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

[@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.

[@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

[@duodecaquark3186]

2 key problems I have with this analysis are failure to adjust seat height with crank length and the acceleration analysis assuming consistent velocity. When crank length is reduced by any measure, if saddle height is adjusted such that the bottom of the pedal stroke remains consistent, the effective change at the top of the pedal stroke is doubled. Also, there are several studies that show that reducing crank length results in increased preferred cadence. This would mitigate some if not all of the acceleration difference we see in your graphs. I also suspect that most cyclist's have a preferred/threshold of force per pedal stroke, and this is why we see higher preferred cadence for higher wattage.

[@gadenhertog]

I was thinking the same thing regarding the lever x velocity; that the increased cadance to compensate for the shorter arm would mitigate (part of) the acceleration difference. Curious how much it actually is.

[@fredEVOIX]

far from you I know but I got 155mm short cranks for my...eMTB cough...anyway I had to raise saddle and handlebar or there was no way I could pedal in OFF mode properly and I try to do all flat and slight uphills in OFF as much as I can, I'm not a road cyclist but most of my muscular XC rides are on roads lots of cat1/HC climbs alongside road cyclists I know what pedaling up Mont Ventoux is and how important proper pedaling is especially when you do 8hrs on a saddle

[@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.

[@TheCharnwoodCyclist]

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 👍

[@marcvb3364]

4 years later, the short crank hype is in full swing (I fully subscribe to it), and this is still the best youtube video on the topic. Have revisited it for the 4th time now, great work!

[@old_papa]

I was a competitive skier and blew my ACL 40 years ago - back when surgical techniques were a little different than today. Anyway, now in my 60's - with minimal cartilage and arthritis in my right knee, shorter cranks make one difference that no other bike adjustment can make. Smaller circles means that your knee is less flexed at the top of the pedal stroke - this is a huge difference for anybody suffering from knee problems. For me, moving from 175 to 170 cranks means not just more comfort when cycling - it means I can walk normally the day after a long ride.

[@kenjurish2170]

Been riding for 30+ years. Always had 170s, am 5' 7", and noticed that I always had trouble accelerating from a dead stop, or ever, as compared to my taller compadres. Switched to 165s about 10 years ago and, man, what difference! Changed everything. And now I know why. Thanks so much for this.

[@Dmxravin]

its because when your thigh or femur is slightly lower it will produce way more force compared to get femur being at 90 degrees relative to upper body. Its like doing half deadlifts. Also whenever I watch taller rider their knees never go up the same amount as it does on shorter riders. Not fair.

[@dannyshearer]

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

[@Bultish]

pen is working

[@glenspringle7337]

Best quantitative explanation I have come across. My background is on robotics and controls. I’ve analyzed the Puma (swing, shoulder, elbow, spherical wrist) geometries “many years ago”. I have not come across a proper linkage/geometric analysis like you did here. Thanks for creating intelligent, analytical content!

[@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!

[@Hou5eSounds]

Oh man this brings back dynamics and kinematics studies from uni... Love it.

[@glennoc8585]

I touched on dynamics at high school, remember fluid dynamics in physics class. I remember we had to calculate the torque and kilowatts required for an oil pump to deliver X amount of litres per hour at a given RPM. The fluid viscosity was set and the pump volume per cycle was also set. Pretty simple with given data much harder to engineer.

[@jameswitte5676]

Subjective comment: I’m 5’9” / 175 cm and in my late 50’s. I live on a small mountain range where my climbs average 5 to 9% with gradients of 14 to 18% common. I’ve had several road bikes in size 56 cm and they all came with 172.5 mm cranks. After reading and watching about crank length I went with 170 mm on my last bike purchase. I couldn’t be happier. The difference when climbing was immediately noticeable. I wasn’t straining as much, but I was spinning faster. My times going up remained about the same, but I feel better at the top. Thanks for the informative video.

[@tomrafferty2562]

Hi James I've always thought getting the right riding position is an art, but the science is all new to me. I would have immediately guessed that a longer lever / crank would make spinning easier, but I suppose that's where ergonomics comes in! Found your comment very interesting 👍🏾

[@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.

