Many models come with air fork caps from the factory and S&W, among others, made aftermarket caps. IMO it was a cheap fix to make dampening rod forks "feel" better. I have a Yamaha XT350 dual sport that needs 7+psi in the fork to handle correctly.
Part of the compression dampening of a fork is controlled by the amount of air above the oil, hence many fork oil specs are measured in mm from the top of the tube, not cc's. As the fork compresses over a bump the oil is forced up in the leg, "squishing" the air above. Oil can't be compressed, while the air can. Adding air pressure - usually no more than 10 PSI - makes the fork compress more slowly, and adds some faster rebound. It's usually a noticeable improvement. Air should never be added with a compressor, only with a small pump designed for the application.
Perhaps someone can chime in with an appropriate PSI setting?
Those against air caps say the pressure causes seals to fail. I find this to be highly unlikely and more likely caused by a mechanical reason or excessive pressure.
I hope this makes sense and helps!
