Col des Hirondelles

Overview

The Col des Hirondelles is a giant. Over 33.6 km of continuous climbing it lifts you from 617 m at its base to a summit of 2478 m, a net gain of 1861 m earned one pedal stroke at a time. At an average gradient of 5.54% it never looks brutal on paper, but length is its weapon: this is an hors catégorie ascent, the hardest classification there is, and it demands respect long before the numbers start to hurt. The total ascent of 1828 m — slightly less than the net figure — tells you the road climbs almost without pause, with only the faintest relief along the way.

How the Gradient Unfolds

Do not be lulled by that 5.54% average. On a climb of 33.6 km an average is a blend, and here it hides a maximum pitch of 13.8% — well more than double the mean. That gap is the whole story of the Hirondelles. Long, sustainable stretches at or below the average are punctuated by ramps that spike toward the mid-teens, and each of those steep sections lands harder because it sits on top of an ever-growing pile of vertical metres. With the summit at 2478 m, the closing kilometres also unfold in thinning air, where the same wattage buys you less speed. Expect a rhythm-based effort broken by moments that force you out of the saddle.

Pacing Notes

On a climb this long, discipline beats bravado. The single biggest mistake is starting too hard on the gentler lower slopes and paying for it near the 2478 m top. Anchor your effort to your FTP and hold the early gradients slightly under threshold, banking energy for the 13.8% ramps and the altitude in the final third. If you are unsure of your threshold, run the numbers through the FTP estimator before you commit to a target power. Because the total ascent is 1828 m, sustained hours of climbing are guaranteed — think of it as a long tempo effort with surges, not a series of short attacks. Fuel early and often; the metabolic bill for 1861 m of net gain comes due whether or not you feel it at the start.

Gearing

Come geared for the long haul. The 5.54% average is manageable, but the 13.8% maximum arriving deep into a 33.6 km climb is where compact-friendly ratios save your legs. A compact chainring paired with a wide cassette is the sensible baseline: a 34×32 will get most fit riders over the steepest pitches, but on a climb with this much accumulated fatigue a 34×34 gives you a genuinely lower gear to spin rather than grind. If your setup allows, err toward the easier option — you will never regret an extra gear on an HC monster, and cadence preservation is what keeps you moving at 2478 m.

Summary

The Col des Hirondelles is a full-day objective: 33.6 km, an HC rating, and a summit high enough to change how your body responds. Pace it patiently, gear it generously, and the numbers become an achievement rather than a punishment.

Length: 33.6 km
Average gradient: 5.54%
Max gradient: 13.8%
Start: 617 m
Summit: 2478 m
Net gain: 1861 m
Total ascent: 1828 m
Category: HC