Unstable handling / weave

This is the first thing to check for instability, since it's free, immediate, and very often the culprit. Inflation pressure gives the tire its shape and stiffness, and directly governs handling. An underinflated tire deforms under load: the bike becomes soft, heavy, and vague through direction changes, weaves, and the tire overheats and wears abnormally (on the shoulders). An overinflated tire reduces the contact patch: the bike becomes twitchy, skittish, and loses grip and stability over bumps. An imbalance between front and rear pressure also upsets handling. Correct pressures are given by the manufacturer (a sticker near the swingarm or the manual) and differ front/rear and by load (passenger, luggage). The check must be done cold (riding heats the air and skews the reading), with a reliable gauge, and it's a basic job. Simply resetting pressures to spec very often restores healthy handling. If a tire keeps losing pressure, it has a leak (valve, rim, puncture) to address. Important: if pressure is correct but instability persists, look further (worn/deformed tires, steering bearings, chassis). Checking pressure regularly (every two weeks) is a fundamental safety habit.

Since tires are the only contact with the road, their condition directly governs stability. A worn tire often takes on a "squared-off" profile (flattened in the center from motorway miles): the bike becomes reluctant to lean, stands up in corners, and feels unstable at the tipping point. Facet or scalloped wear (from incorrect pressure, tired suspension, or misalignment) causes weaving and vibration. A deformation (a bulge after an impact), a cut, or rubber hardened by age (beyond roughly 5 years, even with little tread wear) all degrade grip and handling. This is a common cause of instability, especially on a bike that's covered a lot of miles or whose tires are old. Visual inspection is accessible (tread depth against the wear indicator, profile, deformations, age via the DOT code), but replacement (fitting + balancing) needs machinery and is done by a professional: hence the intermediate rating. Follow the recommended sizes, ratings, and type, and both tires are often replaced together for consistent behavior. A worn, deformed, or too-old tire should be replaced without delay, both for stability and for grip and braking. Distinguish this from simple incorrect pressure (rule that out first): here, even properly inflated, the tire is worn out or damaged.

Warning: the steering head bearings connect the fork triple clamp to the frame and let the handlebar pivot smoothly and without play. From constant straight-line riding, the balls or rollers mark the raceways at the center position: a "notch" forms at center (the steering catches in a straight line and wants to stay there), a source of tank-slapping and vagueness. Conversely, loose bearings (insufficient preload, or wear) create play that's felt under braking (a jolt through the fork) and encourages tank-slapping. This is a classic cause of instability at low speed and when the bars are lightly released. Diagnosis is accessible: with the front wheel raised, slowly turn the bars to feel for a notch or binding at center, and rock the fork back and forth to detect play. Because the steering is a major safety component (the expert-only safety lock rule), the repair — adjusting preload to spec, or replacing the bearings (removing the top clamp, races, fitting and greasing new ones, adjusting) — affects steering geometry and needs tools, method, and precise torque specs: it's rated expert-only and left to a professional. As long as the steering binds or tank-slaps noticeably, be very cautious, or don't ride at all: a tank-slapper at high speed is dangerous.

Warning: the chassis and suspension are major safety components, and their diagnosis is reserved for a professional (expert-only). Several faults in this family destabilize the bike: a leaking fork (seals) or a bent one, a dead rear shock (hydraulics gone, sagged spring) that no longer controls the wheel, play in the swingarm or its bearings/linkages, or misaligned geometry / a bent frame (often after a crash, even a minor one). Symptoms show up mainly over bumps, in hard cornering, and under braking: the bike bounces, wallows, lacks precision, or weaves. Suspension's job is to keep the tire in contact with the road and absorb irregularities; when it no longer does that, handling and grip degrade dangerously, and tires can develop facet wear. Diagnosis (measuring play, checking suspension condition, verifying alignment and geometry) and repairs (servicing/replacing the fork or shock, swingarm bearings, checking the frame) require specialized tools and experience: leave it to a professional. Before anything else, rule out the simple causes (tire pressure and condition, steering bearings). Chassis-related instability should never be ignored: it can make the bike dangerous, especially at high speed and on poor road surfaces.

Repair to be left to a professional.