With the rapid takeover of automatic transmissions and modern comfort systems in recent cars, such as the CVT gearbox, vehicles’ character and soul are on the verge of fading out. The feeling and the connection a driver/mechanic has with their classic cars remains unmatched, thanks to the relatively more straightforward mechanical engineering utilized. Some believe that while simpler and older designs introduce limitations, they also grant the driver a unique experience as they require the owner to be present and more in touch with the mechanical details of their ride.

That said, getting familiar with different mechanical systems commonly used in cars might not be necessary, but it sure is beneficial, especially if you like tuning, modifying, and building cars. And one of the vital mechanical sections of a vehicle is the transmission. And if you love driving cars, you definitely know how manual gearboxes make a massive difference in the driving experience. However, not all manual transmissions work in the same way, and not all serve the same goal.

While there are countless ways to design a gearbox, each meant for specific usage and outcome; there are manual transmission types that are the most common. They include sequential manual transmissions, unsynchronized transmissions, and constant mesh gearboxes. Let’s take a look at how they differ from each other and what their features are, starting with the sequential.

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Unsynchronized Transmissions Also Known As Crash Gearboxes

FIAT 500L gear shifter
Petar Milošević Via Wikimedia Commons

The name unsynchronized transmission, or as it is sometimes called, “non-synchronous transmission,” might not sound as familiar as other types mentioned above. The reason is that very few unsynchronized transmissions are used in modern cars.

What differentiates unsynchronized transmissions from synchronized ones is the fact that they do not feature any synchronizing mechanisms, meaning they require the driver to mind the engine RPM and the speed of the car to decide on the correct gear ratio; otherwise, the grinding of the gears spinning at unmatched speeds can damage the gearbox, hence called the crash gearbox or “rock crushers!”

In conclusion, driving vehicles with unsynchronized transmission require skill and experience, which is why they slowly disappeared as drivetrains met features that help ease things off and give the driver a practical and comfortable experience. They are still found in older trucks from the 1960s since they were sturdy and were easy-to-fix.

Non-synchronous transmission birthed shifting techniques, such as double-clutching and float shifting, are done by experienced drivers like long-haul truckers. Float shifting is when the driver does not hold the clutch pedal down when going in and out of gear since they would synchronize the RPM and the drivetrain speed quite accurately when shifting.

Double-clutching is self-explanatory in that it is pressing down the clutch pedal twice when shifting. The reason for doing so is to either let the engine slow down or manually speed it up to synchronize it with the speed of the drivetrain.

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How A Constant Mesh Gearbox Works

TREMEC TR-6070 7-speed manual transmission
MeMike200 Via Wikimedia Commons

The invention of the constant mesh or synchronized gearbox significantly improved the crash gearbox. The constant mesh setup found its place in most cars starting from the late 1950s, and a modified form of it is present in most modern manual transmissions.

The identifying characteristic of the constant mesh gearbox is all gears on both shafts are constantly moving, and while the layshaft holds its gears fixed on the output shaft, the gears can move since they sit on bearings.

The “dog clutch” plays a vital role in modern synchronized transmissions. In short, it is a selector mechanism controlled by a fork that helps it slide on the shaft and fit its teeth into splines on the shaft. The locking mechanism of the dog clutch makes it almost impossible for slips to happen. Needless to say, this makes it useless for cases that require intentional slipping.

The synchromesh mechanism, first used on production cars by Cadillac, consists of synchronizer rings used on forward gears to smooth out the shifting. This mechanism matches the speed of the input shaft and the selected gear, and it is part of the constant mesh design that differentiates it from the unsynchronized transmission.

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Why Sequential Transmissions Are Great For Racing

Lexus LFA Sequential manual Gearbox Drivetrain
Tennen-Gas Via Wikimedia Commons

You are more than likely to have heard of sequential transmission and have a basic know-how of how it works. Yet, they are quite interesting setups since they are a type of non-synchronous transmission but also can be constant mesh transmissions. They lack the synchromesh mechanic and only allow the driver to shift into the next gear or the previous one.

The sequential gearbox design is relatively simple and grants fast shifting, which is why it is quite popular in motorsport applications. Most motorcycles also feature this transmission type, hence the upshift and downshift buttons.