Hunterdog

I had fitting today on Retul bike. Fitter (who was great) shortened crank length to 160 and comfort level increased as did RPMs by about 15. I am 58, stiff but fit, with 29 inch inseam. Any thoughts?

McBTC

For some even with longer legs the tradeoffs favor shorter cranks than what off-the-shelf bikes usually come with. 165s are about as short a crank as the big part makers support so going shorter will increase your cost as that moves you into a specialty market.

ThermionicScott 

Yep, if you're still in square-taper land like me, your cost might increase all the way to $45 or even $70 for 160mm cranks.

AnthonyG

With a 29" inseam you could go to even shorter cranks of say 155mm or 150mm. They're not easy to find but they are out there. I built a custom bike around 140mm cranks and I've gone even shorter with the cranks since then. As well as making it easier to spin higher cadences, shorter cranks reduce leg movement which reduces the rubbing against the saddle. If a frame is designed properly for shorter cranks then the seat tube angle will end up being more relaxed than currently used for small frames, which will move the riders weight backwards over the rear wheel, which will take weight off the riders hands and shoulders.

Anthony

Leisesturm

Cranks, gears and wheels/tires are a system. Any gear calculator that does not ask you to input all three of these parameters is not going to be able to tell you exactly what your feet are going to be feeling in a given gear. Chainwheels and cassettes are still sold with ~160mm - ~175mm cranks in mind. Going higher or lower shifts your torque curve correspondingly. As I understand it, 130mm is the shortest practical crank. Even at 140mm the average rider is struggling to turn gears that are no issue for a longer crank. So gear down, duh... ok so lets do that. The average gearing is set up so you have big jumps at the low end that get smaller and smaller as you go to higher gears. This works well with the human torque curve. You will be miserable with a gear system that forces you to remain in the lowest gears of the range where all the increments are 15% and more. There is evidence that, for recumbents, shorter cranks can be useful. You will lose torque and possibly, speed, but recumbents are more aerodynamic so there is a trade-off. With DF bikes there is no such trade-off. You lose torque, speed and you still have the frontal area of a DF so unless you constantly spin like a madman you go nowhere fast. It might be instructive that despite all the excitement in America over short cranks, Europeans mostly don't touch them. Not even recumbent riders. Even in velomobiles where space for feet is often lacking the standard crank remains 175mm. Just throwing a whole bunch of stuff out there. There is a point in there too but I've forgotten what it is. FWIW.

AnthonyG

Yes its a connected system, and you have to look at the whole system including the rider.

Lets say for the sake of an argument you have two bike riders with identical leg strength but different leg lengths. Lets say they both have the same thigh and lower leg proportions. Due to the lever of the thigh bone, the rider with the SHORTER leg, will be exerting more force at the knee but with slower leg speed at the knee. The rider with the longer leg length will have less force at the knee but with greater leg speed at the knee.

The lower leg is effectively a connecting Rod (Con Rod) connecting the thigh to the cranks.

It balances out. Riders with shorter legs are capable of pushing gears that are effectively bigger, and as such, shorter cranks are not a disadvantage in any way.

What's needed here is parts that match each other, just as in a car engine. You don't put V8 Con Rods in a 4 cylinder engine. You don't put a crank the dimensions required for a 7 litre V8 in a 1.5 litre 4 cylinder engine.

Humans are much more flexible than a car engine so we get by with things that don't really suit us but even humans are better off when things are the right size for us.

pakossa

I read an on-line article once claiming that 150 mm cranks would be the best length for nearly all riders, regardless of leg length. (Something about power being the highest -- albeit very slightly -- at that length, as well as allowing for a much larger bar drop -- hip angle not closed up as much -- before any power loss occurs.) Any know more about that?

AnthonyG

Shorter cranks DO open up the hip angles, and if the frame is designed properly for shorter cranks then the seat tube angle will be more relaxed, with greater saddle setback, which makes it FAR easier to adopt a low, aerodynamic riding position for a greater period of time.

