Bicycle Crank Length Debunked: Position and motion analysis using 4-bar linkage.

Trying to take some of the guess work and 'tradition' out of cycling.
For enquiries: [e-mail]

[@PeakTorque]

Thanks for all the comments so far. Some assumptions/approximations for the dynamic model: 1. The 'tibia' or shin length of the link is assumed to be the hypotenuse between the actual tibia length and ankle to cleat measurement. This is an approximation to make it work in the four bar analysis. Ankle flexion assumed constant. 2. Crank angular velocity assumed constant. This is imperfect as the crank vel varies with load in 1 rotation.

[@eduardobueno5040]

See this for some information on the ankle rotation during the pedal stroke: cds-0.blogspot.com/2011/09/pedaling-model-i.html?m=1

[@bikescience1]

Eduardo Bueno What’s interesting is if you repeated that mathematical process with 10 different riders (or even the left hand side of the same rider) you’d more than likely get a different result. Again, it’s pretty easy with the right tools to measure the angles your study has extracted. Studies have shown that more elite riders have a tendency to drop the heel less in the first phase of the stroke. The pedal stroke dynamics of individual riders is learnt (by huge amounts of repetition) and is very individual in itself. The real world challenge for bike fitters is to allow for individual movement patterns and more often than not, asymmetries in those patterns.

[@PeakTorque]

@@JerrysScenicCycling really interesting results, thanks for sharing.

[@lyanbv]

@@JerrysScenicCycling I would imagine this to be the case indeed. Cleats position affects muscle recruitment. Far too forward requires more calf to stabilize the foot for pedaling. Another issue is the shoe. Sometimes, larger-size shoes are purchased to deal with wide feet. unfortunately further moving the cleat screw-in position forward (as opposed to buying actual wide-style shoes).

[@bikescience1]

Lyan Villacorta Such an important point. I see so many bike fit clients who turn up with big shoes because they have wide feet. There are some great wide fit shoes on the market now, so if you have wide feet it’s really worth taking the time to get the right shoe. Different brands have different hole positioning relative to their last too. The first Fizik shoes that were released 10 years or so ago had holes REALLY far forward. Bont have them pretty far back. Do your homework before buying.

[@dannyshearer]

As soon as I heard PowerPoint, I was waiting for 'aged 5' to appear 😂. Crackin' vid 👍🏻

[@Bultish]

pen is working

[@davidgeorge9233]

Worst thing about this video was that it ended! Really informative and thoughtful analysis of something I’m currently trying to get my head around. Thanks 👍

[@BDGRanger]

"Let's put some numbers to it" thank you! The lack of data and sound reasoning behind most discussions/debates in cycling has driven me nuts!

[@jscottfree]

Fascinating study you’ve done here. I’m very interested in following your progress. My academic background was in Human Factors Engineering and Ergonomics many years ago before I started cycling. I have been searching for an analysis just like yours. I like that you also mentioned leg mass as well. I am about 185 cm tall with large feet (size 47), and big legs. I also have fairly long tibias. I do not use a power meter but I do get the feeling that I cycle better with shorter cranks. The joint angle I am concerned with is the knee. There is a very powerful fulcrum there as well and I want to continue to preserve my 65 year old knees. I haven’t measure power but is seems to me I am able to generate more power at the pedal with the more open knee angle a shorter crank provides. Knee angle and hip angle are related but I haven’t paid attention to hip angle. I look forward to seeing your next video on the subject. Thanks!

[@tomkunich9401]

As I aged, I noticed that my warm-up for rides to the point where I could accelerate with the group was taking longer and longer. I am 6'4" tall and have used 175 cranks forever. I decided to try 172.5's, and that warm-up period more or less disappeared. Also, my pedal stroke up very steep hills smoothed out. Since I am about to turn 79 (tomorrow) and over the last two years, my power has almost disappeared, so every little bit helps. Over the past 12 months I've ridden 5,000 miles and climbed 80,000 feet which is about one third my normal climbing (last winter destroyed many of the hill roads) I have been unable to easily find the climbs best suited to me. Plus, with my degradation of power, I am too slow to ride with the group and so have to ride solo much of the time. But the improvement in power with the shorter cranks. I am able to continue even on the worst sections, though I am crawling over the steep sections.

[@Fastfitnesstips]

Beautifully done; loved the analysis!

[@zzhughesd]

FFT!!! Could you add a video for all us fast-tempo long distance for-fun-pleasure riders. Less so than this video for explosive-power-racing types !!!! I'd love to think I'm fast and unlike Alehandro I was still fast at 40 and now 43 I've lost power and now just enjoy zone 4 no zone 5's

[@athaliawhitworth]

The analysis of the bottom of the pedal stroke affirms what I've wondered. After making all other adjustments to seat position, my hunch was shorter cranks would make my hips happier by smoothing my extension. Data says yes. Thanks so much for this in depth explanation.

[@PeakTorque]

Glad it helped!

[@floydie9255]

I went from 170mm cranks down to 165mm in an effort to improve pedal stroke smoothness when in the aero tuck position. Now I know why it works!

[@manuelmalagon210]

Floydie I did the same! I even feel more confortable breathing while riding on the drops.

