Manual transmissions are the original kind of transmission, as ancient as the first cars produced. However, their design changed massively over the years. Even so, manufacturers and people alike wish to phase them out, thanks to the convenience of automatics and the emergence of electric vehicles.
Two or Three Shafts
Firstly, let’s analyze the rough design of a manual transmission. It either has 2 or 3 shafts, an input shaft, an output shaft, and an optional countershaft. This choice stems from the engine’s layout, be it longitudinal or transverse. We’ll analyze the 3-shaft gearbox thanks to the myriad of resources available. The input shaft holds the clutch and the first cog, built and cut into the shaft itself. Torque is permanently sent via this cog downwards on another cog found on the countershaft.
This countershaft has several cogs assembled onto the shaft either with roller bearings so they are free to rotate around the shaft, or via rectangular slots so the cogs are fixed to the shaft. The torque that arrived earlier from the input shaft flows down the shaft until it meets the first fixed cog, sending said torque upwards via that gear onto the output shaft.
The output shaft is built similarly to the countershaft, with cogs assembled via roller bearings or via rectangular slots. However, there’s a caveat. These cogs are mirrored to those found on the countershaft. If a cog on the countershaft is mounted via bearings, then the cog mounted on the output shaft is fixed via slots and vice versa. This shaft sits coaxial to the input shaft, or it sits at the same height and is positioned after it if you will. One end of the output shaft is resting inside the input shaft’s pinion cog, in a hole into which a thin roller bearing, called a needle bearing, sits. This bearing allows the input shaft and output shaft to rotate at different speeds.
Synchronizer Hubs and Collars
Moreover, synchronizer hubs are mounted on the output and countershaft only via rectangular slots. Consequently, these hubs always rotate at the same rate as the shaft. You can think of these hubs as smaller drums, smaller than the cogs themselves, with slots on their circumference.
Synchronizer collars mate with these hubs. They are sleeves that can grasp both the hub and the cogs themselves thanks to some clever bit of engineering. These collars push a brass ring with small teeth into a conjugate ring cut into the cog. This brass ring, called a synchronizer ring, works like a conical clutch, allowing the cog itself to slowly catch up in terms of rotational speed to the hub, and consequently the shaft.
These collars move when you select a particular gear, allowing said cog to move at the exact same speed as that particular shaft. Furthermore, thanks to the modular design, only that cog must move in tandem with the shaft, while any other cog can rotate at a different speed thanks to those bearings. Thus, these collars allow the driver to choose when a cog is fixed to the shaft and when they aren’t, so we can shift gears (1).
When you move your shifter, you move those collars thanks to some rods with forks at the end. Each line in the H-pattern corresponds to a rod. First gear and second gear share a synchronizer collar, same with third and fourth, same with fifth and sixth. Consequently, the hubs sit in between neighboring cogs, and moreover, these cogs are mounted on roller bearings, otherwise we couldn’t engage them as explained above. These collars don’t have to sit on the same shaft. You could have the first-second collar on the countershaft, with the third-fourth and fifth-sixth collars on the output shaft.
Shift Those Gears
Now, let’s give an example with some figures so it’s easy to follow, with the caveat that two cogs meshing form a gear, and that a gear increases torque while reducing speed, generally anyway. Let’s assume that the engine spins at 4000 RPM and we’re in neutral. The first-second gear collar sits on the countershaft, the other two collars are on the output shaft. We wish to engage first gear. The input shaft always spins at the same rate as the engine, in our case 4000 RPM. The input shaft transmits the torque and rotation via the pinion found on the input shaft down on the countershaft via a constant gear explained earlier.
This gear reduces the speed once, to let’s assume 2000 RPM, as such the countershaft spins at 2000 RPM. Moreover, the synchronizer hubs rotate at 2000 RPM and the same can be mentioned about the cogs that create the third, fourth, fifth, and sixth gear considering that they are fixed via slots and not bearings.
Consequently, the cogs that make the first and second gear spin freely at any RPM. The output shaft doesn’t spin at all, considering that we want to engage first gear and we wish to start moving from a standstill. As such, all the cogs found on the output shaft don’t spin at all, either because they are mounted on roller bearings or they are meshing with other gears that sit on roller bearings.
We engage first gear, so, the first-second collar moves to the first gear, forcing the first gear cog to move at the exact same speed as the countershaft, that is 2000 RPM. Now, torque must move upward via the first gear, so its speed is reduced again, to let’s say 1000 RPM. That happens because the first gear’s cog found on the output shaft is mounted via slots, not bearings, so it doesn’t spin freely, forcing the shaft to spin at the exact same rate as that cog. Now, that speed can flow from the transmission to the differential and wheels. Finally, we engaged first gear (2)!
To engage, let’s assume second gear, we must take it out of first gear, so the output shaft spins at 1000 RPM, not from standstill anymore. When we engage second gear, we can reach higher speeds, so the gear ratio is smaller. Consequently, the countershaft speed will decrease as well, and instead of 2000 RPM as before, now we will have 1500 RPM (presumably). Remember that, initially, we had 4000 engine RPM reduced to 2000 RPM on the countershaft, so the RPM was halved. As such, now that we have 1500 RPM on the countershaft, the engine will spin at just 3000 RPM. Those 500 RPMs on the countershaft were absorbed by the synchronizer. Without it, you would have to press the clutch down and keep the engine speed around 3000 RPM, a process known as rev-matching!
Role of the Clutch
You may ask, how do you start moving from a standstill? That’s thanks to the clutch! You can disconnect the transmission from the engine entirely by pressing the clutch down, or you can slowly start moving the car by letting the clutch slip a little, just enough to let the car catch speed. It might seem extreme, but the clutch pads are incredibly abrasive, and the spring that pushes the clutch has plenty of force, so it certainly is doable (3).
The Much-Loved Manual Transmission
And that’s a brief explanation of how a manual transmission works. Why is it so loved and seemingly needlessly complicated? The manual is adored because it is fun and engaging. Shifting gears allows a control unlike any other, from crawling to clutch kicks and even brisk starts.
Is it needlessly complicated? It might seem so, but multiple automatic transmissions, like the dual-clutch transmission, work exactly like the manual. The only difference is that the shifts are electronically controlled, not human-operated. Are there any advantages?
Fuel economy, it still being king if driven properly. Resilience is another seeing how manuals are quite bulletproof. Last but not the least, due to ease of repair, seeing how incredibly modular they are.
(1) Tremec. Manual Transmission Synchronizers 101. [Online] January 31, 2019. [Cited: January 05, 2022.].
(2) John D. Kelly at Weber State University. Manual Transmission Operation. [Online] September 23, 2012. [Cited: January 05, 2022.].
(3) Thomas Schwenke. How a clutch works! (Animation). [Online] September 24, 2013. [Cited: January 05, 2022.].
See Also — How Does a CVT Work?