I don't want to claim that shorter cranks give you MORE power, but they certainly don't reduce your power.

Anthony

Hunterdog

Thanks for the responses. Finding a 160 crank is difficult and they look expensive. 165 crank arms are available at reasonable cost. My current crankset (FSA Gossamer) has 172.5 crank arms. Is the additional 5mm really worth the money?

gregf83 

What's the reason for the slacker seat tube angle? I've got short legs/long torso and would be interested in exploring going shorter than the 170mm I'm using now. I have my seat back as far as possible on a setback post and think it would be good to go back further. Any bikes you're aware of that have slacker seat tubes or is this a custom option only?

Leisesturm

Shorter cranks don't reduce your power, they decimate your power. Unless you are a trained cyclist that is used to cadences up to and exceeding 100rpm. Most regular cyclists find cadences over 80RPM mentally fatiguing even if not physically so. As to frames properly designed to use short cranks... which would those be? All production frames assume the use of cranks considered "normal". That would be around 165mm on the short end and 175mm on the long end. Not that stops anyone from using shorter or longer cranks as they see fit. As I understand it, a slack seat tube, set back seat post configuration emphasizes low rpm torque, and such a configuration assumes the use of longer, not shorter crank lengths. The ultimate and polar opposite configuration would be a track bike. If you can describe a lower and more aerodynamic position than a 75* seat tube angle track bike with a -15* stem and Pista bars I'd like to see it. Swap out the Pista bars for Triatholon bars and you can maintain that low, aero position more or less comfortably for hours.

AnthonyG

Bike manufacturers have gone beyond the concept of making bikes that fit people. The game these days is make the minimum number of different size bikes that the maximum number of people find "acceptable" fit wise. In truth a particular bike fits no one perfectly, but its close enough for many. Its not even about the average person anymore. They've gone beyond averages.

When you shorten the cranks, you need to move the saddle backwards to maintain your KOPS position, otherwise your knee will move in front of KOPS. Now strict KOPS isn't the only way to do things but its a great way to find the ball park. Moving your weight rearwards, which takes the weight off your hands and shoulders is also greatly beneficial for most people.

The seat tube angles on modern small frames is WAY too steep for human beings. It doesn't fit anyone properly but it makes the bike acceptable enough for long enough for you to buy the bike in the first place. The thing is, when you are riding a bike with cranks that are TOO long for you it does help to be in front of KOPS as it opens your leg angles up.

I'm old enough to remember child/adolescent bikes from the early 70's and even late 60's. They had VERY short cranks, 110mm, VERY relaxed seat tube angles, 69 degrees, small wheels and very short front centre distances.

That's what it took to fit a child/short person on a bike. I had one of these child bikes at the same time thatI had a custom 27" wheeled racing bike, and while the racing bike was faster, I was MUCH more comfortable on the child's bike and could ride MUCH further on it. The custom racing bike had an INSANE 78 degree seat tube angle to make it work.

If only I knew then what I know now.

Anthony

CliffordK

These Origin8 Cranks come in 145, 155, and 165, and are reasonably cheap.
Amazon.com : Origin8 Triple Alloy Crankarms, Silver : Sports & Outdoors

It appears to be 130 bcd, so not super small chainrings (other than the triple)

Oh, and some Origin8 160x110 cranks (also see other links on the Niagara site).
Bulletproof BMX Crankarm Set 160mm 110 BCD Forged Black

I believe torque and stroke circumference is a linear function with the length of crank. So, shortening the crank will naturally decrease torque and increase cadence. Lengthening the crank will increase torque and decrease cadence. And, as others have mentioned, you can't just look at cadence.

I'm reasonably flexible, so I tend to like the longer cranks, but am not sure how much I can tell the difference between 170, 172.5, 175, or 180mm cranks, other than saddle height.

gregf83 

Thanks for the explanation. I was hoping one of the manufacturers had a bike that might fit me a little better. I suppose a seatpost with more setback might help but I had trouble finding one of those last I looked. Or a custom frame.

