Maintenance Q&A

Tire pressure, wheel

June 2019

Paul Abelson

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Q: In your February article, you mentioned air pressure for small (low profile) tires, but I run tall rubber. What is the correct pressure for my 11R24.5 tires? I’ve been running at 105 all around because that’s molded on the tire.

A: You’ve been running greatly overinflated, which accounts for the harsh ride and uneven tire wear you mentioned on the phone. The pressure on the sidewall is the absolute maximum for the tire, not the recommended. Your tires are load range G. At 105 psi, each steer tire can support just more than

6,000 pounds, 12,000 on the axle. For higher front axle weight, you’d need an H range tire.

With 12,000 on steer tires, you should have no more than 34,000 on each tandem. That works out to 4,250 on each of the eight tires in each tandem. The load-inflations tables suggest 75 psi per nonsteer tire. For some tire manufacturers, that could be as low as 70 psi. If you want a safety margin without sacrificing too much tire wear or ride comfort, go to 80 psi, but no more.

Remember, always check tire pressures “cold” before starting a trip. In just a short distance, the friction from tires flexing will heat the tires and the air inside and give you a false reading.

Air in a tire supports the truck and cushions the ride. It also maintains the tire contact patch as the manufacturer designed it. If air pressure is too high, the contact patch will be shorter and narrower, resulting in more center tread wear and less traction for accelerating, turning or braking. With a soft tire, the length of the footprint gets longer but the center can lift, promoting greater edge wear.

Ride quality is affected if air pressure is too high. Air acts like a spring to absorb bumps and cushion the truck and its contents. You felt every tar strip and bump on the road with your tires being around 30 psi over the recommended level. You’ll get a better ride, less uneven wear, longer tire life and better traction by keeping tire pressures at the levels recommended by tire engineers. You can find them on the internet by searching “truck tire load inflation tables.” You may have to scroll down past the ads. In the future, you may want to consider a smaller tire, perhaps a 275/70R22.5 or a 295/60R22.5 to get a broader range of loads if you often run at less than 80,000 pounds gross. If you change sizes, make sure your speedometer calibration is adjusted accordingly.


Q: I’m a Land Line reader and father of a trucker. I’m a fan of your columns, so I’m turning to you for answers. How often does this happen, why does it happen, and how can we prevent wheels flying off and killing innocent people? There are several websites with rants about a logging truck that lost two wheels and killed a Maine state trooper. TV news interviewed truckers who had no idea how to prevent these. There are lots of videos of tires flying down roads inflicting grief and destruction. What preventative maintenance tasks or inspections would stop such things?

A: Thank you for an interesting question and phone conversation. Before calling, I reviewed the video links you included. Most of the wheel-off incidents were from light-duty SUV and pick-ups or from outside North America – mostly Russian. Only two involved heavy trucks, but even two are too many.

A major reason we have so few has to do with Ontario, Canada’s tough position on wheel-off incidents. With any wheel separation, there is “absolute liability,” an automatic presumption of fault with fines up to $50,000 in Canadian dollars. Their laws greatly increased operator awareness.

To understand the fix, you should understand how threaded fasteners work. They are elastic and can be stretched beyond their original length, although not by much before they become permanently deformed and may even break. The stretching is done by applying force to the threads using an inclined ramp. It takes less force to move an object up a distance when that force is applied over a greater length. A threaded fastener has its “ramp” wrapped around its length many times, allowing less force to be used to stretch the fastener as desired. The head of the bolt or the base of a stud exert a clamping force at one end, while the nut, to which the force is applied, is the clamp at the other end. The stretched fastener wants to return to its original size. It pulls the two ends together, clamping things in place.

In the case of a wheel end assembly, the stud clamps the wheels to the brake drum or hub. Engineers have determined just how much force is needed to hold the assembly together. They specify what metallurgy should be used in the fasteners, how they should be hardened and tempered, and what forces should be applied to the nuts. These are expressed to us as the sizes and grades of the components and the torque specifications. Light-duty vehicles may take as little as 85 ft-lb, while our trucks may require as much as 550 ft-lb on the nuts.

While poor lubrication or impact damage account for some wheel-off incidents, the vast majority are due to over torquing fasteners. Too many mechanics use air impact wrenches when replacing wheels. Some reuse fasteners from previous jobs without knowing if they have been distorted. Too much torque on these components stretches them beyond their ability to spring back. They can’t apply the needed clamping force. They allow the wheels and drums to move against each other, enlarging bolt holes and allowing movement that increases wear. Bolts and studs break and wheel assemblies fail.

All this is well known, taught in trade schools, and included in manufacturers’ literature. Until we come up with a way to police mechanics who take short cuts by using air wrenches instead of hand tightening with calibrated torque wrenches, the problem will continue. LL

 

Send your maintenance question to Land Line Magazine, PO Box 1000, Grain Valley, MO 64029; email them to Maintenance@LandLineMag.com or fax questions to 816-443-2227. Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.

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Paul Abelson, senior technical consultant, is a longtime contributor to Land Line. He’s a member of the Society of Automotive Engineers and a member of the Technology and Maintenance Council. In 1995, TMC awarded him its Silver Spark Plug award. In 2006, he received the Lifetime Achievement Award from the Truck Writers of North America. Although he’s “retired,” he still makes a popular contribution to Land Line readers with “Maintenance Q and A.”