mstateglfr 

your best is not good.

Moisture

I've stated all the reasons for why I would never, ever go back to 19 or 20% after trying 21.5%.

Koyote

All you guys are a hoot. You let a doofus suck you back into a zombie thread.

shelbyfv

Maybe just giving him more rope.

Moisture

I ended up writing a detailed response to your question yesterday before it all got wiped away just as I was finishing. I will briefly summarize.

These are observations based on a rider who supposedly spins at a lower cadence than most of you.

I thought that smaller crank arms would help be spin faster. They didn't. My logic is that your nervous system is pressured into having less time to time your spin, more chances of accidentally overshooting the 6 o clock position as you spin past the dead zone. Going from 165 to 175 for example allowed me to spin at a similar cadence, just smoother while being given the potential to apply more leverage without any drawbacks. There is a fine line between being able to apply as much leverage as reasonably possible without affecting your average cadence or the smoothness of your spin. I think that 21.6% is that fine line. 21% seems to me like it would be offer a slightly faster spin.

I'll say it again, 21% is not a definitive way of calculating crank arm length, but I think that it is a good starting point.

smashndash

This is great real-world anecdata. I feel like there's not enough information on the downsides of shorter cranks. Tandem applications is definitely one of them. Shorter crank users need to use easier gears and higher cadences. However, I would challenge your assertion that this uses much more energy. The equivalent cadence on a shorter crank uses the same foot speed - ie a higher cadence becomes easier. The difference in energy usage due to this is probably not significant. You can say that there is more overhead due to more repetitions of pedaling through the deadzone. However, it's been shown that shorter cranks have significantly smaller deadzones as well. So overall it's probably a wash.

The biggest difference is probably neurological. You have to fire your muscles with a higher frequency. I can personally spin up to 190+rpm seated and produce real-world useful power upto 140ish rpm seated. I have no issues cruising at 100rpm on my 165mm cranks. I'm definitely just as strong on the climbs compared to when I had 170s, especially seated. But for someone who struggles to spin 90rpm, going from 170mm to 150mm would be a shock because the required firing frequency is much higher. However, I think this can be trained pretty easily. It just takes time and an understanding that 100rpm now is not the same as 100rpm before.

That being said, there are so many benefits to shorter cranks, such as aero, pedal clearance, comfort and injury prevention, that what little tradeoffs exist (which are hard to prove) are probably worth eating. Not saying that's the case for the tandem necessarily, but I'd be curious to see what happens if you put those 151mm cranks on her single bike.

Moisture

smashndash the 165 and 170 arms.. what percentage of your inseam is that?

So I'm seeing preferences towards shorter arms for a faster spin, and there are riders out there which perform better at lower cadences outputting more leverage. I think this should give us a better idea what an ideal percentile range would be for the average rider.

By this point, I think this is mostly a matter of changes in gearing as well as fit optimization, although it can help you output a bit more (or less power, depending on how the change in leverage over the crankset affects the quality of your cadence.

Carbonfiberboy 

Uh, yup. My 71 y.o. wife has trouble spinning over 95. You're an exceptionally fit person, though you could try this: On your trainer, pick some moderate power, like 70% FTP and spin 80. Note your HR. Now produce the same power at 120 rpm. I betcha your HR goes up, not as much as a "normal" person's would have, but some. That's the "leg thrash" penalty. The older/weaker the rider is, the worse the penalty. On our tandem, I'm running 175s as I said. That's 23% of my inseam. I can spin those at 100 no problem, but not climbing. Maybe 85 upper climbing limit for producing serious power w/o spiking the HR. t don't think there have ever been any captain's sets made with 165 cranks. Plus I'd really rather not rejigger my cadence at my age. My best climbing cadence has been 78 for 25 years, though I'm working on bringing that up. The other minor issue is that I produce double my stoker's power at the same effort level, so there's that. Plus she cramped so bad after Sunday's ride that she can't do intervals with me today. Yeah, piss and moan, piss and moan. We're going out for a 37 mile 2000' group ride this coming Sunday, that's the good news!

