How Tires Work

I am thumbing though a DOT publication on pneumatic tires that is close to 1,000 pages long and has some interesting things in it on tires. I am passing along some parts of it that were interesting to me.

It is abundantly obvious that this material has limited interest. I hope it does help explain some of the choices made by the tire manufacturers. If you can stomach this post, it may help you make some decisions next time. (or, you can do what I do - ask the dealer for advice).

In an undeflected tire, the cords are tensioned by the excess pressure of the inflating gas over the external or atmospheric pressure. The tire casing takes up its equilibrium shape, which is determined primarily by the cord paths, somewhat modified by the other components of the tire. As the tire is pressed against a flat roadway, the tread rubber is compressed and at the same time the tire casing locally loses its axial symmetry and takes on a substantially flattened contact patch. If there were no tread rubber, the casing would be flat to the ground in the area of the contact patch.

The bottom sidewalls of the tire do not "push upward" to keep the wheel rim off the ground. The wheel rim HANGS in the bead coils, which in turn hang in the tensioned casing cords, which have lower tension in the contact patch than elsewhere. The sidewalls of the tire are in tension. The contact patch has reduced tension because of the weight of the vehicle.

The tire contact area's shape depends on the tire cross section shape and structure. The relationship between tire contact area and tire deflection is nearly linear, as you would expect. Tire deflection is the most important variable governing the area of contact between the tire and the roadway. If inflation pressure and load are varied simultaneously, the contact area will remain effectively constant. There are few studies on the effect of velocity on contact area, but experimental data seems to indicate that velocity slightly increases contact area. Fluid contaminants, such as water, oil, slush or mud reduce the contact area due to the persistence of a a fluid film in portions of the formerly dry contact area. This resistance of the fluid to expulsion is tied to the viscous and inertial forces of the fluid as it is wedged and/or squeezed between the tire and the pavement. This results in a loss of dry contact area.

Tests of tires on wet surfaces using photographs taken through glass in the "pavement" surface have analyzed the variables in determining the effect of the fluid on the tire. The extent to which the fluid will persist along the centerline of the contact patch is determined by whether the fluid inertial forces are large enough to bend the tire surface inward along the footprint centerline. Tires with rigid carcass construction, high inflation pressures, and large tread element size will tend to work better in wet conditions. Tires with flexible carcass constructions, low inflation pressures, and narrow, widely spaced grooves, will tend to lose contact along the centerline in wet conditions.

Even when fluid is insufficient to flood the major groove network, it can reduce traction by persisting in individual areas. Contact is established first around the perimiter of the contact area, trapping the fluid in the central area under pressure. The sipes of the tread element and microtexture of the surfaces of the tire and pavement allow the fluid to escape (if it does so).

Contact area on dry areas is still not completely understood, and there are different conflicting theories. Experimental data indicates that the net pressure distribution at any point in the contact patch depends primarily upon tire pressure. Small blocks of tread design often demonstrate markedly different distributions on different areas of the same block. While in theory the smooth tire gives the best traction, pressure levels in blocked patterns exceed those in smooth areas by about 25%. Maximum tire pressure (on the contact area) occurs in the middle of the projection, being slightly shifted to the "toe" of the blocks, in the direction of vehicle travel, due to the increased stiffness of the sheared and compressed rubber in that region. Uneven tire wear and moulding irregularities including seams, can result in quite different distributions at different points in the life of a tire.

The tire structural elements which most effect the amount of pressure in the contact area are:
- Elastic support of the tread by the sidewall
- bending of the tread
- "Snap through" buckling of the tread, defined as the tendency of the tread to seek a deformed equilibrium position due to membrane compression
- normal compliance or stiffness of the tread.

Slip between the tire and the roadway is important with reference to the contact patch. When a tire is brought into contact with a flat roadway, it can only form a flat contact area by simultaneous bending and compression of the tire surface. This means that tread elements in contact with the roadway will generally undergo a small amount of deformation in the plane of the carcass as such tread elements pass through the contact patch of the rolling tire and exit out the trailing edge. This process takes place in every tire.

It is possible to minimize the membrane stretching and contraction by making the carcass structure of the tire as rigid as possible. This is generally the principal behind radial tire construction. It is also possible to minimize such membrane distortions by use of extremely stiff materials for the tire carcass, such as steel wire. Since the membrane deformations actually take place in the carcass of the tire, it should be possible to prevent motion of the tread on the contact surface by choice of an appropriately soft tread compound.

Last update: 06:30 PM Sunday, September 26, 2004

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