CDL: Shifting Gears

 

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Shifting Gears: Manual, Automatic, and Auxiliary Systems

Shifting gears correctly isn't just about smoothness — it's about control. A driver who can't get a commercial vehicle into the right gear at the right time has less command over the vehicle, especially in situations like climbing a grade or approaching a curve. This article covers manual double-clutching, multi-speed rear axles and auxiliary transmissions, automatic transmissions, and retarders.

Manual Transmissions

Most heavy vehicles with manual transmissions require a technique called double clutching to shift gears properly.

Double clutching takes practice to master. If you linger in neutral too long, you may find it difficult to engage the next gear. If that happens, don't force it — return to neutral, release the clutch, bring the engine speed up to match the road speed, and try shifting again.

Knowing When to Shift Up

There are two main ways to judge when it's time to shift:

• Engine speed (rpm): Get familiar with your specific vehicle's operating rpm range and watch the tachometer, shifting up once the engine reaches the top of that range. Some newer vehicles use "progressive" shifting, where the rpm point for shifting rises as you move up through the gears — know what applies to the vehicle you're driving.
• Road speed (mph): Learn which speeds correspond to which gears, then use the speedometer as your guide for when to shift up.

With practice, many drivers eventually learn to judge shift points by the sound of the engine alone, regardless of which method they started with.

Just like shifting up, downshifting requires knowing the right moment to act — using either the tachometer or speedometer as your guide. There are two specific situations where downshifting ahead of time is especially important:

• Before starting down a hill: Slow down and shift into a low enough gear that you can control your speed without relying heavily on the brakes. Riding the brakes on a downgrade can cause them to overheat and lose stopping power. Shift down before the descent begins, and choose a gear that's usually lower than what you'd need to climb the same hill.
• Before entering a curve: Slow to a safe speed and downshift into the right gear before you reach the curve itself. This lets you apply some power through the turn, which helps keep the vehicle stable, and also sets you up to accelerate again as soon as you exit the curve.

Multi-Speed Rear Axles and Auxiliary Transmissions

Many commercial vehicles are equipped with multi-speed rear axles or auxiliary transmissions to provide additional gear range. These are typically controlled through a selector knob or switch mounted on the main transmission's gearshift lever. Because shift patterns can vary widely between vehicles, it's important to learn the specific pattern for whatever vehicle you'll be operating.

Automatic Transmissions

Vehicles with automatic transmissions often allow the driver to select a low range, which increases engine braking on downgrades. Selecting a lower range prevents the transmission from shifting up past that point, unless engine rpm exceeds the governed limit. Using this braking effect is an important tool when descending grades.

Retarders

Retarders are devices designed to help slow a vehicle down, reducing how much a driver needs to rely on the brakes. In addition to giving drivers another way to control speed, retarders reduce wear on the brakes themselves. There are four basic types: exhaust, engine, hydraulic, and electric. All types can be switched on or off by the driver, and on some vehicles, the strength of the retarding effect can be adjusted.

When a retarder is switched on, it applies braking power to the drive wheels any time the driver fully releases the accelerator pedal. Because retarders can be noisy, it's worth knowing where their use is permitted before relying on them.

Understanding how your specific vehicle shifts — whether that's a manual transmission requiring double clutching, a multi-speed axle with its own shift pattern, an automatic with selectable low ranges, or a retarder system — is essential to maintaining control in everyday driving and in more demanding situations like hills and curves.