Mountain bike suspension forks come in a lot of sizes and brands but they have some fairly universal characteristics. They have a one-piece lower fork leg assembly forged in alloy, with sliders for the an upper assembly. The uppers have stanchions (tubes) that slide into the lowers, and a crown that attaches the stanchions to the steerer tube. The springs and dampers are both located inside the upper tubes and are operated by a piston shaft from the end of the lowers. The external seals you see between the lowers and the stanchions are dust wipers and sometimes oil seals, but hold no pressure.

There's a lot of press, yearly updates, and aftermarket modifications for premium forks like the Rockshox Yari and Pike and Sid. But those are premium mountain bike racing products. Stuff in our price range is simpler and the marketers don't pretend it's better every year. Still it's nice to know how it works.

The least expensive forks, usually from SR Suntour, have steel stanchions, and steerer, and a coil spring. They may have a cartridge that provides lockout or damping, but then again they might not. (There is a rubber part called a damper in their diagrams, but it seems to be a silencer to keep the spring from rattling.) There are a few smaller brands seldom seen in this price range like RST and Zoom. Upgrades from here are mostly about weight (air spring, aluminum parts), and adding damping with adjustments. In top end commuter bikes or midgrade mountain bikes, most likely a buyer will find Rock Shox, and there are some better Suntour forks that show up too.



Coil springs perform really well. Specifically, for this application, they do not pressurize any seals, so they have almost no running friction. But they are a big piece of steel, so for weight savings most nicer forks use air springs. Air springs usually have at least two chambers, positive and negative, on either side of the piston. In this price range they have a mechanism to equalize the negative chamber to the positive at top-out, so they can be pumped from the positive side alone. Air forks are tuned with pressure to suit the rider's weight and preference, just like tires, which is a benefit over coils, which need to be swapped if they are incorrect strength.



Damping, if present, is achieved by running oil through small orifices. Compression damping (when the fork moves up on impact) and rebound damping (when the fork springs back out) are usually done with separate "circuits" where the oil is metered for the desired force in one direction and WFO in the other. It seems intuitive that both would be on the piston, but in Rockshox and most other forks they are separated. The rebound circuit is on the piston, freely moving up and metered going down. The compression circuit is in the top end of the stanchion and meters the oil displaced up by the incoming piston shaft. Most forks with damping come with rebound adjustment at a minimum. Rebound is the most important adjustment, because it needs to be matched to the spring rate and that keeps the chassis under control. However, because it's the second thing that happens, it has less influence on the feel of the fork, which is why there is a lot more attention paid to engineering and marketing the compression damping.

If the fork has a lockout function, it usually closes the compression damper down or off. (The Suntour HLO damper cartridge also blocks an oil path but does not provide any useful damping.) The lockout function may have a cable operated remote on the handlebar.

Air forks, and rear shocks too, are pressurized with a shock pump. This has a Schrader valve with a screw-on chuck that seals before it pushes the valve, to prevent leakage, so the pressure is correct. It has a small piston to achieve high pressure with precision. It could potentially be used on a tube but it would take a lot of pumping.