (longitudinal when accelerating and decelerating, transversal when steering).  The load on each tire varies according to different operating conditions. When cornering at high speed, the car tilts laterally and the outer wheels support most of the weight of the vehicle. In extreme cases, the inner tires may even lift from the ground.While braking, due to the load transfer (which depends on the height of the vehicle’s centre of gravity  and its wheelbase), the weight exercised on the front wheels increases; to a similar extent that on the rear wheels decreases. The exact opposite occurs during acceleration. These phenomena are particularly evident on motorcycles; it’s enough to watch a race to see how, when a rider opens up the throttle at the start of the race or after a corner, very often the front wheel lifts off the track (modern traction control systems greatly limit this problem, and sometimes can almost completely eliminate it). When the bike rears, all the weigh shifts to the rear wheel.These forces are downloaded on the ground in basically in two ways: by molecular adhesion and mechanical hold of the elastomer (any polymer having the property of natural rubber), which literally " penetrates " the textured surface of the road. Grip is the end result of this equation. Molecular adhesion exploits the Van der Waals forces and operates where the rubber meets the surface of the road, where bonds are formed , broken and reformed continuously. Mechanical hold takes place due to the asphalt roughness and the deformation of the tread compound. The tire’s grip is therefore influenced by the size of the footprint on the ground, the contact pressure and the surface conditions of the asphalt.

 

The tread is deformed and  during the rotation, a new portion of the elastomer comes into contact with the ground , is compressed and subjected to  forces that deform it , bending it ( backward in braking and forward during acceleration ), moving  to a limited extent before detaching at the end of the footprint.To ensure a good grip , in addition to the compound characteristics, we have to consider the load exercised on the tire . And it’s precisely to increase this load , and thus improve grip , that racing cars are extensively equipped with airfoils able to generate a great down-force.The maximum available grip can be represented graphically by the so-called contact patch, the radius of which indicates how much force can be transmitted to the ground in different directions. It is important to emphasize that if this force is exploited to a certain extent transversally, it proportionally decreases in the other direction or longitudinally and vice versa. In other words , if the total traction available is used to brake or to accelerate,   what is left to steer the vehicle is greatly reduced. Similarly, while cornering the capacity to brake or accelerate is reduced because the longitudinal grip decreases.

 

In order to download these forces effectively we have to consider the tire’s scrolling. The optimum, as far as  grip is concerned, is between 5 and 15% for tires intended for normal road use ( in the case of competition tires we talk of approximately 2-3%). If a 10% slip occurs during  acceleration, when the average speed of the vehicle is about 50 km / h, the speed of the driving wheels is 55 km / h.

Similarly, when braking the speed of the wheels is lower than that of the vehicle.

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