Eli5: How do standard transmissions work, specifically downshifting?
In: Engineering
Do you have a multi-speed bike? It’s the same thing really. The lowest gears are “easy” to pedal….but you pedal like crazy to go very slowly.
You shift gears and your pedaling equals higher road speeds. That’s “up-shifting”,
You come to a hill and peddling gets hard (Lugging your engine) so you “down-shift” to make it easier on your “engine”. You pedal faster (rev higher) but easier.
In a car….your engine likes to be in a certain RPM range to deliver maximum power…..and another RPM range to deliver maximum economy. Shifting the gears makes your best attempt to stay within those ranges depending on road speed and load and elevation changes.
Your engine can only supply a limited amount of torque and can only spin to a limited rpm speed. The transmission uses 1st gear to get you moving from stopped, until you hit high rpms, then the 2nd gear takes low rpm to high rpm again and so on.
It’s just like a bike, your legs can only move so fast and you can only pedal so hard, so as you go faster, you change gears when your legs cant move fast enough, then you pedal slow to fast as you move through the gear.
For down shifting.
When you start moving slower and need to go to a lower gear, your engine is at its slowest rpm at the bottom of the gear, and you need to shift to the top of the next gear, so you need to shift out of the gear then speed the engine up, then shift into the lower gear.
1st (low rpm) –> (high rpm) /shift
2nd (low rpm) –> (high rpm) / shift
3rd (low rpm) –> (high rpm)
So for down shifting, you have to follow the same flow but in reverse, and you rev match to the high rpm of the top of the gear you are down shifting into.
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.
A clutch is basically a disk brake that connects the engines’ shaft with the gear box. If you kick down the clutch pedal, you loosen the “brake” and decouple the rotation of the engine shaft from the drivetrain and have no torque applied to the gears, which allows you to pull them apart and set together another pair at different RPM. If you slowly take your foot off the clutch pedal, the disk brake in the clutch starts syncing the RPM of the engine with that of the selected gear by friction.
By ‘standard’ transmission, are you talking about a manual transmission (a car that has a third pedal) or an automatic transmission? They are different but similar. Basically, the ‘gear’ is a drive ratio, how many times does the crankshaft do a complete rotation for the driving wheels to do a full rotation. If you do more than one motor rotation per rotation of the wheels, you are in lower gears. This allows more torque (twisting force) at lower speeds. Great for getting off the line or going up a hill. If the ratio is 1:1, that is normally your ‘drive’ gear. If the ratio is higher, so less than one rotation on the engine for a full rotation of the wheels, is ‘overdrive’. Modern cars have several overdrive gears for efficiency.
Now imagine you are driving along and you start going up a hill, you don’t shift down and you mash the gas, you might notice the car ‘slipping’ (this can be called ‘lugging’ as well). What is happening is the engine doesn’t have enough twisting power to maintain the drive ratio the driver has selected. To remedy this, the driver will select a ‘lower’ gear, one that makes the rotation of the engine higher than the rotation of the wheels. Allowing the car to drive the same speed with higher engine RPMs, well inside the ‘torque band’ of the motor.
How the transmission *actually works* is a different story, and if you are talking about an automatic with a planetary gearset, this is way beyond the understanding of a 5 year old. Manual transmission is a little easier to understand, there is a friction plate (a rough surfaced disk that connects the engine to the transmission, a clutch (a very powerful spring you articulate with your foot) and a lever that, once the clutch is engaged, can easily move the selected gear into the active drivetrain. The ‘gear’ is exactly that, a toothed gear that determines the drive ration. It is similar in idea to a bike ‘gear’, but technically a bike is a sprocket and not a gear. A gear turns another gear, the a bike chain ring connects the toothed gear to a chain, hence it being a sprocket.
Latest Answers