As someone who works in the power transmission business, I deal with torque and horsepower on a daily basis. I am curious why more triathletes don’t go with longer cranks. Torque is equal to force (however hard you can pedal) times radius (in this case crank length). If you pedal at a given force, you will simply be more powerful with a slightly longer crank. I have no idea to what degree the advantage would be, but with all the talk of drag reduction and being more aero (which is certainly important) I’m surprised people don’t look at going with longer cranks to gain power. Certainly cranks between 170 and 175mm might be perfect for someone with a certain inseam, but if someone is taller than 5’10", a longer crank could be an advantage that would offset the drag they gain by sitting up higher than the sub 5’10" folks. Of course this would be based on inseam and not overall height. Leonard Zinn makes custom cranks to 220mm on zinncycles.com. Now, I realize he is making custom bikes for seven footers, but the cranks could work for people shorter than that. He uses a formula of inseam(mm) times .216(some constant) to get ideal crank length. In the mountains you would probably go on the shorter side, but most triathlons don’t have mountains to climb. Why don’t bike builders raise the bottom bracket height and use longer cranks on their larger size frames? Anyone?
Sorry, there is no free lunch. Yes, you get more torque for a given amount of force on the pedal, but you have to move the pedal through a longer distance. Work is equal to force times distance. Thus it is more work to apply the same force around a longer circle.
In the case of a crank, if you apply the same force around a crank arm twice as long, you will need to apply this force over twice the distance as well in order to keep the same cadence. This will be twice as much work and will fry you quickly.
As someone who works in the power transmission business, I deal with torque and horsepower on a daily basis.
Perhaps you should take a little time off and go to a night course in engineering or physics. No need to repeat what Art wrote, but I will anyway.
Power is force (torque in this case) times speed. No free lunch. To get more power from a longer crank, you have to spin it around just as fast as the shorter one. Sounds like a simple challenge, yes? No. In real life, people turn the longer cranks more slowly.
This is a can of worms that big bike and component manufacturers have no desire to open. If the Leonard Zinn formula (I don’t know that he came up with it because I read it from someone else before him) is correct, then a lot of people are using cranks that are too short or long. Raising the bottom bracket and making longer cranks is going to cut into profit and as long as that is the case, no one is going to want to do it.
You would be surprised at how many people think that riding 180mm cranks is crazy. Now that I have some 190s (using the .216 x inseam in mm) I can’t wait to hear from them. It’s true there is no such thing as free lunch, but if our legs are most efficient in a certain range of motion, then there should definately be a greater range of crankarm sizes.
It comes down to money and companies like shimano, campy, trek and others are not interested in building bikes and components that fit 100 percent of the population. What they make now fits a lot of people and the odd-bodies out there just have to make do or put up the cash for custom frames and cranks.
Chad
Perhaps you should take a little time off and go to a night course in engineering or physics. No need to repeat what Art wrote, but I will anyway.
Why did you have to include the first paragraph? To be a dick? The second paragraph would have suited just fine.
Look at it this way…in order to move the pedal around a circle at a certain rpm, the pedal must move faster at a given rpm as the pedal is located further away from the axle. Seeing as how the pedaller has a couple of axis of his own (hip, knee, ankle), it begins to be apparent that faster movement of the joints around these axes consumes more energy. Sure, TORQUE is increased with longer levers, but not (horse)power (to use the automotive terminology). Sort of like high torque (but low horse-power) from a tractor engine compared to the relatively high wattage (but low torque) from a motorcycle engine.
The faster your foot has to move, the more energy it takes just to move the foot quickly. There IS a sweet-spot in there somewhere, depends upon the rider’s current physiolgical and kinesthetic conditions.
I believe it is the nature of slowtwitchers to be antagonistic. Just take it in stride…
Having ridden tall frames, and various crank arm lengths, the old track guys would say leg speed is everything, being able to spin up quickly to a hi cadence, which work for short bursts of power. For a tall rider hi cadence just Reduces blood flow.
Raising the bottom bracket screws up the center of gravity and handling . I have a 67 cm road bike that is about 2 cm taller than I need, with 180 crank arms which help noticeably in climbs, but flat ground 175 - 180 mm hard to tell. Even the 170s on the mountain bike is livable. , 10 - 15 mm change would be noticable . But a small market for large sizes keeps to a custom builder level or lucky find . Zinn’s fitt for me would be 202 mm crank arms.
