Suspension oils

Two oils, two functions in the same fork

A suspension unit contains oil in two completely different places, and that’s the first thing to understand before ordering.

• The bath oil, in the lower legs, lubricates the stanchions, bushings and dust seals. It provides no damping. This is the oil replaced at the more frequent service, which restores a fork’s original smooth action.
• The damper cartridge oil, in the hydraulic circuit, is what provides damping. Its viscosity determines compression and rebound speed; changing it alters how the suspension behaves.

WT is not a standard

That’s the trap when choosing from this range. The viscosity rating — 2.5, 3, 5, 7, 10, 15 or 30 WT — is not comparable between brands : each manufacturer has its own measurement method, and one brand’s 5 WT oil can be noticeably thicker than another’s. The only reliable reference is the oil specified in your model’s manual, for the relevant model year. Choosing an approximately similar viscosity just because it has the same number is a sure way to end up with suspension that no longer responds as it did before.

Special cases

• Forks with air spring from some brands use a dedicated air-chamber oil in very small quantities; it is neither bath oil nor damper cartridge oil.
• The rear shocks have their own specified oils, often different from those for the fork on the same bike.
• The hydraulic dropper seatposts require a specific fluid — fork oil is not a substitute.
• Some forks with semi-open bath use motor oil of a specific viscosity, as specified by the manufacturer.

What size to buy

The bottles of 120 ml are enough for a lower-leg service or a top-up. The litre is for workshop use, for anyone maintaining several bikes or carrying out full services in succession — provided it’s used within a reasonable time, as oil ages once opened.

How often

Manufacturers specify service intervals in riding hours, not kilometres: around 50 hours for the bath oil and 100 to 200 hours for the damper cartridge, with much shorter intervals in muddy conditions. A timely lower-leg service costs little and prevents bushing wear, which is expensive to put right.

To find out more: How to choose your suspension oil

See also: Suspension tools · Suspension pumps · Bike lubricants and greases