Wikipedia has a good animation of a manual transmission, but something irks me.

enter image description here

If I understood correctly, in the animation, the input shaft is in green, and the output shaft is in cyan. The dog clutches (synchromesh in modern cars) are therefore between the free gears and the output shaft. This forces both the engine and the gear assembly to change speed in a gear shift. If the synchromesh were between the input shaft and the free gears, only the input shaft (therefore the engine) would need to change speed during a gear change. I don't see anything after a quick Google that suggests things are different in many other gearboxes.

What is the reason for this that I'm apparently missing? Acting as a flywheel is the only that crosses my mind.


2 Answers 2


Honda B Series

Image from https://www.j-k-tuning.com/Transmission/BTrans2.php

You can put them anywhere you want to (and people do). But it is easier to put them on big gears than little ones. If your four speed was working the other way around with the green as an output shaft, all the gears would have to have the inverse ratios, and you would have all your syncros on little gears.

In the image above, you can see both shafts carry shift forks in this model.


That is a simple animated drawing, getting a gearbox out of a recent car and stripping it would show you that most gearboxes rarely have dog clutches.

They have synchromesh rings - usually a bronze based material that has a conical surface to match a cone machined on the steel gear - this works to match the gears to the same speed by friction as the gears are brought into mesh.

Synchros have been used for many years and were, for a long time, only fitted to third and fourth gears. Commonly called “crash” boxes. As manufacturng processes, costs and the driving population changed then synchro was fitted to all gears except reverse.

The synchro works and has no effect on the engine / flywheel because the clutch has disconnected the engine form the input shaft during the gear change.

Edit to answer based on change to question: Then the output shaft is solidly connected to the axle / wheels (unless it is a Saab of a certain vintage) so the synchro makes sure the internals change speed to match the output shaft by friction as mentioned before.

  • $\begingroup$ Good detail to add - at least that informs me about the title. But I'm afraid the question is about why most manual transmissions have these clutches where they are. $\endgroup$
    – setun-90
    Commented Apr 3, 2020 at 7:35
  • $\begingroup$ You have completely changed the question... $\endgroup$
    – Solar Mike
    Commented Apr 3, 2020 at 8:05
  • $\begingroup$ What I understand is that the synchromesh and dog clutches are in the same places on synchronized and unsynchronized transmissions. Since the question is about where they are, whether the transmission is synchronized or not, I don't see how this is that different... And I'm also proposing this go to chat if it gets long. $\endgroup$
    – setun-90
    Commented Apr 3, 2020 at 8:11
  • $\begingroup$ The design of the synchro ring has a friction surface to match the speed and then "teeth" to provide a means of aligning the gears as they engage. I don't have time for a chat. $\endgroup$
    – Solar Mike
    Commented Apr 3, 2020 at 8:15
  • $\begingroup$ Is there a common name for these two elements? Based on the fact that they (dis)connect the output shaft and the free gears? $\endgroup$
    – setun-90
    Commented Apr 3, 2020 at 8:46

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