AnthonyG

Sorry, your wrong. Some basics.

Power = torque x rpm.

Longer cranks may increase your torque, just when your cranks are level, but then reduce your torque when you are trying to get through top dead centre. Longer cranks increase the degree of movement that your leg needs to go through in the whole cycle making you less efficient.

Shorter cranks may reduce your maximum torque when the cranks are level but make it FAR easier to get through top dead centre giving you more torque in the TDC position than if you had longer cranks.. Shorter cranks reduce the degree of movement in you legs making you more efficient, particularly if you have your seat positioned correctly.

Shorter cranks smooth out your torque delivery and allow you to spin higher cadences, giving you just as much power, as longer cranks. Short people who are struggling to deal with cranks that are really too long for them will produce more power with shorter cranks. If your tall and you fiddling around with 5-10mm crank length differences that it might not matter to you.

EDIT; I don't have any problems with high cadences. I find 90rpm to be very relaxing. 110-120rpm is fine if I'm working hard and I get to over 140rpm in a sprint. I don't have to think about it at all.

Anthony

McBTC

True and a consideration that many do not seem to appreciate is sort of the reverse side of cadence in that while shorter cranks work to increase RPMs -- so, going to shorter cranks, it might seem may hit up against some kind of natural performance barrier that limits what may be attainable by all but elite riders -- there also is the issue of pedal speed: longer cranks result in increased pedal speed at any given RPM.

As far as what feels more natural to the average rider, and the most efficient combination of factors in most riding situations, pedal speed may be as important and possibly a more important factor than simply looking at RPMs. In other words, at 75-80 RPM, the pedal speed will be faster for longer cranks -- and, faster pedal speeds may feel unnatural -- such that shorter cranks at a higher gear and RPM at a given inches per second but lower pedal speed may actually be the higher performance option for some riders.

Leisesturm

Leverage is leverage. Torque is torque. A longer lever will allow the same rider to push the same gear through TDC with greater perceived "ease" than a shorter lever. This is not debatable. It is why you use a 14" crescent wrench on a frozen bolt and not an 8". If you just want to spin a bolt around then the 14" is going to be unwieldy and you will get much better results twirling a loose bolt with a much shorter wrench. When using shorter cranks you must gear down, and when you do so, of course the gears will be easier to spin, they are lower gears. And you seem to think that shortening cranks raises cadence in and of itself. They do not. They require increased cadence if you are going to keep up with your riding buddies. A cadence of 100RPM will be easier to maintain with shorter cranks than longer ones but you still have to be willing to go there. Also there are plenty of riders, this one included. That are well able to spin "normal" cranks well in excess of 100RPM. And 100RPM is still 100RPM. Just because the cranks are shorter doesn't make it the same thing as a longer crank at 60RPM. Its still a big deal.

Leisesturm

You and Anthony both need to get out on your bikes and observe gears, terrain and resistances in the real world. Not physics textbooks. Both of you make your assumptions from the rider outward. Reality is actually the reverse. Shorter cranks require shorter gears. Thats why mountain bikes have shorter gears than road bikes. They always have shorter cranks and shorter gears together and longer cranks and taller gears together. If you go shorter than the MTB standard of 160/165 you must made still another reduction in the gearing. This, however is never done. People are using 150/140, even 130 with the same cassettes and chainrings meant for use with 165 cranks and calling the results a success. I'm not sure how that can be but people are contrary and who knows what criteria they are using to define success anyway.

Leisesturm

The modern sloping top tube is the main innovation that allows bike manufacturers to reduce the need for so many increments of sizes. Seat tube angles are more about achieving a given chain-stay length than the need to punish modern bicycle customers. If you want KOPS you can get it on just about any bicycle produced, whether it is the right size for you or not. Zero setback, 25mm setback... 2" setback and more. Seats with extra long rails. Anything is possible. Worrying about the lack of relaxed seattube angles in the modern bike marketplace seems a baffling waste of energy to me.

