Behind the Wheel of a Big Rig

Double-clutching is a skill that was, decades ago, virtually mandatory for all drivers—especially performance and race drivers—but is now about as critical as being able to rapid fire a muzzle-loading musket. Except when you're driving a big-rig.


The procedure for double-clutching: push in the clutch, move the shift lever to neutral, release the clutch, match engine speed and those of the transmission input and output shafts by either pressing the accelerator (if downshifting) or allowing engine speed to fall (if upshifting), push the clutch in, move the lever to the next gear.


Remain in neutral too long or hurry between gears and you'll grind cogs and wind up in neutral. When not in gear, the truck will either lose speed (if on level ground) or gain speed (if going downhill).


Grades imperceptible in cars become Everest-like when you're pulling 80,000 pounds. Or, worse, the weight is pushing you. If the rig loses or gains speed by a few mph, you'll have to give up chasing the gear you were after and hunt for another. It may be up or down. It may be two up or two down. Shifting without touching the clutch can be easier, but you still must match engine and transmission speed.

Splitting Gears
Heavy trucks have multiple-ratio drive-axles, much like high- and low-range found on most four-wheel drives, except that you actually use both during everyday driving. Truckers must manipulate a switch or switches to change ratios; it's called splitting. Some axles are split after the driver rows through the conventional "H" pattern positions. Others demand a split between every gear. For the former, the split comes after, usually, Fifth gear. Flip the switch and the gear position that formerly was First now is Sixth, Second becomes Seventh and so on. With a split-every-gear system, to shift from First to Second, the gear lever is left stationary and a switch on the lever is thrown. When it's time to go to Third, the switch is moved back to its original position and the shift lever is moved back to the 3-4 position. A flip of the switch gives you Fourth. Repeat through all gears. There will be a quiz. That's not true: There will be many quizzes.


A popular heavy truck powerplant is Detroit Diesel's turbocharged 12.7-liter inline six-cylinder. It makes 380 horsepower at just 1,800 rpm and a gargantuan 1,350 pound-feet of torque at a very low 1,200 rpm. (For comparison, the Dodge Viper boasts 500 horsepower at 5,600 rpm and 525 pound-feet of torque at 4,100.) A diesel offers so much torque that to get 40 tons rolling all you have to do is release the clutch. No throttle is needed. In fact that's contraindicated. However, the diesel's rev limiter kicks in at about 2,200 rpm. Depending on the transmission and load, the driver must go through nine or more gears to reach 45 mph.