Col du Blancsex

Overview

The Col du Blancsex is a hors catégorie climb, the hardest rating there is, and the numbers explain why. Over 13.3 km the road lifts you from 390 m at its base to a 1559 m summit, a net gain of 1169 m. The average gradient is a relentless 8.77%, and it spikes as high as 15.15%. This is not a climb you ease into and settle. From the first kilometre the slope is already steep, and it stays steep for the better part of an hour. With 1148 m of total ascent packed into a short 13.3 km, there is almost no false flat or recovery to break the effort, which is exactly what earns a road the HC label.

How the Gradient Unfolds

Compare the two elevation figures and the story becomes clear: the net gain of 1169 m is larger than the total ascent of 1148 m, which tells you the road climbs almost without pause. There are no meaningful descents or flat sections to give your legs a break. The 8.77% average is the whole climb in a single number, but the 15.15% maximum warns that some pitches are far harder than the mean. Expect the gradient to swing between sustained high-single-digit ramps and short walls approaching that 15.15% ceiling. On a climb this uniform, the danger is going too hard early and having nothing left when a steep ramp arrives late.

Pacing Notes

The smart way to ride the Blancsex is to hold a steady, sustainable effort tied to your threshold rather than chasing the steep ramps. Set your target around your FTP and let the steep pitches push your power up briefly without redlining. If you do not know your threshold power, run the numbers through the FTP estimator before you commit to a pace. On a 13.3 km wall averaging 8.77%, going five percent too hard in the opening kilometres will cost you far more than five percent later. Aim to arrive at the steepest sections with something in reserve, because a 15.15% ramp late in the climb will punish anyone who has already emptied the tank.

Gearing

Gearing is not optional here, it is survival. For most riders a compact 34-tooth chainring paired with a wide cassette is the baseline. A 34×32 is the minimum I would take onto a climb averaging 8.77% with pitches at 15.15%, and even strong riders will be grateful for it on the walls. If you climb at a lower cadence, carry extra fatigue, or simply want to protect your knees over 1169 m of gaining, step up to a 34×34. The extra range costs you nothing on the steep sections and buys you the ability to spin rather than grind, which matters over a full hour of climbing.

Summary

The Col du Blancsex is a pure HC test: short, steep, and unbroken. There is no rhythm section to hide in and no descent to recover on, just 8.77% on average for 13.3 km with ramps to 15.15%. Pace it to your threshold, gear low, and respect the summit.

Length: 13.3 km
Average gradient: 8.77%
Max gradient: 15.15%
Start: 390 m
Summit: 1559 m
Net gain: 1169 m
Total ascent: 1148 m
Category: HC