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Seems like an on-going debate whether us taller riders really benefit from using longer cranks.
My bikes are pretty much stock, so have 175's on Stumpjumper FS and 180's on Stumpjumper HT.....As some other's have mentioned, you just get used to what size you ride.
About the only thing I'm sure of is that your possibility of pedal-strikes increases with longer lengths, especially on a FS bike.
 

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I run 180s on my mountain bikes, 170mm on my fixed gear, and looking at running 180mm SRAM cranks on my upcoming cross/touring bike build.

Do it! You can get Truvativ Stylos for cheap to see if you like the longer crankarms.
 

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rock crusher
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Surly Mr. Whirly crank arms are available in 185's as well, and at a reasonable price.

Personally, I'd imagine that there is a rule of diminishing return with really big crank arms, say >190mm. You will need to have a high bb to accomodate the arms, or pedal/crank strikes will happen all the time. But, the high bb may throw off bike handling/geometry, and any technical riding will suffer IMO. 180 is the biggest I have gone, but already bash my stuff all the time in the local rocks.
 

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Discussion Starter · #7 ·
I appreciate everyone's input. After a bit of fit theory and crank research, number crunching and biasing for ride type, seat angle; rail position, shoe size, cleat position, XX gearing, and bottom bracket height, I've determined that I need 200mm cranks. The most compelling literature I found was written by Zinn. http://zinncycles.com/Zinn/index.php/archives/1399

Also, as some have pointed out, we are guilty of becoming accustomed to what we ride and changing will take a little getting used to. Therefore, it may be a little while until I get my bike dialed and I adjust to the changes; I'll try to provide an objective ride report when I can.

Ride On! :thumbsup:
 

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burgundy snake said:
I appreciate everyone's input. After a bit of fit theory and crank research, number crunching and biasing for ride type, seat angle; rail position, shoe size, cleat position, XX gearing, and bottom bracket height, I've determined that I need 200mm cranks. The most compelling literature I found was written by Zinn. http://zinncycles.com/Zinn/index.php/archives/1399

Also, as some have pointed out, we are guilty of becoming accustomed to what we ride and changing will take a little getting used to. Therefore, it may be a little while until I get my bike dialed and I adjust to the changes; I'll try to provide an objective ride report when I can.

Ride On! :thumbsup:
From the above article:
3. A stock mass-produced mountain bike will probably not have a high enough bottom bracket to ride on technical trails with a crank any longer than 175mm or perhaps 180mm.
 

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turtles make me hot
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Jeff in Bend said:
From the above article:
3. A stock mass-produced mountain bike will probably not have a high enough bottom bracket to ride on technical trails with a crank any longer than 175mm or perhaps 180mm.
True. My bike came with 175s and I swapped on 180's. Doesn't sound like much, but I definitely have more strikes.
 

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turtles make me hot
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Specialized says the BB height is 328 mm. It's a full suspension bike, so it changes constantly. I used to run Crank Bros Mallets on the bike, once I went to the 180's, and started hitting the ground, I switched to Eggbeaters and changed my riding style a little. I really hit the ground very little any more. Mostly in turns if at all any more.
I did notice an increase in mechanical advantage. I climb a little easier. I'll not be going any shorter ever again. I have to check the BB height on my Kona HT. If it's a little higher, I may try some 185's.
 

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rock crusher
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x2 on pedal selection dictating the frequency of pedal strikes... but not crank arm strikes. I ran platforms and Mallets for a while, but not anymore.

Tire selection can also change your bb height substantially as well. I run some large volume tires (2.4 Purgatories and Ardents among them), that can easily raise your bb height by 5mm or more to accommodate longer cranks.
 

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turtles make me hot
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Pitch said:
x2 on pedal selection dictating the frequency of pedal strikes... but not crank arm strikes. I ran platforms and Mallets for a while, but not anymore.

