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I was thinking it would be cool to have a bike where the crank arms can be adjusted automatically "on the fly", depending on load. If that is not so safe then maybe a manual adjustment that the rider could make from a control mounted on the handlebars while riding. For example, when extra torque is needed, the crank arms could be extended to maybe 180mm. When cruising along on flat pavement on a windless day (or with wind at the riders back), shorter crank arms might be desirable to save on excessive leg motion. Having adjustable crank arm lengths might actually extended the effective gear range even wider. For example, suppose the crank arm length adjustment is between 180mm and 140mm and is currently set in the mid portion of 160mm. The rider then attempts to ride up one of those spiral walkways that are part of a pedestrian bridge over a roadway. If the lowest gear has already been selected but more hill climbing torque is needed, extending the crank arms from 160mm to 180mm should help. Even if 180mm is excessively long for the rider, it is only temporary until they crest the hill, then they can be reset back to 160mm. I think that would be another fun thing to play with on a bike.

So my question is would this be beneficial to a rider and if so, why don't I see these available? Has anyone attempted to make this, either homemade or available to the general public? From an Engineering viewpoint, can it be done? If so, how?

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    Manually-adjustable crank arms have been produced, but I haven't seen mention of any for 20 years or so. They were/are for people with one leg shorter than the other or some other physical limitation. In terms of gear ratio, there's nothing the adjustable arm would do for you that a larger/smaller sprocket wouldn't.
    – Daniel R Hicks
    Dec 10, 2016 at 12:54
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    The engineering would be hard (which means heavy) because the cranks rotate, so you'd have to couple the control action. It would also have to be rock solid - more weight. Better would be a system where you got off the bike to set the length. This could be useful for rented bikes, gym trainers etc.
    – Chris H
    Dec 10, 2016 at 12:58
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    The maximum useable crank lenght depends on the distance of the bottom bracket to the ground.
    – gschenk
    Dec 10, 2016 at 17:48
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    Adjustements of such a crank might be nice for finding the optimum length. For any serious riding it will not be very useful. The crank has to be very stiff. This makes it heavy as it is, even in high-end road groupsets (eg Sram Red ca 550 g). Introducing moveable parts and keeping it stiff will be very costly. Without any benefit, when one has found the optimum crank length, there is no advantage in changing it. Simply get the right cranks in the first place.
    – gschenk
    Dec 10, 2016 at 17:51
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    Another factor to consider is changing crank length also typically requires a change in saddle height, and sometimes a change in handlebar height/reach. As others have said, just use the gears you already have on your bike. Adjustable length cranks do exist, e.g. I used to have a set of adjustable length SRM power meter cranks on my ergometer which I used with clients doing tests or efforts. There is sufficient research to inform us that changing CL has almost no impact on our ability to generate and sustain power. Torque generation isn't our limiter, sustainable power production is.
    – alexsimmons
    Dec 11, 2016 at 2:42

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There is great possibility in this question. I agree with the technical assessments already given here but wouldn't it be interesting to simply dump the derailleur and change the amount of torque by varying the crank arm length? More weight? Sure but you loose 2 derailleurs, a freewheel and it might even out. The engineering would be in the bottom bracket. A synchronized gearbox that lets the crank arms move as the torque goes down might do the trick.

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    You have no conception of how "mechanical advantage" works.
    – Daniel R Hicks
    Sep 4, 2017 at 21:15
  • When I ride a bike with 140mm crank arms, I feel as if I cannot get much power with them. 175s on the other hand really "crank" out the power. If a biker stands up on the pedals, longer crank arms will increase the torque. Being able to change the length of the crank arms would have multiple advantages if it could be made to work. One of which would be to give your legs a rest if they have been working hard or maybe just to use your muscles in a slightly different way.
    – David
    Sep 21, 2017 at 3:46
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The system you propose is just a third way to change gear, in a mechanically very complicated way. Any benefit that could be obtained by lengthening the cranks can already be obtained by moving to an easier gear and a bike with two or even three front chain rings and anywhere between five and eleven rear sprockets has plenty enough gear ratios available to cover all situations. And if the range of gears available isn't wide enough, replacing the chainrings and/or rear cassette is a much simpler fix.