[@rickmancini772]

I also went from 170 to 165 at 5’8”. It made a world of difference, The typical crank lengths are wrong.

[@keirfarnum6811]

When I was racing off road in the 90s, everyone was using 175+mm cranks, but being under 5.5 ft tall with short legs, I found 175 cranks just too long. I switched to 170s and never looked back.

[@njm3211]

I switched from 172.5 to 165 cranks a few years back for the reasons you so ably have shown in your presentation. Your presentation is the best I have encountered so far on this topic and validates my decision to switch to shorter cranks. Good to know that moving seat forward also helps to open hip angle. Maybe you could have a look at oval chainrings too, another controversial topic. Cheers

[@PeakTorque]

Thanks Norman. There are lots of assumptions and its a simplified model. Mixing engineering and biological/chemcia/neurologicall functions will never truly align.

[@martinanderson1737]

Great video ! With the reduction of crank length, should your model not also increase the seat height since the maximum extension of the leg is correspondingly shortened when the shorter crank is a the bottom of the rotation? That would open up the hips even more.

[@jonridesbikes3853]

Martin Anderson very true 👍

[@laurynasjagelo5075]

1:30 and 16:30 mentions that.

[@glennoc8585]

It does but you must consider that as one raises the saddle the setback incrementally increases too if the axis point is fixed so if you bring the saddle forward your hip angle is the same.

[@troyvantrienen9418]

I would love to see how an oval chainring plays into this equation and also the other interest would be how cadence speed would it should change to create the same output. Great clip.

[@brucecunningham6718]

Well present video, thank you. As a touring unicycle rider the subject of crank length is always interesting. Road unicycles always use much shorter cranks than bicycles, mostly as being fixed gear it gives the best compromise of torque and speed. Gearing can't be changed but crank length allows for at least a change of leverage for different riding conditions, styles and rider's strength/skill. I run 110 mm cranks on a 29" unicycle and 127 mm on a 36" unicycle, others go much shorter down to 89 mm. It makes hills harder but cruising far less taxing and even a little faster, perhaps because the angular velocity allows for an easier passing of the dead spot at the bottom on the crank stoke. The issue I find hard to compare is rider position. There are no "bike fits" for unicycle riders! The basic position is much more directly over the crank hub with little room to vary seat position fore or aft and a very open "hip angle". I believe it's still be same 4-bar linkage so perhaps the mechanics are the same as on a bicycle. Maybe you can comment on that? Again, thanks for interesting video.

[@nwimpney]

I don't think short cranks make unicycling any easer/faster due to the dead spots, I think it's simply that you are limited more by cadence and how fast you can move your feet, than you are by the amount of force you can apply to the cranks. to do 20km/h on a 29" unicycle you're going to need to do around 145rpm. You don't need much more power to do it than you would on a bicycle. If you try and ride your bicycle at 20km/h with a 1:1 gear ratio and the same 145rpm, you would be happy to have tiny cranks there too.

[@thegearboxman]

Don't forget the beer belly effect! For a lot of riders I see, reducing the hip angle means that the knees don't hit the gut LOL.

[@andywebb3568]

Being a shorter rider (173cm) I changed from 170 cranks to 165 a few years ago, it changed my life on the bike. It isn't just about the 4 bar. Shorter cranks allow the saddle to be moved up and back a few millimeters, for the same closed hip angle as the longer cranks, which unloads the weight on the hands, in my case it stopped numbness in my hands.

[@sisterfixit4017]

Same here.

[@martinbogadomartinesi5135]

lol if you consider yourself short, idk what I am (165cm), even 51cm frames feel big to me

[@Skooteh]

it's neat to see the benefits of shorter crank lengths and I'd be interested to see how this could be adapted for MTB use where shorter cranks have inherent value anyways- fewer pedal strikes/lower BB for cornering aggressively. I wouldn't be entirely surprised if this is the next portion of geometry that gets pushed to the practical limit (like head angle, bar length, top tube length and stem length already have).

[@James-zu1ij]

When you said inertia, my ears pricked up. I am only 6'2" but my legs are massive (and long). I have gone from 175 to 172.5 cranks and have my seat slammed forward. I feel I get a more integrated comfortable feel. The problem is when im at battle speed, im often at 105 - 110rpm for long periods. This has got to take a tole on my huge legs. Interestingly the only time Iv been able to finish with the chain gang at the end, after 40 minute averaging 27mph, I was running 170 cranks on a single speed with a bigish gear.

[@James-zu1ij]

@Alien On a Bike lol poor thing. It's people like you who get sheltered all the way across France, being dragged along by decent sized men up front. Only kidding I'm just jealous 🤣

[@InfoAction_]

This is a very interesting analysis thats is very well done, I understand the constant angle flection assumption as adding more DOF can drastically change the complexity of the analysis. One thing I was thinking whist watching is that muscles have different force production profiles depending on the amount of flection at the joint (amount of stretch on the muscle), and although such impediments as hip impingement is a first priority, to maximise performance with a force analysis, consideration to each muscles force production profile is needed for an accurate depiction. I have an inkling that the ratio of force production between the glutes and quads at their respective angles would contribute towards a total picture. Again great work, and thanks for the video.