The secret to the world discovering that high cadence is "better" is of course the demo by Lance that spinning faster works better if your red blood cell count is really, really high. That "trickled down" on a lot of people.

Toespeas

you trying to pole vault over the guy in front LOLOLOL ROFL !!!!!

Moisture

Do you think that 23% of inseam is too much? Have you tried arms smaller than 175 before?

AnthonyG

Where this thread is going wrong, in my view, is the notion that what's best for recreational riders is also best for elite racers and visa versa.

Now I guess where this topic really goes off the rails is in the initial suggestion that 21.6% inseam(not from the OP), IS the magic number for racing.

In elite racing, aerodynamics are very important, however since its a team sport there are always exceptions. A team leader that's a hillclimber isn't expected to be breaking the wind or going solo on flat stages so the requirements are different. They will have team mates to break the wind for them on high speed and flat stages. A sprinter who only has to get out into the wind for a few hundred meters usually may be different however one of the "Manx Missiles" advantages was that he was sprinting in a very low/aerodynamic position.

If your a recreational rider who likes longer cranks then good for you. All is good, but you haven't rediscovered some magic number that works for all athletes. Elite athletes have to have VERY strong cardio-vascular systems and 4-5 hours in the saddle is par for the course. 1 hour races are very short sprints in this game.

The requirements of recreational riders and elite athletes are just different.

Moisture

Crank arm length is mainly a factor of the correct fit and ideal gearing in accordance to your power output and application. Whatever the ideal figure may be according to each specific rider, regardless of what sort of riding you do, this has more to do with getting 1cm lower in your race tuck.

AnthonyG

Here's the thing Moisture. You don't actually have a clue, so stop talking like you do.
At the elite, elite end of cycle racing, athletes train for hours in wind tunnels to make tiny improvements to their aerodynamics.

Moisture

Why would I care about elite cycling? Why should any of us care? How is your condescending input with regards to elite riding; something none of us are talking about, or concerned about, pertain to this thread?

smashndash

Just measured my inseam - looks like it's around 31.5 inches (800mm) floor to the floor of my pelvis. 165mm is around 20.6% and 170mm is around 21.8%. So pretty close to your rule. Honestly, I don't think I'd want to go any shorter. I could live with 170s if I had to but 165s are just way better. I also have a strong preference for spinning. When I was strong/fast I was regularly hammering 110+rpm up hard climbs. I also really like to get low when riding, regardless of aero. Getting forward and low helps with weighting the pedals, which helps light riders like me put out big power. That was the main motivation for getting 165s. I was running into a lot of hip impingement despite having a really high and forward saddle. I was being forced to ride on the rivet to get really low. Now I can get low and stay farther back on the saddle.

If I were just riding casually, I'd probably be fine with 170s. Especially since I wouldn't have to spin as franctically.

LOL I had no idea what your ages were. Maybe it might be unreasonable to expect everyone of all ages to be able to spin up.

OK so for a given crank, if I start spinning at a higher cadence, that would increase my foot speed. Assuming the power output is the same, this usually means a smaller anaerobic load and a larger aerobic load, which means your oxygen consumption goes up, but your legs fatigue less. This doesn't mean you're using more energy, it just means more of your energy is coming from your aerobic system and thus your oxygen consumption is going up. "spin to win" comes from the fact that you can trade aerobic suffering for leg fatigue. If oxygen usage were our only metric, we'd all be pushing 55:11 gears. I will acknowledge that spinning up a given crank faster has increased overhead (more instances of deadspots, more reciprocations), so you're definitely using more energy, but I think it's yet to be seen whether the requisite cadence increase from moving to an appropriately short crank actually causes you to be less efficient. There are other factors to consider, such as pelvic stability, comfort, muscle activation etc. Subjectively, I find it way easier to spin circles with a 165 than 170.

