If you've ever had a professional bike fit, you'll know the ritual: saddle height first, then setback, then reach, then the fine-tuning. So when Phil Burt — the man who fitted Team Sky and British Cycling through their most dominant years — told me the first thing he changes on a struggling rider is the crank length, it stopped me. That's not where anyone starts. It's barely where most fitters finish.
But the more he explained it, the more it reframed how I think about the whole bike. Crank length isn't a detail you tidy up at the end. It's the foundation everything else is built on, and most amateurs are standing on the wrong one.
Why the crank comes first
Burt's logic is structural. "I would go crank length first every time," he said, "because I'm fixing everything else around it if I don't get the crank length right."
Think about what the crank actually determines. It sets the arc your foot travels through — how high your knee comes at the top of the stroke, how far your leg extends at the bottom. Every other measurement on the bike is a response to that arc. Your saddle height is set relative to the bottom of the stroke. How far your hip closes is set by the top of it. Your reach, your comfort, the pressure on the front of your hip — all of it is downstream of the crank. Get the crank wrong and you don't get one wrong measurement; you get a whole fit built to compensate for a bad foundation. That's why he refuses to fix anything else until the crank is right. It's the same principle that underpins our full bike-fit guide: solve the structural thing first, and the details fall into place.
Almost everyone is on cranks that are too long
Here's the claim that should make you look down at your own bike. Burt is convinced most riders are running cranks that are too long, and he doesn't hedge it. "I don't know any reason to put someone on a longer crank," he said. "I can definitely give you ten reasons to drop someone's crank length."
Why are we all over-cranked? History and inertia, mostly. Crank lengths were standardised decades ago around frame size, not rider physiology, and the bike industry has been slow to move. Most stock bikes still ship with 170, 172.5 or 175mm cranks regardless of who's going to ride them, and most riders never question it because they don't know it's a variable they can change. So a generation of amateurs grinds around on cranks built for a sizing chart rather than for their hips.
The fix Burt describes is liberating precisely because it's so unglamorous. "If you drop crank length 5mm, the window gets bigger," he said. By "the window" he means the range of saddle heights and positions that actually work for you. A long crank gives you a narrow, fussy window where everything has to be perfect or something hurts. A shorter crank opens that window up, giving you room to get the rest of the fit comfortable. You're not losing anything meaningful in power — for an amateur the difference is negligible — and you're buying yourself a far more forgiving position.
The back-pain test
The moment from the conversation that made the case most vividly was a thought experiment I put to him. If he had one go, wasn't allowed to see the patient, and was just told the rider had low back pain — what single change would he make in the dark?
His answer was immediate: "I would say drop their crank."
Sit with how confident that is. No assessment, no video, no measurements — just the single highest-probability fix for one of the most common complaints in cycling. And the reasoning holds up. A crank that's too long forces your hip to close harder at the top of every pedal stroke. Multiply that by thousands of revolutions an hour and the closure pulls on the lower back, rounds the spine under load, and grinds away until it aches. Shorten the crank and you open the hip angle at the top, the back stops being forced into that repeated compression, and the pain very often eases. It's not the only cause of back pain — core strength and mobility matter too — but it's the one Burt would bet on blind.
How to tell if your crank is too long
You can't measure hip closure on a club run, but the symptoms of an over-long crank are recognisable once you know what you're feeling for. The classic signs are discomfort or pinching at the very top of the pedal stroke, a feeling that your knees are coming up too high or crowding your stomach, hips that rock on the saddle to get over the top, and lower-back ache that builds over the hours rather than hitting you early. Numbness and saddle discomfort can trace back to it too, because a hip forced closed at the top tilts the pelvis and changes how you sit. If a fit has chased your saddle around for months without ever settling, the foundation underneath it is a prime suspect.
This is also where Burt's point connects to the wider set of fixable mistakes he talks about. Crank length is the structural one, but it sits alongside the familiar culprits — saddles set too high, cleats in the wrong place, reach that's too long and stretched out. The reason he leads with the crank is that several of those other problems are downstream of it. Fix a too-long crank and the saddle height that never felt right often falls into place, because you've finally given the rest of the fit a stable base to work from.
Why this matters even more after 40
There's a particular group for whom Burt's advice is close to essential: masters riders. As we get older, hip mobility and flexibility quietly decline, which means the hip can't close as far at the top of the stroke without strain. A crank length that was merely suboptimal at 30 becomes a genuine source of pain at 55, because the body no longer has the range to absorb it.
That's why so many older riders develop nagging back and hip issues they blame on age or "just getting old," when a meaningful part of the problem is a crank that their hips can no longer accommodate. Dropping it gives back some of the range that the years took away. Combined with off-bike work on core strength and the mobility we cover in the cycling-after-40 guide, it's one of the highest-value changes a masters rider can make — and unlike the mobility work, it's a one-off fix rather than a daily discipline.
What to do with this
You don't need to rush out and rebuild your bike. But Burt's insight should change two things about how you think.
First, if something hurts — your lower back, the front of your hips, that pinch at the top of the stroke — don't assume the answer is a saddle tweak or more stretching. Consider that the crank underneath it all might be too long, especially if you're a shorter rider, a masters rider whose hips have lost some range, or anyone who's never had crank length questioned. Our piece on the five fixable reasons you're getting dropped on climbs touches on how position quietly costs you, and this is the most overlooked example.
Second, if you're booking a fit, find a fitter who treats crank length as a real variable rather than a fixed given. A lot don't, because changing a crank means selling and swapping a part rather than just sliding a saddle. But a fitter who starts where Burt starts — with the foundation, not the finish — is one who understands that the most important number on your bike might be the one nobody ever checks.
The pros have known this for years; it's why you now see World Tour riders on cranks far shorter than their frame size would once have dictated. Burt helped bring that thinking to the top of the sport. The good news for the rest of us is that it costs a single part and a bit of humility, and it might fix the thing you've been stretching and foam-rolling around for years.
Hear Phil Burt's full breakdown of the fixable bike-fit mistakes on the Roadman podcast. For the complete picture, read our bike-fit guide, and bring your fit questions to the community on Skool.