Ideal Crank Length

I haven't read all of your links....

I had the chance to meet John Cobb at a LBS recently where he was doing some rudimentary fitting on a fit bike with an adjustable crank. We started with something similar to my current bike fit with the crank at 170 mm. Fine, but I have never been that comfortable staying in aero for long periods. He then left the fit the same but adjusted the cranks all of the way down to 155. I was immediately significantly more comfortable and my pedaling felt smoother. Cobb told me he has put six foot tall folks on 155s and almost no one ever goes back to the longer cranks.

So, I was convinced and got my hands on a set of 155 cranks. I am seven rides in with the new 155s, all on the trainer. I can stay in aero on the trainer for an hour, easily. This is something I could never have done with my old set up. My power is as good or better than before. It is pretty early in my time with them but I can't see going back. I only wish I wouldn't have waited so long. A fitter suggested I try shorter cranks almost two years ago but I was reluctant to spend the $.

FWIW, Cobb sells short cranks on his wbsite for a very reasonable price. He is also one of the nicest, most approachable guys you will ever meet, and has probably forgotten more about aerodynamics, bike fitting and cycling in general than I will ever know. If you call the number on the Cobb website it is highly likely that you can talk to the Man himself and pick his brain about the subject. And no, I have absolutely no financial incentive in this. Not sponsored or anything. Just a satisfied customer multiple times over.

Thanks Jason. I almost added the Cobb link. How great to talk to him! I'm very happy to hear about your switch and results. I'm trying to decide on whether to go to 165 or shorter. Did you stay compact? How tall are you?

http://www.cobbcycling.com/crank-length-coming-full-circle/

Compact.

I am 5' 7". Not tall.

I was very anxious about going that short but I was also concerned that if I only dropped to 165 that it wouldn't be enough to make it worth my while. 160 is probably what I would have gone with but I had a line on a great deal for the 155s so I bit the bullet.

Well generally speaking, I'd say that triathletes should spend less time worrying about their gear and more time pushing their pedals. But as the previous poster points out, there are some exceptions to the rule.

Since you have no clue of my specific training regimen, I'll venture a guess that this is an attempt to stand on a soapbox. As the OP, I'd personally be happy for you to do it elsewhere. I'm sorry you didn't get your SlowTwitch card yet, but with some practice and work on more biting comments, you'll get there, Ziggie (Mark), I'm sure.

There are so many small benefits from equipment that are fairly important to triathlon: helmets/tires/wheels/disc covers etc. I believe smaller cranks may fit into this category. It always starts with training and any idiot knows that so I'm not sure how uneducated you actually are about the benefits of using aero equipment and holding proper aero form. Smaller cranks allows one to get lower and maintain a higher cadence. It may be at the expense of power or not. I personally believe shorter cranks will become a trend, but we'll see.

Of course, for the people who cannot the hold aero position for the extreme majority of the long course race, well, that loss of aero position and becoming a human parachute will likely negate MUCH of the benefit of pricy equipment.

http://www.aeroweenie.com/assets/backup/cervelo_frames/cervelo_posi...

Edited by dtoce 2016-04-02 8:12 PM

Dale,

Personally I think they help a lot. I have been a big proponent for a while. I think the whole cadence/torque thing is a bit of a red herring (IMO)

It should be discussed with a good fitter is you plan to go really short. Remember that theoretically your seat goes up and back and cockpit comes back (theoretically) so can be big changes.

It really depends on what your current fit limiters are.

Thanks, Marc. I was fairly certain you felt this way, especially with your work on/with LS. I was just recently re-fit and and am going through the adjustment period. Because I am racing IM again in July (at the tender age of 55 this year), I was thinking about long term changes I want to make and shorter cranks seem like something which may fit my style.

Since there is a cost and the gain is unknown, I'm thinking the next best step is going to be to go back to the fitbike and try the various crank sizes and really pay attention to comfort, cadence and power. I'm unsure of where the benefit falls off and since I am currently at 172.5, I want it to be worth it and only make one change if I can. I was thinking that I'd push a tiny bit shorter than whatever I found reasonably comfortably/efficient. I'll likely order Cobb cranks and may go with the 165 compact unless even shorter is better in testing.

At my fit, I had more focus on getting lower in the front this time and dropped the spacer I had since my beach accident in 2012. I've had neck and shoulder limitations since the injuries, but my flexibility is improving.

