ROCHESTER HILLS, Mich. (July 16, 2007) — You don't have to be older than dirt—but it helps—to remember when most trucks ran on bias-ply tires and tire fires were a fairly common occurrence.
Thirty years ago most commercial trucks ran on standard bias tires and many still used tubes. Excessive heat was the greatest enemy of high-speed bias truck tire durability. It didn't take much to set bias tires on fire. Brake heat, wheel bearing heat or just spinning the tires on dirt or ice could set them ablaze. Heck, just driving by a restaurant serving hot wings was enough to touch them off!
Things changed in the 1980s. The tire industry developed new casing designs, lighter-weight constructions and new rubber compounds that reduced heat effectively and virtually eliminated tire fires. For years you never heard of a radial tire catching fire unless the truck blew up and burned the tires with it.
But in the last 10 years tires and equipment have continued to evolve. Like a flashback to the good old days, there are more and more reports of tires catching fire and burning the whole trailer loaded with “good stuff.”
Lately, bus fires seem to have grabbed the headlines.
The story that really brought this subject to the nation's attention was the bus fire that occurred on Sept. 23, 2005, when motorcoaches from all over the nation rushed to the aid of Hurricane Katrina victims and helped evacuate the population of New Orleans. One bus, loaded with nursing home evacuees, caught fire in Wilmer, Texas, on Interstate 45 between Houston and Dallas. In that incident, 23 old and infirm passengers lost their lives because they could not be removed from the bus fast enough before fire and smoke enveloped the vehicle and prevented their escape. Since then, many more bus fires have made the news.
Buses aren't the only vehicles scorching the highways. Trucks, tractors and trailers also are experiencing more tire fires lately than I can remember in a long time. What's going on? Several things.
The first is that tires have continued to evolve. Standard radials such as the 11R22.5 and 11R24.5 have given way to low-profile tires. Most fleets that used to be on size 11R24.5 tires converted to the smaller rim diameter 22.5-inch low-profile tires.
Many fleets now use 19.5-inch tires. While these smaller rim diameters effectively reduce fuel consumption due to their lower rolling resistance and improved aerodynamics (less space between the ground and the truck undercarriage), they reduce the open area between the rim and brake components that in the past allowed for air flow which, in turn, cooled the axle end. So tires now tend to run hotter.
Vehicle design has changed, too. In the quest for improved fuel economy, engineers have improved the aerodynamics of trucks and tractors. The intent of vehicle design has been to smooth out the airflow around a vehicle to reduce drag. The most turbulent air is underneath the vehicle and around tire/wheel assemblies. Trucks are now made with larger fenders, fairings and other components that result in tighter enclosures around the axle ends.
Since air flow is reduced, tire heat can build up with no place to go.
Further, in the last five years engine compartment temperatures have risen dramatically. Since the advent of the 2002 low-emission engine, fleet maintenance managers have been complaining about the under-hood temperatures resulting from these new engines that exceed 325 F and have been melting hoses, belts and plastic windshield washer containers.
In addition, over the past two decades the federal government has required shorter and shorter stopping distances for over-the-road vehicles. This has resulted in larger brake packages that have reduced the space for air circulation inside the wheel.
Many people think that vehicle maintenance also has fallen off.
While no one has actual data to substantiate this claim, anecdotally there does seem to be more wheel bearings freezing up, resulting in excessive heat and more seized or dragging brakes that also produce blazing temperatures. These problems and running on underinflated or flat tires appear to result in elevated wheel component temperatures. That can lead to the ignition of various sources of “fuel” in the wheel/tire area—such as tires or bearing oil in the case of dragging brakes or underinflated tires—and are the initiators of tire fires in most instances.
The National Transportation Safety Board (NTSB) investigated the Wilmer bus fire. It reported that the right rear tire experienced a blowout earlier on the trip. Because the flat tire occurred during the night and in the middle of the hurricane evacuation traffic congestion, the driver and the technician who provided roadside assistance would not necessarily have been aware of the tire marks left by a locked wheel, which indicate a more serious mechanical problem.
