Tire Spec’ing

Overdrive Staff | August 07, 2011

Keep your No. 2 variable expense to a minimum by spec’ing right.

Just as a truck with improperly geared rears will incur extra costs for its owner, so, too, will a truck with the wrong tires. There are many more factors than size that account for having the right tire for a specific truck in a specific application. The variables to consider when spec’ing tires are many. Among them are the size and number of plies and related weight rating, tread design, tread depth, tread compound and wheel position.

All play a role in proper tire spec’ing, and smart spec’ing can save costs in two ways: lowering fuel cost per mile, and lowering tire cost per mile. Attributes that lower one cost can have the opposite effect on the other cost, so spec’ing needs to be done carefully. That’s easier said than done in an era where specialization abounds. Tire makers are developing products that are focused to precisely meet specific service options.

Just as a truck with improperly geared rears will incur extra costs for its owner, so, too, will a truck with the wrong tires. There are many more factors than size that account for having the right tire for a specific truck in a specific application. The variables to consider when spec’ing tires are many. Among them are the size and number of plies and related weight rating, tread design, tread depth, tread compound and wheel position.

All play a role in proper tire spec’ing, and smart spec’ing can save costs in two ways: lowering fuel cost per mile, and lowering tire cost per mile. Attributes that lower one cost can have the opposite effect on the other cost, so spec’ing needs to be done carefully. That’s easier said than done in an era where specialization abounds. Tire makers are developing products that are focused to precisely meet specific service options.

Since longer tire life means lower costs, the effort put into careful spec’ing will pay off well. The four main tire applications are long haul, regional, on/off-road and urban. The different applications mean very different tire lives, from as little as 20,000 miles for urban to more than 200,000 miles for a long-haul team. Specific steer, drive and trailer tires are available for each application.

If you’re spec’ing a new truck, you can pick any tire size suitable to your application. A new truck’s drivetrain and engine controls are set according to the spec’d tire size.

Consequently, replacement tires of different specs can require drivetrain and engine adjustments. Changing tire sizes also might cause clearance issues.

More isn’t always better. Rather than last longer, tires with deeper tread might wear faster, burn more fuel and make driving harder if they’re wrongly applied. And the traditional miles-until-removal measure of tire performance more often now plays second fiddle in long haul to picking tires based on fuel economy. Some regional fleets are going to long-haul steers for low rolling resistance rather than tires with more scrub resistance designed for regional operation, for instance. The tires may wear faster, but they save enough in fuel to more than cover the extra tire replacement costs.

Consider this common tractor-only scenario: The steers have a 125 rolling-resistance rating, the drives are at 137, and the truck gets 6 mpg. Replacing steers and drives with lowest possible alternatives for RR rating (97 and 86) improves fuel economy to 6.52 mpg.

TYPICAL (HIGH) ROLLING RESISTANCE TIRES

Fuel consumption:

125,000 miles/6 mpg = 20,833 gallons

Fuel cost:

20,833 gallons x $4 = $83,332

LOWEST POSSIBLE ROLLING RESISTANCE TIRES

Fuel consumption:

125,000 miles/6.52 mpg = 19,177 gallons

Fuel cost:

19,177 gallons x $4 = $76,708

ANNUAL SAVINGS:

$83,332 – $76,708 = $6,624

In addition to the basic spec areas involving tire position, tire life and fuel efficiency, here are other factors to consider when choosing tires:

SHOP FOR THE BEST VALUE. It’s tempting to shop primarily by sticker price, but cost per mile and retreading potential are more important considerations. Making an informed choice means keeping written and dated records of purchase, fuel mileage and tread depth, then comparing records of models you’ve used. Also, consider warranty in any value calculation.

USE RETREADS. Premium tires cost $300 to $600 new, while retreads average about $200. Retread casings are tested via road use and by shearography, laser scanning that detects virtually any flaw. The same research and development that keeps new tires ever more durable and fuel-efficient is being introduced to retreads.

Premium tire makers design casings for retread use. Theoretically, undamaged casings can be often retreaded, although the tire maker’s casing warranty might expire at, for example, 750,000 miles or after three retreadings. On the downside, some carriers and state governments restrict retread use on steer axles. And not all retreads are created equal. If the retread isn’t from a reputable dealer, or if its markings have been scrubbed off the side, think twice before buying.

USE WIDE SINGLES. Wide singles weigh less and save fuel. They also cost less than the two tires they’re designed to replace, although you have to buy new rims to get in the game. Wide singles are retreadable, too.

A retrofit of wide singles on your tractor’s drives will cost an estimated $4,400. That covers four new wheels at $350 or more each and tires that run around $750 apiece. With a fuel-efficiency gain of 4 percent due to the reduced rolling resistance singles offer, though, the fuel savings will pay for the new wheels and tires in about two years. If you’re in a payload-sensitive application, the weight savings could yield bigger revenue, reducing that time period dramatically.