[@nlfiets]

Good analysis. I ride long cranks (old school 185 and 180mm). Recently I have a bike with shorter (175mm) cranks. The feeling is very different. My second ride with the bike resulted in "exploded" legs after 75km. You really need to adjust to it timing wise. My cadence did not change (94rpm average). In several studies they changed crank length and tested in a short period of time. That would never give a good result.

[@palicar]

I rotated my handlebars to have the drops closer to horizontal like you suggested in another video and I am now more confident and comfortable in the drops and use them far more often. Thanks, dude!

[@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

[@jeffr119]

Watched this video and would like to comment on your CAD model/dynamic simulation. In that model the angle between your ankle and lower leg is held constant, which you do mention at approximately 12:25. For your CAD model what you should do is constrain the angle of the foot to the ground, not to the lower leg. If you watch riders from a side view you will see that through a full pedal rotation, the foot is held at 20-30 degrees to the road surface. This varies throughout the stroke, and if you could model this correctly in your CAD skeleton, you would see different results in hip angle especially at the top of the stroke (330°-60°). See if you can find videos of experienced cyclists on trainers from a side view and you will see what I mean.

[@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.

[@MoMadNU]

The time spent reading or watching how to improve my performance normally yields a negative return on investment vs spending the time actually training. This video is an exception. It inspires informed experimentation which may indeed uncover flaws in my current setup. Being able to present significant ME theory to the point of almost sounding sexy is quite a testament to your overall skill set. 😊

[@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!

[@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!

[@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.

[@drapetomaniack]

By surveying some decent bike fitters… that fore aft seat position should not be used to open hip angle because moving seat forward means your torso can become unbalanced as it is more cantilevered over the bars.. this results in saddle sores and numb hands.. therefor fore aft position should be driven by balancing upper body weight.. I’ve experimented with this (and reach.. discovered that handlebar width increases reach!) and my numb hands and saddle sores are GONE!! Much better to use crank length since it doesn’t throw off everything else.

[@bobbyatopk]

Very interesting. One idea I’ve got about the limitations of short cranks is that you spend less time in the down stroke of the pedal (due to shorter range and higher RPM). This gives less time for RFD (rate of force development). RFD is basically how quickly and simultaneously your neuromuscular system can recruit high threshold motor units. To demonstrate this, flick your hand with your finger. Supporting the finger at the nail with your thumb gives time for RFD. If you try flick by just extending the finger un supported, you can’t generate as much force.

[@bobbyatopk]

Secondly, overly long cranks would put you out of the range of motion where you have maximum Actin and myosin cross bridging. This would also limit your force production. Example, flex your wrist (bringing palm towards your forearm) and then try to clench your fist. You have less strength than wrist in a neutral position because you’ve pre shortened the muscle and put it out of the range of maximal cross bridges

[@PeakTorque]

Exactly right with RFD. I think i mentioned that somewhere short may limit that window for torque deployment for bigger riders.

[@PeakTorque]

Thanks for the comments btw. I'm no expert on biomechanics/biology.

[@alexanderhardy971]

So would shorter cranks be better as your legs would be closer to resting neutral position? I think of long cranks as having to do a deeper squat. I could squat way more weight if I didn't have to go down as far.

[@bobbyatopk]

Alexander Hardy yeah, if you had unlimited time to generate force you can achieve the maximum force in a narrow range of Motion (like the top of a squat). But, because the pedal stroke is high velocity, a shorter range of motion reduces the time you have available to generate force (it takes time for your CNS to send the signals and activate all your motor units). Realistically though, I don’t know how much difference it makes between 165mm crank and 170. I think track riders tend to use shorter cranks and they achieve very high peak power output? However, they are very explosive athletes and do a lot of weight training so they would be very good at quickly generating high forces. It’s possible that as you decrease your explosive strength deficit (the difference between your maximum force to the max force you can generate in the time limits of a particular movement), shorter cranks would become more useful to you? If you have low RFD maybe you need a Lower cadence and longer pedal stroke? Sorry not very helpful there, just thinking out loud.