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  • But another advantage of the longer crank arms is it would extend the gear range and make the bike more flexible. For example, if you know you will be doing a lot of high speed road riding, maybe shorter is better for you. If I knew I would be riding in the grass a lot, I would want a lot of torque to power thru it and want maybe 175 or 180mm (I am 5' 9" tall). It would also make the bike a lot of fun but I guess manufacturers are worried about weak points in the cranks by doing that. It would be a nice solution to allow quick release cranks so the rider can change them very fast.
    – David
    Dec 11, 2016 at 3:29
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    @David no. It would not add any flexibility except the bad kind (flex in the drive train). Changing the crank arms would mean you would have to learn to pedal from scratch again
    – Aron
    Dec 11, 2016 at 3:42
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    Actually, pros use longer cranks for time trials and shorter ones for track. They also don't change cranks but have specific bikes for these disciplines.
    – ojs
    Dec 11, 2016 at 10:19
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    If you really can't get enough range with the chainrings and cassette, add hub gears as well. Someone (probably Sheldon Brown) built a 72 speed this way. By mismatching modern components you could probably get 99 (3x3x11) with a simple hub gear, over 200 different gears with the right hub. And this would extend the range a lot. But it would be pointless.
    – Chris H
    Dec 11, 2016 at 16:26
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    @David "It would be cool on a tandem bike to offset the crank arms so one rider is in the power stroke when the other one is not so it helps smooth out the power delivery" While there is an advantage in out of phase pedalling, it is usually not practical, since it makes it difficult to have both sets of pedals in a safe position when banking. Tandemers seem to prefer to be slightly out of phase at most, such that both can get the inside pedal up when cornering.
    – gschenk
    Dec 11, 2016 at 16:35
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The research on subject (source: Wilson & Papadopoulos: Bicycling Science, MIT Press 2004) shows that crank length has very little effect on pedaling efficiency or or maximum power output. There is a small increase in maximum power output with shorter cranks and and in efficiency with longer cranks, and racers who optimize to the last percent do use different crank lengths for event-specific bikes.

On the other hand, experiments showed that tests subjects had individual preferences in crank length even though efficiency was not affected. The crank lengths in tests done on recumbent bike ranged between 110 and 230 mm, where lengths above 180mm showed significant decrease in maximum power output.

As already discussed in comments, an adjustable length crank would be heavy, complex and prone to failure, and these drawbacks would overweigh the small benefits.

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  • Well I had an experience recently. I rode a bike that had 140mm crank arms and I am used to 170mm on my MTB. It seems like the power went down on the 140s. I wouldn't say a lower gear isn't the same as a higher gear with longer cranks. They both increase torque at the rear wheel but in different ways. I feel the crank arms need to be matched to the specific rider for maximum power, then the proper gear ranges selected. Also, I suspect that "too long" cranks, while having less maximum power, have more torque but less speed (in RPMs) due to more leg motion required.
    – David
    Dec 11, 2016 at 3:13
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    Did you do any actual power measurements? Did you repeat the experiment? Did you have other test subjects? If not any of above, you are not at the level of existing research.
    – ojs
    Dec 11, 2016 at 10:08
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    Personal testing is all very well but unless you really control it properly you end up just testing which configuration best matches your learned preference.
    – Chris H
    Dec 11, 2016 at 16:21
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    @David "... one day I want to ride street and another day more offroading and no single setup can be best for both." A decent offroad bike is not the best bike on the road, and vice versa. There may be good reason to have different crank lengths on a time trial bike and a BMX bike. However, the step in efficiency going from 165 mm to 190 mm on a hardtail 29er MTB is small compared to that going from said MTB to a dedicated road bike. Geometry, wind resistance, ergonomics, transmission, and rolling resistance are all different.
    – gschenk
    Dec 11, 2016 at 16:48
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    @David "Suppose someone had a 1 speed cruiser [..] [t]o compensate for the lack of gears, instead have variable crank arms that would change automatically based on cadence." By all means, design it then! Just, be aware of competition. A 3-speed hub costs a tenner or two, a 8-speed hub costs manufacturers well below 100 bucks. Changing the crank length from 200 mm to 150 mm would change the force to provide a given torque by 1/4 (down) or 1/3. A step up or down from the second gear on a standard Shimano 3-speed is about 1/4.
    – gschenk
    Dec 11, 2016 at 17:01
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There are crankarms with multiple attachment points. Obviously this doesn't allow changing leverage on the fly.

My own understanding, ojs' post notwithstanding, is that crankarm length should be proportional to femur length, so unless you've got variable-length femurs, there's not much benefit in variable-length crankarms.

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  • There's also the Look Zed cranks, which have the little trilobe inserts so you can adjust them from 170 to 172.5 to 174 mm. Again you can't change them on the go though.
    – ilikeprogramming
    Sep 20, 2017 at 8:57
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I can understand and agree with your idea. I have and ride several types of bikes, from BMX racing with 180 mm cranks and fixed gearing (because of the sport, I can reach pretty high revs) to my road bike 170 and my all in mtb one 175mm. The fact is of course, crank length is not only about gearing but is an anatomical connection to your bike, so it is linked to your legs length, but also to the use, so yes, the more RPM, the shorter the crank is the rule. I have some ideas about dynamic length crank, but is a field already researched and you can find some ideas at the web. But the fact is cranks are a very stressed part of transmission and will mean a big weight adding to have a working system which will be modifying the balance in each stroke.

