hey cool thread!

a trail rating system is a current topic here in Germany/Austria. the VertRiders, a group of Freeriders in Innsbruck Austria, namely one guy Willi Hofer who wrote a guide book, developed a singletrail rating system for extremely technical trails a few years ago, with levels 0-10, where most people would consider levels above 3 "unrideable".

now this system has been adapted and in cooperation with one of the two big MTB magazines, BIKE they're trying to make it a standard in the Alps/Europe.

the link (in German) is: http://www.singletrail-skala.de/. although the photos are a help even if you can't read German.

basically the idea is such:
* as opposed to a ski trail rating (Green/Blue/Black in NorthAmerica and Blue/Red/Black in Europe) where the rating covers "everything", the rating is only a rating of the TECHNICAL difficulty similar to a rock climbing rating (various systems like UIAA 1-11 but all basically same where approach, exposure, length, safety and existing placements are separately dealt with and not in the rating) the Single-Trail-Raing DOES NOT rate the exposure, danger, etc. or the extra aspects like length, time, heat, water, etc., but merely rates the TECHNICAL difficulty, levels S-0 to S-5, where a street, fireroad or doubletrail does not get a rating as it's not a singletrail!

* S-0 - flowing singletrail with no major obstacles
* S-1 - singletrail with root and rocks, but maximum steepnes 40% and no hairpin curves, no extemely loose gravel, no drops, etc.
* S-2 - singletrail with big rocks, sharp curves, many roots, loose gravel, stairs or small ledges, max steepnes 70%, many sharp curves
* S-3 - steep with major obstacles, big rocks and boulders, many exposed roots, big drops and ledges, steepness often over 70%, loose and eroded ground, sharp curves, steep off-camber sections, etc.
* S-4 - very steep with multiple major obstacles: very steep and blocked trail with very technical root and ledge sections; trials maneuvers such as hops and/or side-setting of rear wheels are necessary to clear obstacles or sharp corners
* S-5 - "unrideable": large drops in series, big drops with sharp curves, eroded, loose and exposed ground, VERY steep (cliffs), multiple obstacles, boulders, cliffs, etc.

this can be roughly translated to the Green/Blue/Black scheme:
S-0, S-1 = Green
S-2, S-3 = Blue
S-4, S-5 = Black

Note that traditional XC ends with S-2 after which a trail is no longer flowing
but here a BLACK is REALLY hard!!

the scale is also "open-ended" meaning that as riders improve and previously "unrideable" trails become rideable, the scale can be extended to handle it (i.e. S-6 could be added and a distinction between S-5 and S-6 made while existing S-0 through S-4 trails do not need to be changed)

the system is designed with the idea of being used in conjunction with a text description of the trail describing length, exposure, danger, aerobic challenge (uphill riding) with the S-rating describing the TECHNICAL difficulty of the SINGLETRAIL sections - with a general rating plus a section ratings: e.g. trail is S-2 with 3 short S-4 passages.

the scale is also bascially designed for technical trails and does not really attempt to rate jumps or stunts although it can be extended for basic skinnies, ladders, drops and such.

My personal note is that such a system on an International scale would be awesome as i could go to Moab or Whistler and get a description of "extreme" trails and use the scale as a reference what the person meant by "extreme" and thus find something rideable (for me drops over 3ft and cliffs are not good (heavy S-4/S-5), but "unrideable" technical trails with huge boulders, rock gardens and hairpin curves - S-3/border S-4 - is what i love!)