[@PeakTorque]

Hi, thanks for the comment. You're completely correct I think. It just highlights how complicated an optimisation problem it is, there are sooo many variables.

[@FrankMOrtiz]

Earlier this year, after watching this video, I went and changed the crackset on the cheap on one of my bikes from 172.5 to 165 mm (for about $40). I tried this out for a few weeks and it felt less fatiguing overall and enabled pedaling at about 10 rpm faster. It also alleviated saddle discomfort for me, especially at higher cadences.

[@willappleton]

Hey I'm now considering the 165 for that exact reason.. may I ask how tall you are?!

[@FrankMOrtiz]

@@willappleton I'm 5'10" w/ 32' inseam and my shins are longer than my tibias, 17' vs 15'. Sorry for the late reply.

[@Varaxis]

I commissioned a custom-built frame with 80° STA and paired it with a 152mm crank. It took a little getting used to, but found no downside other than it forced my seat a bit higher, which made the seat-to-grip height difference a bit worse. I also had to switch from using RPM to using "foot velocity" instead. Thought cornering would be worse, but instead it actually felt like I got more consistent at them. In my head, I thought my legs were doing more of a rapid "piston-like" movement, like stair climbing, and speculated that this kind of position would benefit my fitness on foot (running), which made me wonder if triathletes were going after the same thing. Made me question the whole idea of going for different positions to rest overused muscles was even wise, compared to just giving the most used muscles even more training.

[@waynosfotos]

Very good, and yes people have many myths surrounding shorter cranks, but there is more; Shorter cranks require a higher seat position, this may be desirable for a person using aerobars to get the back flatter Shorter cranks means a slower foot speed, therefore you can increase your cadence easily, this may also be advantageous For track, 165mm is used for clearance, but they run many different fixed gears, so does it make that much of a difference? Your power remains the same as as you said, you have gears to compensate for the torque advantage at the crank, the whole action needs to be considered. Some proclaim shorter cranks are always better, but there is disagreement in the industry. It is clear that you need to study a number of factors as to what you are trying to achieve before making an adjustment. A “general” statement, (if there is such a thing relating to cranks) if using aerobars a shorter crank can be advantageous, for road riding it isn’t so important

[@peterdobos1606]

Noice! Have you considered or already done the 5 linkage version, adding in the ankle-to-cleat link? The whole "cleat under ball of foot" rule of thumb has also been subjected to actually doing the science and thinking about it from a biomechanical POV, with cleats being closer to the midfoot showing some real benefits, such as increased foot stability and reduced calf loads/fatigue.

[@p3trinho]

The first online evidence based biomechanical analysis I met online. Cheers dude. Great job. The tool seems really interesting.

[@glenslough910]

Amazing video! Interesting point about the self selection on gearing, which is something that I’ve had in the back of my mind when considering shorter cranks. It would be nice to know how much of an effect the shorter crank length has on the overall leverage provided, perhaps as a percentage. This could be of interest because it could inform whether or not different chain rings or cassettes, should also be considered, when going to a shorter crank length. I think most people are over geared as it is, but that’s a separate discussion. Thank you for diving so deep into this!

[@CreativeFishDesignsCharlotte]

yours video is still the best today , real data always stands time

[@sabamacx]

I mean, here's a years intro prep work for a sports physiology masters/phd here.

[@dominicbritt]

sabamacx mathematics + physics = engineering ... excellent analysis - would be good to use a model like this to spec setup on bikes when purchasing online - shame that engineers are not listened to more - age old problem.

[@PeakTorque]

@@dominicbritt they may not be listened to by management, but the customers listen. Which is frightfully effective.

[@tobiasbouma4071]

@@dominicbritt Where does that place mathematical physics then? :D

[@sabamacx]

@@tobiasbouma4071 mathematical physics is usually studied in a mathematics department, or the theoretical branch of a physics department. It's not an engineering field.

[@tobiasbouma4071]

@@sabamacx Being in the (MP) field myself, I'm definitely aware of that. I was just surprised at the way the equality was presented given my background, is all.

[@michaelpayne8337]

ST Angle has a larger impact on opening hip angle than crank length. In my experience, determining what hip angle works, or what range works best for a rider is paramount. That is the difference between a bike with some “magic” in how it rides, and one that is just “ok”. The crank length discussion is secondary. You cannot fix a bike with an incorrect seat angle for a rider via crank length changes.

[@CanyonWanderer]

Interesting!, I feel a hobby project coming on that involves Blender 3D, an inverse kinematics rig of my legs to scale and my current bike with correct angles and then loads of tweaking, thanks!

[@PeakTorque]

You're right, it needs inverse kinematics 😉

[@Pillokun]

I went just recently over to 165mm cracks from 175mm cranks that I have used pretty much all my grown life. Well in the first two weeks if felt like the smaller circle you are pedaling in was so restrictive that I was about to change back. You got no power, and it feels like you are on a those tricycles if you remember those from your early childhood. But I noticed that I never ever had any pains or similar issues in the knees and the like, and even though I still am lacking in power it seems I am adapting to this as well. Oh yeah, I am 172 or 5,6 or 5,7 or something like that.

