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Going from 170 to 172.5 Crankarms

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Going from 170 to 172.5 Crankarms

Old 10-29-10, 10:52 AM
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Going from 170 to 172.5 Crankarms

hello out there. Im the new guy ....I was thinking of getting a set of new cranks i have been riding with 170mm for 20+ years now and was looking at a pair of 172.5mm....there the SRAM Red Compact Crankset at a real good price...........any one out there have any advice for me...............


thanks rick the bike ...........out on a ride
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Old 10-29-10, 12:38 PM
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2.5mm---thats less than 1/8th of an inch. Don't think you will notice a startling difference but the Sram red is probably an upgrade to what you were using. That is where the difference "May" come in.

Have to admit that I have bikes with cranks from 165 to 175. 165 compact crank on the road bike that I use more often and for my short legs-(30" inseam) it does seem to work. Then the hills bike with compact again at 170 and the offroad Tandem with 175. (Can't afford to change these out but the length does not cause a problem).

But each bike feels right. That is all you have to think about. May have to change saddle height (One of the places where 2.5mm could be critical) but I doubt you will notice the difference after you have stopped looking for one.
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Old 10-29-10, 01:01 PM
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I made the switch from 172.5mm to 175mm cranks on my road bikes about 5 years ago, and I would never go back (though I do still have 170mm cranks on a couple of my fixed gear bikes). Sure 2 1/2 mm is a very small difference, but for me the increase in power and endurance was immediately noticeable, especially climbing. Of course it depends on your height and inseam. I am 6' 2" and ride about a 60cm road bike, so the longer cranks make sense for me.

These are not set in stone, but here are some general guidelines for choosing length based on your inseam that might help: https://www.cptips.com/crnklth.htm
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Old 10-29-10, 01:17 PM
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Originally Posted by James_T
I made the switch from 172.5mm to 175mm cranks on my road bikes about 5 years ago, and I would never go back (though I do still have 170mm cranks on a couple of my fixed gear bikes). Sure 2 1/2 mm is a very small difference, but for me the increase in power and endurance was immediately noticeable, especially climbing. Of course it depends on your height and inseam. I am 6' 2" and ride about a 60cm road bike, so the longer cranks make sense for me.

These are not set in stone, but here are some general guidelines for choosing length based on your inseam that might help: https://www.cptips.com/crnklth.htm
How did you measure the data?
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Old 10-29-10, 01:31 PM
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Originally Posted by tagaproject6
How did you measure the data?
I didn't measure it. Noticeable was the key word in my previous statement, but I probably should have said that I felt stronger and more efficient to be clear.

I will add that I have seen differing results on the issue of power output from crank length, especially very small differences. You can read about some of those opposing results in this one page from Bicycling Science. Click through to the next page as well. The chart is pretty interesting.

Last edited by James_T; 10-29-10 at 01:40 PM.
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Old 10-29-10, 02:00 PM
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I've got 170, 175 and 180 on various bikes , I ride them all just fine,
seat post extends a little less on the 180.
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Old 10-29-10, 04:18 PM
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I have 165 on my fg and 175 on my road bike. I scarcely notice a difference. However, comparing the two bikes is like comparing an apple to an orange.
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Old 10-29-10, 04:34 PM
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Ditto - if there is a difference, I'm not really sensitive to it. I have a surly with 170's and a old road bike with 175's. I ride both bikes just fine and don't really even think about it.
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Old 10-30-10, 04:37 AM
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You may find that you will need to adjust the height of your seat slightly to compensate -- downward if you are going to longer cranks. I say may. The effect of not changing the seat height may be frontal knee pain.
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Old 10-30-10, 08:12 AM
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I ride 175mm cranks on upright bikes - my recumbents have 170mm. Of course, too many other differences for a valid comparison.
Looking at 2.5mm on a ruler, I wonder how that (or even 5mm) could make any real world difference.
There are proponents of using cranks as short as 150mm on recumbents. Don't know what kind of data supports that.
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Old 10-30-10, 10:52 AM
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Framebuilder Peter White has a good section on crank length on his 'fitting' page.

https://www.peterwhitecycles.com/fitting.htm
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Old 10-30-10, 11:42 AM
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Peter builds wheels and assembles components on a few brands of frames
built by other people, to correct James.

He is also exclusive importer NA distributor of German Schmidt dynohubs and lights.
And Tout Terrain Frames, a German frame shop.
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Old 10-30-10, 11:46 AM
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re 170 to 172.5 crankarms

thanks all for the input. I went back to the store and they had sold the sram cranks

lol....so i got Full Speed Ahead Gossamer Compact With MegaExo BB. ???

I hope these work ok for me. I want to put a
Shimano Dura-Ace Bottom Bracket Cups 7900 on it but i am not sure it will work with the FSA stuff...........any one no if i can do that...........thanks all.............rickthebike out on a ride
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Old 11-01-10, 06:13 AM
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Originally Posted by fietsbob
Peter builds wheels and assembles components on a few brands of frames
built by other people, to correct James.
Fietsbob, he is also well known as a framebuilder, even though that is not the current focus of his business.

https://www.peterwhitecycles.com/bicycles.htm

I could have called him an ex-framebuilder, but either way, he knows a lot about designing and building frames and complete bikes.

Last edited by James_T; 11-01-10 at 06:16 AM.
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Old 11-01-10, 01:01 PM
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Went from 170mm to 165mm cranks. My femurs are longer than my absolute inseam would suggest, so my knee articulates a bit differently, and as such, I needed to go down in crankarm length so that my knee didn't have to compress as much to make the revolution. My right knee is a chronic problem, and since I've run out of parts to change to address it, I am in PT right now, which will hopefully do the trick.
If my femurs were not so long, I would expect that I would not be sensitive to small changes in crankarm length.
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Old 11-01-10, 01:05 PM
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You can use whatever you want, but how tall are you? What is your inseam in comparison to overall height? these types of things MIGHT factor into a sizing change notice. Other than that as posted above, you probably won't notice much.
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Old 11-01-10, 01:08 PM
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Originally Posted by rickthebike57
thanks all for the input. I went back to the store and they had sold the sram cranks

lol....so i got Full Speed Ahead Gossamer Compact With MegaExo BB. ???

I hope these work ok for me. I want to put a
Shimano Dura-Ace Bottom Bracket Cups 7900 on it but i am not sure it will work with the FSA stuff...........any one no if i can do that...........thanks all.............rickthebike out on a ride
It should work, but going with same brand of components always helps promote a more synchronized drive system.
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