[@cuebj]

I'm 69 years old, 180 cm tall. I have 2.5 cm longer legs and shorter torso than average for a male of my height. Usually ridden 172.5 mm as that's what the bikes came with. But my Specialized Steel Langster had shorter cranks (165mm?) and I absolutely loved it, rode it very hard up and down hills in traffic and in rural areas. I had a chance to get shorter cranks when my 2018 Ultegra cranks were recalled and replaced last year... but I forgot! Now about £250 to get shorter cranks and that is a long way my pension budget list of desirable items

[@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.

[@JonathanPowPhoto]

I went for 220mm cranks, from 200 originally then 215mm, reason being, for me they feel comfortable. Like having a nice stretch or walking with a longer stride. I do have a 39 inch inside leg length though.

[@LoscoeLad]

they've found bones of your ancestors, according to late night History Channel

[@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.

[@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.

[@jamieb2865]

5' 7" (30" inseam) and I find 170mm cranks too long. I've been experimenting with shorter kids bike cranks, 152mm to 130mm, and find that 1mm ends up equaling about 1RPM of perceived cadence. (85RPM @170mm feels like 103RPM @152mm) The cadence where I can hold my FTP at the lowest heart rate is about 95RPM. That seems to line up with the dynamic model that I should probably use 160mm cranks. Too bad they're such a PIA to find.

[@PeakTorque]

Very interesting experiments. Good to see someone trying the extremes. Cheers

[@jamieb2865]

@@PeakTorque I managed to get some 160mm 105 Cranks. Instantly increased my average cadence by 8 rpm. Power is down a little because I'm not conditioned to the faster cadence. (Fast twitch fibers in the quads are on fire). Way more comfort, and simpler to find a good fit.

[@cccpkingu]

It is a downright shame how many females don't come in metric.

[@PeakTorque]

Who wants a fraction of a girlfriend?

[@shibaburn7725]

Too many imperial women out there?

[@fredhubbard7210]

@@shibaburn7725 Way too many "Imperial women." It is the pathology of Disney and feminism telling little girls that they are princesses.

[@borano2031]

@@PeakTorque Sometimes, a fraction of a girlfriend is enough... Rgr

[@occyman]

ALL women come in metric ....

[@CsabaTothMr]

So longer crank means more torque, we can agree on that - it's physics. 4:53 shows "They do not increase power". Just because there's "no data, no proof", why would we conclude that? Maybe it's that I'm not looking at this from an aero TT perspective, but when you climb a long steep ascent, and you are running out of gears, don't tell me that an extra torque wouldn't help... I'm 6'4" with extra long legs (opposite ape index than people like Michael Phelps, so my wingspan is 4" shorter than my height, my torso is not XL just L, my legs are XXL long). I have trouble finding long enough crank arms and it just bogs my mind how can people even gravitate towards shorter... maybe it works for short people I guess?

[@PeakTorque]

Crank length is just part of the whole gear system that is: 1. Front chain wheel number of teeth 2. Rear cog number of teeth 3. Crank length 4. Rear wheel outer diameter of the tyre. Increasing crank length can help you in your bottom gear because you're changing the total gear ratio. Max torque thus power comes from the human. You can't magic it from nowhere!

[@CsabaTothMr]

@@PeakTorque For sure. I understand that the main focus of your current analysis was fit. I cannot wait to see power analysis. If we wouldn't focus on fit, then it'd be an interesting question what would be the power outcome once you have a given cog set and platters and your wheel size is given as well, only variable would be the crank length. Referring to the "They do not increase power" claim. I understand though that in the end the power comes from the human, and if someone has short legs, they won't be able to ergonomically apply the power to a long crank.

[@grantbeerling4396]

Real world experience; Male, 177/2.5mm v 165mm height, height;177, long femur (52/48ratio, women legs). 45 years of riding and racing. Longer cranks are more fatiguing, as most long rides have long spells of 160 watt output, just spinning. So small circles are less fatiguing and more efficient due to the flattening of the curve (as shown). I discovered this years ago and my climbing improved ( 1hour plus road mountain climbs) again less chance of peddling in squares and using gears...Also moved saddle forward, more 130mm stem for stable mountain descending (which I love, completely changed the handling for the better), which again helped climbing in a smooth manner, revs the standard 75-80rpm (very few can rev at 90 efficiently and blow). This all came form that original ironman triathlete paper concerning 140mm cranks as an extreme, about 10 years ago.. Conclusion; less fatigue, better run off the bike, faster time.

