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I’ve done some research about crank length. When I’m riding full-gas my knee is starting to hurt but when I’m just chilling in group rides I feel alright.
My crank length at the moment is 172.5mm and I’m thinking upgrading to a shorter crankset. My inseam is 30 inches (762mm) and by the formula I found on the internet by GCN, I should have a 160mm crank length.
Is 160mm too short or should I just go buy the 165mm?
160mm crank is abnormally short. This doesn't mean that you shouldn't but as Axemasta mentions, you should have a bike fit before deciding what crank arm length you should have. Plus you've not given any details about your bike or it's size.
There are so many variables involved in knee pain, it would be foolish for anyone to suggest your first course of action to be crank arm length.
Bikes related variables for knee pain:
Body related variables for knee pain:
These are just examples of a much larger list.
So, in my opinion, without finding the root cause of your issue, buying extremely short cranks should only be done if you are 100% that is what you need. And I don't think (with the information you have given) that will be what you need.
The formulas are a bit rubbish sadly. Based on those, my crank length should be 200mm and that's simply not available. Feel free to disregard those as "average-sized person fairy tales"
On the other hand, I do run 165mm cranks on my recumbent without issue. They're hard to find but do exist. You can also get a machinist or engineering shop to drill and tap some existing crank arms with new pedal holes.
I have a few bikes with cranks ranging from 165mm to 175mm and don't find that it makes much of a difference (although perhaps that changes if you have known issues with knee-flexibility or stability).
I would first make sure that you're pedaling at reasonable cadences and not mashing at high torque, lower rpm. I also think incorrect saddle height (even pretty small changes of a few mm) matter more for me than the crank length does.
165 is not really that short, all things considered.
If you do some research you will find there is a school of thought who prefer shorter cranks or who even break with standard bike-fitting orthodoxy and claim that standard size cranks are too long. I have no opinion whether they are right or not, but just know that running short cranks is not that "crazy" of an idea.
Also apparently there is a market for abnormally short cranks for those with knee problems or limited motion, so some bike shops have made a niche service of shortening cranks.
If you want very short cranks you may consider looking into unicycle cranks. They fit normal square taper cranks but are available in extremely short lengths down to 75mm. Some adaptation would be needed.
Also, Answer makes mini BMX cranks down to abnormally short sizes for 4-6 year old BMX racers. Considering what BMX racers put them through, I would guess they are strong enough for normal use especially considering they are shorter. That's another possible source of short cranks.
I prefer slightly shorter than normal cranks (170mm despite 6'1" height) for simple reason of ground and tire clearance, and have done so forever.
You should watch this, crank length is another myth someone came up with a long time ago.
