WHEN I LEARNED to drive a truck in the 1980s, I spent the first one-third of the program in the yard, learning how to shift gears. Unlike automobile transmissions, a truck's standard transmission must be double-clutched. They do not automatically match gears and vehicle speeds as cars' do.
That's easy enough to do, until you get on a hill. Then, vehicle speeds change more quickly, requiring faster reaction time. Get it right, and you keep control. Get it wrong, and you could find yourself in neutral, unable to get back in a gear. That could be the start of a runaway truck situation or a crash.
Big rigs depend on the engine to help keep control. Diesel engines' higher compression ratios help retard the truck, and engine brakes, also called Jake Brakes, help on steeper grades. But they cannot work if the truck is in neutral. Experienced truckers know when they can complete a shift and when they might get in trouble. But no one starts out as an experienced driver. We all make mistakes, and if we survive, we learn from them.
When stuck in neutral, it is best to pull onto the shoulder, stop the truck, and start over, even if it means creeping up or down a hill in first or second gear. But that assumes you can stop, that your brakes can handle the truck's weight, or the steepness of the grade.
Accurate shifting isn't the only skill experienced drivers develop. Starting on grades with minimal rollback prevents crashes that, while minor, happen too frequently and end up affecting insurance rates and safety records.
Skilled drivers learn when and how to work the clutch and throttle in order to creep forward in snarled traffic or back gently up to load or creep down into a pit. For drivers who earn their livelihood with dump trucks, ready-mix trucks, and heavy-haul rigs, coordinating three pedals with only two feet can be very demanding.
Shortly after World War II, Allison adapted parent company General Motors' Hydramatic fully automatic transmissions for commercial truck duty. They were heavier and far more expensive than the manual transmissions they were designed to replace, and slippage in the torque converters affected fuel economy. These problems have been minimized over the years with lock-up mechanisms, better engineering, and lighterweight materials. But Allison transmissions still cost more than standard transmissions.
Eaton Corp. introduced automation to transmissions in the 1990s. Initially, they were designed to reduce driver fatigue. Research showed long-haul drivers made at least 90% of their shifts between the top two gears, so the top two had the first automation applied to manual transmissions. It helped keep drivers alert but solved few other transmission-related problems. It used the computer to shift betweeen 9th and 10th gears. Meritor countered with the ZF-based Freedom Line that shifts a full synchromesh transmission by computer.
Although not recommended by transmission manufacturers, many experienced drivers “float” the gears, shifting without using the clutch. They lift the throttle to reduce torque and then nudge the shift lever into neutral. They then adjust engine rpm to match vehicle speed in the next gear, and, at the instant when everything aligns, they nudge the lever into the next gear. With enough experience, they usually get it right. When they don't, the gears grind and they go back to the clutch.
To eliminate this practice, Eaton engineers developed computer programs to simulate floating the gears: the AutoShift. Instead of a shift lever, the engineers used actuators. This leaves the clutch for starting and stopping, enabling the computer do the rest.