By dropping the pads, the theoretical movement of the cockpit towards the back sounds right, but I was moved more forward on the seat and the seat was dropped down slightly-at least from my prior position. My hip angle is still pretty consistent and slightly improved. I am currently 103 degrees for a hip angle (and 83 degrees for my shoulders as a double check-goal of 80-85 by FIST). My saddle height achieves a knee angle of 150 degrees now.

I can say that when I went shorter crank I didn't immediately find it that different. It's when I rode them for a few weeks then tried coming back to 172.5 that I really noticed. I went 172.5 to 165.
When we set Lionel's cranks to 165 (from 172.5) he said it made an immediate difference.

The whole cadence thing...ignore it....just shift gears and it all evens out.

In theory you up the seat to preserve the same distance at 12 and 6 o'clock.
In theory you bring the seat back to preserve knee over pedal spindle at 3 o'clock (which many will debate)
If you bring the seat back, cockpit comes back

But that's theoretically, It may just be a solution for stretching you out, creating more drop, creating more knee angle.....so all part of the bigger picture.

Edited by marcag 2016-04-03 8:27 AM

Just want to put it out there that Marc was a huge help in making my decision to try out the shorter cranks. It was nice to get some more reassurance before I took the plunge.

To echo what Marc said, so far I have not noticed any difference in my cadence. I naturally seem to settle in at the high 80s but my coach has me doing some low cadence stuff and the cranks haven't been an impediment to that. Really, the only thing that I have noticed is how much more comfortable I feel staying in aero (while on the trainer).

There is not a lot of good research on crank length and submaximal exercise performance. Crank length probably does matter somewhat, but not in the tiny 2.5mm increments that most cranks come in. Whatever cranks that you are used to, sure, if you change your crank length by 2.5 CENTIMETERS (instead of 2.5 millimeters), you will indeed feel a difference. But then there is the idea of relative size. For some people, 165mm cranks are really short. But for others (very tall riders), 180mm cranks can be even shorter. I recently read this on the web, and I find the argument fairly compelling:
http://bicyclecranklength.blogspot.com/

All that said, if you are shorter than average, look at cranks smaller than average. If you are taller than average, look at cranks longer than average. Going shorter can also help if you have a fairly extreme aero position. And if you typically ride flat terrain, you will also be fine with a shorter range of cranks. However, if you ride a lot of hills, going longer might be better, many riders report struggling with (relatively) shorter cranks on climbs. Another thing, if you're typically a slower cadence rider, go longer. If you're typically a higher cadence rider, go shorter.

Whatever you do, you must adjust your seat height with any change in crank length. Anyway, lots of generalizations above, but test things out and go with what seems to work best for you.



Edited by DarkSpeedWorks 2016-04-03 9:24 AM

just to add to the confusion... going longer is what I needed to become more aero, but I agree for most people you can get more aero with shorter cranks if you need/want to preserve the hip angle (open'ish)

I think most people could ride a pretty wide range and not be able to tell the difference.

Yes, I've heard from a few others that being able to hold aero position on the longer rides is a bit easier on the shorter cranks and if you can't hold it for the entire race, there is a big cost in terms of drag. The key is of course to have the 'biggest crank' you can comfortably turn and hold aero and generate the most power-and that comes with testing. My fitter also commented that my ankling position tends to have a slight toe point. I always suspected this was from old soccer injuries and the total lack of ankle flexibility but do wonder if the shorter crank will help or hurt due to this peculiarity.

I'm still a believer that higher bike cadence bodes better for running due to neuromuscular entrainment and higher running cadence helps minimize landing shock. These may be more personal beliefs, though.

Thank you all for your responses. Marc, I do understand what you are saying about the movement of the cockpit backwards. That's a nice summary. Also, can I ask how tall you are?

I do some over- and under- cadence work in training, just to be able to use it to better shift. I am not a good rider and all this bike stuff was new a few years ago. Marc was instrumental, along with Shane, in helping me understand the power meter. Now that is a valuable training tool! I still use HR also, but it is all just another metric. Going 'naked' , without looking at the Garmin, is fun too. Gotta develop that sense of 'feel' that I lack.

DarkSpeedWorks (Greg), I appreciate the link and have actually read that article a while back and it was good to reread it and print it. It's nice to confirm that there just isn't a lot of data out there yet on optimal crank length and performance, but I do see a trend over the past few years with riders, especially some Pros, going shorter with the cranks. (I also wanted to comment that I enjoy your posts very much-when I do read BT, which is far less frequently of late.)

