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Front fork oil leak

You notice a greasy film of oil on the shiny fork tube, a deposit running down toward the caliper, or a front wheel that dives and rebounds abnormally under braking. The telescopic fork is a hydraulic damper: inside each leg, oil flows through valves to slow the wheel's movement, and a fork seal (oil seal) keeps that oil from escaping along the stanchion. When the seal hardens with age, gets nicked on a spot of rust on the tube, or lets oil past because a grain of grit has lodged under its lip, oil weeps out: the fork loses damping, dives more under braking, bottoms out on bumps and — more seriously — throws oil onto the disc and pads, dangerously reducing braking. A leaking fork is an MOT failure and a real hazard: treat it quickly. Replacing the fork seals is within reach of a methodical DIYer on many models, but a pitted or bent tube (after a crash) is a job for a professional.

Possible causes: front fork oil leak

These are the most common causes, from most to least likely. The guided diagnosis above helps you find yours, test by test.

  1. Worn fork seals

    Intermediate

    The fork seals (oil seals) cap the top of each fork leg and rub constantly on the chromed stanchion that slides inside. Over the miles, UV and compression cycles, the rubber lip hardens and loses its seal; a single speck of dust under the lip is also enough to cause a sudden leak even when the seal is still good. Typical signs: an oil film running down the tube, a dust boot soaked with oil, a level that drops in one leg (the fork sags on one side), soft damping. First free reflex before dismantling: clean the tube, slide a cleaning film (a 'seal mate' tool or the corner of a piece of photographic film) under the lip to dislodge the debris, compress the fork several times and re-test — sometimes the leak stops. Otherwise, replace the fork seals and dust boots, and change the fork oil.

  2. Pitted or scratched fork tube

    Intermediate

    The stanchion must be perfectly smooth and chromed: it's where the fork seal makes its seal. A pit of corrosion (a bike stored outside, flaking chrome), a vertical scratch (a flung stone, a dust boot holding grit like a file) or an impact damage that surface, and no new seal will hold on it for long: the leak comes back after a few hundred kilometres. Locate the defect by running a fingernail down the tube with the seal removed: if the nail catches, the tube is at fault. An isolated micro-pit can sometimes be salvaged (special resin filling, gentle polishing), but degraded chrome means replacing the tube or re-chroming it — a workshop job.

  3. Worn or incorrect fork oil

    Intermediate

    Fork oil oxidises and fills with wear particles over time; it loses its viscosity and no longer damps properly, giving a fork that pumps, bottoms out or bounces. A level that's too low (after an untreated leak) produces the same effects and a sucking noise. It isn't strictly a leak, but it's the common cause of a 'soft' fork with no visible oil: on many bikes the oil has never been changed since the factory. A fork oil change with the manufacturer's viscosity and volume (often SAE 10 or 15, level measured in mm from the top of the compressed tube) restores like-new damping.

  4. Bent fork or triple clamp (after a crash)

    Expert

    After a crash or a frontal impact, a fork leg, the wheel axle or the triple clamp (the part linking the two tubes to the steering column) can be slightly bent. The signs: handlebar out of line with the wheel, tubes that no longer slide freely, a leak on one side only, abnormal tyre wear, a bike that 'pulls' to one side hands-free. It's a critical safety point: a bent fork degrades handling and can fail. Geometry checking and replacement of bent parts must be done by a properly equipped professional — never try to straighten a tube yourself.

    Repair to be left to a professional.