170 mm versus 175 mm cranks

hobbers · Sep 16, 2006

I did a search on 170 and 175, nothing came up. Does anyone have an opinion on 170 mm versus 175 mm cranks? The 5 mm longer crank is not THAT much of a difference. But, it would give you more leverage. However, it would also drop lower on the bottom, so your top tube would come slightly closer to your crotch, and you might have to lower your seat the same 5 mm if you are going from 170 mm to 175 mm. Also, dropping the 5 mm lower on the bottom would hit rocks that you barely cleared with 170 mm cranks.

Are there any actual drive train implications for 170 mm or 175 mm cranks? Or is just merely the size of the lever arm (crank arm)?

Grabtindy · Sep 16, 2006

I think its really just a function of leg length. I have a 34" inseam and 170mm cranks feel awkward to me.

jeffscott · Sep 16, 2006

I made that change in june i think.

You get about one extra tooth of torque in granny.
It was quite the difference. First ride backs of both knees hurt right away, needed to streach out the IT bands.
Then seat came down and a little forward (this was what i needed).
Then a little longer stem, and it really helped climbing alot.
I don,t think it hurt top end.
5-11 32 inch inseam.

hobbers · Sep 17, 2006

I was reading elsewhere and came across another implication. The dead spot at the top of your pedal stroke will be longer with a longer crank. I found this general recommendation:

* inseam < 29 inches - 165 mm crank
* inseam 29 - 32 inches - 170 mm crank
* inseam 32 - 34 inches - 172.5 mm crank
* inseam > 34 inches - 175 mm crank

shiggy · Sep 17, 2006

hobbers said:
I was reading elsewhere and came across another implication. The dead spot at the top of your pedal stroke will be longer with a longer crank. I found this general recommendation:

* inseam < 29 inches - 165 mm crank
* inseam 29 - 32 inches - 170 mm crank
* inseam 32 - 34 inches - 172.5 mm crank
* inseam > 34 inches - 175 mm crank

And the power stroke is longer, too.

With a smooth, round pedal stroke the dead spot is not noticeable.

I find 175s (the "standard" mtb length) to be too short. 180 is better. 182-185mm works best for me.

95Stumpy · Sep 17, 2006

I went through this dilema about a month or two ago. I used the websites below to determine what crankarm I should be at. I went from 175 to 170. My inseam was about a 31. What made me think about going shorter was that I have a hardtail with 170's and my FS was 175(bigger frame 17.5 compared to my 15.5 HT). I noticed the difference. So, I decided to look into it a little further. I did all my calculations and it came out that I needed a crank arm length of 165.xx. They recommend that you add 2.5-5mm for mountain biking so I went with the 170's.

http://www.velonews.com/tech/report/articles/5257.0.html

http://www.sheldonbrown.com/cranks.html

jeffscott · Sep 17, 2006

I cam out at a recommended 175mm from 95stumpy articles.

xKREDx · Sep 17, 2006

Go with the 175mm.

Gregg K · Sep 17, 2006

I have both 175 and 180. I cannot get on the 180 without feeling like it's a clown bike. Way too much leg movement. I'm 5'8".

175, for me. But it's how YOU feel. I suggest trying it before committing.

shiggy · Sep 17, 2006

Gregg K said:
I have both 175 and 180. I cannot get on the 180 without feeling like it's a clown bike. Way too much leg movement. I'm 5'8".

175, for me. But it's how YOU feel. I suggest trying it before committing.

It also can take a few weeks to adapt to longer cranks. You need to think about lifting your knee over the top of the pedal stroke at first.

It is easier to go from longer cranks to shorter than shorter to longer. Once you get use to the longer cranks switching back and forth is reasonably easy though I feel like I am shuffling my feet with 175 and shorter cranks.

Gregg K · Sep 17, 2006

Shiggy's got a good point-

I forgot to mention, I rode the 180mm cranks from 1985 to 2001. And I loved them.

170 mm versus 175 mm cranks

hobbers

Grabtindy

jeffscott

hobbers

shiggy

95Stumpy

jeffscott

xKREDx

Gregg K

shiggy

Gregg K

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