How Does a Manual Transmission Work?

Your car’s transmission is among its most vital elements. It connects the engine to the drivetrain and governs how much power you use from moment to moment. Yet for most people, it’s a complete mystery how it works. The prominence of automatic transmissions has lessened the need to understand how this magical box beneath our feet functions. We’re more than willing to let the computers handle it so we can keep our focus elsewhere (hopefully on the road itself).

At Leith Ford, we think there’s a lot to be said for understanding the basic functions of your car. In many instances, it can help you take better care of your vehicle, which hopefully means it will last longer. In this series, we’re going to teach you the basics of how a transmission works. First we’ll cover how a manual transmission works, then we’ll talk about how automatic transmissions work, and finally we’ll compare the two, discussing the pros and cons of each.

How does a manual transmission work?

If you’ve driven a car with manual transmission, or if you’ve ridden in a manual transmission car, or if you’ve seen a decent action movie with a car chase scene (no one drives automatics in movies), then you know about the clutch pedal and the gear shifter. These are the two inputs by which a driver operates a manual transmission, though if we’re being technical the shifter is the only piece of this whole puzzle that is operated manually (i.e. by hand).

Gearbox and clutch cross-section.

Underneath all that is what appears to be an elaborate mechanism – a whirring array of shafts and gears that somehow translate into forward (or reversed) momentum. Though the diagrams might be intimidating, a transmission is a deceptively simple piece of machinery. All you have to do is break it down into its basic components.

What is the clutch?

The transmission housing contains three shafts interacting with one another. One of them is attached to the engine (the input shaft), one is attached to the differential (the output shaft), and the third shaft, often called the layshaft or the countershaft, interacts with the other two via a system of gears. While your car is on, the engine shaft is always turning, even while stopped. It has to keep going otherwise the engine doesn’t work.

When you step on the clutch pedal, you’re activating the friction clutch, which is situated between the engine’s flywheel and the input shaft. The purpose of the clutch is to decouple the engine from the transmission. While the pedal is depressed, the engine and the transmission both continue to spin, but they spin independently of one another, with no torque transferring from the engine to the gearbox. This is what enables you to change gears. Without a friction clutch and a means to decouple these two systems, everything would break.

Because it uses friction to operate, if you keep your car long enough, you’ll wind up needing to replace the clutch. It’s similar to replacing brake pads, wherein the friction materials simply wear down over time. You can extend the life of your clutch if you’ve had plenty of practice with manuals and can avoid abrupt shifting and aggressive driving.

What happens when I move the gear shifter?

gear shift lever

The countershaft and the output shaft interact via a system of interlocking gears. The difference between these is the gears on the countershaft are fixed and spin with the shaft itself, while the gears on the output shaft are not fixed and spin freely without turning the shaft. This allows the car to idle in neutral without moving forward. The gears themselves are paired in different sizes, creating different gear ratios. The exact ratios vary, but you will know them more commonly as first gear, second gear, and so on.

The gear shifter is responsible for physically engaging the gears on the output shaft, locking them in place so that they turn the shaft and send torque to the drive wheels. This is where visuals are really useful.

manual transmission shifting

Image: Source

Moving the shifter into position engages the gear selector forks. Those forks are in turn connected to a series of dog clutches (not to be confused with the friction clutch) that are responsible for actuating each gear.

Modern transmissions are equipped with synchronization systems that prevent the teeth of the dog clutch from scraping against a gear that might be turning at a different speed. Synchronizing rings were developed to make operating a manual transmission easier and to eliminate the terrible grinding noise that used to happen when the teeth of the dog clutch would clatter against the gear wheels.

All of this happens in an instant. Once you take your foot off the clutch pedal, energy is able to travel from the engine, through the transmission, and to the drive wheels, propelling your vehicle forward. As the engine approaches the limits of its RPM band, you shift up to a higher gear ratio in order to stay within the most effective range.

 

That wraps up our explanation of a manual transmission. If you’re more of a visual learner (don’t worry, we are, too), we’ve embedded a couple of videos below that will show you all the moving parts. Sites like HowStuffWorks are also great about providing details and diagrams.

The next part of this series will explain how automatic transmissions work, and check back later for the final part when we compare manuals and automatics.

If you’re a manual enthusiast, let us know the next time you call or visit one of our dealerships. Every Leith Ford employee would love to help you into any manual transmission vehicle in our inventory.

Great visualization, funny accent.

Fantastic old-school explanation.

Finally, a Lego representation. Because it’s awesome.

How Does a Manual Transmission Work? was last modified: May 12th, 2015 by Leith Ford
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