Wheel end fires, while uncommon, are not rare. They usually make the papers and the TV news websites, though. They can produce dramatic footage of thick black smoke and flames consuming an entire trailer – and the tractor, if the ­driver can’t get the two units unhooked fast enough.

The resulting damage often makes a forensic examination impossible. Was the fire caused by a brake, bearing, or tire problem?

The underlying cause for these fires is always excessive heat that manages to ignite a tire or some stray oil or grease at the wheel end.

In a presentation last September to the American Trucking Associations’ Technology and Maintenance Council, Michelin field engineer Phil Arnold said that rubber compounds begin to break down when tire temperatures exceed 250 F. At 500-550 F flammable vapors develop. And if an ignition source is present, the rubber will start burning at 650-700 F. Spontaneous combustion will occur at 850-900 F.

To put that into perspective, the normal operating temperature range for tires is a between 100 and 150 F.

“Tires contain a great deal of potential energy,” he said. “They are like high-grade coal when they start to burn, and they are very difficult to extinguish.”

When a tire comes apart while driving, it’s usually because of underinflation. Heat generated within the sidewalls of a flexing tire weakens the casing’s steel belts, softening the rubber to the point that it eventually breaks down and the tire blows apart. With tire fires, the heat source is the wheel end. Heat travels through the hub’s metal and through the wheel, where it contacts the tire bead. Since the bead is built differently than the upper sidewall, its rubber just gets hotter and hotter until at some point it begins to burn.

The source of that heat is always ­friction, caused by something like ­dragging brakes.

“There are a lot of different reasons a brake can drag to a point of a thermal event occurring,” says Joseph Kay, Meritor’s director of brake engineering. “In general, it takes a rather large force applied to the brake shoes on a drum brake, or brake pads in a disc brake, to generate substantial rubbing forces that cause the brake system to get hotter and hotter as the vehicle is driven.”

Kay points to several potential sources, including: driving with the parking brake applied; a failed parking chamber diaphragm not compressing the parking spring; brakes not releasing after a brake application; corrosion-related binding of the camshaft or disc brake caliper; malfunctioning slack adjusters; or excessive swelling of the brake linings.

“In most cases the driver will not be able to detect one or maybe two brakes that are dragging because of the engine power and weight of the vehicle,” Kay says. “This is where the driver needs to be aware of excessive smoking from the brakes or any handling differences, such as unusual pull or deceleration.”

Trent Siemens, general manager of Oak Point Service in Winnipeg, agrees. “I’ve known drivers to mistake a dragging brake for a heavier-than-usual load, a headwind, or even terrain that seems to make the engine work a little harder.”

Siemens says the brake pedal itself could be the problem, noting that the truck treadle valve or the brake pedal hinges (in floor-mounted pedals) can stick or seize. “I’ve seen those seize up over time, and the driver won’t always know unless someone flags him down to let him know his brake lights are always on. Keep the brake pedal clear of debris and well lubricated.”

Literally any moving brake part could be a suspect. Valves might not fully exhaust their brake application pressure because of fouling or corrosion. Broken parking brake springs might keep the push rod from fully retracting. S-cams and slack adjusters might need lubrication.

“Proper preventive maintenance and inspection of all wheel end components is critical, as is the proper specification and condition of brake linings,” says Keith McComsey, director of marketing and customer solutions – wheel ends, at Bendix Spicer Foundation Brake.

In other words, each wheel end’s braking system needs to be carefully inspected and confirmed to be in working order, Siemens says.

“If you find any auto-slack-equipped brake out of adjustment, do not just adjust them up and kick it out the door. If they are out of adjustment, diagnose [a] root cause for why the adjuster is over-stroking. It’s probably not the slack adjuster that is at fault.”

Wheel bearings

The interface of the axle and hub is another potential heat source. High-quality wheel bearings, properly installed, properly lubricated, and operated according to the product specifications, seldom fail on their own. Unfortunately, much can go wrong down there due to neglect, oversight, or even the best intentions gone wrong.

Wheel-bearing-related fires can almost always be traced back to a lack of lubrication or improper maintenance – whether the lube is lost due to an outright seal failure, or a seal damaged by water or debris in the bearing well.

Any situation that might increase friction between the axle spindle and the hub needs to be addressed during the installation and maintenance of the wheel end assembly.

“Over tightening the bearing can limit the lube film, which will generate heat, though perhaps not to the level where there’s a risk of a fire,” says Ean Dickerhoof, an application engineer for mobile on-highway products at Timken. “Conversely, excessive endplay can affect seal alignment, compromising seal life, which can allow debris to enter the system or the lubricant to leak out.”

Obviously, seals and lubes need to be inspected at regular intervals, Dickerhoof adds.

“Some people think packing the cavity full of grease is better than partially filling it,” says Michael Gromosiak, Timken’s chief application engineer for mobile products. “There’s a certain percentage fill that’s recommended. If you overfill the cavity you can overheat the bearing because there’s no heat dissipation.”

Timken says bearings typically run less than 175 F under normal driving conditions. Running at 250 F or higher for extended periods of time increases the risk of bearing damage. In the case of complete lube depletion, temperatures will continue to rise, causing a series of damage until the wheel end assembly either fails completely and separates from the truck, or it heats the surrounding materials to a point where the tire ­catches fire.

PM and the driver

While procedures differ for various wheel end assemblies and lubricants, technicians and drivers should at the bare minimum watch for signs of leaking lubricant. Oil-lubricated hubs should be checked every time a trailer enters the shop. The lube should be checked for signs of water contamination (a milky appearance), and smelled to see if it has been subject to high temperatures (it will smell burnt). Also, the condition of the sight glass should be checked.

When working with grease-filled hubs where the lubricant is not visible, the wheel should be jacked up and rotated to check for signs of rough rotation, stiffness or looseness. If the hub cap is removed, verify the correct lube levels to ensure they’re not over-filled, and check for contamination and corrosion on the outer bearing.

Drivers are the last line of defense against wheel end fires, but they can’t be expected to notice everything or even know what to look for without proper training. It also takes extra time for ­drivers to complete thorough pre-trip, post-trip, and en-route inspections.

They should be instructed to touch and smell the wheel hub area to spot excessive temperatures and telltale odors linked to overheated brakes or wheel ends.

Read more wheel maintenance tips