The radiator is the most exposed fragile part on a liquid-cooled motorbike: a thin aluminium honeycomb mounted at the front, directly in the path of everything the wheel and traffic throw forward. A stone, a stick, or years of mud are all it takes.
What a guard does
A mesh or perforated guard sits in front of the radiator and takes the impacts while passing cooling air. A pierced radiator in Vietnamese traffic means a slow limp, an overheating engine and an expensive roadside bill — the failure chain is described in motorbike-overheating-causes, and the fluid-side care in coolant-radiator-care-liquid-cooled-motorbike.
Who needs one
Riders on gravel or rural routes absolutely (gravel-roads-motorbike-vietnam is the argument in riding form); city riders of liquid-cooled scooters mostly for mud and road debris, which matters more than it sounds — mud packed into radiator fins insulates, and an insulated radiator is a slowly overheating one. Underbody skid plates (skid-plates-underbody-protection) complete the protection story for the low-slung parts.
Choosing and fitting
Look for guards shaped for your model, or universal mesh trimmed to fit; the mount must not vibrate against the radiator (that frets holes in the very thing you are protecting) and must be removable for cleaning — a guard you cannot easily remove will not get cleaned, and a mud-packed guard blocks more airflow than no guard. Fit it with proper fasteners and check it after the first few rides.
Maintenance
Rinse the guard and radiator gently with low-pressure water regularly, especially in the muddy season; never blast high-pressure water into the fins, which bend and close off. Guard, clean fluid, and a working fan — that combination is the whole cooling story.