Tire selection can also change your bb height substantially as well. I run some large volume tires (2.4 Purgatories and Ardents among them), that can easily raise your bb height by 5mm or more to accommodate longer cranks.
That's a great point. I just swapped the rims on my bike to Salsa Gordos and they changed the profile of the tire significantly. The bike is absolutely a little higher. I just had my first ride in over a month today. Too much snow. I didn't realize it at the time, but I had zero strikes.
 
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burgundy snake said:
I appreciate everyone's input. After a bit of fit theory and crank research, number crunching and biasing for ride type, seat angle; rail position, shoe size, cleat position, XX gearing, and bottom bracket height, I've determined that I need 200mm cranks. The most compelling literature I found was written by Zinn. http://zinncycles.com/Zinn/index.php/archives/1399

Also, as some have pointed out, we are guilty of becoming accustomed to what we ride and changing will take a little getting used to. Therefore, it may be a little while until I get my bike dialed and I adjust to the changes; I'll try to provide an objective ride report when I can.

Ride On! :thumbsup:
Zinn has sold you a bill of goods and he makes a good living doing it. If you consider his "literature" compelling, why haven't you looked for even the most rudimentary data that suggests it is true? Zinn won't offer it no matter how badly he wants to. Other than formulas that have no data behind them, what has led you to determine a length of 200mm?

I agree with Pitch who said there is "a rule of diminishing returns with really big crank arms". Testing has shown "really big" to be about 150mm surprisingly. We all get long enough crank arms stock on every bike. There's a lot of confirmation bias regarding long crank arms since there's so many claims that proportional cranks are needed. You won't find a single objective test that shows any value to long cranks though.
 

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B_S - I'm taller than you also on a Sultan and I run 180mm Shimano XT cranks. They do feel better than the 175s I had on my old bike. I am very tempted to try the 185 Mr Whirlys as I think I could just about get away with that on a stock frame - and my other bikes wouldn't feel too odd when swapping back to them.

You'd be dropping quite a lot of cash on some 200mm cranks, then buying new long forks - that would be a lot of money for an experiment that would probably not work. You really need a custom frame for cranks that long. I would suggest a 185 Mr Whirly crank set first and see whether 10mm makes a big enough difference for you - it probably will. Less risk as you'll always be able to sell them on.
 

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craigsj said:
Zinn has sold you a bill of goods and he makes a good living doing it. If you consider his "literature" compelling, why haven't you looked for even the most rudimentary data that suggests it is true? Zinn won't offer it no matter how badly he wants to. Other than formulas that have no data behind them, what has led you to determine a length of 200mm?

I agree with Pitch who said there is "a rule of diminishing returns with really big crank arms". Testing has shown "really big" to be about 150mm surprisingly. We all get long enough crank arms stock on every bike. There's a lot of confirmation bias regarding long crank arms since there's so many claims that proportional cranks are needed. You won't find a single objective test that shows any value to long cranks though.
Can you provide where we could find these test that say 150mm crank arms are really big?
I would like to read them. I don't think I would ever go to what Zinn recommends for my leg length because I don't want a BB that's 15" high. I'm going to try some 180's this season and see if I feel an advantage, but I'm keeping my 175's just in case. I have a 38" inseam.
 
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Jeff in Bend said:
Can you provide where we could find these test that say 150mm crank arms are really big?
I would like to read them. I don't think I would ever go to what Zinn recommends for my leg length because I don't want a BB that's 15" high. I'm going to try some 180's this season and see if I feel an advantage, but I'm keeping my 175's just in case. I have a 38" inseam.
Here is something I've linked to a number of times: Martin crank length presentation. This is a slide presentation that summarizes some studies on crank length.

Regarding "really big" I obviously took liberties with that term. ;) Pitch said there must be a point of diminishing returns and Martin has shown that for virtually all adult riders, "long enough" occurs at least by 150mm. He personally believes that number is due to his testing methodology and thinks it could be lower still. Finding his personal comments on the matter takes some digging on the slowtwitch forums.

The TLDR version, crank length doesn't matter. Choose it for whatever reasons you have, ground clearance being a really good one, but don't choose it believing your performance will be better as Zinn says. It won't be.

The beauty of crank length making no difference is it allows people's biases to be sustained. No matter what you personally believe, your own testing will confirm it. That seems to carry great weight with cyclists.

Regarding your reluctance to consider extra long cranks with a higher bottom bracket, I think that's wise. A very long crank will also put the pedal further forward meaning that you will be either more upright or further forward on the seat. It's unlikely that would work well with overall fit. If I were to do a custom frame to accommodate and unusual crank, I'd go extra short. I don't think it's worth the effort though.
 

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craigsj said:
Here is something I've linked to a number of times: Martin crank length presentation. This is a slide presentation that summarizes some studies on crank length.
That presentation wasn't put together very well, but I got the jist of it. What I've seemed to notice that all the data gets measured for peak efficiency or output but I've not seen any tests were someone is measureing power at real slow cadences were no one reaches max cadence output or pedal speed. Like grinding up a steep hill in your lowest gear.

This is what seems logical to me. If I have a 38" inseam compared to someone with a 32" I would use less% of my total range of motion if we use the same crank length. If we both used the same % of our range of motion my cranks would be longer. A longer crank would give more leverage than a shorter crank and more tourqe to the rear wheel when in the same gear.
 
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Jeff in Bend said:
That presentation wasn't put together very well, but I got the jist of it. What I've seemed to notice that all the data gets measured for peak efficiency or output but I've not seen any tests were someone is measureing power at real slow cadences were no one reaches max cadence output or pedal speed. Like grinding up a steep hill in your lowest gear.
One of the studies quoted looked at metabolic costs at submaximal loads (30, 60, and 90 percent). The test methodogy included allowing cyclists to pick their preferred cadence to avoid biasing against short cranks (a failure in some testing). That means, of course, that there's an assumption that you will have the right gear for the task, not one that doesn't allow for the best cadence. I'm not sure what it says about running out of low gears, but what it says to me is that I shouldn't let that happen. I know that shorter cranks need lower gearing so I provide it.

Martin is mostly concerned with road racing and sprinting so he studies peak power, but most cyclists who talk about long cranks and leverage are talking about peak power too. It doesn't seem important what crank you choose when you're just cruising along. ;)

Jeff in Bend said:
This is what seems logical to me. If I have a 38" inseam compared to someone with a 32" I would use less% of my total range of motion if we use the same crank length. If we both used the same % of our range of motion my cranks would be longer. A longer crank would give more leverage than a shorter crank and more tourqe to the rear wheel when in the same gear.
Regarding leverage, read what Sheldon Brown says. I've tried to explain this a number of times here on MTBR but it's never received well. Sheldon Brown is more respected. ;) To summarize, longer cranks do not provide better leverage or more torque in the same gear, what they do is change what that "gear" is.

What really matters is how fast your feet move around the circle. When you equalize foot speed, maximum power equalizes as well. The reason a longer crank puts down more power for a given gear at a given cadence is that the legs produce more because the feet travel further (power is force times distance over time). What's important to understand is that legs produce power, not cranks. A longer crank can't make more power because cranks don't make any.

Regarding your other point, the biomechanical aspects are what's really interesting and where you'd think the real differences in crank length would be. The Martin study showed, though, that it barely matters at all. To quote from his slide, "170mm cranks will compromise power of the tallest and shortest riders by at most 0.5%".

Martin, in discussing his results on the slowtwitch forums, pointed out that his tests did reveal an optimum crank length computable from rider dimensions (and even gave a formula), but the thing is that ignoring it comes at no cost. It's not that there isn't a best answer, it's just that it doesn't matter. Formulas like Zinn's are merely extrapolated from what has been traditionally done, there's absolutely no reason to believe any of it regardless of how reasonable the explanations seem to be. Zinn has tried to perform testing to verify his claims but his results have never been published because, and this comes from Zinn himself, his data wouldn't pass peer review and contradicted his own advice. In other words, he knows it's wrong but he makes money selling it.
 
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