To classify a type of trail, we use the STS scale used internationally. This scale takes into account the type of terrain, obstacles, slopes, curves and the rider's driving ability.
Below is the STS SCALE:
S0 - a single trail that does not present any major difficulties. These are smooth paths through undergrowth, with firm ground and generally good grip. The slope of the trail varies from slight to moderate and the bends are quite wide. These types of trails can be ridden without the need of any particular technical skills.
S1 - Trails with small obstacles, roots and stones. The ground is not very compact and the slopes can reach 40%. The curves are tight and the obstacles can all be overcome with basic riding skills.
S2 - routes with larger roots, stones, steps and stairs in a sequence with little challenge. You will often encounter tight bends and slopes that can reach 70% in some sections. Obstacles can only be overcome with a fair amount of riding skill. The ability to brake at any time is necessary and also to measure it out precisely for this level.
S3 - single trails with many tricky parts. You will often encounter high steps, hairpin bends and difficult slopes and rarely smooth passages. The ground may be slippery and loose. These trails do not yet require the use of trialist technique, however, excellent bike control and continuous concentration are necessary. Good balance is essential.
S4 - very technical trails with particularly difficult root passages on terrain that is generally not very compact. The slope in some sections is almost extreme, with narrow bends and steps. Trial techniques, perfect braking technique and excellent balance are absolutely necessary.
S5 - Single tracks with very technical terrain with counter slopes, slippery surface, very tight hairpin bends, steps and obstacles such as fallen trees. All on slopes that are often extreme. Whoever rides this type of trail is a very experienced rider.