[@h.p.734]

This was hilarious AND informative. I wish I could like this twice! Please do more of these Myth-debunking analysis videos for other topics too!

[@Alan_Hans__]

Excellent video. Love hearing the workings of someone who knows his stuff. Had a quick look through the title of previous videos to see if there's interesting stuff there. Saw the name Hambini so instantly subscribed.

[@stevecrabb1]

Very good analysis, real food for thought, thank you. looking forward to the next one.

[@philflip1963]

The dynamical analysis of power generation in a cyclic crank and chainwheel arrangement on a bicycle is a fascinating but VERY complex matter. As you intimated at the end of the video the most practical way of ascertaining optimal crank length would be to carry out some test, ;rides' with different crank lenghts to find out what suits a particular rider. The proof may well be in the pudding!

[@rcarlmoy5795]

This a very interesting presentation. Have you considered expanding your analysis to include consideration of how changing the angular sweep of the thigh effects the ability of the associated muscles to output power? The analysis seems to assume that the leg muscles will be able to cope with all the resulting angular sweeps equally well.

[@PeakTorque]

That would be really interesting to do. Im trying to formulate some inverse kinematic model to deal with this but would also need some electro-gram medical grade equipment!

[@LoscoeLad]

@@PeakTorque would be most beneficial to this analysis, I;m sure

[@jangrzybek3036]

Great video. I just measured that my old bike has 70.0deg seat tube angle and probably this is why I feel so stretched and uncomfy on it.

[@PeakTorque]

That is very slack!

[@bikescience1]

Nice presentation. Other key points I would consider.... One of the major reasons given for shortening cranks is the theory that it helps in more endurance focused events where power outputs tend to be relatively steady state (again, IM distance triathlon is a good example). There isn’t a huge amount of scientific evidence to back this up, as anything that involves testing whether a bike fit or equipment change increases failure time is inherently really difficult to test (there are just too many other external factors that might influence fatigue rates). It is pretty easy to dynamically measure closed hip angle (most bike fit motion capture tools do this). It’s all very well measuring the angle (or calculating what it might be with a tool like CAD) but that doesn’t tell you what hip angle any given rider can cope with over the course of their chosen event. It is possible to measure how the rider’s centre of pressure moves in the saddle throughout the pedal stroke (the pelvis can become more unstable in cases where any link in the kinetic chain (hip, knee or ankle) reaches, or is close to reaching the limit of its range of motion). Changes in pelvic stability due to any bike fit change including crank length can also be assessed using Inertial Measurement devices (mini gyroscopes and accelerometers) attached to the pelvis. These are especially handy as they can be used out on the road, not just in the lab/fit studio. Another hugely important thing to consider (and this is where using CAD or such like really starts to struggle) is that the human body (even of a highly trained elite athlete) is far from being a perfect machine. Accurately measure knee, hip and ankle angles for 100 individual pedal strokes and you won’t see two pedal strokes the same. Throw into that mix that a key component of fatigue (as well as reduction in force production) is a loss of motor skills (the ability to fire the right muscles in the right order) and it’s easy to appreciate that rider motion can be very different at the end of a long ride than at the start. This is why we feel like we’re pedalling squares when heavy fatigue sets in. A practical way of testing what crank length might work for you is to find a bike fitter with a fit bike that allows seat angle, bar position and crank length changes to be made. Once a position is found that’s approaching “optimal” (sorry for using that word as I’m a firm believer in that it’s unlikely there is a single fixed position that’s actually optimal). Pedalling one legged while varying crank length (and altering seat height to maintain the same leg extension at each selected length) will give a good guide as to what length “works”. The rider should be able to maintain a fluid pedal stroke when riding at low cadence without having to excessively rock the pelvis. Most people have one side that’s more mobile than the other. The trend in bike fitting at the moment is towards practical outdoor testing. Expect to see more fitters offering the opportunity to test crank length changes out on the road as part of their bike fit services. Sorry for rambling. Hope this helps.

[@PeakTorque]

Cheers, great comment and insight.

[@bryngreen2301]

A guru bike fit can adjust all these metrics.

[@bikescience1]

bryn green Choose a fitter as opposed to a branded system when you’re looking at bike fit options.

[@richarddechau4200]

I am retired and have ridden for over 50 years on the road. I still get 50 to 70 miles a week most of the year. But I am old school and know little about the reasons for crank length change. Old school was you don't want your knee in front of the axle when your peddle is at the 3 o'clock position. But I can see the advantage of moving the seat forward and up. When you get you knee in front do you loose any watts in your out put power? I would think as you move forward you affectedly in-gauge the peddle stroke a bit latter. Thanks for the video. I need all I can get ,it's tought keeping up with the young. LOL Rick D.

[@tednruth453]

Then add in the ankle and foot/ shoe plate dimensions......and constantly varying cadence.....shifting from hood to tops to drops.....shuffling forward and back on the saddle as speed and gradient changes/as fatigue sets in/as head wind changes...and effort level/ power output...

[@simonalexandercritchley439]

Thanks,I shall follow with interest. Another aspect: what if you have different leg length's and/or variances in tibea/ femur? E.G: my right femur is 22 mm longer than left,while the right tibea is shorter. Overall my right leg is 37mm shorter. To complicate matters my right ankle has a fusion and alignment issues,making for complicated bio-mechanics.

