Why do manual cars/motorcycles stall if you don’t give them gas as you let go of the clutch?
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You don’t necessarily have to give it gas, but it does make it easier to get going.
Stalling happens when the engine slows down to a speed (RPM) that isn’t fast enough to get the piston to the next power stroke (when fuel is burned to generate power). The piston already has to work against exhaust and air-fuel pressure, releasing the clutch adds the load of moving the car to this
The clutch is a pair of plates that can be separated by pressing the pedal and releasing it lets them touch again. Releasing further increases the pressure between the plates, which increases friction, making them stick together more. This forces the speed of the engine and the input of the gearbox to match
Each gear in a gearbox has a movement speed associated to an engine speed (RPM) and when you change gear, you change the ratio between those 2 speeds. The input speed comes from the engine, it’s converted to the output speed, which determines how fast the car goes.
With these 3 bits of information, we can put it together:
When the clutch is completely released, the engine and gearbox input speeds are forced to be the same. This results in an output speed that moves the car. If we have a stationary car, which would do 10km/h at idle in first gear, then the moment you drop the clutch, it should instantly be moving at that 10km/h speed. What happens instead is that the resistance coming from the car stops the engine. You can stall in a moving car too, if the engine is forced to turn slower than it needs to reach the next power stroke.
The way slowly releasing the clutch helps is by decreasing the pressure between the clutch plates. This allows them to have a speed difference, essentially converting some of the engine’s power to heat instead of movement. This also means that part of the force that would stop the engine is converted to heat, letting the engine keep turning easier. While the clutch is allowed to slip, the car is less efficient and wears the clutch out, but with normal operation, this wear takes decades to become noticeable. Pressing the gas while getting moving gives more power to the engine, letting you put more load on it but heating the clutch up more in the process (which is fine if you don’t go crazy with the gas). If your engine is more powerful than the grip of your tires, then at high enough RPM, it will be able to handle the clutch being dropped instantly, but in that case the wheels will spin (which also converts mechanical resistance to heat, just on the tires instead of the clutch) and the whole drivetrain will get a shock and can be damaged, so again, not a good idea.
All this works pretty much the same way on a bike as well. They use slightly different gearboxes which can shift without using the clutch at high speeds, but the rest is the same
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