Rock Products

MAY 2019

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40 • ROCK products • May 2019 What's more, drivers must keep their eyes (and fingers) on the tires to inspect for wear – a weekly fingertip diagnos- tic (simply running a hand over the tread) can detect signs of irregular wear. Should toe or feather wear be found, for example, it could mean an alignment problem, suspension problem, or bent tie rod. If drivers detect early signs of irregular wear, your fleet can fix the underlying problem and continue with the evalua- tion. Be sure to keep record of any wear issues and vehicle adjustments. Evaluation Periods Evaluation units should be checked at intervals that cor- respond to about 20% of typical tire life. The main item to analyze is tread wear, and for this you need a quality tread depth gauge that measures to 1/32nd or 1 mm. Electronic gauges with digital readouts are easiest to read. When checking the tire, you should have three points for gauging across the tread face – the outside, middle and inside of the tread. Again, a fingertip diagnostic should be conducted as well and any signs of irregular wear reported. Irregular wear could indicate a problem with the vehicle, as mentioned earlier. If the irregular wear is found to be severe, in the drive or trailer position, rotate the tires in cross axle design (for example, LRI & O to RFI & O). However, don't rotate side-to-side. Since deeper tread squirms, wear rates will be faster at the start of the evaluation when the tread is near full depth, then level out toward the end. Projecting Tread Wear After two to three checks, you should have solid wear data on your tires. With this wear data in hand, you can now make projections on how many more miles the tires will last before they're pulled for retreading. This will give you an apples-to- apples mileage comparison and provide the basis for your first cost-per-mile figure on virgin rubber, once you factor in the purchase price. Now that you have data on how much tread is worn on your tires, how do you make your calculations? First, calculate tread wear rate in miles per 1/32nd of tread – miles traveled divided by tread consumed. Tread wear rate = (current mileage – installation mileage)/(original tread depth – current tread depth). As an example, the first test truck's current mileage is 124,000 miles and tires were installed at 100,000 miles. The original tread depth was 32/32nds and the current depth is 26/32nds. So, 6/32nds were worn off. Thus, the wear rate is 24,000 miles divided by 6, or 4,000 miles per 32nd. Next, calculate projected miles to removal. You'll need the original tread depth of the tire, and the pull tread depth. For drive positions, a typical pull depth might be 4/32nds, and in our example, the evaluation drive had 32/32nds of tread. Projected total miles to removal = wear rate x (original tread depth – pull point tread depth). In our example: 4,000 x (32-4) = 4,000 x 28 = 112,000 miles. Provided that your operation is not so severe that you lose many tires to damage, the projections give a good measure of relative performance and a good basis for calculating costs per mile. Casings and Retreads With projected mileage in hand, the next step is to understand and get a feel for the tire's next life. And, that's retreading. If time weren't a factor, you could run a full-blown evalua- tion getting true mileage figures on your evaluation tires, plus the number of times the tire could be retreaded. This is cradle-to-grave documentation, which is good to maintain as part of normal operations. But, that could mean years to gain quantifiable data. Instead, we recommend two things: First: Look at the warranty on your evaluation tire and compare it against the tire you're currently running. The manufacturer should offer a casing warranty in the event that the tire is not retreadable for certain reasons (but not for damage caused by severe operations). The warranties typically range from four to seven years from the date of manufacture, and offer casing credits ranging from about $40 to $130 on the first retread. Most manufacturers also offer a reduced casing credit for the second retread. This warranty comparison should give you a feel for how well the manufacturers back their products. Second: Talk with your tire dealer or retreader and ask about retreadability of the evaluation tires compared to your cur- rent tires. The retreader may give you actual rejection rates for the two brands (also known as RAR or "returned as received"), but be sure the information relates to operations similar to yours. Also, ask them what they will pay for a virgin casing of each brand. Your tire dealer may also be able to refer you to other fleets that are running the tires you are considering. You could ask them what they're experiencing in retreadability. Determining Cost-Per-Mile You have your data, now you can crunch some numbers. The original tire cost-per-mile is a simple calculation. Take the purchase price of your evaluation tire and divide by your projected mileage. As an example, you may be paying $400 for your evalua- tion drive tire and your projected mileage is 112,000 miles. So, divide your cost by the mileage and you come up with $0.003571 per mile (or $3.57 per 1,000 miles). If your current EvaluaƟng Tires

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