Crank length?

Ron

Well-Known Member
14 yo girl. 29” inseam, roughly.

This is on a mountain bike.

165mm or 170mm? I am thinking 165mm

Thank you
 
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tonyride

Don't piss off the red guy
I don't think crank length depends on the rider but rather the bike's BB height and the terrain. If it is rider dependent then the crank length would vary according to frame size from the factory but it doesn't. No matter what make and model bike you pick, every size from S to XL will most likely have the exact same crank arm length. It doesn't matter what your inseam (or age either) is. Best way to determine the correct length is to ride what you have and see if you get many pedal strikes. If so then consider going 5mm shorter. Some pedal strikes here and there is normal and wouldn't require any changes but if you get a lot of strikes and hard enough to throw you off course then definitely go shorter. There's no set rule of thumb just going by age and inseam length. How's that for a winded long non-answer?
 
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Ron

Well-Known Member
Sorry for the fat finger on my original post. I made the correction.

@tonyride and @Ironjunk The bike presently has 175mm, stock cranks. But feel 170mm would be way better for her.
 

tonyride

Don't piss off the red guy
Sorry for the fat finger on my original post. I made the correction.

@tonyride and @Ironjunk The bike presently has 175mm, stock cranks. But feel 170mm would be way better for her.
I guess I'm trying to understand the situation, is she experiencing pedal strikes or she just feels the there's too much rotation when she pedals?
 
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Ron

Well-Known Member
I guess I'm trying to understand the situation, is she experiencing pedal strikes or she just feels the there's too much rotation when she pedals?
under knee cap and hip joint discomfort. Videos I have watch and what I have read say this is a condition of not the down stroke but knee too high on the up stroke.
 

tonyride

Don't piss off the red guy
under knee cap and hip joint discomfort. Videos I have watch and what I have read say this is a condition of not the down stroke but knee too high on the up stroke.
Gotcha. Initial assessment may suggest the crank arm length is the culprit and it could very well be but my wife has around the same inseam and she's been fine with 175mm cranks for years. I guess it wouldn't hurt to try 170 or even 165 to play it safe.
 
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mattybfat

The Opinion Police
Team MTBNJ Halter's
5'11" and been rocking 160s for a while.
Eliminated that hip pointer pain and more importantly the pedal strikes have immensely decreased.
 

soundz

The Hat
Team MTBNJ Halter's
I'm sure there is some science behind it but like tyre pressures and handlebar lengths, I feel this is one of those things that are best measured by keep going shorter (or longer) until it feels best for you in the areas you are riding on the said bike.
 
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pkovo

Well-Known Member
That's close to my 13 year old daughter's inseam and she is on 170mm cranks on both her mountain bikes and 165s on her BMX Race bikes.

Until recently, she had one mountain bike and I had 160mm cranks on it. Bought her a second bike with 170's fully expecting to swap them for 165s, but after riding the bike with 170s she told me she prefers the 170s. It seems like this should be too much crank for her, but but she isn't wagging her hips riding them (a sign they are too long). She isn't getting any discomfort, and seems faster so I'm leaving them. So she's 5'2 and I'm 5'11 and we ride the same cranks.

So I guess my point is, they may work fine for someone with that inseam. I think it depends a bit on the individual. My daughter is into martial arts and super flexible, where I'm old and NOT flexible, so I think that makes a big difference.
 

John the Plumber

Well-Known Member
I guess I'm trying to understand the situation, is she experiencing pedal strikes or she just feels the there's too much rotation when she pedals?
I am riding 172.5 thinking of 170. What I have read on different occasions is that with a shorter crank you can accelerate faster and avoid pedal strikes,too.
 
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Pearl

THIS CHANGES EVERYTHING
Mandi is 4'11 and rides 165s on road/cx (and mtb I think, I can't imagine it being more than that)
 
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