[@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.

[@philipa1842]

Looking at oval chainrings in conjunction with this analysis would also be quite interesting.

[@keanoscarermitano619]

Looking forward to this as well

[@keacoq]

Great video. Another step to trying shorter cranks. Now 175mm for 170 cm rider. Amazing sound quality as well. Good sound quality not essential but adds to the authoritative impression!

[@jjhammond1]

It doesn’t have to be that complicated. Think about rolling up a car window with a hand crank. The only two considerations are the length of the crank (leverage) and the weight of the window (resistance/gravity). If the window weights next to nothing, there is little resistance (like riding the flats) so the main difference is that you would have to roll the window up faster and make smaller circles to match the time it would take to roll it up with a longer crank, which you would have to roll more slowly and make bigger circles to roll the window up at the same rate. The added leverage with using a longer crank wouldn’t be significant with a light window offering little resistance. Where things really start changing is when the window gets heavier. Then, the short crank loses leverage exponentially as the window gets heavier (as the grade increases on a climb). Then, it’s much harder to roll up the window with a shorter crank than the longer crank giving you more leverage. In fact, it would get to the point where you would have to turn the shorter crank more slowly and apply more force due to the lack of leverage, and you may be able to turn the longer crank faster due to the increased leverage and less force you’d have on the heavier window. So the takeaway is that you have to ride shorter cranks differently. On the flats you can spin up the cadence and be more efficient, but when you ride uphill, you lose leverage more quickly with shorter crank arms so you have to compensate by shifting more frequently as the grade increases and use easier gears.

[@CreativeFishDesignsCharlotte]

you need to rethink that , its so not as simple as you would like to think. i don't want to call names but your view is honestly infantile

[@daisyseptimus561]

Yup. That's what I experience when I change to 165mm crank and rode it for more than 8 months. Change back to 170mm and it's like night and day. 165mm is not for me. And I don't have any problem riding high cadence on 175mm on my Full Sus MTB. And I'm only 169cm.

[@HADDEN67]

I’ve been using 145mm and 150mm cranks for 15 years. I measured power versus speed for several lengths before switching.

[@obikedog]

Enlightening! This is the best analysis of crank length found thus far.

[@VankruCycling]

Great video! I would like to add a few comments and thoughts though. With the shorter crank installed, the saddle should be raised at least the equivalent amount of difference. This would open up the hip and knee angle further. Additionally, the angles can be opened up with a more rearward cleat position. The other factor to consider, on a road bike, is depending how far the saddle is moved forward, this can result in pressure and discomfort in the hands, wrists, shoulders and neck and effect breathing. There of course many other changes to be made, as a result of what I have listed above - "knock on effects" Enjoying the videos, keep them coming!

[@CreativeFishDesignsCharlotte]

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

[@OpenMTBMap]

Great analysis. I think the main problem is that road bikes are still way too conservative on their geometry. Crank length is one point - but the much larger problem is that road bikes seem archaic when it comes to findings on what is faster on mountainbikes, think Mondraker Forward Geometry or Pole bikes - mow more or less implemented in all bikes. Ideally we want much steeper seat tubes - maybe 76-77°, maybe even 78° as on Pole bikes (though then the more you use your bike for climbing - the steeper you like your seat angle), longer reach but shorter stems, and much slacker head angles (and yeah wheel base will rocket from the paltry 99cm to 110-115cm or so - notice enduro mountainbikes have arrived at 130cm already for a size large). The main problem is the handlebar width - going 10-20cm wider would likely be really beneficial fore most going uphill, but really inefficient in the wind. This would resolve a lot of too much weight on hands/shoulders problems. Someone really needs to think forward, forget UCI norms, and look at things that made mountainbikes fast. Then get people to train on a new geometry for some months and compare speed and comfort. Some things from the mountainbike side may not work on roadbikes (for sure no 75-80cm wide handlebars) but a lot will. To counter the slacker head angle and weight distribution - the chainstay length will need to grow too - however road bikes likely need much less weight on the front wheel vs mountainbikes - so shorter CS = less weight on your arms = more comfortable. And yeah if crankarms reduce in size, the long needed evolution of chainring sizes going smaller needs to continue too. Average Joes tend to run way too big rings up front instead of spinning faster and trying to choose a small ring that allows them never to drop below 80RPM on their steepest common climbs. It's crazy how conservate and stupid a lot of the bikefitting is on roadbikes - trying to fit people onto frame geometries that are simply wrong from the start. And yeah it needs a radical departure from the status quo. Most of bike fitting is simply an reaction to wrong bike geometries.

