VegasJen

All my bikes are pretty much off-the-shelf offerings, so I think they're all standard cranks (170mm?). I don't think I'm particularly short, but I do have a shorter leg to torso ratio. My inseam is something ridiculous like 26". And all my bikes are on the small frame size anyway, 49 or 50cm for my road bikes. My two tri bikes are 51cm, which I think is about as small a frame as I've found in a tri bike that still runs a 700 wheel.
But I have to wonder how do I know if I need shorter crank arms? Like 165 or even 160mm? Short of dropping a few hundred dollars just to try it out, is there something I could measure that might inform me? And what are the advantages/disadvantages to going with shorter crank arms?

Eds0123

If your inseam is 26" pretty good chance you might benefit from 165 mm or even 160 mm crank arm lenght. This video might shed some light on the subject.

BobbyG

I have five bikes, all with drop bars and with different length crank arms, and I no longer feel any meaningful difference. My mountain bike has the longest crank arms, and when it still had straight bars and I rode it in a more upright position, after I entered my 50s, I started to have sore hips post-ride. But since converting it to drop-bars the hip soreness disappeared, even though it still has the longest cranks.

To test out different crank lengths on the cheap you could bring a measuring tape and test ride bikes similar to yours, but with different cranks.

Or, if you're lucky there's a bike co-op or sympathetic bike shop near you where you can get a cheap used set of cranks installed cheaply to test them out on your bike.

JohnDThompson 

You may want to consider shorter crank arms if your knees are giving you problems while riding, if you have trouble reaching or maintaining high pedal cadence, if you have problems with pedal/wheel overlap, or if you have problems with pedal strike when cornering at speed (mostly a fixed gear issue).

In general, people with shorter inseam measurement find shorter crank arms helpful. If your bike is in the 48-52cm frame size, it may already have 165mm arms. Arm length is often marked on the back side of the arm, or you can measure it directly: center of bottom bracket axis to center of pedal spindle axis.

Iride01 

I too will tend to think that if you are having knee pain that you can't solve by changing up your position or just learning to use a gear that doesn't have you mashing and putting excessive strain on your knees, then you might benefit from a shorter crank. Also might be a little more comfortable if you regularly can keep a very aerodynamic position for a long time.

I use 165mm cranks and I have a 34.5" inseam. Didn't care for the longer cranks when I tried them. IMO, the formulas that calculate the crank length you should use aren't telling you what length you must use, they are telling you the length of crank you shouldn't exceed. But shorter is okay if you like them. Depending on your bike and your wallet size, it's not usually an inexpensive experiment to see what you prefer.

bargainguy

I'm a shorty at 5'4, and also into folding bikes. As an experiment, I had Mark Stonich of bikesmithdesign.com shorten a couple cranks for me maybe 15 years ago.

He took a couple 170mm cranksets (105 double and an Ultegra triple) and shortened them both to 148mm. I tried each one on different bikes and came to the following conclusion.

Spinning is obviously much easier - your feet are pedaling smaller circles - but any gain there is lost once you hit a hill or a strong headwind. Then, the lack of leverage really compromises any traction that you'd have with a longer crankarm.

I still have both cranks, but these days I'm running 165 or 170 on all my bikes.

KerryIrons

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

Fredo76

When you scrape your pedals at the velodrome!

veganbikes

Do you need them? If the answer is yes then there you go! If not then don't worry about it. I think people put a lot of importance on crank length. The only time to really worry is if you are scraping. I went to shorter cranks on my main commuter purely for scraping on my other bikes I really don't care unless I scrape. If my fitter were to say something, I might consider thinking about it but would probably ask some questions on why and what benefits it would have. In the end unless I was running something ridiculously long like a 190mm or something I wouldn't ever worry about it.

VegasJen

To my knowledge, and I have asked around, I am not lucky enough to have a co-op near me.
As stated above, all my bikes range between 49 and 51cm. But I did just check and my Trek WSD, Spec Roubiax and my Ridley Cheetah all have 170 cranks. Oddly, my 49cm K2 has 175 cranks. Maybe that has something to do with why I feel that's potentially my fastest bike? The K2 is also high on my favorite list, but I don't really think the crank length has anything to do with it. More just an overall feel to that bike.
Ya, I'm afraid of giving up leverage with the shorter crank arms. Seems like that might be a well-founded concern.
I don't have any scraping that I'm aware of. What I do get on occasion is some knee to boob interference, and sometimes, particularly mashing up a steep hill, I'll hit the front wheel with a toe.

veganbikes

That is not ideal but that could be an incorrectly sized bike or a very aggressive position. A little toe overlap especially on a road bike isn't uncommon but usually at slower speeds or while turning but it can happen under those conditions obviously. You could try something a little shorter and see if that helps a little.

