MacGyverBurrito

what determines the length you use.
I have a shimano fc 6206 triple crankset I wanted to use, but it has 180mm crank arms. I've always used 170mm lengths because thats what always comes with the bikes I get. the crankset came off a bike that was pretty tall. around 62cm. do taller frames need longer crank arms? or do taller riders need longer crank arms? the frame I have is a 53cm and I'm 5'8 with a 29" inseam. does that matter or is it pure preference. I also dont want to be scraping the ground on turns, but 10mm doesn't seem like it would be that much of a difference.

jeirvine 

I'd say other than ground clearance, it's really just a matter of preference. Through a lot of trial and error I've settled on 172.5 - it just feels right, though I have 170's and 175's on a few bikes, and it doesn't bother me. I've never ridden a 180.

lostarchitect 

Some people notice it a lot, some less. I notice it. I have short legs and strongly prefer 165. I have ridden really short cranks (155) and they are good, too. I find 170 ok, and 175 almost unrideable.

Based on your size--similar to me, I might add--I would start with 165 if at all possible. 180 is, IMO, nuts for a person of your leg length.

Edit: Peter White suggests using 18.5% from the top of your femur to the floor to determine length. This works well for me and puts my ideal length close to 165.

tarwheel 

I've got 172.5 cranks on all five of my bikes. I'm afraid to use anything different because my knees seem to be very sensitive to changes in saddle heights, so I figure the same thing would apply to crank arms. Unfortunately, 172.5 cranks seem so be harder to find than 170 and 175 these days.

MacGyverBurrito

that's what I was thinking too. I just wanted others opinions. I might put these up for trade here or sell them on eBay so.I can buy a new crankset. maybe its because I've never used anything else, but I'm quite comfortable with 170mm length.

rhm

All the evidence is that crank arm length does not matter much. My wife is much shorter than I, and uses longer crank arms.

Most frames are designed for crank arms around 170 mm. If you want much shorter than that, you'll be getting into problems with geometry. Note, 165 is not much shorter than 170; it's just a tiny bit shorter and will make no perceptible difference.

That said, I pretty much ride the shortest "period correct" cranks I can find. 160's on a couple bikes, 165's on most of the others, 170's on a couple where I haven't yet found shorter ones. In my humble opinion most road cyclists use cranks that are too long. I believe the shorter arms allow me to spin more. A tiny little bit more, that is.

lostarchitect 

Your wife is shorter--but how long are her legs compared to yours? My GF is about the same height as me, but her legs are a lot longer.

For me, that 5mm at the crankarm makes a difference of between 1 and 2 cm in saddle height. It's not a huge problem, but it can make the fit a little wonky on a frame that's already on the large side for me. But the difference is perceptible. It wouldn't be the first time I've been overly sensitive about something, though.

lasauge 

I would expect that you would find the 180mm cranks noticeably too long, the difference crank length makes is subtle, but coming from 170mm you'd be making your cranks almost 10% longer, a major change.

I have a 31" inseam and tried riding with 180mm cranks a couple years ago, I liked how they let me generate more torque, which was quite useful for riding off road, but I didn't keep them because I found it hard to spin them at a decent RPM for very long - I could do it in bursts if rocked in the saddle, but if I tried to ride normally it was fatiguing to have to 'stretch' into every part of the pedal stroke. Personally I find that 172.5 is perfect, 170 or 175 acceptable, any shorter and I'm constantly spinning a ridiculous cadence that ultimately limits my ability to accelerate the bike at will.

fietsbob

have an assortment between 170 & 180. .. saddle lowers when the crank arm is longer so the length is equal.

bradtx

MacGyverBurrito, Struggling to keep a short concise reply. Basically I've read many of the recommendations for crank arm length and I've found personal preference and cadence to mean as much as all of the 'science' on the subject. It won't hurt to experiment using the 180 mm long crank arms.

Brad

Salubrious

The longer the crank arm, the less you can spin. This is why track cranks tend to be 165mm. If you are a masher, you will prefer longer cranks. But keep in mind that mashing is known to be hard on knees.

