inseam to crank arm length...

[soulchild]

I have been reading up on crank arm length and inseam calculations and I'm curious as to what exactly the effects would be of going from say a 175mm to a shorter 170mm arm length. I ask because I'm 5'9 and my inseam (I'm pretty certain) is a 30 and have read conflicting information in relation to others with a 30 inseam and anywhere from 170-175 crank arm lengths, but then read of others with 32+ inseams using a 175.

What exactly would be the pros/cons of going with a shorter crank arm? And what are you all using?

 

[soulchild]

thanks Chris & Ben...

 

[walter]

you should have just bought a frame and gone from there

 

[soulchild]

naa... I'm good... stay tuned but that is the route I will take in the future

 

[MTBTyler]

Its said that in some cases that getting smaller cranks can help people with knee pain.

 

[walter]

naa... I'm good... stay tuned but that is the route I will take in the future

lemme guess, uuummmmm XTR?

 

[soulchild]

lemme guess, uuummmmm XTR?

XTR?!? Walter I'm offended... you really don't know me at all do you wait... I'll give you a hint... follow the crumbs

 

[walter]

XTR?!? Walter I'm offended... you really don't know me at all do you

Haha, OK. Lemme see here, I see something carbon in your future

 

[soulchild]

Haha, OK. Lemme see here, I see something carbon in your future

what is this carbon you speak of?

 

[soulchild]

Its said that in some cases that getting smaller cranks can help people with knee pain.

thanks for actually answering... appreciated...

 

[monteverest]

I'm about your height (5-9/5-10, 31ish inseam) and went from 175 crank to 170 cranks this year. Of course I also changed bikes so this comparison is highly contaminated.

Here are my thoughts: 170s spin faster and pedaling motion is more fluid. I don't really feel that deadzone as much. It seems I could get more leverage on the 175, enough that I unconsciously shift forward to the nose of the saddle when climbing with the 170s, something I didn't have to do often with the 175s. Steep hills are tougher with the 170s.

My knees (front of knee) actually began hurting when I switched to 170s so I raised the seat. Found out my saddle actually sits higher when pedaling the 170s than with the 175s, which is not good for the gnar stuff. Now I constantly raise and drop the saddle - probably invest in a gravity dropper soon.

All in all, I'd go back to 175 but I'm already pedal spanking like crazy so it's out of the question.

 

[smufguy]

Maybe I can contribute something to this thread.

Normally selecting the crank arm length is done by multiplying 5.48 and your inseam (in inches) to provide the crank arm length in mm. I am a 32inch inseam and I get a 175.36mm crank arm and run a 175mm arm with no problem. My XC bike and my AM bike both run the same 175mm arm.

power on both crank arms (170 or 175mm) remain the same, but the torque applied varies and obviously greater with 175mm being a larger moment arm.

The change in knee angle is only afftected from the 90deg forward motion to the bottom of the down stroke. Knee angle is greater with the longer moment arm, but depending on riders, body manages to cope with the stress and thus build up muscle and tensors in your knee. Your inseam is also related to your overall body height which should be properly matched to your bike frame. That said, the crank arm length is indirectly related to the bike frame size. Altering it might work for some, maybe for few with some adaptation.

 

[soulchild]

thanks all for the input...

 

[monteverest]

Let me add to smufguy's already astute observation.

Torque is proportional to crank length, thought the property of proportionality is unlikely to be direct but rather exponential or logarithmic, as t increases at a steeper rate than x, yielding something along the lines of t = ya^2 or t = k(log)a(x) where t is torque.

The above assumes seat height is not constant but increases or decreases to maintain a contant leg extension, which is likely to be a more dispositive expresion of power than inseam. Leg extension is measured by the length from the seat to contact point with pedal when crank is at the nadir position.

 

[jdog]

I have short(er) legs for my height. 5'-10" and I buy jeans in a 30" length.

I ride

172.5 on the road bike

175 on the geared mtb

180 on the ss MTB

I go between the 3 lengths w/o thinking about it.

Here is more info to confuse you:

http://velonews.competitor.com/2003...-lennard-zinn-a-question-of-crank-length_5257

 

[soulchild]

thanks guys... I was really just concerned about trying out a 170 vs a 175 and whether or not I would encounter any knee pains as a result... after talking to Chris and Ben at Halters last week I got the answers I was looking for...

 

[Yo Eddy]

Here's an interesting perspective:

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

 

[Steve Vai]

I run 170's on the road and cross bikes and 175's on the MTB. I can't feel a blind bit of difference in either my pedal stroke or power output. It's 5mm.

An example. A bolt is stuck in something and no matter how hard you try it just won't budge with the wrench your using. Now pretend you have a wrench that's 5mm longer. Do you really think that's gonna make a difference?

During the Hunter Allen seminar I went to, he went over crank length to wattage output in some detail. Brought up a test they did on the track and had hundreds of WKO+ files proving it makes no difference at all, even going from a 140 to a 200mm crank arm.

-Jim.

 

[JerseyPete]

Using Google I found this thread and wanted to know if people still felt the same about 170 vs 175. I am short and have always had 175.
Will having 170 make a difference vs 175?

 

[jShort]

Using Google I found this thread and wanted to know if people still felt the same about 170 vs 175. I am short and have always had 175.
Will having 170 make a difference vs 175?

Maybe 172.5 would workn