How Synchromesh Works
by Bill Kelly, firstname.lastname@example.org
Let’s say you’re shifting from neutral into 3rd. Clutch is in, and you’re doing 40 mph. The input shaft/countershaft/gear train are oozing toward stopping in the gear oil. The driven clutch plate too. The output shaft is spinning with the rear wheels, driving the selector hubs via their splines. The syncros are trapped in the hubs, so they’re spinning with the output shaft too.
As you push the lever toward the 3rd position, the hub gets pulled back toward the real 3rd gear, pushing the syncro ring ahead of it. By friction, 3rd gear is dragged to match the speed of the mainshaft. By constant mesh, it takes the countershaft etc. with it. If you don’t believe me about the friction, jam one of your syncros against the gear and try spinning it (I’m assuming your gearbox is apart).
Then the spring-loaded balls inside the hub let go. This takes the pressure off the friction surfaces, but by then your whole gear train is spinning at the right speed for 3rd gear,