How Synchromesh Works

by Bill Kelly,

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, even though you’re not really in 3rd yet. With the friction reduced, things are free to move a little, if necessary, for the hub to slide over those little teeth, which I have heard are called the “dog gear”, BTW. Notice how pointy the teeth are in the direction facing the hub?

There’s a vulnerability in the system here, if you stop moving the lever forward after the syncro balls release but before the dog gear is engaged. The countershaft et. al. are going to slow down from the weight of the gear oil. Count up to 3, push the lever the rest of the way, and experience that grinding noise, and the jolt that goes clear up to your shoulder!

Yes, those teeth take all 110 horses, but then the hub is engaging all of them simultaneously. The wider “ratio” teeth are only making contact on a couple of teeth at a time.

At least that’s how the Herald gearbox works.

Bill Kelly
’62 Herald Convertible, in restoration, with its gearbox all over my basement!