[@hardtailparty]

Fantastic analysis, thank you for posting this.

[@alcrane2289]

Something may looks perfectly correct on paper, but in RL we have 3D space... how about changing Q-factor instead of crank length? In most cyclists pedals are not very well aligned with hips vertically by z-axis, obviously.

[@markmarlatt1105]

I had always wondered the effect of crank length for my neck! I'm fairly flexible but my neck has issues and wondered it crank length would help. And I'm a bit cheap on getting a proper bike fit.

[@mikekrasovec6390]

Changing the crank length for neck issues is wishful thinking and expensive. First assuming that we're talking about road bikes if you have a visor on your helmet - remove it. Now. Visors are fine for mountain biking and such where you have a more upright posture but not for a road bike. After that I'd look into handlebar position. Usually this means that you want your handlebars to be moved higher and/or back.

[@jamesmedina2062]

How is your neck doing? I used to have more frequent neck aches than today. I find swimming really helps a lot to make it limber and any cycling helps the back overall if done for enough time.

[@markmarlatt1105]

@James Medina unfortunately I don't get on the bike as much as needed to properly adjust. The neck has gotten better as I adjust my fit. I've found that a zero setback sestpost and shorter stem are pretty much a must.

[@jamesmedina2062]

@@markmarlatt1105 give things time. You could get more limber or tighter, but one things for sure : you could probably ride more👍

[@feedbackzaloop]

Sir, your acceleration graphs are (as we have a proverb in Russia) written with a pitchfork on water. Firstly, - beyond being poorly scaled - "analysed" disregarding the sign - your main talking point is femur deceleration at "9 o'clock" position -, but mainly because the differentiation was done with constant crank angular velocity.

[@PeakTorque]

Yes i forgot to mention constant crank velocity which was simplification. I don't have the load profile to determine the true crank velocity in 1 revolution. Deceleration/acceleration i ignored the sign for simplification as both are taxing on the muscle required.

[@ProffessorSeen1]

Coming back to the video after a long look at my own position. I think one major thing you failed to capture is when you shorten crank length you must increase saddle height to match the previous leg extension. This will open the hip angle another degree or two. Run 165mm (5’6” and 31” inseam). Have problems with tight hips. Going to try 155 and 160 crank length.

[@TheKryztiandivor]

@PeakTorque Could You make a follow-up for this video, regarding how one should choose crank lenght? I'm 189 cm with 91 cm inseam (long limbs, shorter torso). I have both 172,5 mm and 175 mm, i "feel" that I can produce power more easily with shorter ones (even with such minor difference...).

[@richardwright2000]

The concern with acceleration prompts the question - so what - . At peak power the the crank is horizontal and acceleration is zero. Acceleration peak occurs on the bottom, 6 o’clock crank position. No work is being done. Force due to peak acceleration (reversing the rotation of the thigh), is reacted statically by the bearings and through the saddle and hip. There is no muscular force involved. You can coast through the 6 o’clock position without effort and acceleration will be as high. Fatigue may not be an issue. For balance, I don’t know if anyone notices a couple due to the reversal of thigh angular velocity. What’s noticed is the couple between the pedal and the wheel plane, watch the sprinters throw the bike from side to side.

[@Da5idc]

This was a very useful exercise - it would be great if you did a follow up or two I'm particularly interested to see a comparison with Greg Lemond's method of setting saddle height from the BB

[@RobinHood-yk8og]

You should add a 3rd dimension to the 4-Bars, to show what happens laterally when one of your bars, the cranks, is too long for the other fixed points. It's not pretty and in the long term it's going to lead to debilitating pain. I've been there and it's no fun. I moved from 175mm through 170mm and ended up at 165mm cranks in order to overcome a deadspot at the top of the up-stroke where previously I had to swing my knee out to the side to make it over the top. At 5' 9" I would be prescribed 170mm+ cranks by any 'Crank Length Calculator' you can find, and I don't have any flexibility issues. If in doubt, go small. You might not be at optimal performance, but you also wont be destroying your knees.

[@davidwebber8028]

Great insights. I've been using "trail and error" method - with my smart rollers and Zwift. Each time I tweak one piece, saddle, seat post, cleats, bars, oval rings, I was seeing up to 20 watts + on FTP. And then you have different bikes frame geometry too. I am hoping I have close to ideal for my body now - but always interested to explore more.

[@ds6914]

Love to see this for riding out the saddle.

[@xantostest]

I'm not sure about the observation that "torque doesn't matter because you can always change gears". Would it be more accurate to say that "pedal pressure" doesn't matter because it can be compensated by the gear selection. Let's say that I want to output 200 watts. With a smaller crank arm and similar pressure on the pedals, I would (?) have to spin faster in a larger gear at the rear in order to produce the same number of watts. Although the amount of "work" is the same, this doesn't change the fact that I am spinning (i.e., exerting pressure) thru a lever that is longer with a 172.5mm compared to a 165mm crank, which is the definition of "torque". The way humans are engineered, maybe the overall torque is similar when averaged over the pedal stroke - smaller cranks delivering the torque without the "peaks" of the longer cranks. But theoretically, if our power delivery was uniform throughout the stoke, wouldn't the longer cranks would have more torque?

