Coll de So

Overview

Coll de So is a hors catégorie climb, the hardest rating on the scale, and its numbers explain why. Over 14.4 km the road hauls you from a start elevation of 738 m to a summit at 1983 m — a net elevation gain of 1245 m. The average gradient sits at a punishing 8.67%, and that figure is not softened by long flat interludes: this is a sustained, relentless ascent where the slope rarely gives you room to breathe. With a maximum gradient of 17.43%, the climb reserves the right to hurt you at any moment. Total ascent measured across the route comes to 1220 m, confirming that almost every metre of the 14.4 km is spent going up.

How the Gradient Unfolds

An 8.67% average over 14.4 km is the kind of number that reshapes a ride. Because the mean is already so steep, the flatter sections — if you can find them — are still hard work, and the presence of a 17.43% max means there are ramps where the gradient nearly doubles the average. That 8.76-point gap between average and maximum is the story of the climb: expect stretches where you settle into a rhythm, punctuated by pitches that force you out of the saddle or deep into your reserves. With 1245 m of net gain packed into 14.4 km, the thin air near the 1983 m summit adds its own tax on the closing kilometres.

Pacing Notes

On a climb this long and steep, pacing is everything. Ride the first kilometres well below your limit — the FTP you can hold on the flat is not the effort you want to chase here, because 14.4 km at 8.67% will expose any early overreach. Aim to keep a steady power on the sustained sections and treat the 17.43% ramps as brief surges, not as your new baseline. If you do not know your threshold, run the numbers through the FTP estimator before you go, then leave a comfortable margin. Climbing 1245 m from 738 m to 1983 m rewards patience: the rider who paces the lower slopes conservatively is the one still turning the pedals cleanly near the top.

Gearing

Gear for survival, not for pride. On a 14.4 km HC climb averaging 8.67% with 17.43% ramps, a compact 34-tooth chainring paired with a wide cassette is the sensible baseline. A 34×32 will get most fit riders up the sustained sections, but the steepest pitches are where it starts to feel marginal. If you have any doubt about your climbing legs, fit a 34×34 (or wider) so you have a bailout gear for the 17.43% walls and the thin air near 1983 m. The extra ratio costs you nothing on the average slope and saves your knees when the road kicks up.

Summary

Coll de So is a proper HC test: long, steep, and unforgiving from 738 m to 1983 m. Respect the 8.67% average, fear the 17.43% ramps, and gear low enough to keep spinning through all 1245 m of climbing.

Length: 14.4 km
Average gradient: 8.67%
Max gradient: 17.43%
Start: 738 m
Summit: 1983 m
Net gain: 1245 m
Total ascent: 1220 m
Category: HC