Maintainenece Q&A

Wheel fire, electric trucks

1. I had a wheel fire this week on the driver’s side. The tire burned up and ruined the brakes and hub. It even melted some bearing metal. My mechanic wants to replace the entire axle, including the brakes on both slides. Do I need to do all that, or is he trying to pad my bill?
2. As with many maintenance issues, the answer is “maybe.”

Here are some of the factors to be considered. The first task in any repair operation is to determine the initial cause of the damage. If you don’t correct the cause, you’re liable to see the problem recur. With wheel ends, replacement on both sides is recommended. It keeps traction, braking and wear even.

Fires – all fires – need three things: fuel, oxygen and ignition. With wheel fires, the tire, brake lining or even lubricating oil or grease can be the primary fuel, followed by the tire itself. Those are ever-present, so the real question is what started it.

Yours is a timely question. Because of an increase in “thermal events,” TMC recently conducted a survey to determine the causes, frequency and severity of fires. This is a first step in forming a task force. Preliminary results indicate two primary causes of wheel end and tire fires: brakes and wheel bearings.

Brake problems arise when a shoe does not fully disengage after application. That causes continuing friction, causing the drum to heat above the tire rubber’s ignition point. If the drum and wheel can’t dissipate the heat fast enough, drum temperature rises enough to ignite the tire. This brake drag can be caused by any number of things. Bushings in the mechanism may be worn. Seals or air lines may leak.

Remember, while increased air pressure applies the service brakes, high pressure air keeps the parking brakes released. A leak in an air line may allow the spring to overcome release pressure resulting in brake drag. There have been many reports of air lines damaged by debris thrown up when run over by a truck. That’s why it’s so important to listen closely for any sounds of air leakage during pre- and post-trip inspections. Today’s trucks are so powerful that they can easily overcome the drag caused by a damaged brake system, especially when in cruise control. Be alert for smoke.

Another leading cause of wheel end fires is improper bearing adjustment. Adjusting wheel end bearings is a job for skilled technicians. Don’t try it unless you have experience in maintenance. It requires specialized tools, including specific seating tools, dial indicators and gauge blocks. The procedures are laid out in TMC RP618, Wheel Bearing Adjustment Procedures. They should be followed precisely. The only exceptions are with preadjusted or unitized wheel ends that come adjusted from the factory. Always make sure there is adequate lubrication to the wheel ends. And never give the wrench an extra push “just to make sure it’s snug,” as some mechanics like to do. Over-tightened bearings can create friction heat of 1,300 degrees, more than enough to ignite lubricants and tires.

1. I drive a dedicated run from Cleveland to San Diego, out one week and back the next. I have been reading a great deal about all the new electric trucks that will be out soon. I will be ready to buy a new truck in two to three years. Do you think I should consider an electric truck?
2. You ask a very intriguing question but one best answered when you are closer to buying your truck. At several major trade shows around the world, some very interesting technologies were introduced that will affect the future of electric trucks. At the same time, all the major truck makers and several upstarts like Tesla and Nikola announced prototype Class 8 trucks for regional use.

There are still many uncertainties about electric rigs. What shall the power source be, batteries, hydrogen or something else? What will be the layout? A central motor, a motor on each axle, or one at each wheel end? How far can they go on a charge? How will power be replenished? What sort of maintenance, if any, will be needed? Answers to these questions may change over the next few years.

Granted, electric power does have many advantages. Some prototypes have as much as 500 hp at each wheel-end (2,000 hp total). Electric motor peal torque is at zero rpm. No hydrocarbons are burned (at least not on the truck), so no costly exhaust aftertreatment is needed. Because power and torque curves match performance demands, no heavy transmission is needed. Nor is a heavy drive train, because power will be transmitted by relatively light wires.

Great progress has been made in battery capacity, but prototype trucks are still limited to local pickup and delivery or regional haul, around 300 miles under optimum conditions, not the long haul you do.

There’s an infrastructure consideration. How much electric capacity is there to recharge all the batteries that will be in service? Environmentalists demand cleaner vehicles but resist the building of more generating plants. Local and regional carriers can use available power at night with capacity from existing sources, but where will you recharge when driving out west? It will take years to create charging capability comparable to what we have with diesel today.

Even with hydrogen cell technology, we will need new infrastructure to produce, store and deliver the gas. I think there is a great future for electric trucks, including Class 8 long haul, but that future is more than three years away. Until then, my opinion is that you should stay with a tried-and-true diesel, and let the major fleets do the testing. LL