0

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?

David
  • 818
  • 1
  • 7
  • 23
  • 6
    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 '16 at 12:54
  • 2
    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 '16 at 12:58
  • As a compromise, I would be happy with adjustable crank arms that required the rider to first step off the bike, make the adjustments, then resume riding. It would be nice if the crank arms had some removable pin (perhaps a Clevis pin) with cotter pin for quick adjustments. I think it would be fun to try all the lengths from say 140mm to 180mm in 5mm increments to see which is best on an average length trip for maximum comfort. Once that is determined, the other settings would be used for special purposes. I wonder what the maximum usable crank length is for a 6 foot man. 200mm perhaps? – David Dec 10 '16 at 13:12
  • 1
    The maximum useable crank lenght depends on the distance of the bottom bracket to the ground. – gschenk Dec 10 '16 at 17:48
  • 2
    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 '16 at 17:51
  • 1
    Crank shorteners are already available. I installed some on a bike recently that are actually very easy to adjust across quite a range - loosen two hex bolts each side, slide to position, tighten bolts. – mattnz Dec 10 '16 at 21:18
  • 4
    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 '16 at 02:42
  • @DanielRHicks - I slightly disagree with your statement. Longer cranks can effectively give you more torque without having to use a lower gear. Gears affect speed. For example, suppose on your daily ride you encounter a hill and know which gear you need to use to climb it. Suppose you have 170mm crank arms. Now imagine if you instead had 180mm crank arms and hit that same hill. Would you need the same gear or could you maybe go 1 gear higher? Suppose 1 gear higher. Are we looking at "apples and apples" here? No! The longer crank setup may allow you more speed up the hill. – David Dec 11 '16 at 14:46
  • 1
    @David To use a longer crank you must have more leg movement. Movement takes time. – Daniel R Hicks Dec 11 '16 at 15:02
  • @DanielRHicks - yes it does but that is like saying a longer golf club wont hit as far because you have to swing it on a farther arc. I am simply suggesting longer cranks will help with torque if the rider has the ability (range of motion) to allow it. This could be done experimentally or more scientifically such as when someone is properly fitted for golf clubs. I have 3 bikes now and they have 170mm, 165mm, and 140mm crank arms. The 140s are on my future wifes bike and are WAY too short for me. I like 165s and 170s but can take more. 200s would be fun but might scrape on turns.... – David Dec 11 '16 at 15:09
  • You've got a good point: Why doesn't a golfer just change gears, rather than using a longer club? – Daniel R Hicks Dec 11 '16 at 15:28
  • A bike is about simplicity, not complexity. You should try riding a fixie for the ultimate minimalist experience on a bike. – Criggie Dec 11 '16 at 19:04
  • If your real question is whether some cyclists might benefit from a longer or sorter cranks, the answer is definitely yes. When I first got into cycling nearly 40 years ago there was a lot written about choosing the correct length cranks, but you see very little mention of that anymore. Mostly this is because bike building and fitting is no longer a craft, but rather a mass-marketing activity, run by multi-national corporations. – Daniel R Hicks Dec 11 '16 at 23:19
  • The OP might have been conflating gearing and crankarm length. I'm not aware of any science on varying the crankarm length over the pedal stroke. Others have said this could create some bike fit issues. However, there's at least some science on oval chainrings, which vary your gearing across the pedal stroke - higher gear in the region where you're pushing down on the pedals, easier gear when you're in the recovery stroke. There's no clear benefit, however. https://bicycles.stackexchange.com/questions/2623/is-there-a-practical-gain-from-elliptical-oval-chainrings – Weiwen Ng Nov 19 '19 at 17:28

7 Answers7

15

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.

David Richerby
  • 16,963
  • 4
  • 53
  • 83
  • 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 '16 at 03:29
  • 1
    @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 '16 at 03:42
  • @Aron - That seems like a biased comment to me. I can change bikes with different crank arms and I don't have to relearn squat. I am talking about just casual riders, not racers. I could probably ride 2 bikes going from the extreme crank lengths of 110 to 230mm. Sure would be fun to try. I seem to prefer 170mm. 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. Perhaps offset the crank positions by 90 degrees for the 2 riders. – David Dec 11 '16 at 03:54
  • "But another advantage of the longer crank arms is it would extend the gear range and make the bike more flexible." I already explained how to extend the gear range of a bike: replace the chain-rings and rear cassette. And note that this is exactly what the pros do: they don't even replace one fixed-length crank with another of a different length; they replace the gears. Neither serious nor casual riders have any interest in changing crank length from ride to ride. – David Richerby Dec 11 '16 at 09:39
  • 1
    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 '16 at 10:19
  • 2
    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 '16 at 16:26
  • 3
    @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 '16 at 16:35
  • The one I was thinking was was actually only 63 speed – Chris H Dec 13 '16 at 09:24
10

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.

