Road Cycling - Crank Length revisited

Found a site that talks about crank length..However, when I do the calculation, I get a crank length of 182, waaaay longer than what I use. The author says he writes based on 1970's data, so that may be the problem. However, he has a section on old wives tales, which made me think twice about dumping the site.

Here's the link,


Crank Length (http://www.nettally.com/palmk/Crankset.html)

Originally posted by fubar5
Found a site that talks about crank length..However, when I do the calculation, I get a crank length of 182, waaaay longer than what I use. The author says he writes based on 1970's data, so that may be the problem. However, he has a section on old wives tales, which made me think twice about dumping the site.

Here's the link,


Crank Length (http://www.nettally.com/palmk/Crankset.html)

According to that calculation, I should be using a crank length of 189mm. Hmmm, I have to agree with you.

I currently use 175mm, and some sources say that I should be using 172.5 to 170mm.

I ride 175's and according to that web site I'd need 165's. I'll keep my 175's thank you very much :)

Zack

There's a great article on bike fitting that actually disparages calculations the whole way through, with one exception. The author suggests calculating the crank length, and said that he was extremely happy when he first listened to the numbers.

It's a good read all around, discussing body mechanics and such, but the article is long. Search for the word "femur" to go right to the part about cranks.

How to fit a bicycle (http://www.peterwhitecycles.com/fitting.htm)

One thing that gets little attention in the longer crank advocates' position is that to have the same travel of the rear wheel for a given movement of the leg, or conversely the same speed for the same pedal pressure, you need proportionlly larger rings, or smaller sprockets. for example, if your crak is now 170 mm and you go to 187, then if your largest chainring were 50 teeth, it should go to 55.

Further, in order to have the same pedal ground clearance, the bottom bracket should be 17 mm higher, and the seat tube 17 mm shorter, and you might need front wheel 17 mm further forward, which might imply a shallower head tube angle, and might need chainstays 17 mm longer, if you are riding with panniers. Merely changing the crank length by a significant factor will throw all the other geometry into question. At the very least you probably would need to move the seat down, which would make the top tube length feel shorter, because the handle bar would be relatively higher. I agree that longer cranks with proportionally larger geometry is appealing, but appears to be unavailable in production bikes.

All that said, I did increase my crank length by 5 mm, to 175 mm, on my old schwinn le tour a while back, and small changes in that range appear not to make much difference in bike handling and performance, while making pedaling more comfortable.

Originally posted by ZackJones
I ride 175's and according to that web site I'd need 165's. I'll keep my 175's thank you very much :)

Zack

If you read the reasoning portion of that article the author states that the constants might actually be low, which would mean your 175 is just fine.

The real fuss is about longer legged cyclists' crank length, which according to the article should be much longer.

the formula is derived from the assumption that a 170mm crank is perfect for someone with a 31" inseam....

If you ask me, back engineering from that assumption alone is not conclusive... If you study it more closely, you would be able to see more variables... Emperical formulas are not based on experiments alone, and derived from a single assumed perfect situation... Besides, he would need more data to conclude that his assumption is correct. He must have performed controlled experiments, with emphasis on control...

There are more things to be considered than inseam alone... as mentioned above:


One thing that gets little attention in the longer crank advocates' position is that to have the same travel of the rear wheel for a given movement of the leg, or conversely the same speed for the same pedal pressure, you need proportionlly larger rings, or smaller sprockets. for example, if your crak is now 170 mm and you go to 187, then if your largest chainring were 50 teeth, it should go to 55.

Yeah, it seemed like the calculation was bogus. Bike fit is a very interesting topic though, so I figured I'd bring that calulation up for discussion.

Originally posted by dexmax
the formula is derived from the assumption that a 170mm crank is perfect for someone with a 31" inseam....

If you ask me, back engineering from that assumption alone is not conclusive... Besides, he would need more data to conclude that his assumption is correct. He must have performed controlled experiments, with emphasis on control...

There are more things to be considered than inseam alone... as mentioned above:

All too true- the formula depends on the assumption that a 170 mm crank is perfect fopr a 31 inch inseam- a very difficult assumption to support. The other key assumption appears to make a lot of sense, and I would like to see a reason to reject it- that assumption is that the optimal crank length is proportional to body size if the relative dimensions of bones is constant. I don't see any evidence to reject the second assumption, after reading the links in the article. The basis for the assumption relating 31 to 170 is observation of successful cyclists.

The issue as to whether the author needs to substantiate his assumptions with controlled experiments seems to be a red herring as the current crank lengths were not derived by controlled experiments either. I would bet that the only thing that would alter the current practice would be a highly successful cyclist adopting another approach to crank sizing and receiving attention for his approach. Shaped skis were a good idea, but did not become popular until someone started winning races on them, and the ski company publicized the fact.

In this case, apparently Miguel Indurain did use a similar approach to crank sizing, but for some reason, perhaps economics, the component manufacturers did not pick up on it. Maybe frames sized for longer and shorter cranks can be a new fad after the current "compact frame geometry" runs its course. In that event the manufacturers will be able to sell us all new bikes with "just the right crank length."

I'm pretty happy with my 172.5's.

Originally posted by FOG
All too true- the formula depends on the assumption that a 170 mm crank is perfect fopr a 31 inch inseam- a very difficult assumption to support. The other key assumption appears to make a lot of sense, and I would like to see a reason to reject it- that assumption is that the optimal crank length is proportional to body size if the relative dimensions of bones is constant. I don't see any evidence to reject the second assumption, after reading the links in the article. The basis for the assumption relating 31 to 170 is observation of successful cyclists.

