Failure modes for suspension forks, air and coil

Question

An air suspension fork on one of my bikes starts showing its age: noticeable play, scratches on stanchions etc. It has not become less smooth and does not leak air so far, however.

I am starting to wonder how safe it is to continue using it, and what will happen when it "fails".

Possible outcomes of a non-catastrophic suspension fork failure that I can imagine are:

1. All air leaks out and it compresses fully, becoming very short
2. All air leaks out and it expands fully, becoming at its full extension
3. Air air blows out, making stanchions loose in the legs
4. The fork just locks at random compression position.
5. Stanchions become so loose with so much play that it becomes noticeable at braking and steering.

What of these options are real, both for air forks and coil forks? Are there other failure scenarios resulting from a fork's old age but not a crash?

Answer 1

Basically the air and oil seals get worn and leaky. You'll get less air spring pressure, droop and increasingly poor damping.

Of your options, 2 and 3 are not realistic. If the air spring pressure drops to zero (relative to ambient atmospheric pressure), there is no force to extend the fork. With no air spring pressure the stanchions will not slide freely on the legs because the oil based damping system is still working.

Answer 2

The air spring operates on the inside of the stanchion tubes, so is not affected by the scratches you can see on the out side surfaces. However the damage to the outside of the stanchions will allow dirt past the seals and increase the speed of wear on the dust seals. The dirt then grinds against the bushings wearing them out along with the stanchions and seals. Eventually that dirt will also make its way to the air spring and wear those seals, and into the damper affecting its performance. The air spring will start to leak and the forks will start to loose pressure slowly. If the leak is fast enough, it might blow the dust seals, although its more likely the air will just escape past the now worn seals.

The most likely scenario is the play in the fork increases to the point you are noticing it while riding, having to top up the air pressure in the shock and poor dampening performance, along with stiction (Forks stick a bit before they start moving). It will become massively unpleasant riding the bike long before risk of a catastrophic failure.

Replacing the stanchions is usually possible, although the cost is often prohibitively close to cost of new forks, especially bushings need replacing as well.

Doing more regular lower leg services and replacing seals will help extend the life of the forks, but once the dirt gets past the seals their life is limited.