Moisture

How accurate you guys think this is? -

Another interesting formula yielding similar results comes from fitguru Bill Boston (www.billbostoncycles.com).He suggests measuring your femur (thighbone) from the center of the hipjoint to the end of the bone in inches. This number will be your cranklength in centimeters.

Moisture

smashndash 21.8% of 80(cm) is 175mm. 21.3% is 170.

so it seems like 21% is roughly an average starting point for the majority of riders, and that you can go maybe .5% in either direction depending on your specific style of cadence. You've really helped to prove how its better to go a little bit too short than too long with this entire crank arm length ordeal. You've also indicated that close to 21.6% may actually be a little bit too long, at least in your case with the 170mm arms, according to your riding style and proportions.

In my case, I dont think that going roughly 20.5% like you did would result in a better spin or agreeable results. I guess I am simply towards the other end of the scale. In the future id choose 185mm (21%)

As for going low - well...what id consider to be low for myself, I find like it really helps with my centre of gravity around tight, quick turns. Ever since I've gotten down to a more reasonable weight level, I've had no problems running my stem a little bit lower than the handlebars without any drawbacks in handling performance or signficant low back fatigue on longer rides. It is really helping me to enjoy the added performance benefits. I feel like the amount of power I can output (whether its sustained or short, hard bursts) easily outweighs any penalties in failing to get super aero, so maybe this is another reason for my preference in using crank arms which are on the higher end of the scale.

I'm curious,.how does your inseam look in proportion to your torso length in percentage? 45% is the average. For me, even 1% longer legs than the average seems to affect my ability to get low. Again,.im not complaining. I guess my relatively short torso should explain my preference for a shorter stem, because I thought that my super long arms would have made up for this.

I am continuing to lower my stem in small increments here and there. I am glad to see myself adapting towards a more performance oriented fit.

smashndash

Sorry for the bad math. You're right. I guess even 170 is under the "rule". Especially considering that I'm slightly underestimating my inseam.

I'm 5' 7" ie 1700mm. My inseam would then be 47-48ish% of my height. I have fairly long legs for my height.

I actually think the best indicator of crank length isn't femur length, inseam or height. It's tibia length. When you're at the top of the pedal stroke, your femur is basically parallel to the ground, so even if it gets longer or shorter, that's just going to affect ankle flexion. Your tibia, however, is still mostly upright. Of course, the longer the femur, the less perpendicular your tibia is to the ground. So maybe crank length should actually be inversely proportional to femur length, but with a small coefficient. Your torso should never have any say over the fit of the power chain.

If you have a long tibia and a short femur, your knee is going to come up higher than someone with the same inseam, shorter tibia and longer femur. I think inseam also misses out on the distance between the pelvic floor and actual hip socket, which is where the femur pivots.

Do you know of any formulas that explicitly prioritize the tibia?

The thing is, this is a really basic geometry problem to solve if you can just test a given persons hip mobility and determine what the maximum knee height can be. This max knee height might be lower for people who are trying to get more aero. See: TT bikes. Then you can work your way down and see how high the foot can go before ankle + knee mobility becomes an issue. That's your crank length. I think it's a bit of a wild goose chase to try to come up with a formula as simple as "21% of inseam" because there are so many variables at play here. And as far as "starting points" go, everyone has to use the cranks that come with their bike anyway. If the crank seems unsuitable for some reason, then you can do that basic geometry.

Basically, crank length is a hockey stick optimization curve. You want it to be as long as possible without hitting the wall (ie exceeding the comfortable range of motion of any joint).

BTW: you are not alone in thinking that 175s are not long enough for some people. I largely align with Adam Hansen's fit philosophy and he runs 180mm cranks at 6' 1"

Carbonfiberboy 

I used to do a good bit of randonneuring. I've observed that randonneurs, in general only, prefer to ride a lower gear, higher cadence for the reasons you give. They don't ride at high effort, so oxygen cost is not a factor, but leg fatigue sure is. The long distance competitive riders I've ridden with are incredibly smooth. However for low VO2max riders like myself trying to ride at high effort, oxygen cost is a big factor, really trying hard to stay aerobic. I did get to be pretty good at long anaerobic efforts, but in the long run that's not sustainable as it gobbles up the glycogen. So that's why I do big climbs on event rides at a relatively low cadence. I do heavy gym work to increase my leg endurance, or did before Covid.