ojs
  • 21,985
  • 5
  • 41
  • 83
  • 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 '16 at 03:13
  • 7
    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 '16 at 10:08
  • And of course, were the bikes you tried otherwise identical? This is one problem with the existing research. – ojs Dec 11 '16 at 10:20
  • It is just common sense that longer extension of your legs will generate more power, like a longer stroke in an engine. Short stroke engines reduce torque but allow more RPMs. The same is true for bicycle cranks. Someone with shorter cranks can spin out higher but will be generating less power per crank rotation compared with much longer cranks. I also remember when I was a kid, I had a BMX type bike with very long cranks and I was able to generate good power with them. I don't remember the length of them but they were quite long for a kid. – David Dec 11 '16 at 14:37
  • There is also such a thing as "seat of the pants" testing. In the motorcycle field we call this the "butt dyno". That is, I don't have to do technical testing for power to see if longer cranks give me more power overall during my ride. What I am interested in is acquiring the best crank arm length to give me the most peak power when I need it but also not excessively long such that it interferes with everyday "normal" riding. This is why I asked about adjustable cranks cuz maybe one day I want to ride street and another day more offroading and no single setup can be best for both. @ojs – David Dec 11 '16 at 14:51
  • Why not just have crank arms that can be quickly replaced with some electric tool in like a minute or less per crank and they will already have the proper pedal mounted on them? That would be really fun to just change them quickly. I would want to try 180 some days and maybe 160s another day. That is a compromise however. Ideally, I would want long cranks arms at low RPMs and then they would automatically shorten as I spin up. I don't care if such a mechanism would be heavy cuz it would be fun to try. I don't care if the bike would weigh 45 or 50 pounds cuz the fun factor would be high. – David Dec 11 '16 at 14:58
  • Here is a good example. Suppose someone had a 1 speed cruiser which is a very simple design and quite popular. To compensate for the lack of gears, instead have variable crank arms that would change automatically based on cadence. At low RPMs, the crank arms would automatically lengthen (perhaps to 180mm) to allow more torque to get the bike rolling. Then as the rider spins up to maybe a mid cadence such as 50 to 60 RPMs, the crank arms would shorten slightly to 170mm. If the rider then approached some fairly high cadence such as 100 RPMs, then maybe 160mm. That's the general idea. – David Dec 11 '16 at 15:04
  • 6
    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 '16 at 16:21
  • 1
    @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 '16 at 16:48
  • 2
    @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 '16 at 17:01
  • Good work ojs! I was hoping I'd find one of their papers to cite, but this is the next best thing. To the OP: the science shows the relationship between power and crank length is not so simple. – andy256 Dec 12 '16 at 00:42
  • Also I wonder if there is a benefit to variable length cranks of less pain while riding. For example, with fixed length cranks, you are using the same muscles in the same way each pedal rotation. However, suppose after a few miles, the rider is having some knee pain raising them to the top of the pedal stroke and/or maybe problems extending their leg all the way to the bottom of the pedal stroke. It seems like if that happened, it would be a good time to shorten the crank arms and lower the gearing about the same. Variable length cranks on a 1 speed is a good idea I think. – David Dec 19 '16 at 13:18
2

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.

Adam Rice
  • 29,997
  • 1
  • 47
  • 102
  • 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 '17 at 08:57
0

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.

Andres Millan
  • 21
  • 3
  • 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 '17 at 12:05
0

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.

Criggie
  • 124,066
  • 14
  • 180
  • 423
Dallin from Alice Springs
  • 11
  • 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 '19 at 16:10
  • 1
    @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 '19 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 '19 at 17:14
  • 2
    @DavidW Well, in this case "you" was the generic "you." Dallin could also search so as not to be misled by your comment. See https://www.ncbi.nlm.nih.gov/pubmed/26559455 and https://www.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 '19 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 '19 at 23:27
-2

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?

Conor
  • 125
  • 3
  • Changing crank arm length does not change "gearing". – Daniel R Hicks Sep 20 '17 at 00:48
  • Daniel R Hicks: Fair point. I'll change the gearing to "gearing". The point still stands? – Conor Sep 20 '17 at 00:49
  • How does "mechanical advantage" change?? – Daniel R Hicks Sep 20 '17 at 00:57
  • @Daniel R Hicks: By giving greater action at a point (lever) on the crank? – Conor Sep 20 '17 at 01:06
  • It doesn't change the amount of energy in the system....EVER! – Conor Sep 20 '17 at 01:13
  • "Greater action"? How can you go "greater" than the length of the leg? – Daniel R Hicks Sep 20 '17 at 01:33
  • @DanielRHicks you might be interested in http://www.sheldonbrown.com/gain.html. The proposed implementation would be heavy and develop play quickly. There's a reason why seat posts and handlebar stems have clamps instead of pins. – ojs Sep 20 '17 at 06:33
  • 2
    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 '17 at 10:05
  • @ojs - Note that Sheldon's "gain ratio", as stated, only works on a penny-farthing. – Daniel R Hicks Sep 20 '17 at 11:48
  • @DanielRHicks the same math works on all systems that drive a wheel with a crank. – ojs Sep 20 '17 at 12:08
  • @ojs - Read his description. Nowhere does it take into account gearing. The wheel can go around one time with the turn of the crank, or twenty. – Daniel R Hicks Sep 20 '17 at 12:15
  • @All@ I'm removing "gearing" from the answer. Gearing was WRONG – Conor Sep 20 '17 at 15:36
  • @ DavidRicherby: The OP asked "how?" I added in the OP questions and directly addressed each. The "how?" is the main focus of the post and has remained as my original answer. – Conor Sep 20 '17 at 15:59
  • @Conor You've directly addressed two of the questions with "Dunno" (i.e., not addressed them at all) and addressed the fourth with "Yes!" followed by a completely speculative proposal, one of whose fundamental components is glossed over by saying "provide a mechanism". That's like answering "How could we put men on the moon?" with "Put them in an air-tight capsule with enough oxygen and then use provide a mechanism to launch that into space (perhaps burning some sort of fuel)." – David Richerby Sep 21 '17 at 10:07
  • @ DavidRicherby Of course, everything you state is correct, but missing a point (a very important point) I don't claim a full historic mastery of the subject (point 3); I don't claim to be an authority to the full breath of all cycle technologies ever tried, or currently available. But the OP DID ask "how?", and I provided that, at least. So rather that spouting some BS on the historic/current view of the technology,The OP asked if variable length pedal cranks could be implemented, and how. – Conor Sep 22 '17 at 22:24
-6

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.

Murphsouth
  • 16
  • 1
    You have no conception of how "mechanical advantage" works. – Daniel R Hicks Sep 04 '17 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 '17 at 03:46