Leegoocrap (Chris)-it's always nice to know there are people on both sides of the Gaussian curve. Optimal power with the best position is and least drag is certainly the goal and I'm glad you found your optimal. Can I also ask, how tall you are?


I suppose the time is now for me to do this, prior to my upcoming big bike volume months, as the race looms ever closer. I doubt it will seriously negatively adversely affect my riding this close to the race...at least I hope...

So, I like the 'assumptions' in the article by Kirby Palm-1)Taller people should use longer cranks , 2)Crank length should be proportional to the size of the cyclist and 3)Crank length should be related to inseam (and therefore height/leg length). I certainly see no reason why all bikes have to have 170 or 172.5 cm cranks.

Palm's blog has the formula of:
optimal crank length = 5.48 x inseam length (in); or 2.16x inseam in (cm)

It would be nice to have more crank options out there to allow some competition to drop the price. I like this adjustable crank but 500 bucks is pricey.
http://www.hscycle.com/Pages/adjustablecrankset.html

I also like the Bike Dynamics guide based on height and inseam*(I copied male numbers-women are ~1-1.5inches shorter on average):
Height Crank Length(cm) Inseam (cm)
6'0" (183 cm) 177.5mm 85
5'11" (180 cm) 175mm 84
5'10" (178 cm) 172.5mm 83
5'9" (175 cm) 170mm 82
5'7" (170 cm) 165mm 79
5'5" (165 cm) 160mm 77

for the link and table:
http://bikedynamics.co.uk/FitGuidecranks.htm

....Or perhaps I'll put it off until after the race and build a bike from the ground up with a friend of mine who loves building bikes. I'd probably get what I want a lot cheaper and could have some cool upgrades like Di2 electronic shifters!

All wonderful stuff, but I don't think most of it addresses the primary reason a triathlete would consider shorter cranks, ie to open up a hip angle closed off being in the aerobars.

Simple : measure the drop between your seat and your pads. What is preventing it from being a big number, ie more drop : it may be your closed hip angle. If so, you're a candidate for shorter cranks, if not, don't bother. All the leg proportion stuff, high torque....yada yada yada, IMO.... noise for us.

7.5mm is not going to change much in terms of torque, cadence....etc. 7.5mm will give you 1.5cm at the top of your pedal stroke and in aero, that is significant.

Many people have tried to give a formula and it just does not seem to work. The formula might work 50% of the time.

Crank length is about fit, unfortunately it is the one thing that most fitters don't touch.
My bike came with 172.5 cranks, your chart would have me on 170mm. I switched 3 years ago to 165, I would have gone shorter but there just was not a lot of people making the shorter cranks. The sub 165 cranks that were out there they wanted a lot of money for, it's changing now and more people are offering shorter cranks.

thanks for sharing!

What a fabulous thread! Thanks for that Dave. I went through some articles all the way back to the 2001 Martin study (only 16 riders) which looked at crank length and power. In that study, the length that stood out as having the most power output was 145 followed closely by 170. The other three were statistically significally below those at: 195, then 120, then 220. Other lengths were not used in that study.

Interesting.

This has the citation of the Martin Study.
http://biketestreviews.com/cranklength/

unfortunately the martin study isn't very helpful for endurance athletes. :/ (at least triathletes / TTers)

Edited by Leegoocrap 2016-04-04 12:23 PM

And, oddly enough, the height or leg length of the test riders was not considered (if I recall correctly).

What would be very interesting to see is, not if there is a relationship between crank length and power, but if there is a relationship between relative crank length and power (i.e., crank length relative to leg length).

and threshold as opposed to max!

That may be true. Or not.

It is kind of like saying that youu could take 10 riders of varying abilities, keep them training the same, and change their head wear all you want (some going bare, some using road helmets, some using aero road helmets, some using basic aero helmets, and some using advanced aero helmets).....they will finish in the same order. That could also be true.

But, I bet in both cases, even if they finish in the same order, they likely won't finish with the same time spreads between each. Especially in the case of crank length, if you test on highly varying terrain (can't just do this test on the flats) and if you use some very short and very tall riders. The short riders will have a tough time with the long cranks and the tall riders will have a tough time with the short cranks. Or, at least, such has been my experience.