Col de Bostan

Overview

Col de Bostan is a hors catégorie giant, the hardest classification a climb can earn. Over 23.6 km it hauls you from a start elevation of 752 m all the way to a summit at 2275 m, a net gain of 1523 m. The average gradient reads 6.47%, but that single figure hides a total ascent of 1535 m across the route's rises and dips, and a maximum pitch of 24.98% that ranks among the steepest sections you will ever meet on a road. This is a climb defined by length as much as steepness: a sustained, high-altitude effort that rewards patience and punishes anyone who goes out too hard.

How the Gradient Unfolds

The 6.47% average makes Bostan sound merely hard rather than brutal, but averages lie on a climb this long. With a net gain of 1523 m yet a total ascent of 1535 m, only about 12 m of the route comes back downhill — this is almost pure climbing, with barely any respite to freewheel and recover. The defining feature is the contrast between the steady mid-single-digit stretches and the ferocious 24.98% ramp, where the gradient nearly quadruples the average. Expect the road to lull you into a rhythm across the long central kilometres, then spike sharply enough to force you out of the saddle. At 23.6 km, the summit at 2275 m also means thinning air in the closing kilometres, so the final third feels harder than the numbers alone suggest.

Pacing Notes

On a climb of this length, discipline is everything. Anchor your effort to your FTP: riding the long steady sections at or just below threshold leaves you the headroom to survive the 24.98% ramp without blowing apart. If you are not sure what your threshold is, run the numbers through the FTP estimator before you tackle Bostan so your power targets are honest. Break the 23.6 km into thirds — settle in early, hold your composure through the middle, and save something for the altitude-sapped finish near 2275 m. Treat the steepest pitch as a moment to survive rather than race; let cadence drop, keep the effort controlled, and recover on the shallower grades that follow.

Gearing

With 1523 m of gain and a 24.98% maximum, gear low and gear generously. A compact 34-tooth chainring is essential here. Pair it with at least a 34×32 for the sustained grind, but given how long the climb runs and how steep that peak ramp gets, a 34×34 is the smarter choice for most riders — the extra sprocket keeps your cadence spinning when the road stands up. If your legs are anything short of professional, err toward the easier 34×34 and thank yourself on the 24.98% wall.

Summary

Col de Bostan is a true hors catégorie test: 23.6 km, 1523 m of net climbing, and a summit at 2275 m, all guarded by a 24.98% maximum gradient. Pace it with your threshold, gear it low, and respect the altitude — it earns its HC billing.

Length: 23.6 km
Average gradient: 6.47%
Max gradient: 24.98%
Start: 752 m
Summit: 2275 m
Net gain: 1523 m
Total ascent: 1535 m
Category: HC