[@vanwilder1101]

Agree, frame geometry needs to be looked at, seat tube angles are far too relaxed. I've always thought we should be far more over the BB in order to sync the lower body. I always slam my seat as fwd as I can and never have pain

[@SharpByCoop]

I'm 5'6 / 165cm with 29" inseam. I modified by machining two new positions on Shimano arms from 172.5 to 155mm. I also lifted my seat by about 15mm. The difference was compelling. For me, MUCH smoother and I stopped rocking on my seat. My Zwift output appears to have gained 5-10w at same heart rate. Too many geometry factors here for me to consider (or understand). My 'seat of the pants' mods tell me I'm OK with the new geometry.

[@seanmclaughlin4039]

I'm running 155 rotor cranks at 6 ft 3 they are great for running of the bike. Maybe losing a little force but over longer tri distances the less stress on the hip flexers out way the trade in my opinion

[@LoscoeLad]

wow

[@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).

[@fieldlab4]

I suspect: What the acceleration peak allows is for you to drop the toe a bit through ankle flex. The toe drop absorbs the acceleration which works perfectly to begin the pull up cycle. If so, that could be an advantage.

[@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.

[@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.

[@ShawnIsBatman]

For me It is all about comfort/position, I moved to 165 recently. I a 57, have hip ROM issues and have had a hip replacement (resurfacing) done on my right hip about 13 years ago. Moving to 165 has not only been more comfortable on the bike, but has also reduced post-ride pain in the hip significantly. Before making the switch myself, I had gone through several bike fitting sessions, these had varied in quality from being purely "visual" where the bike fitter was just 'looking' at my position all the way up through a full video analysis. None of these fit sessions recommended changing my crank length. I believe bike fitters may sometimes avoid this recommendation due to the cost...it may be easier for them to change saddle position/height or recommend replacing a cheaper component like a stem vs. cranks.

[@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 🤣

[@abrahamvega1660]

With regards to increasing the seat tube angle, the more forward your saddle is the more weight you offset towards the hoods (if we’re talking about the road bike position), there is still a limitation to this, so keep note everyone, sometimes a little bit of both reduction of crank length and proper saddle fore aft is key.

[@p3trinho]

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

[@macht4turbo]

I have a question regarding the the change to crank arm length: Why did you not change the seat height by the same amount? It is recommended to change the seat height by the same amount you change the crank length to reach the same effective seat height again. I see the value of isolating a variable in analysis, but in this case it does not seem correct to isolate this change when you will always change seat height at the same time. Further more so as in the next step you talked about the benefit of raising the saddle and setting it further forward. This would be the next logical step after reducing crank length anyway. Seeing bikefit as a system, where one parameter changes others at the same time, it seems to me, that it is not advisable to isolate changes for analysis.

[@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.

[@michaelstoecker4178]

Excellent analysis! Thank you also for the very clear presentation.

[@zamahlplays6585]

Thank you for this content man. This is exactly the thing I've been looking for. Keep it up!! Definitely looking forward to more analysis like this.

[@wtfiswiththosehandles]

I'm 186cm but prefer 170mm cranks to 175mm due to bad knee (yeah, most people say I should use 172.5 - 175mm). Never thought about angles, but a friend recommended that I try shorter cranks to see if it helps with knee pain. I was sceptical at first, because in my simple mind, shorter crank means more torque and worse pain... But it worked! Your video explains a lot why it is so. I might experiment with saddle position as well. Thanks!

