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.

  1. 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: yamaha front clutch

  2. 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: yamaha back 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:

honda front clutch

honda back clutch

  • 1
    Welcome to Motor Vehicle Maintenance & Repair! Oct 18, 2022 at 16:43

1 Answer 1


There are many different models of this type of bike/scooter but they all have an automatic clutch system to allow smooth starts and to prevent the engine from stalling when stopped.

Some are the simple centrifugal clutch with a spinning weight that puts more pressure on a friction surface the faster the engine turns. These are also common used in go-carts and such. Here is an image of one of these:

Centrifugal Clutch

The main drawbacks to this type are limited power handling ability and also there is often a need for a separate transmission that needs to be shifted either automatically or by the user.

Another very common type is the "CVT" style where CVT stands for "continuously variable transmission". These look like this:

CVT clutch and transmission

Not only are these used in scooters but are very common in snowmobiles and they can handle quite a bit of power. There are two spring-loaded pulleys with movable sides and the one attached to the engine has weights that press the sides of the pulley together as the engine speed increases. So at idle there is little pressure and the drive belt just slips. As the engine speed increases the pulley tightens up and engages the belt and things start moving. As the transmission starts going faster it has weights that pull its sides apart and so you get a smooth increase in gear ratio.

Others use a "torque converter" type of system as it common in automobiles although much smaller. It's usually an oil-filled housing with two "turbines" facing each other. As one speeds up there is more and more force on the other one and it starts to move too.

Here is one example:

Torque Converter Clutch

These are more expensive to manufacture and so are generally only seen in high-end "scooters" that are often called "maxi-scooters". I believe the BMW 650cc scooter has one of these but it's more like a motorcycle than a scooter:

BMW 650cc Scooter

Beyond that there may be other systems but I have never seen any of the videos you are referring to that use a "traditional manual clutch". In that case it would NOT be what you appear to be talking about but rather a standard motorcycle clutch system.

In my places, such at the US state where I live, a "scooter" falls into a different legal category than a motorcycle and one of the requirements for it to fall into that category is that it cannot have a manual clutch. If it does, then it's a motorcycle, requiring a motorcycle endorsement on your driver's license vs. a scooter which requires nothing at all, not even a driver's license.

  • Was going to say, pictures are worth a 1000 words. Oct 18, 2022 at 16:42
  • I think your third picture is a friction disk clutch or torque limiter, not a torque converter. Oct 19, 2022 at 1:20
  • Thanks for your reply, and sorry for the lack of information in my question. I thought underbone is popular everywhere, but clearly they're not. In my area, scooter with CVT are more expensive. Underbone is the cheapest kind of "scooter". There are also underbones with slightly bigger engine (cylinder), as well as a clutch lever. I guess the clutch system will be different in those as well.
    – Hp93
    Oct 19, 2022 at 13:48
  • As answered; centrifugal clutches work well . They release at low speed and hold at high RPM. No variable transmission is needed. I had a "Sailsbury'" centrifugal clutch ( no transmission) on my 1950 Whizzer ( about 50 cc), It worked perfectly and was very very simple. Oct 19, 2022 at 14:13

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