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  • I'm not really sure how this answers the question. If crank length is about "an anatomical connection to your bike", I can see how you'd want to have different length cranks on different bikes, but I don't see how that would make you want variable-length cranks on any one bike.
    – David Richerby
    Nov 29, 2017 at 12:05
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I love this concept, and it has been on my mind for the last few days. A shorter crank would make for easier cruising at high RPM without having to increase your pedal speed.

When the torque is needed using the longer crank arm with a slower pedal speed. Gear changing would be done accordingly.

And to answer the response given here several times, changing to a lower gear would not achieve this same result. A lower gear would mean higher RPM therefore higher pedal speed. A shorter crank arm would result in higher RPM and not necessarily a higher pedal speed.

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  • This doesn't make sense. There's no relationship at all between crank length and cadence, neither at a given speed nor a given workload. With a longer crank arm you need less pressure on the pedals to produce a given torque, but you push it through a longer distance. In Work = Force · distance you are decreasing Force but increasing distance to keep Work the same. (The converse is true for shorter cranks.)
    – DavidW
    Nov 14, 2019 at 16:10
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    @DavidW, actually the research shows that at the same power output, crank length and cadence are inversely related. The mechanism is that for differing crank lengths the foot speed differs, so the leg muscles flex and extend at different speeds across different degrees of flexion and extension. You can search on Pubmed for research papers by Jim Martin et al. for this.
    – R. Chung
    Nov 14, 2019 at 16:22
  • @R.Chung "You can search on Pubmed for research papers by Jim Martin et al. for this." Not my job; it's your point so it's up to you to show your work. Plus, when cyclists talk about power output, they mean at the bb or hub.
    – DavidW
    Nov 14, 2019 at 17:14
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    @DavidW Well, in this case "you" was the generic "you." Dallin could also search so as not to be misled by your comment. See ncbi.nlm.nih.gov/pubmed/26559455 and ncbi.nlm.nih.gov/pubmed/11417428 (and related articles) to see that crank length, cadence, and muscle range and shortening velocities are related, as I said. You're welcome.
    – R. Chung
    Nov 14, 2019 at 19:45
  • To me, lots of excuses are given here due to "extra weight". Not everyone that rides bikes cares about weight. What about the fun factor. Imagine being able to go out riding and selecting the crank length you prefer. Even better would be a way to change the length quickly (perhaps with a tool you can carry with the bike). Way too many excuses given here and some people say no benefit, just use gears. If it makes someone happy and increases the fun factor then that IS a benefit! Think about how much technology is thrawrted due to cost or weight. Do the crank thing and I'll lose 10 lbs!
    – David
    Nov 15, 2019 at 23:27
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would this be beneficial to a rider?

Yes, It would give a mechanical advantage (increased torque)

why don't I see these available?

Dunno, maybe the technology would be too flexible or heavy

Has anyone attempted to make this?

Dunno

can it be done?

Yes!

how?

I'm going to stick my neck out here...........

Ridiculous as it may be, consider an oval cross-section crank arms with holes drilled through, and a sleeve (terminated with the pedal) that fits over the crank arm with a cam operated pin that locates the sleeve on the arm, varying the effective crank arm length. Now, provide a mechanism to co-ordinate the two peddles' length (perhaps, a cable passing through a channel up the middle of the crank.

This could work (assuming that it could be made strong and rigid enough).

Would this not provide an effective "Mechanical advantage" method for a fixie?

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  • Changing crank arm length does not change "gearing".
    – Daniel R Hicks
    Sep 20, 2017 at 0:48
  • Daniel R Hicks: Fair point. I'll change the gearing to "gearing". The point still stands?
    – Conor
    Sep 20, 2017 at 0:49
  • How does "mechanical advantage" change??
    – Daniel R Hicks
    Sep 20, 2017 at 0:57
  • @Daniel R Hicks: By giving greater action at a point (lever) on the crank?
    – Conor
    Sep 20, 2017 at 1:06
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    I don't see how this answers the question. All you seem to be doing is suggesting a way of implementing variable length cranks (which isn't what the question asks for) and repeating the observations in the question that longer cranks give more mechanical advantage than shorter cranks.
    – David Richerby
    Sep 20, 2017 at 10:05

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