Unstable handling / weave

Warning: handling is a safety matter — identify the cause before getting back on the road, and when in doubt, always have it checked by a professional. The bike weaves, the handlebar starts shaking on its own (a tank-slapper), it pulls to one side, or it feels vague and imprecise. Several systems can be at fault, and the approach goes from simplest to most serious. The most frequent and free cause to check is incorrect tire pressure (an underinflated tire makes the bike soft and unstable, an overinflated one makes it twitchy). Next come worn or deformed tires (squared-off profile, flat-spotting, a bulge) that cause weaving, worn or notched steering head bearings (tank-slapper, binding spots) — a steering component rated expert-only — and finally chassis problems (a leaking fork, a dead shock, play in the swingarm, misaligned geometry after a crash) reserved for a professional. The distinction comes from the circumstances: instability at low speed or with hands off the bars (tank-slapper: steering, tires) versus instability over bumps and in hard cornering (suspension, chassis), and from tests (pressure, steering-head play with the front wheel off the ground). Urgency threshold: pronounced tank-slap, a bike that weaves, or steering play are major safety defects — don't fight it, slow down gently (never chop the throttle abruptly during a tank-slapper, grip the tank with your knees), and have it checked before riding again. Degraded handling shows no mercy at high speed.