What causes the grinding noise when you shift gears? How does your car recognize the gears when you move the shift knob? What happens if you skip gears, or throw your vehicle into reverse while traveling forward at 60 mph? If you’re driving a car equipped with a manual transmission, you may have asked yourself these questions at some point. We’ll clarify these issues below by taking a brief tour through this assembly. We’ll begin by looking at the reason your vehicle needs a transmission in the first place.

The Role Of Your Transmission

In an internal combustion engine, the valves and piston (and by extension, the crankshaft and other parts) work in unison to move your vehicle forward. If your engine only had a single gear, its rpms would increase with your car’s speed. The problem is, your engine has an optimum rpm range in which it performs efficiently to generate horsepower; outside that range, it loses its efficiency. If the rpms continue to climb, your engine will “redline,” which means it’s on the verge of failing.

Your tranny prevents this from happening. It allows the ratio between the rate at which your engine operates and the rotation of your drive wheels to change with your speed. When you move the gear selector through the characteristic “H” pattern, you’re essentially preventing your engine from redlining.

Most cars with a “stick shift” (that is, equipped with a manual transmission) have five gears. Moving the shift knob changes the gears.

Going Through The Gears

When you press down on the clutch pedal, your engine disengages from your transmission. This allows the engine to idle without your being forced to turn it off – for example, while you’re waiting at a red traffic light. Releasing this pedal reconnects the two assemblies.

A shaft extends from your engine to one of many gears on your tranny. This gear connects to another gear through interlocking teeth. This second gear is connected to a layshaft, which is attached to additional gears. These other gears, in turn, are each connected to one of the five gears of your manual transmission.

These five gears are connected to another shaft, which is connected to your vehicle’s differential. Also, the five gears are separated by a series of collars (usually three of them); these, too, are connected to this shaft.

As your engine turns, the connecting shaft also turns, which cause the layshaft to turn. As the layshaft turns, the interlocking teeth of its connected gears cause the five main gears to turn. Meanwhile, the collars located between the five gears are engaged by a gear selector fork. This fork is controlled through your shift knob. As you move the shift knob through the “H” pattern, the fork engages one of the collars based on the specific gear you select. This causes the collar to connect to the corresponding gear, which then turns the differential, and ultimately, the drive shaft.

Clarifying The Potential For Problems

So, what causes the grinding sound when you shift gears? Each collar is equipped with dog teeth. These teeth allow the collar to connect to the gears on the differential. Many people believe the grinding noise is caused by the gears’ teeth connecting improperly. This is a common misconception. In reality, the noise is caused by the dog teeth failing to find the proper fit in the differential gears.

What about putting the transmission into reverse while traveling at 60 mph? This would be impossible due to an idler gear that sits between the reverse gear and the layshaft gear. The reverse gear rotates in a direction that is opposite the other differential gears, and the idler gear prevents the layshaft gear from engaging it directly. Hence, you’ll hear a loud noise, but there’s no need to worry about accidentally destroying your transmission in this way.

Ultimately, your manual transmission is very similar to an automatic. The main difference is that you control the gear selector fork with the shift knob.

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Category: Automotive
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