Maintenance Q&A
Long life and new life for your tires

By Paul Abelson, Senior technical consultant

After more than 20 years of 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. Is there something I can do to the air suspension to improve the ride?

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 more 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 most likely disappear.

A few months ago, I blew a steer tire. The closest truck stop sent a local tire service company. They found a large nail in the shoulder area. Since the tires had only 6/32 of tread left, I kept the good one for a spare and replaced both. Lately I developed a wobble in my steering. It just felt weird. I took the truck to my regular dealer, and they wanted to replace my front wheels and all the front studs and hardware. Is this legitimate? What caused this? How can I avoid it?

First of all, be sure to have road service done by a well-known company with its own road service trucks. They generally follow approved procedures like those in the Technology and Maintenance Council’s recommended practices. It is very important to make sure wheel fasteners are of good quality and torque limits are not exceeded.

Wheels are held in place by the clamping forces between the studs and wheel nuts. Under proper torque, the studs are stretched within their elastic load limit. Like a rubber band that is stretched, the steel stud wants to spring back to its original length, clamping the wheel in place. Too much torque, especially on substandard studs, stretches them beyond their elastic limits, destroying their ability to spring back and hold clamping force. The wheels loosen, holes are worn out-of-round, and you get the wobbling effect you experienced. Your dealer is probably correct. The only corrective action after damage has occurred is replacement.

To avoid future damage, wheel mounting should be done manually with a torque wrench. Far too many studs and wheels have been ruined by mechanics with air-powered wrenches, especially those out of calibration.

You will also want to make sure to be aware of the shop’s policies and recommendations on retorquing lug nuts after a tire service is completed. Warranties are often voided if you do not have the lug nuts retorqued within a specified number of miles.

With things getting financially tight, I decided to give retread tires a try. When the dealer inspected my tires, he told me he would retread three of the drives, but would give me trade-in credit for the fourth. I still had between 3/32 and 4/32 inches remaining, more than the legal 2/32 limit. He gave me a song and dance about safety margin and inflation, but it didn’t make sense to me. Can you help me out and explain it to me, and maybe help me get some value for my tires?

Yes, to the first part of your question, but sadly no to the second part. Retreading tires has come a long way since the days when you could bring in a used casing and, as long as no cords were showing, they would slap some rubber cement on the tire, wrap new tread rubber on it, put it in an oven-like device to cure it, and send you out the door. Today, we know there’s more to making a safe retreaded tire than just the tread.

First, all reputable retreaders inspect the tires inside and out to make sure there is no damage, internal or external, to the casing. The remaining tread will have to be ground off to make sure the casing is round, the right size to accept its new tread, and in condition to bond properly. They need at least 4/32 of tread remaining, preferably 5/32. That gives them something to work with.

They measure at the lowest portion of the tread, so if alignment issues caused irregular wear, they measure where the wear is greatest. If your casing has enough tread, it then goes through a series of non-destructive tests before they do any work on it. Just as imaging allows doctors to see inside people without resorting to exploratory surgery, computers, lasers and ultrasound images allow retreaders to look inside a tire.

They look for damage to the internal structure such as broken wires, often because of underinflation. That’s one reason why maintaining proper air pressure is so important. When tires are significantly underinflated, often by as little as 15 psi (undetectable by “thumping”), treads flex excessively, generating heat that weakens internal bonds between components. Flexing can also break steel cords.

As for getting value out of your used casings, it’s too late for your current tires, but if you maintain your tires properly, you should be able to get full value out of your next set. 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)