I'm about 5'9-5'10 and i use 172.5mm cranks at the moment. I have a 175mm dura-ace crank however that's brand new laying around my apartment. I've been told that the 175 would hurt me more than help me but i would like some more people to weigh in. My pants are 30 or 32" inseam if that helps any. I Figured i could lower my seat height a little and the 2.5mm crank arm difference wouldn't be that big of a deal?

I'd search for you, but I'm lazy.

try google" site:www.bikeforums.net crank length knee pain"

It's 2.5 mm for goodness' sake, I don't think it's going to make any significant difference.

That's what i thought but the people i have talked to have tons of riding experience and they insisted it would be bad. I personally don't understand why. My thinking is i would need to adjust seat height some and maybe cleat position but overall it would not differ much from my 172.5mm cranks and higher seat height. I thought about just trying it for a few hundred miles to see but i'm lacking a left crank arm so i can't put it on without buying an arm. I don't feel comfortable paying that amount of $$ for a crank arm if the crankset will not work for me because then i'm stuck trying to sell it..

Going to 175 made a difference for me, but I'm 6'2 and have a 34-inch inseam. You shouldn't have to adjust your cleats, but I'd lower your saddle 2.5 mm. And if you haven't done it already, get a professional bike fitting.
I've also experimented with 177.5, but not for a long enough period of time. Hinault's, Eddie B.'s , etc. older books seemed to suggest trying the longer lengths and giving yourself enough time to get acclimated to determine if they really are beneficial for you, but lately the focus seems to be on higher cadence.

It's noticeably easier to spin a 172.5 at high rpm than it is to spin a 177.5, but that could simply be because I haven't taken the time to become accustomed to the latter.

I would imagine your pedaling style will come into play here. If you spin a lot 175 might be a bad choice. When I started riding 175 on mountain it was uncomfortable at first but I adapted. Given the rarity of 175 Road cranks you could probably get more money ebaying them and buying a 172.5 of similar quality and make some cash to boot.

I have ridden on 172.5 for the past 15 years without issue and when I built up my Lynskey last year, opted for 175's. After a full year of pain, I developed plantar faciitis and achillies tendonitis in both legs. Require surgery on one in the coming months. My surgeon, a very avid cyclist with a sports medicine clinic blames the spin diameter and over use directly. Take it for what its worth, just one guys experience.

Here's an interesting U of Utah study on the matter. They varied the cranks over a huge range, 120 to 220 mm. They concerned themselves with effeciency of power transfer etc., not so much on goodness or badness for the knees.

https://www.plan2peak.com/files/32_ar...gTechnique.pdf

2.5mm can make a big difference, but you could also shift your saddle...

i ride both

cant tell the difference

like someone said, its 2.5mm per crank... im sure many of you have that much leeway with your feet in your bike shoes

I'm sure many of you have that much leeway in your butt on the saddle...

I ride 170 and 172.5. Both work fine for me. I prefer 170 and 175 is uncomfortable.

I'm your height, but my legs are shorter.

By the way guys, it's not 2.5mm. It's 5mm. You adjust the saddle down 2.5mm so you can reach the low crank with the same extension, and the high crank will be 2.5mm up as well. That's 5mm more crunch.

As I understand it, most people are okay within 2.5mm of their preferred length. So, if the op really likes 172.5 the 175 probably won't bother him much. If he finds 172.5 to be a touch on the long side and would prefer 170, then the 175 may just be too much.

I've played around with this some, using the three most common sizes, on the same different bikes, over a longish period of time. For me, 6' 1" with a 34" inseam, it was like this:

- I used Ultegra 6500 double cranksets with 53-39 rings and the same Dura Ace 7700 109.5 bottom bracket the whole time. I used an 11-23 cassette the whole time.
- I had 170s first. They, at the time, felt very fast, but I found my self mashing a lot (I was out of shape and had a bad fit).
- I tried some 175s for about six months. They worked better for a while and felt a little easier. ...Then, I started spinning more often.
- I got another bike that was equipped with 170s and decided to use them on the other bike. If felt like I could spin more smoothly, and easier than I could with the 175s
- Soon, I sold that crankset and got another for the new bike that was 172.5. It seemed that my cadence wasn't as smooth now, but still quite smooth.
- During this time, I still had the set of 175s that I put back on the old bike. When making a direct comparison of the two near identical setups, I found that it was the crank that I liked more ...but it was same model crankset, just a different size(?)
- I try another 172.5 on that bike, and pretty much instantly, I find it to mesh better with my riding style and cadence.
- Testing out other 170 cranks on other bikes has led me to believe over time that it is my ideal size of crank arm. The effect is almost intangible, but I plan to replace all my cranksets with ones that have 170mm arms at some point.


Keep in mind that I did this 'experiment' over the past several years and it's intended to be an observation, at best.

That is how I would describe my situation/experience.

If you're really getting into the weeds on this, then lowering your seat 2.5mm would achieve the goal of keeping the same leg extension at the bottom of the crank rotation, but at the top you'd be a full 5mm shorter (2.5mm seat lower+2.5mm crank longer) at the top of the rotation. Since we're not professionals (most of us, anyway?) here, I'm not sure it's that big of a deal, but something to think about.

I rode and raced on 172.5 for 10 years or so (6' and 32 inseam), then started riding mountain bikes with 175 cranks. Then I switched to 175 cranks on the road....and played around with lowering the saddle but found I liked the 175 cranks best with NO CHANGE in seat height. Over the past 5 years I've switched to more spin, less mash, and raised my saddle by about 10 mm. All in all it is not much change and a very personal issue

it's a 1/10th of an inch. there's probably that much variation between the length of each leg.

fortunately, you are about the same size as my fiancee and i have already done "homework" to see if her free 175mm campagnolo cranks would be suitable.

according to lennard zinn and several studies a 175mm crankset is perfect for your 32in inseam.
https://www.zinncycles.com/cranks.php
https://www.nettally.com/palmk/crankset.html


several other sources that i found "less credible" showed 172.5


i say try them and see what you think.

I'm about your size, and I have a 172.5 crank on my solo bike and a 175 on our tandem. I feel like I spin faster on the solo bike and pedal slower but harder on the tandem. That's pretty much what the normal wisdom is.

Since I ride both bikes regularly, I'm used to it. It's like my two cars. They drive differently, but I accommodate both without much thought.

As others have said, give each size a fair try, and then use the size that you like.

more interesting info:

i know inseam is more important than height, but it is harder to find

Graeme Obree 5' 11" 175mm
Lance Armstrong 5' 9.5" 175mm
Jan Ullrich 6' 0" 177.5mm
Marco Pantini 5' 7.5" 170mm (180mm in the mountains)