GO LOW-PRO. For long-haul, on-highway applications, a low-profile tire set-up with a shorter height than width offers fuel-efficiency, handling, weight, overall truck height and tire life advantages over more standard configurations. The aspect ratio – sidewall height measured in a percentage relative to its width at its widest point – of the most common 11R22.5 tires is about 100 percent, meaning the sidewall height and width are the same. Low-pro tires’ aspect ratio is around 80 percent, with size designations such as 295/75R22.5.


SEVEN TIRE KILLERS

Avoid these most-common tire threats to keep costs low.

1. UNDERINFLATION. A tire running on low air works harder than it was intended to. The potential for damage accelerates the further the tire gets from optimum pressure. If it doesn’t fail, the tire could end up being useless for retreading.

2. OVERINFLATION OR OVERLOADING. Overloading a tire produces problems similar to those of underinflation. Never load a tire beyond its rating. Instead, adjust air pressure for the load and/or ambient temperature. Hot temperatures increase air pressure; cold reduces it.

3. POOR WHEEL ALIGNMENT. If wheels aren’t aligned correctly, tires aren’t rolling straight down the road, and wear will accelerate. This can happen to the point where the tread wear cuts into the casing and even the cords. At that point, the tire is nothing but scrap. Keep in mind that total vehicle alignment is important in this regard. A trailer that’s “dog-tracking,” or running with the rear to one side, will wear the tires on the tractor, too.

4. POOR SUSPENSION MAINTENANCE. Neglect of the suspension system will create a snowball effect, with the tire facing uneven stresses that change every few seconds, as well as a suspension system that simply cannot be brought to spec. Key suspension maintenance activities include: frequent greasing (at least twice every oil change) and inspection of tie rods, king pins, bushings and the like for looseness.

5. IMPROPER MOUNTING OR DE-MOUNTING. This would include using starting fluid, tools with rough edges, failing to clean the wheel or inspect it to make certain it hasn’t been damaged or over-stressed, and replacing the core seal rather than re-using the old one.

6. IMPROPER REPAIR. Never allow a rope (patch) repair to serve as a permanent fix to a punctured tire. The tire should be removed soon after and inspected, and the inner liner resealed.

7. TIRE UNSUITED TO APPLICATION. All manufacturers rate their tires as to suitability for position and application. Only certain tires are designed to work in more than one position or application. The wrong tire or tread will not be nearly as durable as the right one.



GOOD HABITS FOR LONG TIRE LIFE

PERFORM ROUTINE MAINTENANCE. Proper maintenance of tires, wheels, axles and the steering column will help keep your tire costs down. Wheels naturally lose alignment and balance. Bushings, shocks, bearings and valve stems wear out. Checking wheel alignment periodically can pay big dividends. Whenever you start seeing a heavy spot or a flat spot, the tire has ceased to be centered on the hub, and it’s beading out of round. Whenever you need to scrap a tire, inspect it to determine the cause of the failure, and record it.

The plungers inside shock absorbers create friction, and friction creates heat, so if the shock – not the outer dust barrel covering the top half of the shock – is hot to the touch after driving, it’s working. If it’s cool to the touch, it’s not working and should be replaced. Make sure bushings at the top and bottom of shocks are inspected and replaced whenever worn. If you can grab the shock absorber and rattle it, the bushings have pounded themselves out.

MAINTAIN PROPER INFLATION. Ironically, maintaining correct inflation is free and relatively easy, yet it’s the highest-saving maintenance you can perform on your truck. Improper inflation is by far the greatest reason why tires fail or wear out prematurely, also wasting fuel. Daily pre- and post-trip inspections give owner-operators the opportunity to check pressures and also look for leaks, punctures, broken valve stems or embedded objects such as nails. Metal valve caps tend to withstand wear and tear better than plastic ones. Even absent damage, truck tires typically lose one to two pounds of pressure a month from normal use. A slow leak such as a puncture might cost you one to three pounds of air a day.

Poor fuel economy isn’t the only cost of underinflation. Manufacturers design big-truck tires for a specific pressure, usually clearly specified on the sidewall. If the pressure’s too low, the sidewalls overflex and overheat. For each pound of air pressure lost, a tire’s on-road temperature rises about 2 degrees, and hot tires are prone to losing tread more quickly and failing. Don’t overinflate, either. An overinflated tire sustains rapid and irregular wear and is more susceptible to damage from running over debris and scrubbing curbs.

Some operators use aftermarket tire sealant products both as a safeguard against punctures and to help maintain proper inflation pressure. At least one tire manufacturer, too, markets a line of highway truck tires with sealant built-in from the factory. A quality sealant will have been extensively tested, and its dealer should be able to provide testing documentation. If you’re considering such a product, don’t hesitate to ask for this.

EMPLOY GOOD DRIVING HABITS. Speeding, hard braking, curbing and tight turns cause tires to wear faster or even develop irregular wear. On the other hand, steady acceleration, braking and steering can extend tire life. Many of the problems caused by bad driving habits show up as uneven tire wear. If you notice unusual shimmying or repetitive bumps, inspect your tires. Even absent rough driving, look for irregular tread wear in your daily inspections.