How to Double Clutch Big Rigs
Heavy-duty tractor-trailer trucks (often called "big rigs" or "semis") have non-synchronized transmissions to better handle the huge loads they haul. The gear cogs in a passenger car are synchronized or slanted to easily line up when going from one gear to another. In a tractor-trailer transmission, the gear cogs do not automatically line up to slide into gear. Double clutching is required to match the engine speed with the speed of the drive shaft.
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Instructions
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Depress the clutch pedal and push the gear shift lever into neutral. This allows the engine to run freely for a moment and to match the speed at which the drive shaft is turning.
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Release the clutch pedal and glance at the tachometer (it's usually located next to the speedometer and measures the engine speed in revolutions per minute or RPM). Some tachs have markings to indicate the suggested RPM for shifting into each gear. Remain in neutral until the engine speed reaches the suggested RPM. Each engine is different, and each has specific RPM requirements.
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Depress the clutch again and push the shift lever into the next gear. The transmission should shift easily with no crunch or clunk sounds. If it doesn't shift easily, the engine speed and shaft speeds are not matching. Depress the clutch again and either let the engine speed slow or increase the engine speed by gently tapping the accelerator until the truck goes into the next gear smoothly.
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Tips & Warnings
Familiarize yourself with the truck you will be driving before leaving the lot. Go through the gears so you feel where the lever should be for each gear. Read the owner's manual, if you have it, to memorize the optimal RPM for smooth shifting.
Avoid forcing the shift lever into gear or grinding gears, or you will damage the gear cogs.
References
- Photo Credit line of trucks 1 image by Aaron Kohr from Fotolia.com
