Pas de la Combe d'Enfer

Overview

Pas de la Combe d'Enfer is a hors catégorie climb that leaves no room for negotiation. Over 16.1 km it averages 10.14%, hauling you from a start elevation of 732 m to a summit at 2367 m. That is 1635 m of net elevation gain, with a total ascent of 1571 m once the small undulations are accounted for — and a maximum gradient of 31.81% that ranks among the steepest pitches you will ever point a road bike at. This is a climb that rewards patience early and punishes ambition often. At more than double-digit average grade sustained for over an hour of climbing for most riders, it belongs in the same conversation as the hardest categorized ascents anywhere.

How the Gradient Unfolds

The headline number is the 10.14% average, but averages hide the story. A 16.1 km climb that averages that steeply, yet spikes to 31.81%, is a climb built on ramps rather than a smooth escalator. Expect long stretches near or above the average punctuated by savage kicks where the road tilts far past what any seated rhythm can absorb. The gap between the 10.14% average and the 31.81% maximum tells you the surface is uneven in effort: there is no true recovery, only sections that are merely very hard between the sections that are brutal. Because the net gain (1635 m) exceeds the total ascent figure's steadiness, you can treat almost every metre as climbing — this is not a rolling road with a hard finish.

Pacing Notes

The single biggest mistake on the Pas de la Combe d'Enfer is starting too hard. With 16.1 km at 10.14% ahead, your early effort sets a ceiling you will pay for near the 2367 m summit. Anchor your pacing to your FTP: on a climb this long, most riders should sit meaningfully below threshold and save headroom for the 31.81% ramps, where power will spike whether you want it to or not. Ride the steep pitches as short, controlled surges and settle back immediately afterward rather than chasing a lost rhythm. If you are unsure of your numbers, run them through the FTP estimator before you commit to a target — guessing high here means blowing up with hundreds of metres still to climb.

Gearing

Do not under-gear this climb. With a 10.14% average and a 31.81% maximum stretched across 16.1 km, gearing that feels generous on shorter ascents will leave you grinding to a standstill. A compact 34-tooth chainring is the sensible front, and on the cassette a 34×32 is the minimum I would recommend for a fit rider — and even then only if you are confident holding cadence on the steepest ramps. For most riders tackling 1635 m of climbing at this pitch, a 34×34 is the smarter choice, giving you a spinning gear to survive the 31.81% sections without wrenching your knees or stalling. If your bike allows an even easier ratio, this is the climb to fit it.

Summary

The Pas de la Combe d'Enfer is a proper HC test: long, relentless, and steep enough at its worst to stop you dead. Respect the 16.1 km, gear low for the 31.81% ramps, pace off your FTP, and let the 2367 m summit come to you.

Length: 16.1 km
Average gradient: 10.14%
Max gradient: 31.81%
Start: 732 m
Summit: 2367 m
Net gain: 1635 m
Total ascent: 1571 m
Category: HC