[@osh241]

I thought I needed different cranks until I worked on my hip mobility.. a few weeks later no issues and ITS WAS FREE!! 😎

[@Max__apex]

Beautiful Newtonian Mechanical explainstion....

[@James-zu1ij]

Testing cranks with a human can be difficult. The brain has to learn every movement we have made since we are born. We get the physical structure for them in the brain. If we do a new movement we are "blind to it" we literally cant do it. It might not seem like it because we take it for granted. We as adults can access similar movements and modify them reasonably easily with some focus. But the physical structure that control this new movement will be rudimentary until weeks have passed. Testing for differences may be difficult. If you ask how do babies do it. They fumble randomly until positive conditions arise. There is some genetic predisposition to certain movements having said that, other wise foals would not be able to stand up or feed etc

[@tanpanama7563]

James totally agreed. human biomechanics is not as straight forward as mathematical model. especially when comes to skill. how do a person turns the pedal is the missing link in a lot of discussion. if you're turning a full circle for each leg.. then crank velocity after 5 o'clock does play a role. but if you are only turning half a circle.. alternating between legs.. it should not significant.. furthermore if you are generating power at the hip not the ankle.. crank length discussion should be limited to range of motion.. if you have it.. get the longest crank.. if you do not have it.. find a shorter one. and it should be discussed together with hip flexor role at TDC

[@ErcoArgante]

Nice vid! Thx! You suggest that reducing the femur acceleration might be advantageous as it might reduce fatigue or strain. Going for an even lower crank length, let's say 150 mm, reduces femur acceleration even more. You don't seem to address any negative effects of shortening the crank length. Did I miss these? In another YT video about reducing numbness in hand and wrist a bike fitter suggested adjusting the saddle more aft, as this reduces the weight on the arms, without the need to heighten the handlebar and therefore remain aerodynamic. He argued that one should be able to ride the bike with the arms not touching the handlebar for a short while without falling off the bike to the front. According to the bike fitter this was the ultimate way to find out the correct saddle position (and even find out whether your bike frame size was too big for you). However, adjusting the saddle more to the back reduces the effective seat stay angle and also reduces the hip angle. What are your thoughts on that? Again, like in my previous question I'm trying to find counter arguments to increasing the effective seat stay angle and increasing the hip angle.

[@DWMtukwila]

I have found that shorter cranks are so much easier on my joints. I still ride hard in spite of needing two knee replacements. I absolutely cannot tolerate long cranks now. I would expect the same for someone with a bad hip as well

[@khalidfh]

Great engineering here. I was looking for someone to do this using Adams with all forces. Matching the forces with the muscle fiber to show really fatigue. As small fiber will be fatigue faster with similar force compared to large one. Thanks. Enjoyed the video.

[@PeakTorque]

Adams! Gosh. Not used that for years. Forgot it existed. For the motion plots i put the equations into python. Cheers

[@Max__apex]

Do an analysis about seat position forward or back in relation to hip Rotation but relate it to power as the vertical components of pedal stroke EDIT: You did seat angle analysis that show a good understanding from u

[@bikernaut1]

Great work. It shed some light on crank lenghts. Thank you.

[@CyclespeedTours]

Excellent vid. I made one a few years ago trying to debunk the myth of 'longer cranks = more power', but your figures, calcs and schematics make it much clearer!

[@tomstephenson5452]

Do you think 165mm cranks are too short on my tt bike as a 187cm rider

[@DynamiteDB]

I really enjoyed this years Berkshire Crank Length Review with special guest Warren Buffet. Great watch 🤓

[@lyanbv]

Submitting my suggestions - extend the analysis into 1) how the new positions affect muscle group recruitment; 2) ideal cadence range based on leg mass and resulting acceleration profiles

[@PeakTorque]

Not easy! How much u gonna pay me? 😉

[@lyanbv]

@@PeakTorque viewership! hehe you did ask for suggestions :p

[@9psi]

It would be good if you did an analysis of why light bikes feel fast even though the acceleration of the bike plus rider is only 1-2%. I think it is because of the force or impulst which comes back through the seat and handlebars which is due to the lower inertia. I can be bothered working out the maths and I’d probably stuff it up

[@matt.rus1]

Maybe I missed it, but what about overall comfort on the bike. You seem to have focused entirely on force production but that's only useful if youre sitting in a position comfortable enough to sustain that force for the duration of your event. Does increasing the seat tube angle not move the riders centre of gravity further forward over the bottom bracket, causing potentially excess weight bearing on the hands and a poor weight distribution between the 2 wheels?

[@PeakTorque]

Yes. But there is no proven ideal CoG. It changes hugely with the road inclination (climbing or descending).

[@stevetee4145]

I bought a bike with 165 mm cranks. I’m used to 175 mm cranks. The difference was very noticeable. On my indoor bike trainer the power output figures confirmed my feeling. Shorter cranks are harder to pedal for me.