This is all about leverage, but the most important lever most people don't think about has to do with your knees. It is much easier to stand up from a partial crouch than if your knee is bent at 90 degrees. So you can exert more force on a shorter crank arm (within limits). Keep in mind that the seat height will be higher for a shorter crank- you can't just put them side by side. The French believe that the crank arm has a fairly fixed relationship to your leg. I am 6' with a 32-33" inseam; I find that 170 is the upper limit and 165mm to be quite comfortable. I have a crank that is 175mm; it is is clearly too long- riding it on the road beats me up. I can ride longer and faster with 165mm cranks, even if the bike is heavier.

If you have problems with your knees you will find shorter crank arms to be preferable. One should also take it that shorter arms are thus easier on your knees.

I have seen one study online that made the claim that crank arm length makes little difference. However the study was flawed as it had very large jumps in crank arm length, with jumps far larger than just 5mm. A proper study would have done increments from 150mm up to 180mm: practical crank lengths, and done the same range for a number of different people. It is true that the difference is slight, but if you are racing it is the culmination of slight differences (plus rider skill, which is of course the most important) that wins races.

non-fixie 

If you like toe clips and fenders, be aware that longer cranks will cause or increase toe overlap. I have one pair of 180mm cranks and found them OK to ride with ... in a straight line. Cornering in town was nigh impossible.

cyclotoine

Studies show that longer cranks allow you to generate more power, but the trade-off is endurance. I prefer 180, but if I have been training on 175 and suddenly jump on 180s I can tell. Even more so I can tell with my 185s. Last year I trained on 185 and road 180 on fast rides, I felt faster on the 180s. My mountain bikes are 175 though because that is what they come with and I ride more mountain than road now so I can tell when I am on a road bike. According to Zinn I should ride cranks over 200mm.

I think the people who are further outside the median height can really tell because cranks are sort of optimized by trail and error for average height range. Take a big heavy box and put your back against the wall. Try pushing it away from you when your knees are bent at 45 degrees with open hips and then at 90. Note that you will inevitably close the hip angle when you try to bend your needs to 90 and this is also part of the equation and more important than the angle of the knee. Now which position is it easier to push the box away?

If you are tall you will have a very open hip angle at the top of the stroke on a 170 crank (which was the standard for years). If you close the hip angle with a longer crank you will produce more power through the power phase of the stroke. Note that 5mm of added crank length decreases the distance of top to hip joint by about 1cm assuming you lower your saddle 5mm to accommodate the added length. This is the reason why time trail riders tend to run longer cranks on their TT bike then their stage bike. This is why there is more saddle to bar drop on a track bike (to close the hip angle and produce more power (this compensates for the shorter cranks used to maintain high cadence and keep those tall gears moving). The aero position on the TT bike has a two fold purpose. I is aerodynamics but it also closes the hip angle and increases the power.

I love climbing with longer cranks, it feels awesome. I dropped friend who I shouldn't have been able to drop on hurricane ridge running my 185mm cranks. Anecdotal evidence I know.


If you are getting ride of that crank please let me know. I'd be interested in it.

rhm

She's not just shorter than I, she's much shorter than I, and her legs, too, are much shorter. When I ride her bike I look like a circus act.

It mystifies me that such a small bike (a Bianchi Eros with 650c wheels) was designed for 170 mm cranks, but the fact can't be denied. Even more mystifying is the fact that she insists it fits her perfectly; but again, it can't be denied.

Stairs offer a pretty good analogy. People of all sizes, including very small children, use stairs if the same height. It doesn't matter much if we are all able to use them. What's the most efficient, that's another question entirely.

lostarchitect 

Yep. And sometimes when you get used to something the way it is, it then "fits." For instance, my pants.

jimmuller 

What they all said, more or less. I ride 170s but have ridden 165 too. I can't tell the difference easily while riding.

But I can tell how it affects my position on the bike. I originally put 165s on the Bianchi before I had realized its BB height was about 1cm greater than my other bikes. That combination placed me 15mm higher off the ground, and it really felt like I was up in the air. Now I run that crank on the Bertin which has a lower BB. The Bianchi feels better, and the Bertin has a lower risk of pedal strike.