Would work really increase that much? 25.4mm is an inch, so an increase of an inch in either direction does not seem like it would call for such a drastic increase in wattage to maintain the rpm’s. I realize it certainly would if the persons inseam called for the 175mm crank and they started increasing the length. I don’t doubt that there is a point of diminishing returns based on the length of your legs. But, what about a rider who has longer legs? Could he pick up an advantage? I was just curious. I asked the question tonight in my physics and engineering class at the local community college and the teacher wasn’t sure.
That is scary that your physics teacher couldn’t answer. Hopefully, it will sit in his mind until your next class and he will be able to answer then. Otherwise, make sure you have a good textbook, because your teacher is not the sharpest knife in the drawer.
We all try to operate in our sweet spot according to many variables including crank arm length. Find your biomechanical sweet spot, if you can, and stick with it.
You can think of longer cranks exactly the way you would think of a longer lever or a lower gear. You apply less force, but to generate the same power you have to move through a longer distance in the same time.
I have very short legs. 170’s are long for me. I like 165’s, but most vendors don’t make them, so I adapt as best I can to 170’s most of the time.
I dont know how many studies have been done on crank length. The best way to determine yours would be to ride on a dynatron or computrainer at a set wattage (probably best at just below VT) with adjustable length cranks and measure your O2 consumption. You could adjust the crank length and see at what length you used the least O2 for that intensity. My personal guess is that there wouldnt be much difference.
You could also do seperate VO2 max tests with different length cranks and see if it made a difference.
Some of you may be able to answer this for me, my local bike shop sold me my first tri bike, the crank lenghts are 170mm and 175mm on the right hand side, i questioned them about this they said it would make no difference, i have done 2 tri’s now and for the last couple of months i have had chronic knee problems, also when i run i have allot of pain in my hips, a cyclist friend told me this is becuase i do runs off the bike, excuse my ignorance i am new to the sport. hope some one can point me in the right direction.
thanks
rob.
Damn, since someone else has taken my role as being a dick in this thread, i have to be nice.
i switch between 175’s on my mtb, (and my soon to be built cross bike), 172.5 on my road and tt bikes, and 165 on my track bike. I do notice the difference between the 165 and 172.5, i htink mainly becuase i am using the same shoes, pedals, and similar bikes. Also, i notcie it to be a bit harder to start in a big gear on the track than it might with longer cranks, but it is easier to “ankle” the gear in a circle on the third lap of a kilo.
I do not notice the difference in crank length on the mtb, mainly (i think) becuase i use different shoes, different pedals, am in a very different position, and have several inches of suspension. I do however, notice the difference in q factor on all of my bikes.
find a new bike shop, but only after you make them put the same size cranks on each side. Unless you have a major discrepency in foot size or leg length, and the bike was beliberatly fit using different size cranks, your lbs is full of bs.
edit; i think only a foot size discrepenc might warrant a different crank length, a leg length disrepency would more than likely warrant shims under the pedal, not an alternating crank size.
Thanks Ben, I have had the bike for six months now should i just take it back or should i take it to a bike swap some where
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i might go back to the shop, remind them of your situation with the different size cranks that came on a bike you bought there, and that you are having joint and pain issues that might be related to the cranks being different sizes.
It’s a tough call, i would expect them to replace one of the arms, so that they are both the same size. I would expect them to do this for free, becuase it was their screw up. This is an area where tom d might be able to give the best opinion becuase he sees things from a shop perspective, and is an experienced fitter.
One problem with longer cranks is that you have to flex your hip into a tighter angle at the top of the stroke. This is a limiter for a lot of cyclists.
I’m glad the guy’s physics teacher admitted he didn’t know the answer. The problem with using straight physics in a pedalling question is: the biomechanical inefficiencies associated with acceleration of limbs by MUSCLES is not linear as forces from a motor or weight. The latter is more commonly used by high school physics teachers.
Muscles aren’t very efficient compared to motors. Just making a muscle contract more speedily than “normal” uses tremendous amounts of energy compared to making most motors run faster than their “normal” speed. It’s this difference in relative efficiencies that throws the simple lever/torque/speed question of a crankarm out of whack. The teacher is right, he doesn’t know the answer. Few do, and I’m not one of them, either!
I’ll second what Ben said… you’re local shop either A) is stupid or B) just wanted to sell you a bike and lied to you
both are very probable so it’s hard to tell, I’d take it back and speak with the manager/owner who A) still may not know shit or B) may not care or C) hopefully, will change your cranks to the same length for free b/c that would be the right thing to do
Well, if you bring the nonlinearity of muscles into the equation, yes, it gets very complicated. It sounded like the teacher was just commenting on the leverage of the cranks though. That answer is very simple. No free lunch.