ROCHESTER HILLS, Mich. (June 21, 2004) — Ah, springtime—when a young man's fancy turns to thoughts of…tire testing! That is, if he is concerned about running the right tires in his fleet.
Spring is the best time to start tire tests, since the tires will have a chance to wear off the first couple 32nds of tread before going into summer and should still have plenty of rubber on them for the traction needed to take them through the winter months.
This year most fleets are buying new tractors and trailers to replace aging equipment and perhaps in-crease their fleet size. Several tire companies have come out with new models of tires that may well be an improvement over the tires some of your customers have run in the past.
If you have recommended these tires to your fleet accounts, they probably will want to compare them with the model tires they were running previously or with some new tires your competitor has. Either way, they may look to you for help in setting up a good comparison test. You'll want to be involved in this process—especially if your tires are being compared with your competition's products.
A simple test
Let's look at running a simple tread wear test. The first thing to do is to determine what you are testing. It is important to test only one variable at a time.
For example, test only one tire brand vs. another brand of the same type/vocation; one model vs. a newer model of the same brand; one retread brand vs. another brand; a mold cure retread vs. precure retread; a conventional size tire vs. a low-profile tire, etc. Notice, in all cases you are running two samples of tires: the test vs. a control group or another test group.
A real test is not just throwing a few tires on a vehicle and seeing “what they'll do.”
A question people always ask is: “How many tires should I test?”
For steer, drive and trailer tire tests, it is recommended that fleets have a sample of 30 tires for each individual test type or test group at the end of each test. This means that more than 30 tractors and more than 60 tires are needed if you're testing two groups of steer tires. That's a lot of tires and vehicles for some fleets.
Keep in mind that smaller sample sizes can be used in small fleets to indicate differences in tire groups, but the results may not be repeatable. That means the fleet may not see the same results when they actually run the tires for real in their operation. My advice is to use as many tires as possible if an ending number of 30 is out of the question.
If the fleet wants to test retreaded tires, be sure that all test tires have casings made by the same manufacturer. A retread test with “mixed” casings or tread widths may invalidate test results.
Of equal importance is ensuring that all new tires and retreaded casings in a tread wear test are manufactured within one year of each other as designated by the Department of Transportation code on the tire sidewall.
When mounting the tires make sure all wheels in the test are the same width. If the wheels al-ready have been in service, inspect all of them for damage, ex-cessive runout and wobble and in-stall new valve stems and grommets when the tires are mounted. Make sure vehicles are equipped in the same manner with steel and aluminum wheels if the fleet uses a mixture of wheel types.
Inflate the tires to the fleet's operating pressure after you ensure that it is adequate to carry the vehicle load and install metal valve caps. (Inflate-through valve caps are my favorite.)
Installing test tires on new equipment is the optimum situation to have since all the vehicles are usually specified the same. Suspension components on new vehicles are in good shape and are not worn or loose, which could affect tire wear. Tire wear rate and wear uniformity can vary with vehicle type, make, year and major components. The fewer the variables the better the data you will get, so make sure the fleet uses identical vehicles if possible for all test groups.
If vehicles of different specifications must be used, make sure they are equally represented in each test group. If vehicles that have been in service a while are used, the fleet should make every effort before testing begins to replace worn steering and suspension system components that are likely to need replacement.
As you are well aware, axle alignment is the second leading cause of irregular wear after improper inflation and, consequently, is a critical variable in a tire test. Therefore, at the start of the test the fleet should align all vehicles—even brand new units—to normal specifications. New tractors and trailers are not known for being well aligned when they leave their factories. If any unusual wear develops during the course of the test, check and correct vehicle alignment again. Be sure to keep all alignment records.
Usually half the test vehicles are equipped with tires from one test group while the other half has tires from the comparison test group. However, if desired, tire test groups can be mixed on the same vehicle to equalize exposure to random variables. The positions of the test and control tires placed on one vehicle should be mirror images of the tires placed on the second vehicle.
To avoid differences in weather conditions and geography, install all the test tire groups within 30 days. The vehicles should be hauling approximately the same weight, traveling in the same geographic area and the drivers should be representative of the fleet's personnel. Drivers with unusual driving habits should be screened out. To eliminate the geographic variations, use a domiciled fleet if possible. This type of fleet will run locally or regionally and return to the same terminal all the time.
