Since World War II, retreading has become not only an internationally established industry of major proportions, it is also a major recycling activity that conserves scarce raw materials. Aside from retreads' lower prices, compared with comparable new tires, the market demand for retreading is linked to the performance of the product, which must equal or better that of a new tire. This essential factor is directly dependent on the latent strength of the casing once it has ended its original tread life.
Unfortunately, casing availability has begun to be affected by the low replacement rate of radials over the last several years.
It is becoming more difficult to find acceptable numbers of retreadable casings-due perhaps to poorer casing quality (truck casings, in particular, show increased signs of structural defects), to prolonged service on the road or to poor maintenance.
In retreading, the essential basis for high quality production consists of the accuracy and skill with which each processing stage is undertaken.
The initial inspection stage, in which the acceptability of the casing for retreading is determined, is without doubt the most important of all. Thus it calls for highly skilled personnel who have a thorough knowledge of the construction features of the various types of tires.
During this inspection, it is imperative that all possible flaws, both evident and hidden-including manufacturing defects or damage due to improper use-are brought to light. The inspector also must have a good knowledge of tire mechanics and the results of improper use.
The standing wave
For instance, maximum speed limitations are imposed on all types of tires because of a longitudinal distortion known as the standing wave. This name derives from the fact that these waves present a stationary appearance to an observer.
A standing wave is a ripple in the tire formed behind the road contact area, a deviation of tread and sidewalls from the uniform deflected shape of the casing at low speeds.
This ripple behavior varies in accordance with the tire construction. In the bias-ply tire, the sidewall and tread maintain a constant relationship because of the cross-ply construction of the casing. One standing wave affects the entire cross section.
In a radial tire, the wave may be generated either in the tread or in the shoulder due to local buckling of the two-piece construction, or in the sidewalls themselves.
Whatever the type of tire, if it is running under conditions that allow a standing wave to develop, the repeated deformation caused by the wave process results in considerable heat buildup that affects the strength of the tire and may ultimately lead to its destruction.
The areas most likely to suffer a reduction in tensile strength as a result of a standing wave are the upper and lower sidewall areas roughly corresponding to the belt edges and the edges of the bead reinforcements. These areas require close, special attention when examining tires for retreading.
The standing wave problem can be postponed by reducing the load on a tire or by increasing its inflation pressure, either of which will reduce the vertical deflection of the casing. The newer, ``low-profile'' tires also are a step in reducing vertical deflection.
Tire failures are always the final stage in a continuous, mostly irreversible deterioration process, and a visual inspection-no matter how careful-is not enough to guarantee against latent defects.
Even the most sophisticated inspection processes available today are not an absolute guarantee, simply because the deterioration process is both continuous and progressive.
To meet the requirements of today's high-performance commercial vehicles, the modern radial truck tire must continue its technical development.
Trucks today are operating over distances and at average speeds that were unthinkable only a few years ago.
Even the tire manufacturers themselves do not seem to be sure whether they have the final answer and whether the current radial truck tire will remain the same in its present structural and dimensional form.
And though the current technical level of the modern radial is very high, from the standpoint of maintenance, the user is in need of a greater sensibility and knowledge about inflation pressures, loads and the prevention of excessive wear.
A campaign to better educate tire users in proper maintenance would help protect casings and pave the way for longer-lasting, more reliable retreads-which in turn, would be of greater economic benefit to the user.
It would then also be possible to salvage a greater number of retreadable casings, thereby conserving scarce materials, most of which are derived from petroleum.