i’d have to assume that a shorter crank arm can produce the most in the form of not lost and quickest transfer of power. also plays a bit of a role with less weight being less material, right? am i correct? and if so and with an inseam of 32", what woud you think a good crank arm length would be if coming from 175mm? compact 53 x 34, btw. roadie, not tt.
thanks.
I have 32" inseam and use a 175 on my roadie, 172.5 on my tri and 170 on my trainer. To tell you the truth, I can’t tell a difference. I think outside of the extremes, <165 or >180, it is a placebo effect.
I have 32" inseam and use a 175 on my roadie, 172.5 on my tri and 170 on my trainer. To tell you the truth, I can’t tell a difference. I think outside of the extremes, <165 or >180, it is a placebo effect.
thanks for the reply. i just googledcrank arm and got this from wikipedia:
Sizes 
!(/skins-1.5/common/images/magnify-clip.png)A left crank attached with a cotter.
Bicycle cranks can vary in length to accommodate different sized riders. Major manufacturers typically offer crank lengths for adult riders from 165 mm to 180 mm long in 2.5 mm increments, with 170 mm cranks being the most common size. A few small specialty manufacturers also make bicycle cranks in a number of sizes smaller than 165 mm and longer than 180 mm. While logic would suggest that riders with shorter legs should use proportionally shorter cranks and those with longer legs should use proportionally longer cranks, this is not universally accepted. Very few scientific studies have examined the effect of crank length on sustained cycling performance and the studies’ results have been mixed. A simple exploration of the logical basis of proportional bicycle crank length can be found on the web here. Several formulas exist to calculate appropriate crank length for various riders, one from the most often cited crank length website can be found here. However, the exact length an individual cyclist feels most comfortable with may vary depending on the rider’s cycling specialty. Bicycle riders typically prefer shorter cranks for higher cadence cycling such as criterium and track racing, while riders typically prefer longer cranks for lower cadence cycling such as time trial racing and mountain biking.
one of those links is this one. i guess i kinda found my answer and that is that the only real benefit there is to shorter cranks is a slight bit less weight seeing as how the cadence created is due to gearing. http://bicyclecranklength.blogspot.com/
only posting this because it’s pretty uselful and it worked out for myself just now at 32" even by 5.48 ( i dunno where that number came from but it worked) the optimal crank length via leg length is 174.36 or 175mm. talk about your science!
L(mm) = 5.48 x I(in)
Now, multiply the inseam measurement (in inches) by 5.48. This provides a good estimate of proper crank length, in millimeters, for general road cycling or racing.
For those who prefer metric units, simply convert the equation by 2.54 cm/in. L(mm) = 2.16 x I(cm)
I have 32" inseam and use a 175 on my roadie, 172.5 on my tri and 170 on my trainer. To tell you the truth, I can’t tell a difference. I think outside of the extremes, <165 or >180, it is a placebo effect.
With a 32" inseam, Im sure you probably wouldnt feel much of a difference, but with a 30" inseam I guarantee you do 
175’s killed my knees once I started riding more than about 180-200 mpw. Switched to 170’s and the pain just went away.
my first road bike ever and i was riding 200-250 a week. that ther number you threw up i was putting down on my mountain bike. knees feel great, i want to use my power more efficiently is the thing.
I’m not so sure the shorter crank is actually shorter/lighter… seems possible the arms themselves are all formed the same so it’s easier to mass produce, and then the pedal hole is just drilled +/- 2.5mm this way or that. I’ve got an older set of 170s and the pedal hole looks like it’s off-center (towards the spindle end) from the “bulb” shape at the end of the arm. Maybe it varies with the make & model, though… any manufacturing folks out there?
You are correct with Alu crank arms, most carbon forks are really carbon wrapped Alu (or even steel) and the jigs are the same. With real carbon cranks (Campy and others) each length has their own mould for lengths and integrating the BB spindle systems.
I’m not so sure the shorter crank is actually shorter/lighter… seems possible the arms themselves are all formed the same so it’s easier to mass produce, and then the pedal hole is just drilled +/- 2.5mm this way or that. I’ve got an older set of 170s and the pedal hole looks like it’s off-center (towards the spindle end) from the “bulb” shape at the end of the arm. Maybe it varies with the make & model, though… any manufacturing folks out there?
never thought of the arm being the same length and the pedal hole drilled closer. good point. i really wanna go to 170’s and see how that works out. if you read the first link from wikipedia that i posted earlier, the guy goes on about how there may be some positives in arm length according to power. he uses a basic formula with extreme ‘ifs’ (50 mm arm, the normal 170 arm and a 300 mm arm) to prove it, hypothetically.