(I have to wonder, though, why the technician didn't try to turn the hub when he mounted the tire/wheel assembly.)
It was only because a motorist passing by the motorcoach later alerted the driver to the fact that the right-rear tire hub was glowing red that caused him to stop again.
The NTSB concluded that the wheel bearing assembly lacked lubrication, which resulted in high frictional forces and high temperatures that caused the wheel bearings to fail and ignite the tire on the right side tag axle.
NTSB tests found wheel components can reach temperatures of 1,300 F. Tires ignite spontaneously at 800-900 F and will reignite over and over again even when suppressed because of the super heat created by the failed wheel-end components.
No quick fix
So what does this all mean to you? Well, if you run your own fleet of trucks, it means you had better make sure your wheel bearings are properly lubricated and check the sight glass on the axle ends regularly to ensure you have the proper oil level. Also, inspect your brakes regularly to ensure they are properly maintained and balanced and are not generating excessive heat.
Even if you don't run your own trucks, you've got to be careful servicing your customers' equipment. If you approach an axle end that appears hot, be very cautious. Not only check the tires on the vehicle, but check the oil level in the hub sight glass.
If the hub won't turn or is hot to the touch, the vehicle has seized bearings. If the hub is relatively cool, but the wheels are hot, it's probably a case of a seized or dragging brake.
Be very careful in servicing this equipment. Changing the tire may get the vehicle back on the road, but only for a short time until the real cause of the problem sets it ablaze.
Also, protect yourself. Last year I wrote about inspecting wheels for shrunken bead seats. Excessive heat can change the metallurgy of both steel and aluminum, which can cause wheels to lose strength and change dimension. The bead seat and rim flange area of these wheels can actually get smaller and change contour.
As a result, the flange area may no longer have the ability to hold the tire bead on the wheel under pressure. The tire can then blow off the wheel during or after inflation with explosive force and injure or kill you or anyone else who gets in the tire/wheel trajectory zone.
If you suspect that excessive axle-end temperatures were experienced, carefully inspect the wheel for signs of charring or discoloration caused by heat and a rim flange that has a scalloped appearance. The tire beads may be burned, deformed or have left rubber crystallization on the rim flange.
When you remove the wheel from over the brake drum and see that the brake drum has an orange iron oxide color on it, that's a definite sign that the brake drum has been exposed to excessive heat and probably the wheel has been, too. Any time you suspect a tire was run overloaded or underinflated for an extended period of time, pay special attention to inspecting the wheel for heat damage as well.
I wish I could tell you this phenomenon is a passing aberration. However, I think just the opposite is true.
Vehicle designers are still working on improving aerodynamics to improve fuel economy. They have more impetus now that fuel prices are higher than ever and are not expected to come down. While most attention has been on tractors up to this point, expect to see changes in trailer design as trailer skirts are adopted to reduce turbulence under and behind trailers—which will eliminate the air cooling that currently exists for their axle ends.
Further, with the 2007 low-emission engines making their debut, under-hood temperatures are likely to continue rising. And, hold on to your hat, the even lower emission engines beginning in 2010 probably will continue this trend. Can you imagine the temperatures steer tires are going to be exposed to when engine compartment temperatures get even hotter?
But wait, there's more. The federal government is once again changing the required stopping distance for over-the-road trucks, reducing it in the range of 20 to 30 percent from current stopping distances. This will necessitate the use of larger, more aggressive brakes, especially on steer axles. Larger-size brakes not only will generate more heat levels, but also will reduce the space for air circulation inside the wheel.
If someone was smart, they'd invent a way to use this heat buildup for good rather than for evil and destruction. I'll bet someone could figure out a way to temporarily attach a precure tread to a casing, mount it on a wheel installed on a truck/tractor and use this heat in the 300-400 F range to cure the retread while the tire rolls down the road. At these temperatures it would only take about a half hour to cure!
Somebody please stop me!
Peggy can be reached via e-mail at [email protected]. Her previous columns are available at www.tirebusiness.com.