Today's retread, besides offering a cost savings over the price of a new tire, also must be equal in terms of safety, mileage, appearance and road performance. The safety of today's retread results from the availability of more perfect and reliable casings, more efficient equipment for the examination of casings and from more comprehensive and adequate technological experience.
Retread mileage could vary a lot in years past. But today it's predictable. Modern tread compounds are technically comparable to those used in new tires.
Highly automated vulcanizing methods, much like those used in new-tire production, also can provide retreads with strength and mileage equal to, or even better than, some new tires.
Today's retread often is indistinguishable from a new tire-thanks to European equipment manufacturers who led the way in producing equipment designed for radial tires long before some of our domestic manufacturers even realized this was necessary. However, our domestic makers rapidly are catching up.
On the highway, retreads must measure up to the same requirements as new tires when it comes to road adhesion, static and dynamic balance, and lateral and radial runout.
Despite the fact that new-tire designs have been available for years, many retreaders have opted for ``neutral'' tread designs.
In the past, this has created problems in terms of excessive road noise, road adhesion and rolling resistance unsuitable for the radial tire.
When retreads are processed using new-tire designs and rubber compounds similar in all respects to top-quality new tires, there should be no substantial difference between them and new tires where road adhesion is concerned.
Road adhesion is mainly governed by tread design, rubber compound quality and type of casings-all of which are factors generated outside the normal production procedure at the retread shop.
However, characteristics such as balance, runout and roundness are determined by the care and precision with which the retreading process is carried out.
Therefore, in order to produce a retread truly comparable to a new tire in every way, it simply is not enough to use modern equipment and quality materials. Precision in workmanship also is absolutely necessary-and this is where shop production technique becomes an art.
Take a moment to consider how the adoption of a few simple standards and extra attention to detail can have a significant effect on product quality. For example, not all new tires are perfectly balanced even though they are routinely checked for balance at the end of the production line.
Some tires may suffer a slight change in balance due to the casing ``settling down'' after being mounted and run on the road.
However, by the time the tire becomes available for retreading, the casing usually is stable.
Even so, a stable casing does not necessarily mean a balanced tire. Indeed, due to uneven tread wear, the tire may be anything but balanced.
Therefore, it must be buffed perfectly round. This no longer is difficult when using modern buffing equipment, since most tires are mounted on a chuck or hub and inflated.
To ensure the tire is buffed perfectly round, it is only necessary that the tire/chuck assembly be running concentric to the rasp and that a constant inflation pressure is maintained in the casing.
It also is important to maintain a symmetrical buffed profile so there is equal thickness-and therefore weight-of rubber remaining on both shoulders of the casing.
After the buffing stage has been completed, it is a fairly simple matter to conduct an elementary check for static balance.
The lightest spot on the casing can be indicated with a chalk mark on the sidewall. Then, when the new tread is applied, the rubber splice can be positioned so the light spot on the casing counterbalances the heavy spot on the tread.
To obtain the highest degree of roundness and minimum lateral runout, careful attention must be given to buffing symmetry, buffing dimensions, the precision with which the tread is applied and accurate placement of the tire in the matrix.
The maximum diameter of the matrix should be measured, and from this dimension should be subtracted twice the design depth plus twice the required undertread thickness.
By undertread thickness, we are referring to the total thickness of rubber between the base of the tread design and the top ply or belt assembly of the casing.
Undertread thickness varies according to the tire size and tread design. Undertread is essential in protecting the belt assembly against damage and for securely anchoring the various design elements in order to prevent tread separations or local rubber lifting. For this reason, OTR and winter designs require a thicker undertread.
Since it is customary to leave a thin layer of original rubber on the casing after buffing (passenger tires, 2/32-inch; light truck, 3/32; medium truck, 4/32), allowance for this must be made.
Whether cured as a remold (bead-to-bead) or a retread (shoulder-to-shoulder), the tire is fully enclosed in a matrix in European-style equipment. If the casing has been accurately prepared and conforms to the dimensions of the matrix, centering is automatic.