How to be zipper free!

April 1, 2001

Tire safety and tire performance came under severe scrutiny last year with the Firestone tire/Ford Explorer situation. Tire tread separations, roll-overs and blow-outs became household by-words! Tire people were besieged with "what does 26 psi mean?"

The sensitivity of "tire performance" has pervaded all types of tire problems and has heightened interest in all types of tires, cars, sport utility vehicles, pickups, cement mixers, logging trucks, waste haulers, and long and short distance carriers. The problems of sidewall "zipper" ruptures and exploding multi-piece rims has flared up as well.

The Occupational Safety and Health Administration, trade associations and tire mounting safety classes all continue to preach the use of tire safety cages to inflate truck tires. But there always seems to be someone who has not been properly trained, does not have a safety cage, or does not use one in order to "save time," and, as a result, suffers personal injury and sometimes a fatality.

My view of "exploding" truck tires indicates that they have become less numerous in the past few years because of the following

* More truck tire service people use cages.

* More truck tire service people are aware of "zippers."

* More truck tire service people attend tire service training classes, such as the International Tire & Rubber Association's Commercial Tire Service classes.

* More tire dealers scrutinize worn radial truck tires closely for overloading/under-inflation, and sidewall damage.

* More retreaders are using more sophisticated, non-destructive testing equipment.

Truck tire safety primarily, though not completely, centers on the proper mounting of retreaded radial truck tires, used radial truck tires and repaired radial truck tires.

Any of the above types of truck tires with a sidewall weakness may fail with a zipper-type rupture within an inflation pressure range of 60 to 90 psi. At least that is my experience. I have not come across a radial truck tire zipper occurring at less than 60 psi inflation pressure.

My experience with situations where someone has been injured due to radial sidewall ruptures always has been outside of a safety cage. Viewed from the opposite side, I have never had a situation where someone has been injured while inflating a tire in a safety cage.

In any event, during the past summer I have been pleasantly surprised at the increased number of retreaders who are pressure-testing their finished retreads on a pressure tester to verify radial sidewall strength. Pressure-testing a finished retreaded radial tire appears to have several positive benefits:

1. It verifies the hydrostatic sidewall pressure up to 120 psi.

2. The tire service technician will have less fear and apprehension in inflating radial retreads.

3. Customers (truckers) appreciate the idea of pre-testing their tires.

4. The pressure-testing procedure finds and eliminates potential or actual weak radial sidewalls.

The unit for pressure-testing that appears to be most popular is the Matteuzzi G-100, made in Italy. I have not run across any other pressure tester in the United States (Zangl makes a pressure tester for the German market).

The current installed price of a Matteuzzi G-100 is approximately $25,000. The unit is fairly simple to operate and reasonably fast -- my timing of the process from mounting to de-mounting a tested tire was less than two minutes.

The cost per tire for equipment and labor is calculated as follows:

Step one:

Cost of Matteuzzi G-100: $25,000.00

Annual depreciation expense: $2,500.00

Monthly depreciation expense: $208.00

Cost per work day (22 days/month): $9.46

Step 2:

Labor time to process a truck tire for testing: 2 minutes

100 truck tires per day: 200 minutes

Estimated labor cost per hour: $15.00

Labor cost for 3-1/3 hours: $49.50

Estimated equipment depreciation cost per day: $9.46

Estimated labor cost for 100 truck tires: $49.50

Total: $58.96

Estimated cost per tire tested: $0.59

My experience with the Matteuzzi tester in the field is limited to two large radial truck tire retreaders who have been using their test equipment for more than one year. They have had no zipper ruptures while inflating radial truck tires for truck service.

The Matteuzzi tester has three levels of inflation, 25, 65 and 115 psi. Pressure levels are adjustable.

At the 25 or 65 psi level, sometimes distinct ripples, or waves, can be detected in the radial sidewall, indicating a weak steel body-ply structure prior to rupture. When this happens, the test will stop at this level, and the tire is then rejected for any use on a vehicle.

In my observations, a pressure-testing device like the Matteuzzi is a necessary item in today's truck tire market -- not only for retreaded truck tires but also for used high-tread tires and repaired truck tires. In fact, it is my opinion and suggestion that truckers and truck stops use high pressure-testing equipment on used and repaired radial truck tires.

I also talked to truckers who purchase or have their truck tires retreaded as to their opinion on pressure-tested radial retreads. Without exception, they all stated that if it were available in their market they would insist on pressure-testing their tires.

One trucker who has experienced several zipper ruptures claimed he would certainly give his mechanics "peace of mind" to know that the retreads they were mounting had been pressure-tested to 115 or 120 psi.

Curious, I went back to the two retread facilities that were using the pressure testers. I looked over the finished retreads that had been pressure tested and could find no indications, either through a label or branding, that the retread had been pressure tested to 115 psi.

I thought it would have great merchandising potential as well as visual testimony to the actual testing to mark the tires in this way. Both retread plants said they hadn't thought of labeling their tires with a pressure-testing symbol, but would look into it.

I suspect that a retread facility with a pressure testing process on each radial truck tire that had a bright label would certainly attract attention and give the truck tire user a higher degree of confidence in the safety of their radial retread sidewall strength. And I know of a least one retreader who is considering a minimal fee for pressure testing radial truck tires.

In summary

As the technology in non-destructive testing and inspection of worn truck tires progresses, the retread industry will supplement visual tire examination of incoming tires for retreading to a much higher degree of sophistication (see "Few injuries get past casing inspection machinery," Modern Tire Dealer, August 2000). This will allow the industry to eliminate from processing more tires for hidden defects that could prevent the successful performance of another tread.

In addition, the use of a high pressure tester (up to 120 psi) with all processed radial truck tires will, in my opinion, offer the chance for a "zipper-free" retreaded truck tire for the trucking industry.

Photo 1: The Matteuzzi G100, with the safety panel slid back, costs around $25,000 installed.