Col de la Seigne

Overview

Col de la Seigne is a genuine hors catégorie (HC) climb, the hardest rating on the scale before you drop back to categories 1 through 4. The numbers make the case on their own: 27.6 km of continuous ascent at an average gradient of 7.8%, starting at 581 m and topping out at 2735 m. That is a net elevation gain of 2154 m, with 2420 m of total ascent once the small descents and false flats are counted. Very few climbs combine that much length with that steep an average, and the summit altitude alone puts it deep into thin-air territory. This is a full-day objective, not a warm-up.

How the Gradient Unfolds

An average of 7.8% held over 27.6 km is relentless by definition — there is no long valley run-in hiding inside these figures. The gap between the 2154 m net gain and the 2420 m of total ascent is only 266 m, which tells you the profile rarely gives anything back. In practical terms, that means almost no descending and very little genuine flat: what dips exist are brief, and you pay them back immediately. Expect the effort to feel steady rather than punchy, with the sting coming from duration and altitude rather than from short walls. As you climb past 2000 m toward the 2735 m summit, thinner air quietly compounds the sustained 7.8% grade, so pace that feels comfortable low down will cost more near the top.

Pacing Notes

On a climb this long, pacing discipline decides everything. Anchor your effort to your FTP and hold a sustainable percentage of it from the very first kilometre — going out even slightly too hard over 27.6 km compounds into a serious blow-up near the summit. Because the average never relents at 7.8%, treat the whole ascent as one long time-trial effort rather than a series of sprints. If you are unsure of your current threshold, run the numbers first with the FTP estimator so your target power reflects your real fitness. Factor in the altitude too: above 2000 m your usable power drops, so plan to ride the upper slopes a notch below the wattage that feels right lower on the mountain.

Gearing

For an HC climb of 27.6 km averaging 7.8% and finishing at 2735 m, gear low and gear generously — you will be turning the pedals for a long time and fatigue makes every gradient feel steeper. A compact 34-tooth chainring is the sensible front choice. Pair it with at least a 34×32 for a low bail-out ratio, and strongly consider 34×34 if you have it: the extra cog preserves cadence deep into the climb and protects your legs across those final high-altitude kilometres. When in doubt on a climb of this size, err toward the easier 34×34.

Summary

Col de la Seigne is a long, sustained, high-altitude HC test that rewards patience and punishes overreach. Respect the length, meter your effort against your threshold, and carry a low enough gear to keep spinning to the top.

Length: 27.6 km
Average gradient: 7.8%
Max gradient: n/a
Start: 581 m
Summit: 2735 m
Net gain: 2154 m
Total ascent: 2420 m
Category: HC