STS - Single Track Scale
S0 describes a single trail without any particular difficulties. They are mostly forest or meadow paths on a natural surface with good grip, or compact gravel. Steps, rocks or roots are not to be expected. The gradient will be slight to moderate, curves are not tight. Even without any particular MTB technique trails with S0 can be managed.
On a trail marked S1 you will already have to deal with smaller obstacles such as flat roots and small stones. Very often you'll find that the odd gulley or erosion damage is the reason for the raised difficulty rating. The surface may not always be firm. The gradient would have a maximum of 40%. Hairpin turns are not to be expected. On an S1 trail basic MTB technique and constant attention will be required. The trickier passages call for dosed breaking and body displacement. They should generally be ridden in standing. Obstacles can be rolled over.
An S2 difficulty rated trails will most likely contain larger roots and stones. The surface is frequently loose. Steps can be expected. Often there are narrow curves and the gradient can be up to 70% in places. The obstacles require body displacement to be successfully ridden. Readiness to break at all times and the ability to shift your centre of gravity are necessary techniques as well as the ability to regulate the attack of your breaking and permanent body tension.
Blocked single trails with many larger boulders and / or root passages belong to category S3. There are often higher steps, hairpin turns and tricky traverses. The chance for some relaxed rolling is seldom. Often the surface is very slippery and with loose scree. A gradient of over 70% is not unusual. Passages with an S3 grade don't yet require special downhill technique, but do need very good command of biking and unbroken concentration. Exacting breaking and a very good sense of balance is required.
S4 describes very steep and blocked single trails with large boulders and / or demanding root passages, as well as frequently loose scree. Extremely steep slopes, narrow hairpin turns and steps on which the cogs unavoidably touch are a frequent feature of a category S4 trail. In order to ride a category S4 trail, trail techniques such as being able to shift the front and back wheels (e.g. in hairpins) are absolutely essential as well as perfect breaking technique and balance. Only extreme riders and exceptional bikers manage an S4 trail in the saddle. Even carrying a bike up such passages is often not without its dangers.
The difficulty grade S5 is characterized by a blocked terrain with counter climbs, scree slopes and erosion, extremely tight curves, multiple obstacles (e.g. fallen trees) occur without a break – frequently in extreme steepness (cliffs). Very little if any breaking time. Obstacles may at times need to be dealt with in combination. Only a handful of freaks tackle S5 passages. Some obstacles have to be jumped over. Hairpins are so tight that shifting the wheels is hardly possible, even carrying the bike would be almost impossible as you need your hands to hold on or even climb.