Col des Ayes

Overview

The Col des Ayes sits in the Hors Catégorie bracket — the hardest classification a road climb can receive — and the numbers justify it immediately. Starting at 229 m elevation, the road climbs 1309 m net to the summit at 1538 m over 16 km, with a reported total ascent of 1929 m. That gap between net gain and total ascent signals something important: this is not a clean, unbroken ramp. Expect at least some undulation embedded within the overall grind, meaning you will surrender hard-earned elevation at points before the road kicks upward again.

The average gradient of 8.18% is already demanding across 16 km. Sustain that for the full duration and you are looking at a serious FTP-limited effort from the first kilometre. Use the FTP estimator to establish your watt targets before attempting a paced ascent.

Gradient Character

The headline statistic that demands the most respect is the 37.89% maximum gradient. That figure is not a misprint. A 37.89% ramp is beyond what most riders can sustain for more than a few pedal strokes, regardless of fitness. At that pitch, traction, body position, and gearing all become critical simultaneously.

With an 8.18% average spread over 16 km, the arithmetic tells you the climb cannot maintain anything close to 37.89% for extended stretches — the maximum is almost certainly a short, sharp wall, possibly a switchback ramp or a brief urban or village section near the base or mid-climb. Do not let the average lull you into under-gearing for those ramps, however. The difference between 8% and 38% is the difference between a hard tempo effort and a standing scramble.

What to Expect Section by Section

• Lower slopes (0–5 km): Starting at 229 m, the early kilometres set the tone. With a low start elevation, the road likely passes through village or valley terrain before the gradient establishes itself. Use this phase to settle your breathing and resist the urge to overcook the effort.
• Mid-climb (5–11 km): This is where sustained 8%+ pitches are most likely to accumulate. The total ascent figure of 1929 m versus the net 1309 m gain suggests some rolling sections here — treat any brief descent or flat as active recovery, not an invitation to sprint.
• Upper climb (11–16 km): The final 5 km to the 1538 m summit will test accumulated fatigue. If the extreme gradient sections appear here, your legs will already be carrying significant load. Shift to your smallest gear early rather than fighting to change under load.

Pacing & Gearing Notes

Pacing: An HC climb at 8.18% average across 16 km is not a climb to attack from the bottom. Target a power output you can hold for 60–90+ minutes depending on your fitness level, and accept that the maximum gradient sections will spike briefly above any sustainable threshold. The goal is to limit time above threshold everywhere except those unavoidable wall sections.

Gearing: A compact chainset (50/34 or 46/30) paired with an 11-32 or 11-34 cassette is the minimum sensible setup. On a 37.89% ramp, even a 34/34 combination will feel heavy. Riders using 1x drivetrains should confirm their low-end ratio before committing to the climb. Running out of gears on an extreme gradient mid-climb is a race-ending or injury-risking scenario.

Cadence: Aim to keep cadence above 70 rpm on the moderate sections to preserve muscular endurance. On the steepest ramps, a slower, more forceful cadence is inevitable — focus on smooth pedal stroke rather than grinding square circles.

For comparison with other HC climbs in France, see the Alpe d'Huez profile.