Finally, the last and perhaps most important detail to attend to is identifying test tires and test vehicles uniquely.
This is vital since individual tires are often removed from service or moved to another wheel position or vehicle without the knowledge of anyone who cares. While this may not help you locate a missing test tire, it will indicate if tires have been lost or moved from their intended positions as data are collected throughout the test.
Labeling all test wheels and vehicles with a visual identification mark may help alert maintenance personnel and drivers that the wheels and vehicles are part of a test and should be handled appropriately.
Where's the wear?
It's important to know that tire wear rates, irregular wear and removal mileage vary with wheel position.
Typically, the right front tires on commercial trucks wear more slowly than do the left front tires. However, the right side tires are more susceptible to irregular wear conditions that can cause early removal even though the tires may still have an acceptable tread depth.
The reasons for this difference in wear is normally a combination of road crown, vehicle steering geometry, frequency and angle of left and right turns and total vehicle alignment.
On multiple-axle configurations such as the drive and trailer axles, wear rates will vary from axle to axle. The tires on the rear axle will tend to wear faster than the tires on the forward axle since the truck or trailer tends to “pivot” on the forward axle tires during turns and “drag” the rear axle tires—which grinds off rubber.
Due to these differences in wear rates, fleets often rotate tires by position to even out wear. During a test, rotation should be done only if it is the fleet's normal practice. If rotation is desired, tires must be rotated in the same manner and at the same test mileage interval on all vehicles in the test.
Once the test is running, check and correct cold inflation pressures at least monthly—preferably weekly—with a calibrated air gauge. Make sure to keep records that indicate the inflation pressures measured before they were corrected.
Remove any nails that do not cause air leakage that are found embedded in the tread of any tires in order to prevent them from eventually puncturing the tires. Tires that have been punctured should be repaired and returned to their test position with the same direction of rotation if possible. If this is not feasible, the damaged tire is considered “lost” and should be dropped from the test.
Tires that are damaged beyond repair must be dropped from the test, too. And if the damaged tire is mated with another tire of a dual assembly, the mate must also be dropped from the test since differences in tire diameter will affect the test results. When tires are removed from the test, record the reason for removal, tread depth and accumulated mileage.
Check tread depths at specific intervals—such as every 30,000 miles for line-haul operations or at scheduled preventive maintenance intervals. Measurements taken before the first 30,000 miles are not significant since most tires wear more quickly in the first few 32nds of tread life than in their remaining tread life.
For high-wear applications such as pick-up and delivery, construction, mining and refuse operations, use percentages of “expected re-moval”—such as 25, 50 and 75 percent, and “at removal”—as measuring intervals. The percentages may be based on mileage or time depending on the fleet's normal mean for determining tire life.
The minimum number of tread depths needed is one measurement per “major groove” in two “representative” locations about 180 degrees apart. Be sure to take tread depth measurements at the lowest part of each major tread groove and not on a wear indicator. For consistency reasons, it is best if the same person takes the measurements, although this may not always be possible.
While checking the pressures and taking the tread depth measurements, also inspect the test tires for irregular wear, damage such as cuts and snags, stone retention, etc. and any other anomalies. Also note any driver comments on tire performance, such as ride and wet traction.
Although I know you're going to be tempted to compute results as soon as possible, resist this temptation and avoid making mileage projections early in the tire's life. Projections can be made when at least 50 percent tread wear has occurred, but they should only be used as indicators of tire performance. Removal mileage is the final determination of the test.
When the test is complete, analyze the data. Exclude tires removed for road hazards and accidents but include tires removed due to manufacturing anomalies. For steer tires, compute the average mileage for each test group by adding the total removal mileage of all the steer tires and dividing by the number of tires that completed the test from that group.
Average the removal mileage for all drive or all trailer wheel positions. To determine cost per mile, simply divide the original cost of the tire or retread by the average removal mileage.
With the current boom in the freight hauling business, fleet managers are up to their eyeballs with spec'ing new equipment and keeping raggedy old equipment running until trade-in time. If you can assist in setting up their tire tests, you will provide a much-needed and appreciated service.
This spring, not only will a young man's fancy turn to thoughts of tire testing, but also to love… of his tire dealer!