Croix de Bostan

Overview

Croix de Bostan is a hors catégorie monster, the kind of climb that earns its HC rating not through a single trick but through sheer, unrelenting scale. Over 20.3 km it drags you from a valley floor at 416 m up to a summit at 1786 m, a net gain of 1370 m. The average gradient reads 6.76%, a number that looks almost civil on paper and lies completely once you factor in what those averages conceal. This is a climb measured in fractions of an hour, not minutes, and it demands respect before you ever turn a pedal.

How the Gradient Unfolds

The headline figure — 6.76% across 20.3 km — hides enormous variance. The total ascent of 1431 m sits above the net gain of 1370 m, meaning roughly 61 m of the climbing is given back and re-earned across brief descents or false flats that break the rhythm. That is the trap: every metre lost has to be reclaimed, so the effective climbing is harder than a clean 6.76% grind. The defining feature is the max gradient of 30.86%, a wall steep enough to overpower a poorly chosen gear and force riders out of the saddle or, worse, off the bike entirely. Expect the road to lurch between merciful single-digit ramps and vicious spikes, so read the surface ahead and never assume the average is protecting you.

Pacing Notes

On a climb this long, discipline beats bravado. Anchor your effort to your FTP and hold the first two-thirds comfortably below threshold — the summit is at 1786 m and it is a long way from the 416 m start, so blowing up early leaves you nowhere to hide. If you don't know your threshold, run the numbers through the FTP estimator before you go, then set a ceiling wattage you refuse to exceed on the shallow sections. Bank effort on the flatter pitches, and treat the 30.86% max as a survival zone: soft-pedal in, spin what you have, and accept that speed there is irrelevant. With 1431 m of total ascent to absorb, consistency across the full 20.3 km is what gets you to the top intact.

Gearing

Gear for the worst pitch, not the average. A 34×32 is the bare minimum here and will leave you grinding on the steep ramps toward the 30.86% max — fine if you have the legs, punishing if you don't. For most riders a 34×34 is the smarter call, buying enough range to keep your cadence up when the wall arrives and preserving your legs across all 20.3 km. If your setup allows even lower, take it; there is no prize for over-gearing a 1370 m ascent.

Summary

Croix de Bostan is a full-blooded HC test: long, steep, and honest about its difficulty. Pace it against your threshold, gear low, and let the numbers do the intimidating.

Length: 20.3 km
Average gradient: 6.76%
Max gradient: 30.86%
Start: 416 m
Summit: 1786 m
Net gain: 1370 m
Total ascent: 1431 m
Category: HC