Maintenance Q&A
Air dryers, psi and 100-mph tape

Q.How often should I change the (desiccant) air dryer on my truck?

A.Assuming the air dryer is properly spec’d for your truck, the general rule is as follows: every two to three years for on-highway, long-haul trucking; one to two years for pickup and delivery work in urban and suburban areas; and yearly when brake applications are frequent and severe, such as refuse pickup or hauling coal or rock in hilly terrain. 

Factors affecting desiccant cartridge life include how many devices are on your compressed air system and how often you apply your brakes. Your air dryer should be self-flushing, but you want to be sure to open your drain valves at each tank after each day’s run. If you notice an excessive amount of water draining from any of the tanks, review any changes that may have taken place in operating conditions or maintenance procedures. 

If you find oil on the pavement or shop floor under any tank, that may be a sign your compressor needs maintenance. Oil from the compressor can contaminate the desiccant, greatly reducing its ability to attract and hold water and shortening its useful life.

Q.After more than 20 years driving for fleets, I finally bought my own truck and trailer. I have 295/80R22.5 tires all around. I run at between 75,000 and 80,000 pounds about 95 percent of the time. I rarely drive faster than 70 mph.

I kept the tires filled to 130 psi. That’s what’s molded into the sidewall. I got a terrible ride so I softened the tires to 120 psi. I don’t want to go any farther from the factory recommendation. Is there something I can do to the air suspension to improve the ride? The trailer bounces terribly, especially when I deadhead between loads.

A.First of all, the 130 psi molded into the sidewall is the maximum allowable inflation pressure, not the recommended pressure. That’s much less. TMC has a Recommended Practice on the subject, RP235A, Guidelines for Tire Inflation Pressure Maintenance. The RP includes a table for “Pressure Determination for a Given Load.” At 34,000 pounds per tandem, each drive or trailer tire carries 4,250 pounds per wheel.

For that load, the recommended pressure is 90 psi at a maximum of 65 mph. For every 5 mph up to the industry-recommended limit of 75 mph, add 5 psi. Since the steer axle carries 12,000 pounds, or 6,000 per tire, steer tires should be run between 110 and 115 psi. If you keep your tire pressures to between 95 and 100 psi for tandem tires and no more than 120 psi for steers, your ride problems will likely disappear.  

Q.I’m caught in a dispute between my insurance company and my shipper. The shipper loaded my trailer using a fork lift. When the receiver unloaded it, he discovered a tear on the inner liner. The operator immediately stopped and called me over and pointed it out to me so he would not get blamed. I put in a claim with my insurance company and they’re going after the shipper who is blaming the receiver. It’s between the insurance companies and lawyers now.

My question for you is how can I do a temporary repair on the aluminum inside wall? The trailer is insulated but not a reefer. It looks like it might be a few months before this gets settled.

A.Although the damage sounds minor, you probably want a proper repair done by an authorized dealer for appearance and effectiveness, and ultimately resale value. That will involve replacing insulation and the full aluminum panel.

The most expedient temporary repair is with duct tape and foam-in-place insulation. It won’t be pretty, but you won’t have that heat transfer spot in your trailer wall. Don’t count on that lasting more than a few weeks at best. For a longer lasting tape repair, use military grade – often referred to as “100 mile an hour tape.”

For a proper temporary repair, cut out the damaged section. Start by using a hole saw to get radiuses at the corners at the area you are removing. Then connect the holes using a reciprocating saw. That way, you’ll avoid creating sharp interior corners, stress risers (a concentration of stress), that could initiate cracking. 

Cut a patch out of plain aluminum sheet. It should be at least 2 inches larger than the hole it will cover (1 inch on all sides). Round the corners and smooth all the edges. Drill appropriate-sized holes for pop rivets in the patch. If you use small, 1/16-inch rivets, drill more closely together. For larger rivets, 1/4-inch, you won’t need as many. The number will be determined by the size of the patch. 

With the patch drilled and de-burred, lay a strip of room-temperature vulcanizing (RTV) silicone sealer around the outside edges of the patch and the inside edges of the hole. Put the patch in place and hold it for a few minutes until it sets. Use the rivet holes in the patch as a template to drill the holes through the aluminum trailer wall so the holes match. Then pop rivet the patch in place. Your patch should hold as long as you own the trailer or until the lawyers and insurance companies get things straightened out. LL