Koyote

You haven't mentioned any problem with your current crank arms...So why the question?

And you don't even know the lengths of your current crankarms -- yet you're wondering if you need a different size.


​​​​​​​Weird post.

urbanknight

When you get knee pain from the cranks being too long.

rollagain

Think about climbing stairs. The most common stair rise is between 7" and 7 1/2" ... in metric, that's ~178 to 190mm.

veganbikes

Yes but is your saddle height and angle and setback or forward correct? If you say keep the same height and everything from a 165 crank and move to a 180 for instance that will cause problems till you adjust things.

urbanknight

Absolutely. It’s not nearly as simple as my post makes it sound.

veganbikes

Just had to check as that is not uncommon for someone to change one thing and not others that would also affect it.

VegasJen

Since you guys brought up the set back and height issue, it is important to note that I have been playing with that over the last few months. I used to like to ride higher in the seat, probably left over from years of riding whatever size the "adult" bike was from the local X-mart. Invariably too tall for me. I noticed I was doing a lot of rocking side to side and was incurring a lot of hip and back pain on long rides. I didn't like riding with a lower seat position at first, but I'm getting much more comfortable with it now. I have adjusted all my seats so that my hips are pretty well flat throughout the pedal stroke. Still not totally comfortable with the leg extension (or lack thereof), but it's getting more comfortable with every ride. The fact that my back and hips don't hurt as much is going a long way to making that easier to get used to.

LarrySellerz

i've broken 3 sets of pedals in the last year, maybe because of pedal strikes. I bet I could benefit from shorter cranks. Been thinking about it for a while now.

easyupbug 

In my case it was when dealing with cycling ending knee arthritis. The change from 172.5 to 165 cranks has been a relief and that 1/3" change in saddle height in my case has gone unnoticed. That said it may arise in the future as I have been holding at 20+ mile rides for a few months now.

genejockey 

Do you NEED shorter cranks? Obviously not, since you're able to ride with 170s. Might you benefit from shorter cranks? Sure! the only thing you lose by trying them out is whatever they cost.

I'm just shy of 6', and I run 170s-175s, so for you who are considerably smaller, just proportionally shorter cranks make sense - at least to try! In your place I'd scour Ebay for a set of 165s of the same model as one of your current bikes, and make the switch. Remember to adjust the saddle height up a bit, or you'll feel like you're pedaling flatfooted.

I have a bunch of bikes. All the bikes I have that were made after 1990 have 175s. The ones from earlier have either 170s or 172.5s. I notice bit of difference, mostly that shorter seem to encourage higher cadence, but it's only about 5 rpm more. They felt really weird at first, because I was setting my saddle height based on the BB spindle - Oops! When I used the pedal surface at furthest extension instead, that went away, but they still seem to spin faster. I can't tell if they allow me to get lower because the bikes are sufficiently different that I notice a whole lot of other things more prominently.

Calsun

Check with your local bike shops and see if they have a bike with 165mm cranks you can try. Even tall riders have switched to 165mm or shorter cranks and been happy with the results.

VegasJen

I rode my K2 again today. This is the bike with the 175 cranks. This is also maybe my favorite bike, although not because of the crank length. But after talking about this, I was paying more attention to my knees and hips today. I'm arthritic in both hips and knees although it really hasn't been a major concern biking, but today I did notice a little more tension in the left knee at the top of the crank stroke, right when the knee is closed at the tightest angle. I think this bike is going to be my candidate for experimenting with the 165mm crank arms. It's a Ritchey crank set so I'll just have to keep my eyes open for suitable 165 arms.

urbanknight

I read a lot when I chose to go shorter and found that it's not always desirable to change saddle height when going shorter. Leaving the saddle where it is will simply use a less sharp angle at the top of the stroke, and a little less leg extension at the bottom, while keeping the center of your motion the same. Longer cranks, however, do the opposite and that's where you start to see repetitive motion injuries start to creep up. For this reason, I left the saddle where is was when going shorter and didn't have any problems. I could probably lower the saddle the same length I changed the crank length, but if it ain't broke...

Mike sgonre

To determine if you need shorter crank arms, you can consider the following factors:

Leg angle at pedal strike: Are your legs hyperbent or hyperextended when your bike's pedals are perpendicular to the ground? If yes, you might be more comfortable with shorter crank arms.

Flat vs. uphill difficulty: If you're struggling with uphill or you need to use more force to complete the climb, shorter crank arms can make the climb easier.

Comfort: If you experience discomfort in your knees or other joints, it may be because your crank arms are not the right length for your body size and proportions.

If you're not happy with any of the above, you can try shorter crank arms to see if another length works better for you. It's best to consult a professional bike mechanic and ask them to help you measure the crank arm length you need as it will depend on your body measurements, seat height and pedal style etc.