How It Works: Manual transmissions | Shandong Forgingss Group

Manual transmissions are less common these days, but they can still strike fear in a new driver's heart — or bring joy to an enthusiast

Few things can strike fear into a new driver's heart, or joy to an enthusiast, like seeing the three pedals that indicate a manual transmission. While the "stick shift" was once the only type offered, the popularity of the automatic transmission has made the manual a relatively rarity.

Today, manual transmissions are mostly found on performance vehicles – although many of those are switching to automatics now – or on the base trims of some entry-level cars for the lowest-possible advertised price.

As its name suggests, the transmission transmits power from the engine on its way to the wheels. The engine's heavy central crankshaft spins to provide this power, its speed measured in revolutions per minute or "rpm," but drivers need a wider range of vehicle speeds than the engine's spinning motion can supply. The transmission's gears amplify the engine's power for acceleration, or reduce its rpm at highway speeds so the engine isn't overworked. Unlike with an automatic, it's up to the driver to determine when to shift to the appropriate gear. Transmissions are complicated, and this is just a simple overview of how they work.

A modern manual transmission typically contains five or six gear ratios, commonly called speeds, as in a six-speed transmission (reverse and neutral aren't included in that number). Inside the transmission, an input shaft is connected with the engine, which turns the shaft; while an output shaft sends its spinning motion toward the vehicle's wheels to turn them. The input shaft's power drives a set of gears, called the countershaft gears. These are meshed with the output shaft gears, causing them to turn as well.

The size difference between the countershaft gear and its corresponding output shaft gear determines how your vehicle drives. In first gear, a smaller gear drives a larger one, giving the torque needed to accelerate from a stop. At higher speeds, the two gears may be the same size, or a larger gear drives a smaller one, so the vehicle travels at faster speeds while the engine's speed remains relatively low. You can see this on the tachometer: as you press the throttle, the engine's rpm rises, but when you shift up to the next gear, the rpm falls.

To shift gears, the engine must be temporarily disconnected from the transmission, and that's where the clutch comes in. The engine's crankshaft, which is constantly spinning whenever the engine is running, turns a flywheel on the end of it. The clutch is between the flywheel and the transmission, and it either connects or disconnects the two.

The clutch contains a disc with friction material on it, similar to a brake pad. When you press in the clutch pedal, a pressure plate pulls this disc away from the flywheel. The two are now disconnected, and no engine power is going to the transmission. When you let the clutch pedal out after shifting, the clutch disc and pressure plate reconnect to the flywheel, and the engine's spinning power is transmitted to the transmission through the input shaft. Just as with your brakes, that friction material on the disc eventually wears down and needs replacement. It’ll need replacing even sooner if you "ride the clutch," meaning that you keep the clutch pedal pressed halfway. It's a common habit for those who are new to manuals, but it only partially engages the clutch and creates unnecessary friction.

Once the clutch is in, it's time to shift gears. The output gears are separated along their shaft by synchronizers, which ensure a smooth transition from one gear to the next. They also lock the selected gear to the output shaft, so the shaft spins at the appropriate speed to power the wheels (all the output shaft gears are turning whenever the clutch is released, but only one is locked to and actually turning the output shaft at any time). Each synchronizer slides between two gears, engaging one or the other. When you move the gearshift lever, you’re moving metal bars called shift rails back and forth. Forks on the shift rails are attached to the synchronizer's sleeve, and as the gearshift lever moves the rails, the forks slide the synchronizer from one gear to the other to engage it.

In simplest terms, to shift from first to second, you push in the clutch to disconnect the engine and transmission; you move the shift lever, which slides the synchronizer from first gear to second gear and locks it to the shaft; and you let out the clutch, which reconnects the engine again. That second gear turns the output shaft, and power goes to the vehicle's wheels.

Reverse involves turning the output shaft the opposite way. While the forward speeds are a result of the countershaft and output gears meshing directly, reverse is a small gear that fits between them to provide that go-backwards direction. In Neutral, none of the output gears are locked to the shaft, so the engine can run without stalling. There's no Park gear as on an automatic, so whenever you leave your manual-equipped vehicle, always remember to set the parking brake so it won't roll away.

Jil McIntosh specializes in new-car reviews, auto technology and antique cars, including the two 1940s vehicles in her garage.

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