Most of the motorbikes you will see in Vietnam - and many of the ones you can rent in Hanoi - use a gearbox format that confuses first-time visitors: it shifts like a manual motorcycle but has no clutch lever. Here is how the semi-automatic underbone gearbox actually works, using manufacturer-published examples (retrieved 23 September 2026).
The centrifugal clutch
The key component is a centrifugal clutch. Instead of a lever-operated clutch, the engine spins a set of friction plates that are thrown outward by centrifugal force as revs rise: at idle the engine spins freely, past a certain rpm the plates grip and drive the gearbox. Yamaha’s published Sirius specification lists it explicitly - a wet, multi-plate centrifugal clutch. No lever, no friction point to learn.
The rotary four-speed
The other half of the format is the gear pattern. Honda publishes the same layout for the Wave Alpha, Blade, Wave RSX and Future 125 FI: a mechanical four-speed rotary gearbox. The shifter clicks down through the gears and then wraps back around to the top - neutral, first, second, third, fourth and back to neutral - so you never run out of clicks at either end.
What you actually do when riding
Twist the throttle to go. Roll off the throttle and press the shifter with your left toe to change up, heel-down to change down. Roll to a stop and you can click back to neutral anywhere. That is the entire skill - which is why the format dominates driving-school fleets and rental counters.
The trade-offs
Shifting without a clutch is slightly harsher than a manual shift done well, and the clutch cannot be modulated for delicate manoeuvres the way a clutch lever can - see our hill starts guide for how underbone riders handle slopes. Slipping a centrifugal clutch under heavy loads is a real wear mode, covered in our clutch care guide, and hard shifting damages the box - our hard-shifting guide lists the symptoms.
Why this matters for visitors
A semi-automatic is the middle option between a twist-and-go scooter and a full manual bike - our automatic versus manual comparison and scooter versus semi-automatic guide place it in context. If you rent, the Honda Wave and Yamaha Sirius classes are our price-published semi-automatic models at 150,000 VND per day - contact us to confirm current availability.