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-   -   175mm vs 180mm or 190mm Crank arms (https://www.bikeforums.net/mountain-biking/349991-175mm-vs-180mm-190mm-crank-arms.html)

scelia 10-04-07 01:16 AM

175mm vs 180mm or 190mm Crank arms
 
I know there is a mechanical advantage to longer crank arms and I am looking at getting a new crank from Profile (Chrome Moly) and I am interested in opinions of longer crank arms than standard. My bike currently has 175mm and it seems to work well, but would I see enough difference in using the 180mm or 190mm crank arms? What are some of the detriments? I am aware that the 190mm crank arms will place the pedals .56inches closer to the ground when pointing straight down.

Thanks for any (useful) feedback.

-Stephen

mx_599 10-04-07 06:05 AM


Originally Posted by scelia (Post 5388013)
I know there is a mechanical advantage to longer crank arms and I am looking at getting a new crank from Profile (Chrome Moly) and I am interested in opinions of longer crank arms than standard. My bike currently has 175mm and it seems to work well, but would I see enough difference in using the 180mm or 190mm crank arms? What are some of the detriments? I am aware that the 190mm crank arms will place the pedals .56inches closer to the ground when pointing straight down.

Thanks for any (useful) feedback.

-Stephen

this is sort of a complicated subject. i am sure much more relevant to road cyclists. there was a "study" that showed the length didn't really have the effects you would have thought. if i recall correctly, some were the opposite. it seemed that what mattered most was proper fit and what the participants were used to. again, i would imagine this is more important on the road because you are seated longer and end up doing far more pedal strokes.

clearance is one issue as you mention. don't forget that too long a crank or too short a crank could cause orthopedic issues. again, less important off-road.

at any rate, i am not sure there is much to gain by going to the 180-190mm. is the bike a single speed?

bac 10-04-07 06:09 AM


Originally Posted by scelia (Post 5388013)
I know there is a mechanical advantage to longer crank arms and I am looking at getting a new crank from Profile (Chrome Moly) and I am interested in opinions of longer crank arms than standard. My bike currently has 175mm and it seems to work well, but would I see enough difference in using the 180mm or 190mm crank arms? What are some of the detriments? I am aware that the 190mm crank arms will place the pedals .56inches closer to the ground when pointing straight down.

Thanks for any (useful) feedback.

-Stephen

Hey Stephen. If the 175s work for you, stick with them. Yes, you will gain some mechanical advantage with longer crank arms, but you're still doing the same "work", as the total distance your pedals travel will be more. Too long of a crank arm (for your fit) can can lead to injury. Stick with the 175s. :)

... Brad

jm01 10-04-07 08:07 AM

...Also, check for chain stay clearance if moving to longer crank arms...some bikes are not designed for anything over 185mm

Svr 10-04-07 11:20 AM


Originally Posted by scelia (Post 5388013)
I know there is a mechanical advantage to longer crank arms and I am looking at getting a new crank from Profile (Chrome Moly) and I am interested in opinions of longer crank arms than standard. My bike currently has 175mm and it seems to work well, but would I see enough difference in using the 180mm or 190mm crank arms? What are some of the detriments? I am aware that the 190mm crank arms will place the pedals .56inches closer to the ground when pointing straight down.


Longer arms can work well for taller riders who like to stand up and mash (think single speed) rather than sit down and spin. What type of riding is this for?

Downsides would be increased chance for injury during seated efforts, and as you mentioned, ground clearance. You'll be smacking the pedals on the ground often if long cranks are installed on a frame with a low BB height.

Mace 10-04-07 11:43 AM

I was thinking about this the other day. I've never felt comfortable with shorter crank arms. I'm only around 5'5" and 170s are common on bikes for my size. I prefer 180.
Both of my current bikes have different sizes - 170 on my road bike, and 175 that I put on the FR bike. I would put longer cranks on the road bike, but the pedals are close to the ground as it is.
It feels quicker (pedaling) when you need to jump lanes (so to speak) and is more comfortable long distance with 170, but climbing hills and getting speed when you don't have much distance to do so seems easier with 175. This was the same when I had my hardtail and switched from 170 to 175. Overall, I'd say 175 is a sweet spot. 180 is tough on long distances. I'd get what size feels best for the type of riding you do, and your size.

scelia 10-04-07 02:15 PM

Thanks for the wealth of information. My bike is not a SS, but a hard tail GT. I had never thought about orthopedic issues arising from longer arms. I have ridden only 170's and 175's on mountain bikes and find that I like the 175mm cranks better. They seem to fit well. Chain stay clearance will most likely be an issue with 190mm.

Pete: As a Clyde (290lbs) I tend to break items that are not "indestructible". I like to jump, ride some DH and I want to work on riding in the skate park. Although in your eyes my riding might be "very, very tame?", when you factor in my weight it is brutal to the components. My like my current Race Face Crank, but it seems a little flexible when I stand and mash on the pedals hard.

Since I have been most comfortable with 175mm in the past and the risk of orthopedic injury, I think I will just stay with what is most comfortable.

Thanks for all your help.

scelia 10-04-07 02:53 PM


Originally Posted by Pete ***erlin (Post 5392344)
I was merely going by your description of your riding:

"I keep the Town & County's on most the time. I have two teenage boys and they like to ride to the skate park. Most of the other riding I do around here is with the entire family (two boys, 1 girl and my wife) and the wife & daughter like to stay on road. "

I think that other components would destruct before your cranks.



If you're feeling flex when mashing, it's probably not your cranks, but your bike flexing.

When I am mashing and look down at the chain rings, I see them moving out as I press down and the return to normal when the pressure is lifted. I have checked the bottom bracket and crank arm bolts. All are tight and the arms are very solid when pulled/pushed to check if they are on tight and BB is tight. It is only 5-6mm, but looks like more from above. It could be the Race Face SRX BB.

never 10-04-07 04:24 PM


Originally Posted by Pete ***erlin (Post 5392678)
If you're seeing flex when mashing, it's probably not your cranks, but your bike flexing.

+1 on that...this happens easily with my road bike when grinding it out...the rings move laterally and it's just the bike flexing.

mtnbiker66 10-04-07 05:34 PM


Originally Posted by never (Post 5393044)
+1 on that...this happens easily with my road bike .......


Wow, you think you know someone....

scelia 10-04-07 07:06 PM


Originally Posted by Pete ***erlin (Post 5392678)
If you're seeing flex when mashing, it's probably not your cranks, but your bike flexing.

But why does the front dérailleur stay still relative to the chain rings?

never 10-04-07 07:44 PM


Originally Posted by mtnbiker66 (Post 5393453)
Wow, you think you know someone....


Oh yeah, I forgot to tell you...I'm only bringing the road bike!


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