Part 5 of 6 May 1st, 2017
Idler gear. The idler gear (sometimes called “reverse idler gear”) sits between the reverse gear on the output shaft and a gear on the countershaft. The idler gear is what allows your car to go in reverse. The reverse gear is the only gear in a synchronized transmission that isn’t always enmeshed or spinning with a countershaft gear. It only moves whenever you actually shift the vehicle into reverse.
Synchronizer collars/sleeves. Most modern vehicles have a synchronized transmission, meaning the gears that deliver power on the output shaft are constantly enmeshed with gears on the countershaft and are constantly spinning. But you might be thinking, “How can all five gears be constantly enmeshed and constantly spinning, but only one of those gears is actually delivering power to the output shaft?”
The other issue that comes up with the gears always spinning is that the drive gear is often rotating at a different speed than the output shaft that the gear is connected to. How do you sync up a gear spinning at a different rate as the output shaft, and in a smooth way that doesn’t cause a lot of grinding?
The answer to both questions: synchronizer collars.
As mentioned above, gears 1-5 are mounted on the output shaft via ball bearings. This allows all of the gears to freely spin at the same time while the engine is running. To engage one of these gears, we need to firmly connect it to the output shaft, so power is delivered to the output shaft and then to the rest of the drivetrain.
Between each of the gears are rings called synchronizer collars. On a five-speed transmission, there’s a collar between the 1st and 2nd gears, between the 3rd and 4th gears, and between the 5th and reverse gear.
Whenever you shift a car into a gear, the synchronizer collar shifts over to the moving gear you’re looking to engage. On the outside of the gear are a series of cone-shaped teeth. The synchronizer collar has grooves to accept those teeth. Thanks to some excellent mechanical engineering, the synchronizer collar can connect to a gear with very little noise or friction even while the gear is moving, and sync the gear’s speed with the input shaft. Once the synchronizer collar is enmeshed with the driving gear, that driving gear is delivering power to the output shaft.
Whenever a car is “neutral” none of the synchronizer collars are enmeshed with a driving gear.
Synchronizer collars are also something that’s easier to understand visually. Here’s a short little clip that does a great job explaining what’s going on (starts at about 1:59 mark):
Shift rod. The shift rods are what move the synchronizer collars towards the gear you want to engage. On most five-speed vehicles, there are three shift rods. One end of a shift rod is connected to the gearshift. At the other end of the shift rod is a shift fork that holds the synchronizer collar.Gearshift. The gearshift is what you move to put a car into gear.
Shift fork. The shift fork holds the synchronizer collar.
Clutch. The clutch sits between the engine and gearbox of the transmission. When the clutch is disengaged, it disconnects power flow between the engine and transmission gearbox. This disconnection of power allows the engine to continue running even though the rest of the car’s drivetrain isn’t getting any power. With engine power disconnected from the transmission, shifting gears is much easier and prevents damage to the transmission gears. This is why whenever you shift gears, you push the clutch pedal and disengage the clutch.
When the clutch is engaged — your foot comes off the pedal — power between the engine and transmission is restored.