I learned to drive big rigs on a B-73 Mack with a Tri-Plex transmission, which is a 5-speed main box and a 3-speed Brownie in a common case, with two shifters. Once the rig is rolling, NO clutch action is required, on either the main or the Brownie, just a good ear and/or an eye on the tachometer. Once the max engine speed is reached in any gear, just ease off the throttle a little, slip the box out of gear, and let the RPM drop until the shifter will fall into the next gear without grinding. On a downshift, reverse the process- - - -let up on the gas, slip into neutral, goose the gas, and slip into the next lower gear. Here's the shift sequence for a 5-X-3 main/Brownie shift pattern:
Main in 1st., Brownie in underdrive- - - -accelerate- - - -
Ease off throttle, slip Brownie into neutral- - -
Drop RPM, slide Brownie into direct drive- - - -accelerate- - -
Ease off throttle- - - -slip Brownie into neutral- - -
Drop RPM, Brownie into Overdrive- - - -accelerate- - -
Now comes the fun part!
(This works best as a 2-handed shift)
Ease off gas- - - -slip main trans into neutral- - - -
Shift Brownie into underdrive with main in neutral- - - -
Drop RPM, shift main into second- - - -repeat as necessary!
For the first few dozen shifts, you'll wish you had rubber-toothed gears, but you'll get the timing down pat eventually! Once you get the hang of a particular engine/trans/RPM split, no-clutch shifting will become second nature! Clutches are for starting and stopping; use while moving on a big rig is optional!
Jerry
