You’ve heard of “dry-box” exchange, have not you? It is a term that usually appears in the old stories that his grandfather (or his father, depending on his age) tell about how difficult it was to drive the cars of the past. “Dry box” is the popular name for the exchange with non-synchronized gears, and the difficulty in driving these cars is due, in part, to a technique required to handle the dry box: the double-clutch.
To understand what such a double-clutch is, and why it is considered a technique for machine connoisseurs, one must first understand how a manual gearbox works.
The manual shift is based on three independent axes, the input shaft, the output shaft (or pilot shaft) and the auxiliary shaft (or counter shaft). The input shaft is connected to the clutch (and by it to the engine), and drives the engine to the gearbox. Inside this input shaft is connected to the auxiliary shaft, which part transmits the movement of the input shaft to the output shaft.
When we change the gears, what happens there inside the gearshift is the change of the relation between the input shaft and the secondary axis. This ratio change varies the difference between the number of turns of the input shaft (motor speed) and the number of revolutions of the output shaft (transmission speed). Put in a simplified way, it varies the speed of the output shaft.
As we saw in the video, this entire set of gears is constantly rotating, even when not connected to the shaft (the gears that are not in use are only “supported” on it) and so they rotate at different speeds. When changing gears, there is a ring between the coupling sleeve (called the “shell” in the video) and the gearing gear that synchronizes between the gear teeth and the gear teeth so that the gearshift is soft. This is such a synchronizer ring.
So what is double-de-clutching for?
In the “dry-box” type of exchange, this ring does not exist. They were only invented in the 1920s and took another 30 or 40 years to become the industry standard. The synchronization of the gears was made by the driver himself using the double clutch (or double clutch, which has nothing to do with the double clutch gears, logically) in both up and down reductions.
And how I do it?
Although it seems complicated when reading the description, it is a simple technique. As an example, you are in first gear and need to switch to the second gear. Then you step on the clutch, put the gearbox in neutral and release the clutch (engaging the transmission to the engine again). This will cause the engine speed (input shaft) to directly influence the gear speed of the next gear, setting it more or less in the rotation corresponding to the next gear. When the engine reaches that speed, you step on the clutch again and engage the gear (by sliding the coupling sleeve over the next gear).
In the reductions the process has a difference in the second stage. You come in third gear, step on the clutch, put the gearbox in neutral and release the clutch. With the engine and gearshift engaged, you increase the engine speed (and hence the gearshift) to the lowest gear. When you reach that rotation, just clutch, and slow down.
The processes seem slow, but in practice everything is done so quickly and intuitively when the common gearshift, with only one clutch.
Still good for something nowadays?
Although it has fallen into disuse with the popularization of synchronized cambials, the technique is still used by truck drivers (as in the video above) and is even required to qualify for heavy trucks in the US, which generally use unsynchronized rates For the sake of greater strength and durability.
In synchronized exchanges, however, the technique is of little use except to extend the life of the synchronizer rings. That’s why Dom Toretto, fidgeting Brian for not knowing how to use the technique in “Fast and Furious” (2001), ended up making a mistake that few people noticed – especially since the translators of the film should not have the slightest idea of what it is “Double clutching”
In a start the technique is completely unnecessary because the changes are made at the spin limit, and dropping the rotations to change the gear will only waste time. What is used in the starts is “clutch slipping” and “powershifting” (shifting gears without relieving the accelerator).
Now, a fairly common double-debris situation almost without notice, is when you pull the car out of the first one and the reverse gear does not engage at all. This happens at a shift where the reverse is not synchronized, and to engage the reverse, you need to leave the gearbox in the neutral position, release the clutch, clutch again and engage the gear.