Tire footprint

The contact area between a tire and the road is called the tire footprint and is shown by Ap. At any point of a tire footprint , the normal and friction forces are transmitted between the road and tire. The effect of the contact force can be described by a resulting force system including force and torque vectors applied at the center of the tire footprint. The area of the tire footprint is inversely proportional to the tire pressure. Lowering the tire pressure is a technique used for off-road vehicle in sandy, muddy. or snowy areas, and for drag racing. Decreasing the tire pressure causes the tire to slump so more of the tire is in contact with the surface, giving better traction in low friction conditions. It also helps the tire grip small obstacles as the tire conform more to the shape of the obstacle, and makes contact with the object in more places. Low tire pressure increase fuel consumption, tire wear, and tire temperature. In most vehicles, the front and rear tires will wear at different rates. So, it is advised to swap the front and rear tires as they wear down to even out the wear patterns. This is called rotating the tires. Front tire, especially on front-wheel drive vehicles, wear out more quickly than rear tires.

Figure1 Tire footprint**

** Figure Ref. http://www.aircti.com

Tires behavior : Hydroplaning

Hydroplaning is sliding of a tire on a film of water . Hydroplaning can occur when a car drives through standing water and the water cannot totally escape out from under the tire. This causes the tire to lift off the ground and slide on the water. The hydroplaning tire will have little traction and there for, the vehicle will not obey the driver’s command.

Deep grooves running from the center front edge of the footprint to the corners of the back edges, along with a wide central channel help water to escape from under the tire.

There are three type of hydroplaning; dynamic, viscous, and rubber hydroplaning. Dynamic hydroplaning occurs when standing water on a wet road is not displace from under the tire fast enough to allow the tire to make pavement contact over the total footprint. The tire ride on a wedge of water and loses its contact with the road. The speed at which hydroplaning happens is called “Hydroplaning speed (VH) ” that is estimated in knots by equation 1

VH = 9*P^0.5 -------(1)

Where P is tire inflation pressure in psi

For a matric system would be

VH = 5.5753 x 10^-2*P^0.5

Where P is tire inflation pressure in Pa

VH is in m/s

Rubber hydroplaning is generated by superheated steam at high pressure in the tire footprint, which is causes by the friction generated heat in a hard braking.

Viscous hydroplaning occurs when the wet road is covered with a layer of oil, grease, or dust. Viscous hydroplaning happens with less water depth and at a lower speed than dynamic hydroplaning.

Ref. Vehicle Dynamics: Theory and Applications , Reza N. Jazar

Using polyurethane in mining and quarrying industries.

In the mining and quarrying industries, however, polyurethane is being used for many other applications with advantage and replacing rubber or even hard steel. In general these applications are connected with movement of very abrasive slurries of relatively small particle size such as in the ball and rod mills, flotation plant, hydrocyclones and piping.

Ball or rod mills have generally been lined with manganese steel replaceable linings, but in recent years the use of rubber replaceable linings has been developed. Dependent on the condition existing in a given mill these rubber linings can last several times longer than the manganese steel, and polyurethane linings can last considerably longer than the rubber linings. Further advantages are that the tooling cost with polyurethane are less and the fact that, due to the availability of the harder grades, the polyurethane can be used as a structural material and does not require reinforcement by metals.

Similarly in flotation plant the impellers, rotors, stator and scrapers are made of steel centers often covered in conventional elastomers. Polyurethane again can be used in certain cases and will generally outlive natural rubber. The main precaution to be observed are limitation in temperature and also extremes in acidity or alkalinity of the slurries.

Hydrocyclones used in the separation of ores and cray are also subjected to harsh abrasive condition, and polyurethane have been successfully used in these conditions.

In mines there is often a considerable fire risk, and comprehensive precaution are taken to avoid spark which can start a fire or explosion. For this reason the use of track laying vehicle with metal tracks is prohibited. Covering of the tracks with natural rubber goes only part of the way to solving the problem due to highly abrasive conditions causing relatively rapid breakdown. The polyurethane track pad do not wear so rapidly and are being used successfully. A further advantage of polyurethane as opposed to metal is the increase in traction obtained which enable the vehicle to climb steeper slopes.