Maintenance Q&A
Braking bad and specing’s effect on tire life

By Paul Abelson, Senior technical consultant

I have a performance issue I have not been able to solve. I have a 50-ton Rogers lowboy trailer with unknown mileage. I pull this trailer with a 1986 Autocar with about 250,000 miles. That is the only use for the Autocar. Both vehicles were purchased separately.

The trailer brakes are slow to apply and slow to release. The truck brakes seem normal. I thought something must be wrong with the trailer brakes. I replaced the pilot valve, no improvement. I replaced the emergency air line with smaller line to speed up empty and fill, no improvement. I replaced the slack adjusters with ones a half-inch longer to speed up their action, no improvement. I replaced the quick release valves, no improvement. I am stuck. What should I try next?

Unfortunately, you started with the most expensive parts of the brake system – the valves. Then you went to a labor-intensive replacement, the air lines. You kept replacing parts without testing to determine whether the parts were, in fact, defective.

You wrote that the truck (tractor) brakes functioned normally, so you were correct in assuming the problem was isolated to the trailer, but you started too far into the system. Once the problem was narrowed to the trailer, the system starts at the gladhands that connect the air hoses from tractor to trailer. The air hoses could be abraded or may have cracked or deteriorated over time. They are inexpensive and easy to replace, and it never hurts to have a spare set on board. If the hoses check out, check the seals on the gladhands. They are often a source of leaks that will degrade braking performance.

If your Rogers lowboy has an air dryer, it may need replacing. Air dryers can plug with contaminants, cutting air into the system. They are easy to find and replace, if not so equipped, and to splice into the system. Without one, the contaminants that plug them would wind up in your trailer valves.

During our phone follow-up, you stated the problem was the old, deteriorated hoses. Since you owned the trailer and it was always connected, you never thought to check them. They deteriorated over time.

I got a new Peterbilt 587 about a year ago. I spec’d it for the best fuel mileage possible. It has a 450 horsepower Paccar motor with the latest Eaton UltraShift transmission, lightweight components wherever possible, and even smaller fuel tanks to save 280 pounds of fuel per fill-up. I figure with my better fuel mileage, I’ll still be filling up at about the same mileage.

But that’s not my problem. Tire life is. I spec’d a 6x2 with Michelin X One wide-base tires, and my tire life has tanked. I’m getting just a bit over 50,000 miles on the drive tires and not much more on the tag axle. What can I do about this?

A 6x2 configuration is a money saver, as are wide-base tires, but only in specific operations. They are generally meant for long-haul trucking, steady driving on Interstates. City driving, delivering to locations with pits, transitioning from driveways to city streets, and stop-and-go driving wreak havoc with 6x2 tires, especially wide singles.

Some fleets, mostly in regional operations like yours, say the 6x2 average fuel economy gain is

2.5 percent with some getting almost 5 percent. The trade-off is a loss of from one-third to one-half drive tire life and some loss of steer tire life. This prompted the Technology and Maintenance Council (TMC) to issue a new Recommended Practice, RP260, Maintenance Considerations for 6x2 Tractor Tires.

All trucks are designed to pivot around the center of the drive tandem or the driving axle. With the lead axle driving, it effectively shortens the wheelbase, increasing tag axle tire wear significantly and steer tire wear moderately.

The 6x2 configured trucks provide less traction. This is most evident on deep snow, heavy ice, loose gravel and over potholes and in repaving zones, changing road levels. These problems can be reduced, but not eliminated, if the tractor has load-shifting technologies to transfer weight to the drive axle at low speeds. To avoid overcompensating or tire overload, avoid loading the drive axle to more than 20,000 pounds. When weighing, measure each axle individually.

Torque applied through the drive axle will be twice that through 6x4 configuration. That contributes to increased wear, as does compression braking torque. There are other factors that contribute to wear of both drive and tag axle tires. You may have learned to use the same tires on both axles of a tandem, but that applies only to 6x4 applications.

Factors influencing tire selection include load, application, geography traveled and weather. A 6x2 tandem with a drive axle and a free-rolling one will pivot around the drive axle, creating excessive scrub on the free-rolling tag axle. You can equalize wear by rotating tires front-to-rear frequently and choosing a tire designed for high-scrub, for spread axles or an all-position tire.

Assuming you drive 100,000 miles a year, fuel costs an average of $2.50 per gallon and you’ve increased to 10 mpg, you’ll be spending $25,000 annually for fuel. If tires are $700 each and you get only half the tire life you did with a 6x4 configuration, you’ll spend $2,800 more per year on tires, not counting tire rotation and other costs. That will save close to $5,000 or more in fuel. If you get only a 1 mpg improvement with your new truck, you’ll still save more than $3,000.

Work with your tire dealer to determine the best tires for your truck. And if possible, save more money by replacing with quality retreads. LL

(Editor's note: When sending in questions for Maintenance Q&A, please include the make and model of your truck and your vehicle identification number (VIN) as well as your contact information. Paul Abelson tries to respond to every question, whether it's published or not. Send questions to TruckWriter@WowAccess.net)