Fork Seals May Not Be What You Think They Are
It’s easy to imagine a fork seal doing something pretty simple: it presses against the stanchion, creating a barrier that stops the oil leaking out.
In reality it’s not that simple.
A fork seal normally operates with a microscopic film of oil between the lip and the stanchion. Without some lubrication, the lip would run with excessive friction and wear very quickly. The seal’s job isn’t simply to create a barrier; it controls how much oil is transported in each direction.
The exposed part of the stanchion that you can see may look completely dry, but there is normally an extremely thin film of oil on its surface.
What happens when the fork moves?
The thickness of the oil film beneath the seal lip is an important part of how the seal behaves.Film thickness is influenced by several factors, including stanchion velocity, oil viscosity, seal geometry and the pressure with which the lip contacts the stanchion.
As shaft velocity increases, the stanchion tends to drag more oil beneath the lip and the lubricating film becomes thicker.
At lower velocity, the film becomes thinner. As velocity approaches zero, the hydrodynamic contribution to the oil film becomes very small. The seal lip increasingly relies on the small amount of lubricant remaining between the lip and the stanchion, with some direct contact between the surfaces. The lubrication regime therefore moves towards mixed and boundary lubrication.
On a dyno, this can be seen as a turnaround spike in friction as the stanchion stops and changes direction. The effect can be more pronounced in compression, where the direction of movement increases the loading of the lip against the stanchion. At the instant of zero velocity there is no relative motion to generate hydrodynamic lift, so the friction required to restart the stanchion can be considerably higher than the friction while it was moving.
Why does a seal leak?
The lip geometry is designed so that the amount of oil transported inward is greater than the amount transported outward.
