Bonking: the wall is arithmetic — here's yours
The machinery · warning lights · roadside repair · prevention · the boundary
Every cyclist meets the wall once. One minute you're riding, the next your legs answer to nobody, your head is cotton wool, and a highway overpass feels like an alpine col. It arrives like an accident — but it's the most predictable failure in endurance sport: a fuel gauge hitting empty on schedule. This page shows the machinery, the warning lights, the roadside fix — and computes the moment YOUR gauge would hit zero.
What is bonking on a bike?
Bonking is the abrupt collapse in power that hits when your glycogen stores run out mid-ride. With muscle and liver stores empty, blood glucose falls and the brain — which runs on glucose — throttles everything: watts, focus, mood. It's prevented by eating early and often, not by toughness.
The wall: when would YOU hit it?
The Fuel Lab runs the tank curve on your numbers: CP, weight, duration, intensity — the snap mark, the bonk line, and the burn-vs-eat ledger.
Run yours in the Fuel Lab →The machinery (why it happens)
You run on two fuel systems. Fat: a huge tank with a slow tap — it can power easy riding for days. Glycogen: a small tank (~400–600 g across muscles and liver) with a wide-open tap — the racing fuel. The harder you ride, the more the mix shifts toward glycogen; past your CP it's nearly all glycogen, which is why a surgy crit empties you far faster than its average watts suggest.
The collapse itself is a liver story. Your liver holds ~100 g of glycogen whose job is keeping blood glucose stable for the brain. When muscles have drained their stores and the liver runs dry, blood sugar sags — and a brain that runs almost exclusively on glucose starts load-shedding: concentration, coordination, morale. That's why bonking feels like a HEAD problem wearing a legs costume. Keep the two states apart, though: empty legs just make you slow — steady pace survives, the snap doesn't. That's cooked, not bonked. The wall is blood sugar — which is why a coke fixes your head within minutes yet never brings the snap back. Against empty legs the only cure is starting full.
Deep dive: the Fuel Lab basics — engine, mix and gut gates — live on the Fuel page.
The warning lights (you get ~15–20 minutes)
The wall sends scouts ahead. In order: ① same watts, more suffering — effort creeps up with no reason · ② slow thoughts — lap math suddenly needs fingers · ③ a sugar craving that arrives like a command · ④ shakes, cold sweat, empty legs — the wall. From first light to impact is usually a quarter of an hour. Eat at stage one. Stage four is too late to fix, only to survive.
Roadside repair (once you're in it)
Fast carbs, right now, 40–60 g in one go — gel, regular cola, glucose tablets, a banana — then ride EASY (below your CP, where fat carries the load; don't stop and stand). Give the liver 10–20 minutes to restock the bloodstream and the lights come back on. What doesn't come back: the time you lost. Straight talk: a real bonk can't be out-eaten mid-crisis. It's preventable, not curable.
Never again (the actual plan)
Prevention stands on three legs, each with its own tool here: start full (carb loading for anything past ~90 minutes — deep dive: carb loading) · eat by the clock, not by feel (a schedule — every 15–20 minutes, on descents, never mid-climb; hunger and thirst always report late during racing) · train the gut (60 g/h is universal; 90+ is a built skill — deep dive: homemade energy gel). The old "75% rule" the forums quote — eat before three-quarters distance — is folklore pointing at the same truth: by the time it feels urgent, you're behind schedule.
A word with many passports
English riders bonk or hit the wall. The French suffer la fringale — the ravenous ambush. Germans meet the Hungerast, a word so old nobody's sure of its roots. Italians crash into la crisi. When every cycling culture independently coined a word for it, you know it's not a you-problem.
Boundary box (medical gate)
A healthy rider's bonk is miserable, humbling and self-limiting. If you have diabetes or take glucose-affecting medication, exercise-related lows are a conversation for your doctor — this page is sport, not medicine. Recurring weakness episodes without an obvious fueling cause belong in a clinic, not a comment section.
FAQ
How long does it take to recover from bonking?
10–20 minutes to function after fast carbs; the rest of the day your ceiling stays low, and full stores take a day of normal eating.
Is bonking dangerous?
For healthy riders it's mostly pride damage — but reactions dull, so treat traffic with respect. If you have diabetes or take glucose-affecting medication, exercise-related lows are a conversation for your doctor.
What's the 75% rule in cycling?
Folk shorthand for "eat long before it feels necessary" — the version with numbers: every 15–20 minutes, by the clock.
Why did I bonk even though I ate?
Check rate (grams per hour, not "I had something"), your starting tank (loaded or not), and intensity — every spike past your CP burns a multiple, not a little extra.
Compute YOUR wall — CP, weight, race format
The tank curve runs on your numbers in the Fuel Lab: CP, weight, duration, intensity.
All fuel guides →Sources
- Brooks & Mercier (crossover)
- Jeukendrup 2014 (intake ceilings)
- Brooks 2018 (lactate shuttle)
- Bussau et al. 2002 (loading)
Sources as named — title and year are enough to look them up.
Sport, not medicine: this page explains the healthy athlete's bonk — it diagnoses nothing and replaces no medical advice (see the boundary box above). All facts from the sources listed — no AI-generated numbers. This page is an independent project.