[@briansamuel5670]

Easily the best explanation of crank length and hip angles I've ever seen. I'm gonna try these adjustments and comment in how I feel. Thanks man!

[@PeakTorque]

Cheers

[@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.

[@EthanBlock]

16:33 Did you account for the lower cadence longer cranks would permit at the same power output? The primary advantage of a longer crank is the ability to apply less force to achieve the same moment. Put another way: In order to average 200W with 180mm cranks, I would have to apply an average force of 177N at 60 RPM With 165mm cranks, I would either have to apply 193N or more likely increase my cadence to 65.5 RPM by selecting an easier gear. How much would that increase in cadence reduce the difference femurral angular acceleration?

[@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.

[@monty2078]

Wonderful video. I have been considering shorter cranks because of intuition only, now I am starting to understand due to your excellent work

[@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!

[@BFinesilver2]

I use Rotor 155 with Rotor ovals. I also have a 76.5 degree seat tube angle with my saddle 10% down..It's incredible how much more effcient that combination is.

[@ParkerJimmyx]

Really enjoyed this, cracking explanation. Solidly de-mistifying bike fitting. Subscribed!

[@larrylem3582]

I found that cadence and gearing only go so far to compensate for shorter crankarms (having to spin faster to achieve the same power). I failed 3 times to ride to the Mt Baldy Ski Lifts on 150 mm cranks on my recumbent. I switched to 175 mm, spun a little slower with a slightly taller gear and made it to the top. I've experimented with cranks from 115 mm (Sinz) to 200 mm (IRD). Forget the analysis, go try them. Train for a month with each length to acclimate the legs. Think beyond the 165 to 175 mm range. (The goal of my experimentation was to use as short of cranks as possible in a recumbent streamliner. I've settled on 140 mm for that application, a 6-minute effort.)

[@zetka102]

really interesting - also worth considering the positional change associated with shorter cranks, what i mean is; shorter cranks would mean saddle can go higher maintaining the same leg extension angles on the bottom of the pedal stroke - at 6 o'clock, and consequently compounded effect on hip angle of increase saddle high and shorter cranks as explained ?

[@junjunmulleda5467]

Id like to see you do a force vs. Femur/Tibia angle at the pedals. That coupled with the crank length should give you the best idea of optimal crank length to run.

[@rayF4rio]

Interesting. Never thought about it in terms of degrees/s of the femur. I have 175's and am considering 170's for less hip angle. But at age 60, I am more concerned about how raising the seat will then impact saddle to stem height and the changes I may have to make to the front end. One thing leads to another...... 😁😁

[@laszlozoltan5021]

in my experience, it is all a matrix of bike geometry, body and adaption, and cycling environment ( eg. hilly, flats, urban, short circuits vs. endurance) - in other words, numbers (175, 170) are meaningless.

[@jamesmedina2062]

Shorter is easier on the body. ‘MOre comfy

[@jen3800]

at 5'7" with a 29" inseam i find the 165 cranks on a medium frame work for me because my upper body is long so i need to stretch out but the little engine needs those short cranks to spin well. i notice a huge difference between 165 and 172.5 here's the worst... when i was asking about this at a local bike shop ( B.S. shop more like it ) i was told in a dismissive tone ( 172.5 is standard. Same thing when i queried bar width. I didn't even buy my bike there!! Then again, it's Toronto where all shop owners have the attitude of they do not need your shitty money. 165 made the difference for me and I don't even know the math as well as you. I just feel it.

[@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

[@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👍

[@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!

[@hardtailparty]

Fantastic analysis, thank you for posting this.

[@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.

[@FrankMOrtiz]

Peak Torque, I went ahead and dropped my crankset down to 160 mm (from 172.5/175 mm) for all my bikes, as well as my wife's road bike that I built up. I noted quite a few observations, and if you're interested, I'd be happy to share in a YT conversation. Over the past year, I've trained and raced at 160 mm and have a lot to say about it. Both bad and good.

