Why Do Car Gears Grind?

Most of us have heard that painful grinding sound while changing gear and think to ourselves all the damage we’re doing to the gears. However, it’s not actually the gears that you hear grinding, it’s something in the gearbox called the ‘collar’.

Diagram of a gearbox that helps us understand why it grinds when you change gear
Diagram of a gearbox that helps us understand why it grinds when you change gear

Let’s explain in basic terms why car gears grind:

  1. Gears inside the gearbox, 1st up to 5th gear all rotate at different speeds. Gears are always in mesh and are rotating with the layshaft, which in turn is connected to the input shaft.
  2. The input shaft rotates at the speed of the engine.
  3. The output shaft rotates at the speed of the road wheels. Gears, 1st up to 5th rotate freely around the output shaft and are not fused with it.
  4. The collar inside the gearbox is fused to the output shaft and rotates with it.
  5. The driver operate the gearbox to change gear.
  6. This in turn moves the gear selector forks which in turn moves the collar to connect to a gear.
  7. When the collar meshes with a gear, a connection is formed from the input shaft through to the output shaft.

We now know that the part inside the gearbox which grinds is the collar due to it having the job of connecting two shafts that rotate at different speeds. But now the question is why is it grinding?

Sandwiched between the gears and the collar is a synchroniser ring, also called a friction ring. It’s a simple brass ring that presses against the gears before the collar meshes with the gears. When the clutch pedal is pressed down, the engine is then separated from the gearbox. 

This means that by use of friction, the synchroniser ring can first match the speed of the gears (which are connected to the input shaft) to the collar (which is connected to the output shaft). Once the speed is synchronised, the collar connects to the gear. The driver then lifts up the clutch pedal and a continuous link if formed from the engine to the drive wheels.

The synchroniser ring is made from a softer metal than the rest of the gearbox. This is so that the ring will wear out before any of the more complex and expensive parts of the gearbox.

So the reason why your gearbox is grinding, is likely because one of the synchroniser rings is worn and is no longer properly synchronising the gear speed to the collar.

I Keep Grinding Gears

If you keep grinding gears, ensure you depress the clutch fully to the floor and guide the gear lever into its respective slot rather than forcing it. See how to change gears smoothly. If after this you are still grinding gears, there could be a fault with your gearbox or a synchroniser ring requires replacement.

What Happens if You Keep Grinding Gears?

If you keep grinding gears without having the cause of the issue dealt with, you’ll ultimately cause greater and more expensive damage to the gearbox.

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