McBTC

What research on the topic exists is very little but supportive. Moreover, in the real world, those who are testing the traditional paradigms are among the most athletic among us and their anecdotal evidence is supportive.

Mostly what seems most certain is that 165s for a shorter rider but not shorter cranks and 175s for even the tallest riders but not more than 180 is simple and in general does work but is not for that reason alone the most efficient combination of factors given gearing and considering RPM vs. pedal speed and what feels more natural.

Less than 160 even for longer legs may actually be more efficient setup. For some, as low as 145 may be better than the 175s that came on the bike. But, learning that is the case if not easy because changing crank size is not as easy as changing gears. Nevertheless, being ignorant of the facts doesn't make 175s the ideal crank size just because that's what came on the bike.

fietsbob

Origin 8 in their cheap BMX cranks goes down that far , If you paid top dollar for one of those Pro Fittings, that may lack Bling.


Then there is TA of France.
TA Carmina cranksets from Peter White Cycles

Or buying cranks that have gone back to a Skilled Machine Shop and had new Holes bored and threaded into Longer crank-arms.

AnthonyG

Leisesturm, where are you coming from?

Are you an industry insider?

I'm 5'2" with a long torso and short legs. I have owned and ride many km's on bikes with, 110mm cranks, 125mm cranks, 135mm cranks, 140mm cranks, 152mm (6") cranks, 165mm cranks and 170mm cranks. I've ridden bikes that have seat tube angles from 69 degrees to 78 degrees.

When I say that I haven't lost power from shorter cranks, this isn't just theory to me. 110mm cranks probably are a little short, but still WAY better for me than 170mm cranks. When I say that longer cranks make it harder to get through top dead centre, I know from personal experience that this is so. My torque delivery with 165mm and 170mm cranks is VERY choppy. With 125mm and 135mm cranks I can feel the extra torque that I can deliver through the whole stoke.

When I was riding on 165mm cranks and a seat tube angle of 75-78 degrees I couldn't use the drops to save myself. My knees rose into my chest and I couldn't bear my weight on my hands (I do have weak arms and hands). With 125-135mm cranks and a seat tube angle of 70 degrees or so I have NO problems riding along in the drops. in fact I find it comfortable.

And the cold hard truth is, I haven't developed anything new at all. I've owned and seen numerous small vintage bikes that are EXACTLY the same as what I recommend. Short cranks, relaxed seat tube angles, small wheels and short front centre distances.

The knowledge has been around a LONG time. In modern times the knowledge has been deliberately forgotten.

Anthony

Leisesturm

I, I, I... me, me, me... your observations are duly noted. I really don't know why the bicycle industry would have it in for riders on such a massive scale as to deny the general public access to a useful tool. If very short cranks were of massive benefit to the majority of riders they would be available. Sooner or later some manufacturer will come up with an adjustable crank that the rider can set to whatever crank lenght they feel works for them based on complicated biometric analysis or simply how they feel that day. Two of my bikes have quick release seatpost collars. I didn't order them they just came that way. But I do use them. I mess with seat height on those bikes where I pretty much leave it alone on the bikes that require me to go find a crescent wrench to adjust the saddle height.

Years ago Shimano did considerable reasearch into human power delivery through the entire 360* pedal stroke. They observed the fall-off in power delivery, not torque, near TDC. Their answer was to reduced the diameter of the chainwheel near TDC (and BDC) and keep it at full diameter elsewhere. Remember Biopace cranksets? Neither do many people. Interestingly another company, I forget which, came to diametrically opposite conclusions about what to do about human uneven power delivery and their 'biopace' chainrings were small exactly where Shimano's were large and vice versa.

Neither company and many other companies have offered very short (or very long) cranks to the public. I'm not going to worry about it.

trailangel

Children, Children...
get off soap box.
back to OP's question
29 in. inseam..... 170mm cranks..

AnthonyG

I was a student of industrial design. I studied manufacturing economics. The industry does what it does because of money. Not for good fit.
Anthony