[@christophertan3195]

They are that's why you move a gear

[@cccpkingu]

Looking forwards to the force analysis, because I imagine within a range of gears, it needs to also be within range of motion, and in the powerband of extension relating to muscle-groups, for all of what is in this video. I think it isn't discussed, because sweetspots of combinations delivered don't lend themselves to the limited range of not only cranks, but also frame-sizes, made worse by how it sometimes in reality is just one model scaled up, down or both.

[@whokitkat]

very nice! have you looked at the premature wear of the tibia's meniscus wear and displacement of forces with different seat/seat angle/ fore-aft positioning?

[@TheWoogeroo]

I'd be really interested to see you do some analysis on cleat position (and foot size I guess) if possible. I'm the same height as you and have big feet, so cleat position in concert with crank length feels like it makes a big difference - moving them back both allowed me to move my saddle forward and down a bit (no more toe pointing). Discounting contribution to pedaling from my calf muscles, moving cleats back I think has a similar effect to shorter cranks. And on size 14 UK feet, a tiny amount of toe pointing can make a huge difference. I would say it's slightly harder to accelerate hard & sprint, especially off the saddle feels a bit odd - but it's fixed some toe numbness.

[@ronitdebnath]

I think you didnt change seat height in the mechanism when going from 175 to 160 by the difference. Not sure but looked like so.

[@trinerd]

Very well illustrated, love it someone shows the numbers.

[@justicefuree3205]

In my mind shorter cranks are better for flats and less elevation changes , longer cranks benefits more on hills and larger elevation changes.?

[@woodstock5nathan]

I know this vid is two years old... But on the subject of smooth or stochastic power requirements, would that have a very different response for climbing and for travelling at speed on the flat. When travelling at speed you are doing short spikes of power to keep the inertia high of the whole system. While when climbing you put power out though much more of the stroke. It's one of the big muscle differences between climbers and time trialists slash sprinters. How then would shorter cranks effect climber and fast finishers differently???

[@fnm441]

Nice video, may need to experiment with seat position more. Was set by the basic plumb line knee over the pedal method by my local bike shop and hardly altered since.

[@scottdavidson4446]

Do you think there would be a model where all the measurements could be plugged in and that would generate the best system? To include seat angle, leg lengths...

[@nwimpney]

I have a problem with your logic of why taller riders don't need longer cranks. It seems obvious to me that if you scale _everything_ up by the same factor all of your angles will be the same, and all of your velocities and accelerations will be proportional (The same if you normalize them to the scale) If a rider is 10% bigger on a frame that's the same shape, but 10% larger, using cranks that are 10% larger, and the seat height, and reach to the handlebars are 10% larger, all of the angles will be identical. All of the velocities and accellerations will be 10% larger, because the whole system is 10% larger, but your graphs should be identical if scaled appropriately. Should a long legged rider with longer, often stronger, muscles be limited to a (proportionally) narrower range of motion to produce force through? That doesn't really seem logical to me. That's not to say, there aren't tall riders who couldn't benefit from shorter cranks if there's other reasons, but if you're going to analyse the linkages, you should at least see that we are already using proportionally "short" cranks at 175mm. Assuming identical proportions, a 165cm tall rider like your friend, on a set of 160mm cranks will have the same angles, and proportional accelerations and velocities to you or I (I'm also 194cm tall) riding 188mm cranks. Unfortunately, It's difficult and/or expensive to even try it, as the availability of longer cranks is very limited, and the bottom bracket height on most bikes makes it a little iffy too.

[@danielakerman8241]

Isn’t there a knock-on problem or potential issue with moving the seat forward of changing the center of gravity and overloading the quads, as well as putting to much weight on the hands (potentially creating wrist or hand pain, and even neck pain)?

[@ingourban2432]

I did not read through all comments, so I do not know if it was a topic yet. How do you evaluate the effect of adjusting the cleat position instead of investing into short cranks for example? Especially in triathlon the mid foot cleat position is getting very popular. And the 230mm you have in your presentation for the feet seems to be the length of the whole feet and not the length of ankle to contact point shoe/pedal. To shorten the length by bringing back the cleat would effect in an increases of the hip angle too.

[@grahamm2015]

Thanks for this, to the point and well explained not a yawn moment. This is what I love about mathematics, proof. Lots to think about here.

[@PeakTorque]

Thanks Graham. There are lots of assumptions and its a simplified model. Mixing engineering and biological/chemcia/neurologicall functions will never truly align.

[@grahamm2015]

@@PeakTorque correct I should not have used the word proof. I got carried away with not being told what to think. As with all mathematical models they are built on a hypothesis so a form of approximation. I enjoyed this along with your video on frame geometry. Helps cut through the marketing on frame design.

[@milessmith589]

Interesting but flawed. Distance from ankle to ball of my foot is 6 inch/18 cm & I'm a massive toe pointer so it's a five bar model you should be using (if 5mm has such an effect on your model yet you ignore in your calculations if not in in passing mention, an 18 cm link). Additionally saying gearing takes care of the rest is arse about face. IMO you start with your strength/s body type and fit everything to it, so crank length is just one of several variables. For the record I ride with either 175 or 180 cranks, a range of 1x rings usually 70 or 73 ( but go as low as 63 & as high as 80 geography of the ride dependant) with a 11/32 or 11/28 rear cassette (again geography dependant) this allows me to spin at between 60 & 70 all day covering my target mileage of 300M/500Kms per day for multiple consecutive days/weeks. Suggest you re model with foot length included and the full range of gearing available prior to including geography, weather/wind, type of riding being done, TdF sprints, endurance RAAM/TARB, etc . The model is too simplistic & lacks all parameters to be of use 🤔 IMO 👍

[@jkk916]

The most general assumption would be that one is neither toe pointer nor heel dropper so what he did makes perfect sense to me. Also, why would you bring gearing into that discussion? It is separate problem and easily solved if you can find adequate equipment.

