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What Happens When I Change Gear?

To understand what happens when you change gear, let’s first look at the anatomy of a car’s transmission and how it relates to the engine and the wheels.

A 5-Speed Transmission

  • The transmission houses the gears and is connected to the engine through a shaft, which can be disengaged using the clutch.
  • Meshed with the gear on the engine shaft is the first gear on the layshaft, which is a single-piece shaft holding multiple gears of different sizes.
  • These gears mesh with the 5-speed gears that spin freely on the splined shaft.
  • The splined shaft has three “collars” which spin with the shaft and engage the 5-speed gears using “dog teeth.”
  • Connected to the splined shaft is the differential, which drives the wheels.

Changing Gears

For a manual transmission car, this method of changing gear first involves putting the clutch down. Doing so will disconnect the engine shaft from the transmission, which will have the effect of slowing the rotation of the gears within the transmission.

With a slower gear rotation in the transmission, the gear stick can be moved into neutral causing the collar on the spline shaft to disconnect from the engaged gear. As the gear stick moves, the rod connected to it moves and slides the collar away from the gear. If the car is moving at this point then the spline shaft will be spinning at a different rate to the rest of the transmission, including the gears. To engage the desired gear, a synchroniser is used to speed-match the collar with the rotation of the desired gear before the “dog teeth” can connect.

Once the new gear is engaged, the clutch can be lifted to bring back the drive of the engine. When the engine is not engaged during this process, the car slows down slightly as there is no drive to the differential. Only momentum is moving the car along. With a higher gear engaged, the transmission can output a higher RPM than lower gears for the same engine RPM.

In looking at the transmission, you will notice that all the gears are constantly meshed, so what causes the grinding noise when a gear change is not performed correctly? The cause is the teeth of the collar hitting the gears and could mean a trip to the motor repair services.

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