“You can’t have too much horsepower” is a universal truth in the trucking business. Truckers like big, powerful engines. But an important link between all that engine power and the drive wheels is the drivetrain, including clutch, transmission and other components. Without proper maintenance these vital components can fail, and then those 600 horses you’ve got under the hood won’t mean a thing.
Drivetrain basics
At the front of the drivetrain is the clutch. It’s designed to slip so you can get a smooth start. The clutch pressure plate uses heavy springs, and is bolted to the engine flywheel and turns along with it. The driven discs are made of metal, with ceramic or organic facings designed, like brake linings, to slip smoothly and handle high heat without undue wear. When the driver releases the clutch pedal, the pressure plate (and with twin-disc clutches an additional metal plate between the two driven discs) traps the clutch-driven discs between the flywheel and pressure plate. With a skilled driver, the springs soon cause the driven disc to turn along with the flywheel with no slip.
The transmission sits just behind the clutch and allows the driver to change the ratio between engine and driveshaft/wheel rpm. This lets the engine develop sufficient power regardless of vehicle speed. Modern American heavy truck transmissions all use a twin-countershaft design, which ensures great strength and compact construction. The gears are always meshed with their adjacent gears – that’s why the “constant mesh” moniker is attached to truck transmissions. This protects the gear from shifting errors.
When the driver shifts, he is just sliding one of the shift collars splined to the mainshaft to engage a mainshaft gear. Constant-mesh gearboxes take less effort to shift than synchronized transmissions because the diesel does the work instead of the
driver’s arm.
Most truck transmissions are designed so that in top gear the drive passes through two sets of transmission gears and the output shaft turns faster than the input shaft and engine; this is called running in “overdrive.” A few transmissions use direct drive in top gear: Shift collars link the input, main and output shafts together, and the drive passes straight through using no gears. Overdrive top gear ratios are used because, in this way, the driveshafts turn faster, but the torque carried by the drivetrain is a lot lower. Trucks with conservative engine torque ratings sometimes run axles with ratios around 3.00:1 and direct-drive transmissions to save a bit of fuel while maintaining correct cruise rpm. Over-the-road trucks using overdrive typically run rear axle ratios of 3.55:1-3.90:1.
All four rear wheels receive driving power through the use of tandem drive axles. One driveshaft carries power from the transmission output shaft to the forward differential; a second carries power to the rear differential. Both rear axles reduce the rpm of the wheels as compared with the driveshaft with a ratio from 3.00:1 to as high as 4.50:1, depending upon the application. They also allow one wheel to turn faster than the other when the truck turns, through the differential. An inter-axle differential on the front drive axle allows the two axles to rotate at different speeds due to slightly different tire diameters and the slightly different paths taken by the axles during turns.
Powertrain components must be matched. Don’t order a new truck and try to save money by under-spec’ing a component – for example, ordering a 1,450-pounds-feet transmission with an engine that produces 1,550 pounds-feet. Also, be careful when uprating engine power curves to make sure the components, including their coolers, are up to handling the torque and horsepower.
