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I’ve done some research about crank length. When I’m riding foooool 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 172.5 and I’m thinking upgrading to a shorter crankset. My inseam is 30inches 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????

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    Have you had a bike fit? ideally a professional bike fit but there are also apps you can use to help dial in your position on the bike. What size frame do you ride? 172.5 -> 160 is a big jump and you should first check that your saddle is at the correct height. How does the knee pain begin? is it at high / low cadence? Do you ride to power, does pain only happen at higher power? The best thing you can do is go to a bike fitter and abide by their suggestions! I ride a 54cm frame, and recently swapped from 172.5 to 170 and I'm preferring the shorter cranks! – Axemasta Jun 12 at 20:12
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    One related answer is here: bicycles.stackexchange.com/questions/67848/… – Weiwen Ng Jun 12 at 20:25
  • If you don't want to invest in a bike fit, some adjustable crank shortners will let you play with lengths without committing. Thorn make one that is infinetly adjustable but it only fits square taper. Ultimately some people like short cranks, some don't, and the range of human dimensional range far exceeds the cycling industry mandate that cranks are 165mm-175mm. Your not a weirdo liking, let alone trying, short cranks – mattnz Jun 12 at 21:37
  • The chances of a 172.5 crank causing knee pain for someone with a 30" inseam are infinitesimally small. It's much more likely to be technique and/or actual medical issues with your knees. Do your knees bow outward at any point during your pedaling? Do your feet splay inward or outward on the pedals and thereby put twisting pressure on your knees? When you want to generate power to go fast, do you use a big gear and pedal really hard but slow? Any of those can create knee pain. It could be your cranks are too long as everyone is different and you may be the outlier. More data would help – Andrew Henle Jun 13 at 1:23
  • A few years ago I was trying to find shorter cranks, but nothing was available anywhere in any brick-and mortar store, and the online choices were all garbage parts for bicycle-shaped objects. – Kaz Jun 13 at 6:18
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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:

  • Overall wrong size frame
  • Bad saddle position (tilt and position on rail)
  • Stem too long
  • Handle tilt
  • Crank length
  • Pedal choice / fit

Body related variables for knee pain:

  • Incorrect cycling technique
  • Muscle imbalance
  • Muscle tightness
  • Non-optimal nutrition

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.

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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.

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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.

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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.

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  • Good answer with many valid points, however I want to point out a small inconsistency, or at least as I see it. "I would guess they are strong enough for normal use especially considering they are shorter." A 4-6 year old person weights at least 3-4 times less than an adult, and their leg muscles are proportionally weaker. Strictly speaking, children BMX cranks do not need to be as strong as those designed for adults. But again, nothing prevents them to be made as sturdy. – Grigory Rechistov Jun 13 at 5:28

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