There's been a lot of controversy surrounding the use of air shocks for downhill riding. Some people say air isn't reliable enough to use as a spring because it heats up, and others say air shocks aren't reliable enough full stop. However, Fox's DHX 5.0 Air shock proves all these people wrong - it's reliable and it fills a hole in the market.
We've been running a stock DHX Air for some time now on a linkage frame, and we also have one on a single-pivot bike. It has the same features as the coil-sprung version - rebound, ProPedal, bottom out resistance and a position sensitive boost valve - so setting it up is just the same apart from having to fill the shock with air.
One of the main benefits of using an air spring is a substantial loss of weight. The DHX Air weighs about 450g (depending on exact size), whereas the equivalent DHX coil-over unit weighs nearly twice as much. Air also naturally allows the shock to ramp up progressively through its travel, resisting bottoming out.
In use, the shock is almost as plush as a coilover unit, and it doesn't feel a great deal different, which is a good sign. However, thanks to the leverage ratio on the linkage frame we were using, the shock felt a touch linear through the first half of the stroke, whereas on the single-pivot frame the feel was much more progressive, like that of a coilover unit. Through the rest of the stroke, however, the action was silky smooth with a flawless action - very impressive.
Our only doubt was whether the performance would be affected once the air heated up on longer runs, but according to our in-house Stig, there are no problems there. We've also been running the shock on a 6in travel Specialized Enduro Pro bike for quite a while now and we're seriously impressed with its performance.