[@fultonlopez7846]

for the accerelation and speed study, did you use a constant bicycle speed or constant crank rotational speed, meaning usually with a shorter crank, omega would go up

[@ptrbssr]

Great piece of work, thinking and visually attractive presentation. OK it's a model. There are assumptions. But it's interesting to see what can be derived from that, knowing that it's not a perfect repesentation of reality (yet... :-) ). Curious to see what your real world experiences are with the 165mm cranks. Look forward to a follow-up!

[@melbournecyclingsegments7510]

Does a sore hip indicate hip angle issues?

[@rkentwenger5095]

I'd have liked to see what the acceleration was like with a 5 mm lower saddle height, also.

[@cpk2GIRL]

How do Cyclists' hip angles compare (depending on condition) to Track & Field Sprinters???.... ie, can the Optimal peak power range of motion (excursion of max efficiency) help determine the best hip angles & thereby the crank length?

[@nlanni]

Bravo - separating fact from fiction! I'm adjusting my seat height and saddle position tomorrow! But I need your torque analysis - please. I'm 65yrs old, 186cm tall, recovering from covid pneumonia a year ago which totally sacked my lung capacity and muscle mass. About 4 months ago I changed my crank length from my traditional 170mm (sometimes 172.5mm) to 175mm crank. I use a hacked drive train (48/38/28t x 11-50t). I’m not a weight weeny by nature, but I have managed to reduce my Surly Straggler down to 21.6 lbs from 28.5 lbs with phat 38c tires. With the longer crank arm, I feel more power and can accelerate faster in shorter periods, and I can keep peddling through varied flats and steeper slopes more often and for longer periods of time. I watch my cadence and it's easier to keep it in the 85/95 PRM range. The biggest plus of all is I get outside and away from a trainer in a room watching a flat screen, which I hate. So far, I've regained about 75 percent of my former strength. My stamina is still the sh*ts though. I would really appreciate your crank length vs torque analysis. I understand what you said about crank length vs gearing, but I'm convinced a longer lever (crank arm) is a benefit across the entire gear range for sport riding vs endurance/road racing.

[@kineahora8736]

You start out with hip angle-but you seem not to be emphasizing KNEE angle-and opening that up is a big deal if you have knee pain or a knee injury. I went shorter and it was *way* better. I also moved to more of a midfoot cleat position and that also helped.

[@harkat16]

Do you know if 'effective seat tube angle more important than crank length' is still true to the same degree on say a S(52cm) frame rather than L/XL(~58cm)? - In my mind it would be less impactful the smaller the frame, right?

[@bighdd]

Great analisys, would you go shorter for road. Do you use 175 on TT as well now?

[@andrewlipsiner9791]

How does ST angle or more importantly , starting with correct seat angle for geometry of frame and saddle set back and height effect correct crank length , here ?

[@danielebergonzi8461]

Thankyou for the well presented analysis much appreciaated Please let me know if you find a source for 160mm R7000 cranks Best of luck

[@cccpkingu]

You have also assumed the footblade as fixed, but you can decide to run the cleats forwards or aft.

[@ekrano]

Very interesting video. The pointer is working.

[@peterers3]

But wouldnt this imply 17:00 and ongoing that if you want high acceleration such as a sprinter you have an advantage with larger cranks? Meaning you can control your smooth efforts till the sprint and then u can crank out more force more accelerations with less pedal strokes. And if your a climber or flat racing guy you want smaller cranks to smooth ur effort and keep good endurance?

[@XCRiders]

I changed to 170 and just couldn’t hammer as well as my old 175mm Im thinking this new short crank like the no gloves thing, Is just a fad.

[@BioStuff415]

did u leave out the fore aft cleat position, especially for toe down riders? huge difference in my experience

[@hulluviking2055]

Any updates on real world perceived "feeling" vs theoretical with numbers model?

[@williamohara8153]

I have some sort of nerve problem possibly affecting the muscles in my right foot. It needs an orthotic for support. My ankles are the only hypermobile joint. The right hip is bothered for some reason, too. What would happen with a 73° seatpost ? I would go to 165 from 175. My femurs are 52cm long. I'm 189cm tall. Any thoughts? I went from racing cat 4 and 10000km/year to lots of intermittent pain this year.

[@jayleung109162]

stunning presentation and analysis

[@timtaylor9590]

you can travel further with less effort with a longer crank especially when standing. imagine a 10cm long crank try standing and pedaling, there will be more wasted energy compared to a 20cm long crank even with gearssss because its an entire full body effort not just legs. this is also why oval chain rings are useful because they make the less important part of the stroke quicker and easier to get around.