Wear-based maintenance/overhaul of air spring suspension fork

Question

Assuming usage as a mountain bike, an air spring suspension fork on a hardtail is to be maintained every 50 hours of riding.

By comparison it's sufficient, and plenty reasonable, to replace a chain ridden in a clean environment, regularly run through a scrubber and lubricated, after some specific wear (0.5% stretch) is reached, rather than after some riding time or riding distance—even if that stretch is reached after 10,000 or 15,000 km rather than 2,000 or 3,000 km.

For a hardtail ridden more often on snow/ice-covered roads with the suspension locked than on trails with the suspension unlocked, is there a wear indication that will determine when it's time for maintenance or overhaul—an indication that skips the 50-hour recommendation?

Motivation

The motivation for the question is this: riding at a minimum three times a week, the first two for two hours each and the third for four hours, means reaching 50 hours after six weeks.

Now suppose that this is one maintenance task I’m shying away from doing myself (because, as with bleeding hydraulic brakes, I don’t want to find a puddle of oil on my indoor floor—nor do I want to discover what brake/suspension fluids do to the lacquer), does that mean I should go back to the shop every six weeks seeking maintenance (or can I lock out the fork when I either don’t need or want suspension, and extend this interval by relying on some test)?

Answer 1

For starters, I think it's unfair to make a comparison with chains. Your chain is a fairly cheap part that is designed to be replaced frequently. Unless your wallet is overflowing, suspension is generally not regarded in the same way. Another important distinction to be made about any sort of "wear indicator" is that they're retrospective, i.e. they tell you when damage has already been done. Again, this is not something you want to be relying on when your overall goal is to minimize damage to the suspension.

Theoretically, you could indeed determine fork service intervals quantitatively. You could get a bath oil sample and chemically test it for impurities, you could use a very high-quality micrometer on the stanchions and measure for nanometers of wear, you could use a high-power microscope to inspect the seals...I think it's fairly obvious that just servicing the fork every 50 hours or whatever is far easier. You'd have to pull the fork apart to perform those checks anyways, so you might as well just change the oil while you're in there. You don't have to change out all the seals and rebuild the damper every 50h. Cleaning out any dirt and changing the bath oil is perfectly adequate.

You are correct in that the 50 hour thing is a statistical average, which would represent moderate-difficulty mountain biking in a fairly dirty environment and with a maintenance-proactive rider. For your use case, yes, I would be confident extending the service interval due to your riding environment (reduced concentration of particulates).

I'm not sure if riding with the fork locked out is any better than riding with it open. For one, you don't get as thorough splash lubrication of the seals and other internals. Wear would also be concentrated in one spot on the stanchions, and any dirt trapped under the dust seals wouldn't be able to safely fall into the lowers. Also, your damper would see more abrupt forces.

Answer 2

There are no wear indicators on these items. You'd have to pull the lowers/air can and inspect the seals, bushings, and other wear components yourself.

Personally, I keep a little spreadsheet of my MTB rides that includes the conditions of the trail and the "shock time" of the ride. For my purposes, "shock time" is any portion of the ride when the high-speed damping circuits are in use. So if I go on an MTB ride that's 2 hours of spinning up a fireroad and then 45 minutes of riding down the trail, I'll mark that 0.75 hours of shock time. If things are particularly muddy or dusty, I'll fudge that number higher. If I'm riding through hero dirt so good that my chain looks exactly as clean as it was at the start of the ride, I'll fudge those numbers down.

I can't say my system is perfect, but the shop I use for servicing my suspension has always said I brought my bike in at a good time (i.e., before it really needed service).

I'll also note that depending on the fork/shock, riding locked out can be harder on the component than riding with it open. I might partially close the low speed circuits for a two-hour fireroad slog, but there's rarely tangible benefit to fully locking things out in my neck of the woods.

Answer 3

The wear indicator for a fork is the coating on the stanchions has worn though, meaning a very expensive repair requiring new uppers. Unlike a chain, these parts are not considered consumables by most riders, and it can be more economical to replace the fork. The degradation of a fork performance though poor maintenance is significant, but mostly goes unnoticed as they slowly become less compliant as stiction and friction increases. You can only tell if a fork needs a service if you know what it should feel like. If you know what it should feel like, you are probably the kind of rider who services it regularly to maintain its performance. The service is best done to some kind of schedule.

When you don't maintain a chain (which is cheap) it wears a bit faster, gets a bit noisy and shifts become degraded. In the extreme it takes out the cassette, but still relatively cheap and considered a consumable. Unlike a fork, just looking at the chain and spin around a car park will tell you its time for maintenance, Clean and lube it, its pretty much back to go.

Locked out forks on MTB's rarely lock out- there is always a small amount of movement - even if just a few mm. This movement is concentrating all the wear on the stanchions to that small area. Therefore IMHO, locking out forks should not be used as a valid reason to extend the service interval.

How religious you are on servicing interval is really the question - a top rider on expensive forks wanting the very best performance should be servicing regularly, possibly more often than recommended. A casual rider whos less worried about fork performance should still be servicing the forks, but can probably extend it out a bit. I am not going to make a recommendation how much, as it depends on riding conditions, tolerance for replacing expensive parts, how much a service costs (time taken - DIY or taking bike to shop, dollars etc).