Most semitrucks, which haul semitrailers, have heavy-duty manual transmissions which differ from the manual transmissions in light trucks. These transmissions require a special shifting method. There are actually two methods for shifting these transmissions – double-clutching and floating. The heavy-duty transmission in a semitruck cannot be shifted like a regular transmission.
Syncromesh transmissions are found in pickups and light trucks. They allow you to shift from gear to gear with one push of the clutch because they have synchronizer rings in them that cause the different gears to line up when shifting. The exception to this is the transmission with a “granny low” first gear. That gear is not synchronized, and while you can shift up with one push of the clutch, you cannot downshift into it.
While automatic and syncromesh transmissions are starting to appear in heavy semitrucks, non-syncromesh transmissions are the norm. There are no synchronizer rings to line up the gears for a one-clutch shift.
The standard method for shifting a heavy-duty transmission is the double-clutch. This method requires pushing in the clutch, shifting into neutral, letting out the clutch, pushing in the clutch again, shifting into the next gear and letting out the clutch. Obviously, this is a lot of movement for one gear change.
Floating the gears refers to the method of shifting without using the clutch. The clutch is only used for coming to and starting from a dead stop. Shifting is accomplished by feel and engine rpm. You use the clutch to begin moving, then, when you want to shift, you begin to put pressure on the stick shift, let off the accelerator pedal, pull the stick out of gear and move it into the next one as the engine rpm drops. It takes “feel” to accomplish this, and you have to either have or develop a knack for it. Downshifting is accomplished in the same manner, except as you shift into the lower gear, you have to increase engine rpm. Improper implementation of this method can result in damage to the transmission, driveline or rear axle.
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