[@stevecrabb1]

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

[@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...

[@carlfrancis8565]

Hope you'll follow this up with effects of shorter cranks on power output. Have you ever seen the equation Crank length (mm) = 5.48 x inside leg measurement (inches)? I applied it, now use 160mm cranks, and spinning is much easier & smoother.

[@user-rl3ef4ju9k]

Mine would be 203 mm...

[@nwimpney]

anyone know where I can get some 213mm cranks?

[@hdwoernd]

Just when i think about the acceleration peaks (in angles where there is not much force), maybe it explains the tendency to grind more with longer cranks? On the trackbike i reach higher cadance than on the roadbike with the 7,5mm longer cranks. Maybe also „rocking“ the hip left and right can be explained with this phase of low power / high acceleration. Very well made!

[@peteranderson6935]

Really interesting. I'm trying to square it all with the fact that I just perform better on longer cranks (180mm). Anecdotal - I've got a better jump, I don't need to get out of the saddle coming out of tight corners, under effort I think more about muscle recruitment and ankle flexion, I'm thinking round the pedal stroke, ....and I really like the wider range of motion under light load for recovery. I'm going to try the CAD modelling, there might be something else going on if you remove the foot/tibia approximation and make it a 5 bar linkage. I think the ankle acts as a fluid coupling and damps that peak acceleration. Like you say, hip ROM is a red herring, there's lots of other stuff you can do to solve that.

[@tomkunich9401]

While riding 175's my knees began to hurt for the initial hour or so of a ride and would then loosen up for the rest of the ride and give me no more troubles as I reached my 70's. I reduced the crank length to 172.5 and pain was gone. I can't tell any power difference. I'm 6'4" tall and have 35" legs hip to bottom of foot. I pedal circles often.

[@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.

[@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!

[@Ultegra10SPD]

Former Formula 1 driver Alexander Wurz of Austria was asked to have surgery to reduce his length length approx 4-5inches so he'd fit in the cars better/they could make them smaller. He told his team where they could take that idea... -U10

[@EhButU]

Superb. V interesting. Nice CAD idea, first thing I thought was surely it is a 5 bar linkage system ( to include foot link and ankle pivot) and glad you pointed this out at 12.22 that you assumed ankle flexion is constant and I think this is the biggest flaw in this model; would have been interesting to see the numbers where the foot is kept horizontal and ankle flexion allowed - think it would reduce the minimum hip angle differences. Also note how you have the ball of foot pivot is not perpendicularly below knee pivot at the 3 o' clock position of the stroke, as bike fitters often recommend. I will try your CAD idea however - think it is great. Would love to have seen the follow up about Forces investigation but couldn't find. My experience with shorter cranks were worse - you needed to go one bigger rear cog and therefore increase rpm to match the longer crank.

[@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.

[@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 😉

[@cncmillman]

This relates to the Steve Hogg bike fit method. He looks for a constant velocity at the knee and tries to limit high accelerations. Would be interested to see the plot with a seat height 10-15mm lower.

[@PeakTorque]

Very interesting. I need to read about that, I've never heard of him. It certainly makes sense.

[@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

[@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.

[@mreyes2633]

You can go too small on crank arm and run out of gear at which your legs can put pressure on the pedal and get suboptimal power output. 175mm crank arm in a climb where cadence is slower would out perform a 160mm crank given the same gear.... meaning small chainring at front (ex. 34) and big ring cassette on the back (ex. 28). I've tried 170 to 160... now I'm at 165... with subcompact gearing for my climbing bike and 12 speed axs helps widen the gear requirement for all terrain

[@luismanrique5373]

You can add shims to match the length of one leg to the other under your clip shoes.

[@ProWeAreSo]

Moving the seat forward might open the hips, but you need to take into account the effect on quad muscle usage. To maintain the same position with shorter cranks, you need to increase the seat height and move the seat slightly back

[@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.

[@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.

[@SuperJimic]

Another epic video. Love this kind of breakdown from an engineering perspective. Im on a PT binge right now. ha ha.

[@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.