And yet - posts from Slowman himself (and Rappstar and many others) profess going to shorter arms to alter your cockpit…
And yet - posts from Slowman himself (and Rappstar and many others) profess going to shorter arms to alter your cockpit…
not too bright are you? if the length of the arm itself is all the same and the hole is drilled nearer or further from the end giving you whatever reach you want, what in the hell does that have to do with anything other than cockpit stance? that’s an entirely different subject, relative to this one no doubt but still that leaves you not so bright…
and if Rapp and Slowman are both saying go shorter than my instincts are right.
The point is that IF your equation is right (shit, we have used those numbers since the 80’s and even before) - the suggested crank arm length may be now decided by a newer and much more scientific system, rather than a random number from decades past. So, if you use that math (or even the .883 method of frame size) you are by default using figures from an antiquated and now proven inefficient system for many riders. Sure, 2.5mm is not huge - but it is proven that it is something and we now see folks going in recent years from mashing to compact evolving into the new trend of short short arms (160mm and the like) with proven valid success.
I miss BioPace
it’s not my equation but it is the first time i’m seeing it.
It is funny watching history repeat. Cranks alone have had so many theories involved from long to short to biopace, Rotors, Q Rings, Round rings, Q Factors, ISIS, Square Taper, External BB, Cartridge BB’s and on and on…yet we just get no faster. Some years ago I did the “Pepsi Challenge” with Campy Record Au Square taper, Record Carbon ST, Record Carbon Ultra Torque and a 15 year old Shimano 105 Square taper - neither myself nor either of my power meters could tell one crank from another unless I was looking down. Now, that experiment of course has no bearing on the new numbers that we are seeing with the shorter crank arms as all of my cranks are 175mm but for on the Townie and Fixie.
And I know that was not your number - if history serves me well it was either Yamagucci or Tom Kellog who came up with that figure…wait, or was it Ben Serotta? No matter. Oh, the .883 was an Eddy B thing (with one Mr. Greg Lemond who took credit for the figure 
I’m not so sure the shorter crank is actually shorter/lighter… seems possible the arms themselves are all formed the same so it’s easier to mass produce, and then the pedal hole is just drilled +/- 2.5mm this way or that. I’ve got an older set of 170s and the pedal hole looks like it’s off-center (towards the spindle end) from the “bulb” shape at the end of the arm. Maybe it varies with the make & model, though… any manufacturing folks out there?
For Shimano (Ultegra, at least, probably all of them) the arms themselves are shorter. I’ve lined 172.5s and 165s of the same Ultegra model up against one another. The arms is shorter by about the same length as the difference in the pedal holes.
By the way, actually doing that really makes it clear what a tiny physical difference there is even between those two crank lengths. I’m not surprised a lot of people can’t tell the difference.
Yes, the new Hydroformed ones are individual for each size, if you find some forged ones from the past, or one of the many low cost models on entry level bikes you will see the area that they leave so that they can move the CNC tap the variation of 5mm. I am sure that if there is a variation in weight it would probably be well within a spec. for the +/- variations in manufacture of each crank arm…especially in carbon cranks (I have seen Carbon cranks vary 20 grams for the same model/length box to box…)
this turned out to be a good thread. thanks for all the info but if all of this is true and the arms generally don’t do much then i’ll stick to what i was doing on Sora 175’s as a complete newb to people on super record with years of experience, drop 'em. too much science involved for my liking, or my brain can’t handle it all rather.
thanks for the info or the conversation peeps. good stuff.
Don’t just rely on this thread. Use the search function here. There are so many threads on this topic. Search for “Crank Length” and you will find a lot of reading on the subject.
I think that’s a touchy subject and comes with little scientific data to prove one way or another. Personally I hate 172.5s and couldn’t stand them on a bike that I bought. Switched to 175s and it felt so much better. I also played around with longer than 175s (I have the adjustable PCs) but never put too much time into that as I’m very comfortable with the 175s. To me, I feel like the legs go in really small circles when I’m on 172.5s but there are many that believe in smaller crank arm length and some go as far as to state that you can’t notice the difference.
When you switch crank length just make sure you compensate for the saddle height.
I’d either buy some used/cheap crankarms of different sizes or ask friends to test ride their cranks. Just because somebody else says they prefer larger or smaller does not mean that you are comfortable with it - or that you do or do not notice the difference