Eli5: How do standard transmissions work, specifically downshifting?
In: Engineering
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.
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