Upward mobility

| November 03, 2005

Moving heavy freight uphill requires patience and practice to avoid drivetrain problems.

Among trucking’s trickiest maneuvers is starting a heavily loaded truck up a steep hill from a dead stop. It’s relatively common, something drivers might face every day, and the maneuver includes several possible mistakes that are easy to make and can cause serious damage to a truck – even a driveshaft sheared clean in two.

“The transmission might be the weaker link in the drivetrain,” says Schneider Equipment Purchasing Vice President Steve Duley. “You might twist the main shaft in the transmission, and when that happens you have to rebuild the unit. But now we’re getting to where we damage more differentials: most likely it would be a broken ring gear, and that means the gear set has to be replaced.”

“I’ve seen them snap driveshafts,” says owner-operator Robert Lyonaisse of Fat Bob’s Trucking in Manchester, N.H. “I’ve seen them rip the rear end shackles right off. I’ve seen them split differentials and rip them right off the housings. I’ve even seen them bend frames.”

All of these mishaps, save bent frames, disable the truck; it can’t be driven, and expenses for towing, repairs and down time run high.

With 33 years of trucking under his belt, Lyonaisse has been both a driver and mechanic, and he has detailed knowledge of starting a heavy load uphill: the forces at work, how to control them and what can happen if the driver makes even a tiny mistake.

“The diesel engines nowadays put out so much torque,” he says, “and that effect on the drive train is compounded by the heavy load, the steep hill and the start up.”

If you’re at 70,000+ pounds and you come to a stop on a steep incline, don’t despair. Getting that big rig rolling again can be treacherous, especially for inexperienced drivers, but there’s a simple procedure for doing it without mishap. Experienced drivers do it routinely. Doing it correctly just takes time and practice.

A short physics lesson might help to explain the tremendous forces that clash with each other when a heavily loaded truck starts out on a steep hill. One force is gravity, represented by weight and the hill. The opposing force is torque, the twisting power created when all those big diesel horses go to turning the drive shaft and subsequently the drive axles. Torque and gravity meet primarily at the clutch and also between tires and road surface. The driver uses the clutch, throttle and gears to apply torque in controlled and sufficient measures to overcome the gravity and move the load up the hill from a dead stop.

To be sure, technology is simplifying the procedure for doing this and decreasing risks. Automatic transmissions for big trucks make it as easy as taking your left foot off the brakes while gently applying throttle with your right foot. Even some standard-shift transmissions have built-in torque controls that protect drive trains from over stress. As well, motor carriers can spec their trucks to handle max-loaded start-ups on steep hills with fewer problems.

“We spec out our trucks with a transmission that has a good low gear,” Duley says. “They have a heavy-duty drive-line. The engines have good starting torque, and we have our drive train combinations – transmission, drive shaft and differential – balanced pretty well, so none of those three components gets more strain than the others.”

Duley also explained why today’s electronic engines are harder to stall out. “They have what’s called a synchronous governor,” he says. “It automatically adjusts the fuel feed to keep the engine at a specified rpm, usually 600.”

For example, if the engine is idling at 600 rpm and the air conditioner kicks in, the synchronous governor increases fuel to keep the idle at 600 rpm even with the added load.

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