Col du Port de Belhay

Overview

The Col du Port de Belhay is an hors catégorie climb that refuses to negotiate. Over 15.5 km it averages a punishing 9.09%, hauling you from a valley floor at 324 m all the way to a 1729 m summit. That is a net elevation gain of 1405 m, with 1325 m of total ascent packed into a single, relentless push. There is no long false-flat approach to hide behind here — an average approaching double digits over that distance means the road tips up almost immediately and rarely relents. Add a savage 21.09% maximum pitch and you have one of the hardest sustained efforts a cyclist can attempt.

How the Gradient Unfolds

The defining feature of Belhay is its consistency of severity. With an average of 9.09% held across the full 15.5 km, there are few genuine recovery zones — the climb keeps you honest from bottom to top. The gap between the 9.09% average and the 21.09% maximum tells the real story: on top of an already brutal baseline, the road throws ramps that more than double the effort. Those spikes are where races are won and where amateurs are broken. Because the summit sits at 1729 m and the total ascent (1325 m) is slightly below the net gain (1405 m), the profile trends upward almost without pause, offering little in the way of descending relief mid-climb.

Pacing Notes

On a climb this long and this steep, discipline is everything. Anchor your effort to your FTP rather than to how you feel in the first kilometre — the temptation to chase early is what blows most riders up before the halfway mark. Aim to sit at or just below threshold on the sustained 9.09% sections, then accept that the 21.09% ramps will spike you well above it for short bursts; the trick is recovering quickly once each pitch relents rather than trying to smooth them out. If you are unsure where your ceiling sits, run the numbers through the FTP estimator before you commit, and set a wattage cap you can defend for the full 15.5 km.

Gearing

Do not under-gear this climb. On an HC ascent averaging 9.09% with ramps to 21.09%, compact chainrings and a wide-range cassette are non-negotiable for all but the strongest riders. A 34×32 is the minimum sensible setup, and even then you will be grinding on the steepest pitches. For most riders — especially anyone not racing — a 34×34 buys you the extra cadence that keeps your knees and your composure intact when the road hits 21.09%. When in doubt, take the easier gear; spinning a 1405 m ascent is always smarter than muscling it.

Summary

Col du Port de Belhay is a true hors catégorie test: long, steep, and unforgiving from base to summit. Respect the average, fear the maximum, gear conservatively, and pace to your threshold — do that, and the 1729 m summit is yours to earn.

Length: 15.5 km
Average gradient: 9.09%
Max gradient: 21.09%
Start: 324 m
Summit: 1729 m
Net gain: 1405 m
Total ascent: 1325 m
Category: HC