The issue as to whether the author needs to substantiate his assumptions with controlled experiments seems to be a red herring as the current crank lengths were not derived by controlled experiments either. I would bet that the only thing that would alter the current practice would be a highly successful cyclist adopting another approach to crank sizing and receiving attention for his approach. Shaped skis were a good idea, but did not become popular until someone started winning races on them, and the ski company publicized the fact.

In this case, apparently Miguel Indurain did use a similar approach to crank sizing, but for some reason, perhaps economics, the component manufacturers did not pick up on it. Maybe frames sized for longer and shorter cranks can be a new fad after the current "compact frame geometry" runs its course. In that event the manufacturers will be able to sell us all new bikes with "just the right crank length."

yes, we have been taught in "design" to consider economics.. without economics and "practicallity", mass production would be impossible and the product would be too expensive to market...

Im a medium sized person who rides 170mm cranks, but if I was very small or tall, I dont imagine that a few mm difference in crank length would be enough. A very short person riding 170mm cranks is like me riding 200mm cranks. A tall person riding 170s would be like me riding 120mm cranks. I dont see any reason to object to proportional crank sizing.
You can get cranks from proper manufacturers (Specialities-TA) in sizes from 150mm to 185, so there is no reason to limit yourself to the 165/107/175 that many "fitting guides" recommend. Why small CNC shops limit themselves to medium sizes I have no idea.
Its quite true that frames need to be dimensioned to accept cranks. On smaller frames, this would mean a lower BB height and less toe-clip overlap. Both advantageous to small riders. On large frames there is plenty of room for large cranks, and it would not cost any more to raise the BB. Trek vary the BB between their largest and smallest models by a mere 4mm, totally useless for fitting cranks.

Originally posted by dexmax
yes, we have been taught in "design" to consider economics.. without economics and "practicallity", mass production would be impossible and the product would be too expensive to market...

It would be less expensive to manufacture pants in different lengths but one waist size, or different waist sizes but one length, yet somehow we manage to have mass production which covers two variables, yet what I suggested was that the manufacturer, who at present must vary the dimensions of frames, raise the bottom bracket on larger frames, and make the wheelbase longer, and then install one different part, a crank with appropriate crank length and chain rings, on different sized bikes. This would not dramatically affect inventory costs, because the manufacturers seem to know beforehand what mix of bike sizes they will sell. They would have to buy different size cranks, but that seems to be part of the planning process. I think what they do now is convenient but not all that good for the purchaser.

Originally posted by FOG
yet what I suggested was that the manufacturer, who at present must vary the dimensions of frames, raise the bottom bracket on larger frames, and make the wheelbase longer, and then install one different part, a crank with appropriate crank length and chain rings, on different sized bikes. This would not dramatically affect inventory costs, because the manufacturers seem to know beforehand what mix of bike sizes they will sell. They would have to buy different size cranks, but that seems to be part of the planning process. I think what they do now is convenient but not all that good for the purchaser.

Yes, unfortunately high-end manufacturers of cranks don't build frames(shimano,campy). It would be a big step for the frame makers and the component manufacturers... And the change has to be simultaneous, else no one would buy these products... It is really up to them... perhaps in this lifetime we can see changes... or perhaps we won't, until someone wins grandslam recordbreaking races with "non-standard" crank lengths.

Originally posted by dexmax
or perhaps we won't, until someone wins grandslam recordbreaking races with "non-standard" crank lengths.

The problem is that the race rules limit chainrings to a maximum of 53 teeth, and the sprocket to a minimum of 11 teeth for road stages (at least as far as I can interpret the rules, and I don't claim to be on target here) so that a competitor who wants to go a certain distance forward, must now traverse a longer arc with longer cranks, moving further, and faster, but against less resistance. I think the setup might be ok for a climbing stage, but I would bet that a relatively flat stage would put the long crank, with the same gearing, at a disadvantage.

If the cycling race authorities related pedal arc to distance travelled thenwe might see a very different choice among the best riders.

Originally posted by MichaelW
Im a medium sized person who rides 170mm cranks, but if I was very small or tall, I dont imagine that a few mm difference in crank length would be enough. A very short person riding 170mm cranks is like me riding 200mm cranks. A tall person riding 170s would be like me riding 120mm cranks. I dont see any reason to object to proportional crank sizing.
You can get cranks from proper manufacturers (Specialities-TA) in sizes from 150mm to 185, so there is no reason to limit yourself to the 165/107/175 that many "fitting guides" recommend. Why small CNC shops limit themselves to medium sizes I have no idea.


I've just been measured up this Saturday and was told to get 150mm cranks. Unfortunately TA no longer makes short cranks (not below 165mm) as there wasn't enough demand for them.
Yet, I hope to get hold of them as there is still a small stock..

If I like them, I'll stock up with 1 or 2 more...

Why are all good things getting obsolete?

I'm hoping to cure knee pain with short cranks, as she got fond of clipless. Before I always rode with pointy toes which prevents kneepain but isn't effective..

Ivana

PS: the current world-record holder among the men's section of recumbent riders Ymte Sijbrandij uses relatively short cranks.
Another acquaintance of mine, who placed 2nd on yesterday's 3hr TT uses 165mm cranks while he's nearly 2 m tall...

Spoke to both of them after the race (during that event I was measured up too) and was convinced to try those 150mm cranks myself too.. if these tall guys can be so incredibly fast on a recumbent guys with short cranks, I should be able to improve myself with a regular bike too.

Ivana