Eli5: How do standard transmissions work, specifically downshifting?
In: Engineering
To set up the scene, the path from the engine to the wheels has 2 disconnection points: the clutch, and the gears. If both these things are disconnected (clutch pedal down, gear lever in neutral) then what’s in between – the clutch itself and transmission’s input half – are spinning freely and by friction will slow down. However when downshifting you’re moving to a gear where the clutch speed actually needs to come up. And since you’re moving metal bits inside a transmission engaging two metal gears into each other requires them to be moving at about the same speed or you’re just going to grind metal together.
In the old days people were expected to double-clutch… You shift to neutral, rev the engine to the speed you want, and then release the clutch pedal while still in neutral. This connects the clutch and transmission input to the engine and brings them up to speed. Now you can press the clutch pedal again for properly shifting into your new low gear without grinding gears.
Nowadays transmissions have synchronizers on the gears. When you push the lever into a gear there’s a bit of rubbing of the synchronizers to bring the clutch+input to the correct speed before the gear goes in. You may notice that downshifting may involve putting a bit of extra pressure on the gear lever to go in and it takes a moment before it wants to happen – that’s this mechanism. However they are a component that can wear out over time, replacing them is expensive since it involves removing and dismantling the transmission/gearbox itself, and honestly they have limits. They are not meant for shifting from 5th to 2nd while remaining at interstate highway speeds, and they are most certainly not able to handle changing gears without pressing the clutch. Don’t abuse them. Also not all gears have them – commonly reverse may not have any synchronization or it may cheat by borrowing another gear’s synchronizer meaning you should not shift into reverse unless you’re absolutely stopped and the engine is at idle. Older cars may not have one on 1st gear.
Consequently some drivers still choose to perform the double-clutch procedure even on modern cars. The synchronizers will last longer and it should make the gear lever easier to move.
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