Col du Tronc

Overview

The Col du Tronc is an HC — hors catégorie — climb, the hardest rating there is before the numbers even begin. Over 11.3 km the road lifts you from a start elevation of 488 m to a summit at 1620 m, a net elevation gain of 1132 m and a total ascent of 1113 m once the small dips and flattenings are counted. The headline figure is the one that hurts: an average gradient of 10%. On a climb this long, a double-digit average is not a spike you ride through — it is the baseline you live at for the better part of an hour.

How the Gradient Unfolds

Because the average is 10% and the net gain (1132 m) very nearly matches the total ascent (1113 m), you can read the profile honestly: this climb almost never gives elevation back. There are no meaningful descents to recover on, no long false-flats to soft-pedal. What relief exists is only relative — a stretch that eases toward the single digits before the road tilts up again. The maximum gradient of 18.83% tells you where the real teeth are. Ramps approaching that figure are roughly double the average, so even when you settle into a rhythm, the road can nearly halve your speed without warning. Expect the hardest pitches to arrive when your legs are already loaded, and treat every easing as a chance to reset rather than a chance to attack.

Pacing Notes

On an 11.3 km climb at 10%, discipline beats bravado. The safest approach is to anchor your effort to a percentage of your FTP and hold it steady through the shallower sections, saving your matches for the ramps toward 18.83%. Going into the red early on a wall this long is the classic way to blow apart before the summit. If you do not know your threshold, run the numbers through the FTP estimator first so your target wattage reflects reality rather than optimism. A good rule: the steeper the pitch, the more you should let cadence drop and lean on your gearing, keeping power controlled rather than surging.

Gearing

Gear for the max, not the average. With sustained 10% grades and pitches near 18.83%, a compact 34-tooth chainring paired with a wide-range cassette is the sensible floor. A 34×32 will get most fit riders up the shallower two-thirds, but on the steepest ramps you will be grinding at low cadence and high strain. If you have any doubt about your climbing legs — or you want to actually enjoy the summit rather than survive to it — fit a 34×34. That extra sprocket is the difference between spinning over the worst pitches and grinding to a near-track-stand. On a climb with over a kilometre of vertical, protecting your knees and your composure is worth the marginal weight.

Summary

The Col du Tronc is a genuine HC test: long, relentlessly steep, and unforgiving of a bad pacing decision. Respect the average, prepare for the max, and gear conservatively — the summit rewards patience.

Length: 11.3 km
Average gradient: 10%
Max gradient: 18.83%
Start: 488 m
Summit: 1620 m
Net gain: 1132 m
Total ascent: 1113 m
Category: HC