Shifting Gears the Right Way

This topic is mostly concerned with shifting gears and the associated clutch actions after the rig is in motion. To first get the rig started moving, refer to our clutching topic which explains in great detail, what you can do to improve the initial start out.

Torque and Power

To understand shifting, you have to understand the difference between torque and power. Torque is the twisting force that the engine exerts on the drive-line to make the wheels turn. Torque is the holding power of the engine. As you start up a grade, the truck speed slows a little, engine power decreases, but engine torque increases and drags the truck on up the grade

Power on the other hand, is the ability of the engine to accelerate the truck. Engine power is increased by raising the engine RPMs. All engines have more torque at lower RPMs, and less torque at higher RPMs. All engines have less power at lower RPMs, and more power at higher RPMs.

If you don't want to accelerate the truck, then keep the current shift gear and the torque will increase as the engine RPM decreases and the holding power of the engine will be maximized without a gear shift.

Torque Limit Considerations

Ok, so now we all know that holding a gear will get the most torque out of the engine. As the grade gets steeper, the engine RPM will decrease, and the engine torque will continue to build. At some point, the engine torque output matches the load torque requirements and the truck holds its speed, or the RPMs keep falling and a downshift is required. The number of cylinders in an engine will determine the interval between firing pulses. The pressure developed in the engine cylinders is seldom exactly synchronized with the throw vector angle of the crankshaft, which results in a fractional acceleration of the crankshaft above mean crankshaft speed at each firing pulse. These torsional forces due to the power stroke are amplified at slower engine RPMs, when the interval between firing strokes is greatest. These torsional forces can damage the driven components of the drivetrain at high torque, low RPMs.

If you don't downshift, when the RPM is low, the engine can handle the torsional forces but the dirvetrain can't. When you downshift, the engine gains more RPM, and the whole process repeats as the engine either holds its RPM, or continues to drop RPM while torque increases once again. Remember, when you downshift, the truck and its load is moving slower, so less work is being done by the engine per unit of time. The engine torque is multiplied a little more by the transmission downshift, so all these factors work the engine less.

Engine Performance

Let us look at engine power and torque curves for a moment. The Caterpillar 3406E engine is spec'd at 1350 lb/ft @ 1200 RPM, and rated at 355 hp at 1800 RPM. This means that for the fastest acceleration of this rig, you would want to keep the RPM near 1800 RPM. To be precise, you would want to go past 1800 RPM by one-half of the next gear RPM drop. So if the next gear drops 150 RPM, then you would shift at 1875 RPM.

For holding grades, you would let the engine fall down to 1200 RPM before downshifting. This would allow the engine to achieve maximum torque, which may be enough to hold the grade in that gear. If the tach continues to fall below 1200 RPM, then drop that required gear.

Fuel effeciency is usually achieved at the engine's sweet spot. The sweet spot is usually midway between the rated torque RPM and the rated power RPM. For this rig, best fuel effeciency on flat land would be achieved at 1500 RPM.

Double Clutching

Double clutching is the art of using the engine RPMs to control the speed of the transmission input shaft between gears to match gear speed. When it is time to upshift, the transmission input shaft is turning the same speed as the engine. To engage the next upshift gear, the engine RPMs must be lower and the transmission input shaft must also be turning slower RPMs to match the gears inside the transmission.

An upshift double clutch consists of the following steps:

  • Let up on the throttle, and at the same time push the clutch part way to the floor, enough to release the clutch (do not push the clutch all the way to the floor if you have a transmission brake on your transmission)
  • Pull the gear shifter into neutral position and let the clutch back out for an instant as the engine RPMs fall (this slows down the transmission input shaft)
  • Push the clutch back to the release position and pull the gear shifter into the next gear (this engages the next gear), then release the clutch.

A downshift double clutch consists of the following steps:

  • Push the clutch part way to the floor, enough to release the clutch (do not push the clutch all the way to the floor if you have a transmission brake on your transmission), then pull the gear shift into neutral position.
  • Let out the clutch and match the engine RPMs to slightly higher RPM than required for the downshift gear match.
  • Push the clutch part way to the floor again, and push the gear shift into the downshift gear, then release the clutch.

In both cases, the clutch uses the engine RPMs to get the transmission input shaft to the proper RPM for the next shift point, so that the gears easily mesh.

Jake Brake and Clutching

There is a lot of arguing amongst drivers about whether or not to use the jake brake during shifting. During the shifting double clutch, the conditions are correct for jake brake activation. If your gear shifts are close together, then the jake brake will fight you on the engine RPM match while shifting.

However, if the gear shift result in large RPM drops, or if the shift is on a steep grade which results in rapid truck speed loss, then use of the jake brake may very well help you to grab that next gear quicker. In this situation, the truck speed is falling rapidly, and the engine RPMs are falling normally, so during the double clutch, the engine RPMs are yanked lower by the jake brake. This might be helpful to get that next gear in place during a slow and steep grade climb.

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