Carbonfiberboy 

That ratio came from measuring crank lengths of TdF riders many years ago, using inseam lengths. Here's a little viddy that says you have it backwards:
Pantani rode 180s. These GCN guys are a bit sloppy as usual, using rider height instead of inseam length, but still, it's obvious that riders use what they think will produce the most power for the least effort, and especially pro racers would do that, right?

The main events I train for are 10+ hour events. The short form is: what's best for pro riders is best for recreational riders, too. There's absolutely no reason that would not be true.

Carbonfiberboy 

All my road singles have 170s, though my formula length would be 165. 175 is less than 1/4" longer than 170. Frankly, going from bike to bike, I can't tell the difference. The funny thing to me is we get so many people over in the bike fit forum thinking that their knees hurt because their crank length is wrong. It is to laugh. The issue is simply that they're weak and stiff, duh.

Carbonfiberboy 

I think TT and Tri riders opening the hip angle is much more about breathing effort than it is joint mobility. Otherwise I totally agree.

HTupolev

People who fit TT/Tri cyclists tend to talk about it mostly in terms of pedaling mechanics, with an overly-scrunched angle identified by pain or weaker pedaling. For Tri specifically, another major consideration is correlation between non-scrunched hip angle and run performance.

I'm not a TTer, but for me personally, I also find that it's an issue with what my legs are comfortable with. When riding in a really low posture on a road bike, I'll usually* self-select fairly forward sit positions even if my cardiovascular system is staying comfy and even if this forces my upper body to support a bit of weight.

*Descending complicates the issue a lot.

smashndash

I think the complications that arise from a high knee depend on how much pelvic rotation you have. If you rotate really far forward, you will run into hip impingement issues. That's usually solved by unrotating the hips and a rounding of the spine. But then a) you run into reach issues b) you can start running into lower back pain and c) if the knees are coming high enough, then yeah you run into breathing issues.

That being said, I think breathing issues are a much bigger concern for those of us who have belly fat. I've noticed that as I've gotten fatter, my belly impinges on my legs (and thus prevents me from breathing) well before my hip joints run out of mobility. My assumption was that people who are serious enough about TTs would get rid of belly fat before competitions but maybe that isn't a good assumption lol.

Anyway. That's pretty much all I have to say on this subject

base2 

It's not 1cm lower in your tuck as much as it is 1cm lower in relation to the crankset. That is the rub. It's that 1 cm that brings all the range of motion issues to bear...of which "aero" is one.

1cm is a lot. About a cm of suspension post sag (cause I'm a clyde again ) caused me to miss several weeks of "training" because of a scraping sensation behind my patella. It tooks weeks to develop. A week off the bike when I realized what was going on. 2 more weeks of gently working back into the routine. I'm still nowhere near performance in strength or fitness I had near the end of January. Knee angles, hip angles matter.

21.6% of inseam for my 80cm inseam equals 172.8mm.

175mm cranks for me resulted in a psoas tendon & iliotibial band issues arising from a former running addiction. So suffering an injury that took about 18 months to recover from, I switched to 170mm cranks. Whoo boy! My cadence went up once I fugured out how to turn them.. I could/can/& do spin & hold 100+RPM but my average still held ~90 The problem with the shorter cranks is I felt "bound up." I could spin like the devil, but I had no torque. I couldn't muscle myself into a good sprint & when I hit that 28 cog on the cassette that used to be ok, I now felt I needed 1 more. Whereas with the 175mm cranks I had tons more power, my power was chunky, lumpy, slow & resulted in tired hamstrings. (175mm is great for a mountain bike though.)

It turns out that for me, in my particular case, 21.6% is nearly ideal.