MacGyverBurrito

thanks for all the responses guys.definitely a lot of information that I didn't know. I'm going to go with 170mm cranks for this build I'm working on because Im eager to get it rolling and its already a size I use. next build Im going to experiment with the length

rhm

Good plan!

When you get around to experimenting, be sure to try cranks of significantly different lengths. Nuts to 165 and 175, go to the extremes. You can get BMX cranks down to 140 mm. See how that feels for a year or two (I tried it, and it was fine). 5" cranks definitely felt too short, though.

zandoval 

I have the same problem with 172 crank arm length. My toe clip just barely grazes my front tire on a tight turn. I have had to just put up with it cause I need big toe clips for my size 12 shoes. (Yes... I know they have invented Clipless pedals).

I still find it amazing how 1 or 2 mm can make such a change in a ride...

bikeman68

180mm is a huge crank size, even Jens Voight rides cranks shorter than 180, he runs the 177.5mm like Jan Ullrich. For you at your more avge inseam Id use 172.5 at the longest. If you have trouble keeping a smooth fast cadence of 90-100 rpm and even pedal squares at rpms faster than that then you may want to run the old classic 170mm. One method I like for choosing cranks is this great chart I have from a French book on cycling. it reccomends 170mm for inseams from 74cm to 80cm, 172.5 for 81cm - 86cm. 175mm f0r 87cm to 93cm, crotch to floor inseam that is

Italuminium

I don't like 175 mm cranks, but I'm sure I'd fail the pepsi test. I have yet to try anything below 172.5, which suits me fine.

Lascauxcaveman 

You've been up Hurricane Ridge? Cool, that is in my backyard; I go up a couple times a year at least. Next time you go up there, get in touch, you can drop me, too.

Of the five or six bikes I've taken up there, I haven't really paid much attention to the crank arm length, but I'm pretty sure everything I have is between 170 and 175. I do know on that ride I spend more time out of the saddle than average.

verktyg 

Folks, we're talking fractional differences in crank arm length!

170mm vs. 172.5mm = 2.5mm (0.098" or a little over 3/32")
170mm vs. 175mm = 5mm (0.197" or a little over 3/16")
170mm vs. 175mm = 7.5mm (0.295" a hair under 19/64")
170mm vs. 180mm = 10mm (0.394" or a hair over 25/64" - more than 3/8")

165mm long track cranks provide a little more clearance to reduce the chance of hitting the track wit a pedal on a high bank.

The differences in shoe sole thickness between brands and models can be more than 1/4" (6.35mm).

Almost all people have differences between the length of their legs of at least 1/4" (6.35mm) or more. My right leg is 3/4" shorter than the left.

The human body has an amazing ability to accommodate to minor differences: crank arm lengths, shoe sole thicknesses, eye glasses and most other things.

I had a bike that I rode for a while that had a 170mm crank on one side and a 175mm on the other! I'm pretty tuned in to those things but I didn't notice the difference until I overhauled the BB!

Now, some folks with shorter legs can benefit from 165mm cranks. Taller riders, 175mm and the tallest folks 180mm... But why do tall track rides use 165mm cranks?

Back in the 70s I rode 180mm cranks on my 700c off road trekker/cruncher bike. I also built the frame built the frame with 11 1/2" BB height.

I have 175mm cranks on my bikes with triples. The longer crank arms give me a little psychological advantage when climbing.

The fairy tale of the Princess and The Pea comes to mind but some folks are more sensitive to little things like crank arm lengths than others - for me it's seat height and saddles...

So there's no "rule of thumb that applies to everyone!

verktyg

Chas.

jimmuller 

He! This just means that your saddle might be positioned wrong if you change shoes. But it affects the entire pedal stroke, top to bottom, and therefore not the difference between leg straight and knee bent.

I could definitely imagine that I felt the difference between a 165 and a 170. Less so between 170 and 170.5. But then, I haven't been sleeping well lately because the mattress has felt a bit odd.

rhm

"... I could definitely imagine..." "... a psychological advantage...." Ayuh. I can imagine world peace.