Col de l'Au

Overview

Col de l'Au is the real thing: an HC climb that earns its "hors catégorie" label the honest way, through sheer relentlessness. Over 20.6 km it averages 7.9%, dragging you from a valley floor at 384 m all the way to a summit at 2015 m. That is a net elevation gain of 1631 m, with 1608 m of total ascent packed into a single sustained effort. There are no throwaway kilometres here — at nearly 8% average across more than twenty kilometres, this is an ascent that rewards patience and punishes ego.

How the Gradient Unfolds

The number that catches the eye is the 18.47% max gradient, but the truer story is the average. A 7.9% mean over 20.6 km means the road rarely relents into anything resembling recovery. Where the max spikes toward 18.47%, expect short, brutal pitches that force you out of rhythm and up out of the saddle. Elsewhere the grade settles closer to the average, which on this climb still means a wall of steady resistance. With 1631 m of net gain but 1608 m of total ascent, almost every metre climbed stays climbed — there is very little descending to break the effort. Treat every steep ramp as a test of composure rather than a place to attack.

Pacing Notes

On a climb this long and this steep, pacing is everything. Your single best guardrail is your FTP — the sustainable power you can hold for a long effort. A 20.6 km ascent at 7.9% will take most riders well beyond an hour, so plan to sit comfortably below FTP for the early kilometres and keep something in reserve for the 18.47% ramps that will spike your power whether you like it or not. If you do not know your number, run it through the FTP estimator before you go, then set a target you can defend from 384 m all the way to 2015 m. Going out too hard in the first few kilometres is the single fastest way to blow up on a climb with 1631 m still to gain.

Gearing

Do not underestimate the gearing you need here. With a 7.9% average and pitches to 18.47%, a compact chainring paired with a wide-range cassette is the sensible starting point. A 34×32 will get many strong riders up, but on the steepest ramps late in a 20.6 km effort, that ratio can feel punishing once fatigue sets in. If you have any doubt about your climbing legs — or you want to protect your knees over 1608 m of ascent — go with a 34×34. The extra range lets you spin the 18.47% sections rather than grind them, and on a summit finish at 2015 m, cadence you can sustain beats a gear you can barely turn.

Summary

Col de l'Au is a proper HC test: long, steep, and unforgiving from bottom to top. Respect the 7.9% average, fuel for a long effort, gear low, and let the summit come to you.

Length: 20.6 km
Average gradient: 7.9%
Max gradient: 18.47%
Start: 384 m
Summit: 2015 m
Net gain: 1631 m
Total ascent: 1608 m
Category: HC