On any chain-driven bike - the semi-automatic underbones and manuals - the chain is a wearing part, and a loose one is the most common mechanical complaint on a rental. It is also one of the easiest fixes in motorcycling.
What a loose chain feels like
- A clunky surge when you open and close the throttle - slack taking up before the drive lands.
- Chain slap noise over bumps.
- Visually: the chain dips visibly at its lowest run, or you can pull it well off the sprocket with two fingers.
Check at handover: with the bike on its stand, lift the chain’s mid-run - roughly 2-3 cm of total up-down play is right on the small classes. Much more is loose; drum-tight is over-tensioned and just as bad.
Why it is not a “later” problem
A slack chain slaps the swingarm, jumps sprocket teeth under load (which locks the back wheel momentarily - genuinely dangerous), wears the sprockets fast, and can derail entirely. A derail at speed is one of the few mechanical failures that throws a rider. The fix is five minutes; the risk of ignoring it is not five minutes of anything.
What to do on a rental
Report it. Chain adjustment and tensioning is explicitly the owner’s maintenance on a Nguyen Tu rental - take the bike back, or call and describe it, and it is done on the spot at no cost. Do not ignore it for days of riding; do not pay a third-party workshop to adjust the shop’s chain without asking (the outside-repair article covers when third-party work makes sense - adjustments are not it).
On the road, far from the shop
If the chain loosens mid-trip, any roadside mechanic can adjust it for a small cash fee - take the receipt, tell the shop at return. Riding home slack because the adjustment is “the shop’s job” is the wrong reading of whose job it is: the adjustment is the shop’s, the next hundred kilometres on a derailing chain are yours.
The broader habit: when something on the bike feels wrong, report it early - the next article generalises this one.