What is Synchromesh Transmission?

One of the more impressive components in any automobile is the gearbox or “gearbox”. While most cars don’t benefit from the technology, the synchromesh is what the single-clutch manual gearbox is aiming for. At least for now.

What is Synchromesh?

Synchromesh transmissions are a further refined version of the constant mesh system, although less common. How it improves the system is by adding another stage to the process of connecting the gears to the drive shaft through the dog clutch.

A synchromesh is almost like a small clutch that sits on the output shaft between gears, slowing or increasing the required gear’s relative speed to perform a perfect meshing of teeth within the transmission.

It divides the dog clutch into two parts, a gear attached to the drive shaft called the synchromesh hub and a collar around the outside that can slide back and forth and is called the shift sleeve.

A new component was added to the gears themselves, the synchronizer cone, and another moving part called the synchronizer ring was introduced to surround the cone.

This is where things get a little complicated.

The collars or shift sleeves are now the components controlled by the gear stick and they can slide halfway on the synchronizer rings in either direction. This presses the rings against the synchronizer cones attached to the gears and, due to the increased friction caused by the widening cone, can either accelerate or decelerate the gear to match the speed of the shift sleeve and synchronizer hub.

As soon as the speeds are matched precisely enough, the sleeve can slide further over the locking ring and come into direct engagement with the cone and the gear wheel, whereby everything is connected and the power is transmitted to the drive shaft.

Incredibly, it all happens in the fractions of a second it takes to shift gears, giving you an even smoother gear change.


The synchronization action consists of three main parts: the bulk ring and a synchronization unit. The bulk ring has outer teeth that mesh with the Synchromesh teeth, but it also has an inner groove pattern that meshes with the gear that needs to be engaged.

The synchronizer has an inner key that coincides with the output shaft and then an outer spline that allows an inner ring to move within the gear. This outer ring is designed so that it will not engage the bulk ring until its speeds are adjusted and the teeth mesh.

So, when you begin selecting a gear with the gear linkage, the shift forks apply pressure to the bulk ring, which then begins to approach the selected main gear. Fortunately, the gear has a tapered shoulder that creates friction with the bulk ring, which also contains a sleeve that is perfectly shaped to accommodate the shoulder, which slows the gear down. Soon the bulk ring and gear are moving at the same speed and in perfect harmony.

With additional force applied when the physical displacement is carried out by the linkage, the synchronization unit is pushed over the bulk ring, both rotating at the same speed. The inner ring of the synchronizer then allows the outer radius of the synchronizer to fully meet with the main gear, synchronizing their movement with one another and completing the gear change smoothly.

A synchronization effectively enables a gear change to be completed with an actuation of the clutch via the clutch pedal, as a result of which the speed adaptation is essentially forced by the efficient engagement of the teeth. Instead of having to adjust the speed of the clutch disc and flywheel, the sync does all of the work a little further down and has made manual shifting a lot easier than it used to be.

We take it for granted how well auto transmissions do their job these days, especially given the engine power that is now being enforced by modern transmissions. But synchro meshes are like the ligaments on a car, seamlessly connecting the transmission of power from one muscle to the rest of the body.

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