I'm trying to understand how the semi-automatic transmission in an underbone works, specifically why the engine doesn't stall when the bike is not moving.
These vehicles are the motorbikes with small engine (~125cc) and without a clutch lever. But they still have a shift lever and require user to manually changing gear (usually 4 gears), by simply reduce the throttle and push down a shift lever (pedal).
In normal condition, the engine never stall. For example, when the bike is standing still and you shift through different gear, or when you go up hill and the bike is too heavy, it's just stop moving but the engine keep running.
What I've learned about the gear and clutch in these bikes:
When you push the shift lever to change gear, or when you change the gear to neutral, the clutch disks are released at the same time the gear is changed, disengage the engine from the wheel.
And when you release the shift lever, the clutch disks are compressed again, connecting the engine with the wheel.
I've read about centrifugal clutch that only engage when the engine RPM is high enough, which is exactly how these bikes behave.
However, videos from the internet show that these bikes use the traditional manual clutch with a lot of frictional disks and springs.
So what is the magic here?
I've found a videos where they repair the clutch of one Yamaha underbone and found that 1 complete set actually contains 2 separate clutches connected together.
The first one (they call it "front clutch") is a centrifugal clutch that seems to be connected directly to the crankshafts(?). This must be the first clutch that jwh20 mentioned?
Image of the front clutch:
The other one ("back clutch") is a typical manual clutch with frictional disks and springs, connect to the gear box.
Image of both the back and front clutch:
This explain my question and the behavior of the underbone.
I think initially I've confused between the centrifugal clutch and the CVT in scooter, I thought they were the same.
Now thinking again, you can't have gear using centrifugal clutch only, but CVT kind of have an "automatic gear".
It makes sense that go-carts use centrifugal clutch. I've always wish that they have gears so I can go faster.
Some other images of a Honda clutch: