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Old 04-29-07, 03:54 AM   #1
travisthomas
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going up to 180mm cranks?

I'm a tall fellow (6'6" with a 36" inseam) riding a rather tall bike (65cm). I'm currently running 175mm cranks; I ruined my left crank, so they're different makes with approximately the same profile. According to some calculator that someone linked to a while ago, with my inseam my "optimal" crank length is slightly over 180mm. I'm not expecting additional power out of it or anything, but I figure that I can lower my seat a touch with the longer cranks and that might be nice. (To get correct leg extension at the bottom of the pedal cycle, my seat is pretty high. I'm often waiting at lights and would prefer to keep my butt on the seat without feeling like I'm going to fall over in front of a car or something...)

Anyway, I'm content with mismatched cranks for the moment but have been keeping my eyes open for 180mm cranks at LBS... or should I just replace the 175's?
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Old 04-29-07, 04:51 AM   #2
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http://sheldonbrown.com/cranks.html
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Old 04-29-07, 06:32 AM   #3
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Whatever you do, properly match your crank lengths.
At 6'6" you'll be fine with 180mm, and if you can get them cheap enough it's definitely worth experimenting. Most of the tests imply that crank length isn't a huge deal, but Sheldon's "we all use the same staircases" example isn't good at all - we all use the same staircases b/c stairs are made for everyone to use and can't be fine-tuned for the individual person.

I used 180mm cranks on my road bike for a long time, but am just as comfortable with the 170's on my old commuting bike. Currently I've got 175's on my road bike, but I might start using the 180 again sometime. Don't notice much difference.
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Old 04-29-07, 07:13 AM   #4
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Generally you don't change your seat height when you change crank length. Yes, the longer crank will slightly extend your leg at the bottom of the stroke but it will also increase the bend at the top. You don't want to increase the bend at the top.


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I'm often waiting at lights and would prefer to keep my butt on the seat.....
If you can remain seated with your feet on the ground, it's safe to say your seat is WAY too low already.
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Old 04-29-07, 07:17 AM   #5
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Quote:
Originally Posted by HillRider
Generally you don't change your seat height when you change crank length. Yes, the longer crank will slightly extend your leg at the bottom of the stroke but it will also increase the bend at the top. .
you do say "generally", but what if the saddle was the perfect height for the original cranks?

How can someone then start reaching an extra 5mm at the bottom of the stroke if they increase by 5mm?

I someone can go up a length without lowering the saddle then the saddle was too low in the first place

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Old 04-29-07, 07:24 AM   #6
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Quote:
Originally Posted by travisthomas
I'm a tall......
definitely go with the 180s.
I'm 6ft with a 35" inseam; I gleefully ride 175s (sometimes 172.5), and I'm experimenting with some 177.5s.

I had 180s a couple years ago (for about a year), but they ended up being a little too long; it was a bit too hard to start the down-stroke with my knees in my chest, and they gave me some medial knee pain
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Old 04-29-07, 07:43 AM   #7
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Quote:
Originally Posted by 531Aussie
..... it was a bit too hard to start the down-stroke with my knees in my chest, and they gave me some medial knee pain
This is why you don't lower you saddle.
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Old 04-29-07, 07:47 AM   #8
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Originally Posted by HillRider
This is why you don't lower you saddle.
I obviously don't agree. It's one of the potential problems associated with using longer cranks.

With a perfect saddle height, I went from 175s to 180s -- how am I supposed to reach the pedals at the bottom of the stroke without lowering the seat???!

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Old 04-29-07, 07:56 AM   #9
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All of the recommended saddle height procedures I've seen are based on the distance from the bottom bracket's center line to the saddle top and thus, independent of crank length. If your "perfect" saddle height needs to be changed to accomodate longer cranks, the cranks are really too long.

What you gain by lowering the saddle is offset by what you lose by having to bend your leg too much at the top of the stroke. Your own posting above made this point.
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Old 04-29-07, 08:12 AM   #10
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as far as I'm aware, the Hamley method of measuring saddle height, which is 109% of inseam from pedal the top of the saddle, is as common as the ".885%" method

http://72.14.253.104/search?q=cache:...lnk&cd=1&gl=au


As I asked, if my saddle height is perfect for me (about as high as my hamstrings can handle while hammering in the drops for extended periods), how can I then suddenly reach an extra 5mm at the bottom of the stroke?
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Old 04-29-07, 08:14 AM   #11
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I'm not sure if I would go with 180 if I were you, mostly because you would lose some ability to lean into turns while pedaling without scraping the pedals and perhaps even breaking a pedal or crank arm.

If you can't reach the pedals at the "perfect" seat height then the cranks is too long, because if you lower the seat to reach the pedal then on the upstroke the pedal is too high causing your knee to be too high thus increasing the chance for knee pain.

Also for faster cadence the smaller crank is idea, where as if your a slower cadence type of person or need the the longer crank for more torque like in mountain biking then you need a longer crank, but again if you mountain biking the longer 180mm crank is going to limit ground clearance.

If your a high cadence type of person and/or aggressively power through turns I would stay with the 175's.

By the way, you didn't say how you ruined your left crank...how did that happen? Maybe that will explain more.
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Old 04-29-07, 08:21 AM   #12
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Quote:
Originally Posted by froze
If you can't reach the pedals at the "perfect" seat height then the cranks is too long, because if you lower the seat to reach the pedal then on the upstroke the pedal is too high causing your knee to be too high thus increasing the chance for knee pain.
That was my going to be my reply exactly. Thanks for saving me the duplicate typing.
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Old 04-29-07, 08:24 AM   #13
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Quote:
Originally Posted by froze
I'm not sure if I would go with 180 if I were you, mostly because you would lose some ability to lean into turns while pedaling without scraping the pedals and perhaps even breaking a pedal crank arm. .
nah, it doesn't happen with modern pedals. I never hit the ground with my 180s, and I could pedal through corners -- but I'm not saying I was pealling whilst leaning over a 45 degrees. Maybe some bikes with a greater BB drop could present issues, but standard bikes are fine

And I wouldn't be choosing crank length based on cornering.



Quote:
Originally Posted by froze
If you can't reach the pedals at the "perfect" seat height then the cranks is too long, because if you lower the seat to reach the pedal then on the upstroke the pedal is too high causing your knee to be too high thus increasing the chance for knee pain..
this is the whole condundrum with long cranks which can't be avoided: long cranks = more leverage, but at the 'cost' of increased knee flexion

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Old 04-29-07, 08:27 AM   #14
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Quote:
Originally Posted by froze
If you can't reach the pedals at the "perfect" seat height then the cranks is too long, because if you lower the seat to reach the pedal then on the upstroke the pedal is too high causing your knee to be too high thus increasing the chance for knee pain..
Quote:
Originally Posted by HillRider
That was my going to be my reply exactly. Thanks for saving me the duplicate typing.
???????????????????????????????????????

I understand there may be merits for a saddle height method which takes the measurements from the BB, and that it could "expose" the folly of people trying to use ridiculously long cranks, but surely reach has to be catered for??

What if a someone is riding 165s on the track and 172.5s on the road with the same BB reach? It means, with the extra, uncatered for, 7.5mm reach at the bottom of the stroke, one if his bikes is gunna be setup wrong

Or, what if someone rides 170s for mass-start races, and 175 for time-trials and hill stages, all with the same height from the BB, and, therefore, a very different reach for each setup? I say 2 of the bikse will be wrong wrong wrong

What if new shoes or pedals are bought with a different stack height which affects reach? It doesn't make sense

This is where the ".883 of inseam" method falls short

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Old 04-29-07, 11:41 AM   #15
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Quote:
Originally Posted by HillRider
If you can remain seated with your feet on the ground, it's safe to say your seat is WAY too low already.
Of course having both feet on the ground would be silly. I'm saying one foot on pedal, one foot on ground (or rather, the ball of one foot), butt on seat and leaning to one side. The idea was to aim for marginally less lean, not to be flatfooted.

All very interesting, folks. Sounds like the only time people agree, they do so for different reasons. hah. Thanks... I'll keep my eyes open for 180's and update how I feel about them.
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Old 04-29-07, 11:44 AM   #16
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Quote:
Originally Posted by HillRider
Generally you don't change your seat height when you change crank length.
That's funny, I thought it was the exact opposite. Everyone i've talked to has had to lower/raise their seat following crank length change. I know this proof by consensus doesn't work but it should count as evidence for something.

Quote:
. For example, installing a longer crank may require lowering the seat, depending on where the seat was before (when the crank is too short, riders have arrived at different conclusions about what extension should be aimed for). With the correct crank length fitted, the proper extension will be readily apparent on the first ride, since it is more critical than with a crank that's too short. Usually the seat will have to be lowered, often by nearly the same amount as the increase in crank length. In cases of considerable crank length change, this can affect the selection of frame size!
http://www.nettally.com/palmk/crimplic.html

Quote:
If you change your crankarm length on your current set up, don't forget to adjust your seat height as changing crankarm length will change the distance from your seat to the lowest point of the crank cycle.
http://www.cptips.com/crnklth.htm

Quote:
Increasing the crank length requires lowering the saddle the amount of the length change; that’s a given, assuming your seat height was correct before.
http://www.zinncycles.com/cranks.aspx

Anecdotally when I swapped my 165's for 175's, I did have to lower my seat to prevent knee pain. The second to last quote makes sense - increase crank length increases the distance from saddle to the lowest point in pedal stroke.

Last edited by operator; 04-29-07 at 11:50 AM.
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Old 04-29-07, 11:54 AM   #17
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Quote:
Originally Posted by HillRider
Generally you don't change your seat height when you change crank length.
I strongly disagree. Saddle height should be set with respect to the maximum leg extension, which occurs at the bottom of the stroke.

See: http://sheldonbrown.com/saddles.html#height

Quote:
Originally Posted by HillRider
Yes, the longer crank will slightly extend your leg at the bottom of the stroke but it will also increase the bend at the top. You don't want to increase the bend at the top.
Right. That's why abnormally long cranks are a Bad Idea!

http://sheldonbrown.com/cranks

Quote:
Originally Posted by HillRider
If you can remain seated with your feet on the ground, it's safe to say your seat is WAY too low already.
That's correct, for frames of traditional geometry, and most especially for mountain bikes.

See: http://sheldonbrown.com/starting

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Old 04-29-07, 12:30 PM   #18
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Quote:
Originally Posted by 531Aussie
as far as I'm aware, the Hamley method of measuring saddle height, which is 109% of inseam from pedal the top of the saddle, is as common as the ".883% of inseam" method

http://72.14.253.104/search?q=cache:...lnk&cd=1&gl=au
actually, I think what I was thinking of was the Nordeen-Snyder method, which is 107% of inseam from pedal to saddle. Their relevant study suggested that at this height, "measurement of oxygen consumption is at its lowest and muscle recruitment and firing patterns are at their highest at a given moderate workload(80%)" http://www.multisportsolutions.com/news.html

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Old 04-29-07, 12:33 PM   #19
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thank you Operator and Sheldon

I thought I was going nuts there for a minute

Last edited by 531Aussie; 04-30-07 at 12:35 AM.
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Old 04-29-07, 12:33 PM   #20
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Just be sure not to pedal going around corners.
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Old 04-29-07, 12:34 PM   #21
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wow! I've never seen it quantified like this before.
This should have the physicists arguing

"A rider producing 300 watts with 170mm crank arms at 80 rpm will produce 318 watts with 180mm crank arm, an increase of 6%."
http://www.multisportsolutions.com/news.html
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Old 04-29-07, 06:38 PM   #22
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Quote:
Originally Posted by 531Aussie
"A rider producing 300 watts with 170mm crank arms at 80 rpm will produce 318 watts with 180mm crank arm, an increase of 6%."
http://www.multisportsolutions.com/news.html
Great quote...but why did you leave out the rest of the quote following yours? The quote that says: "Of course the downside to increased crank arm length is increased stress on the joints, primarily the knee joint." There is more to power then just power; power won't do you any good if you wreck your knees.
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Old 04-29-07, 06:47 PM   #23
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Quote:
Originally Posted by Sheldon Brown
I strongly disagree. Saddle height should be set with respect to the maximum leg extension, which occurs at the bottom of the stroke.
Ok, I'm convinced. However, my recommendation was based on the warning that having the knee bent too much at the top is as bad as too much extension at the bottom. They both cause problems, just different types.
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Old 04-29-07, 07:05 PM   #24
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I have a 32" inseam and was riding 170's on my work commuter bike. I bought a new bike and it came with 175's. When I first hopped on I immediately noticed the larger circle my foot was making... before I even knew the cranks were actually longer. After riding for several hundred miles I really like the longer cranks and haven't had any knee pain because of them. Longer legs should have longer cranks. Go for it.
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Old 04-29-07, 08:03 PM   #25
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Quote:
Originally Posted by froze
Great quote...but why did you leave out the rest of the quote following yours? The quote that says: "Of course the downside to increased crank arm length is increased stress on the joints, primarily the knee joint." There is more to power then just power; power won't do you any good if you wreck your knees.
I already said above that longer cranks increase knee flexion, no matter what; even if the seat is left where it is. And I also said the 180s hurt my knees a bit. I just put the power measurement there as an interesting aside, because I've never seen it quantified before

I'm actually not a huge fan of very long cranks, and I reckon the Kirby Palm and Zinn "formula" puts people on cranks that are way too big. The "21.6% of inseam" method would have me on 192mm cranks, which is totally crazy. The original poster is 6'6", so I'd say 180s would be fine for him

I also believe there are many, many shorter people riding cranks that are too long, just because they come standard on production bikes. For eg, there are too many 5'3" people riding 170s when they should be on 165s or maybe even smaller.

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