Crank arm length

SnakeCharmer said:
I'm all about slow-techy riding. Downhill speed is less important. I need a quirky, nervous, fast reacting bike for constant angle and pace changes. That's it, I'm moving to Utah! :D
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And you understand that decreased offset which yields increased trail does NOT create quicker, quirkier steering - that it does the opposite, right? Or should I just stop trying to get you to read the various articles on the relationship between offset, trail and bike handling?

Fork Offset.png


Increasing trail improves straight-line stability. The front wheel feels harder to turn but also harder to knock off line.
Decreasing trail improves agility. The front wheel feels easier to turn and it can make a bike feel more nimble.


Luckily for this thread, crank arm length is easier to understand. Shorter cranks are probably easier on the knees and are less prone to pedal strikes, but offer less mechanical advantage (leverage). To me a 5mm decrease in crank arm length feels like a two-tooth increase in chainring size, but without the extra distance traveled.
 
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My E29 came with a 170. I ruined to crank in an oops manuever and my shop at the time accidentally ordered a 175 replacement. I didn't know it when I started riding. I got through an entire ride (or three) and did notice more rock strikes, but also that I was more comfortable when I was putting power down. I eventually looked at the crank arms and saw 175, looked up the spec on the old ones, and realized then that my body prefers the 170.

I think 172.5 is the magic number for me based on having 165-175 now (165, 170, 172.5, and 175).
 
I was mulling over a replacement crank (https://www.jensonusa.com/SRAM-GX-1000-GXP-10-SPEED-Crankset) for a bike that originally had a crank with 175mm arms and which has little trouble with pedal strikes...I was considering 170mm cranks to further reduce pedal strikes (I really dislike those), but I went with 175mm instead because the dropper post on that bike has less travel than I would like, and I do not want to raise it 5mm to compensate for a shorter crank arm length.
 
5 mm of crankarm length doesnt really matter that much, except for ergonomics/comfort relative to your leg length. (or dropper post height!) Too long may stress the knees though. I wouldnt worry about the power/leverage difference. Shorter crankarms will have less leverage on your cassete, but your leg muscles' greater leverage on the shorter crankarms will cancel that out somewhat. Go with whats best for your leg length. I like 170s because i have short legs, but have used 175s too. I doubt i could tell the diffeernce in leverage in a blind test
 
herzalot said:
And you understand that decreased offset which yields increased trail does NOT create quicker, quirkier steering - that it does the opposite, right? Or should I just stop trying to get you to read the various articles on the relationship between offset, trail and bike handling?

View attachment 69529

Increasing trail improves straight-line stability. The front wheel feels harder to turn but also harder to knock off line.
Decreasing trail improves agility. The front wheel feels easier to turn and it can make a bike feel more nimble.


Luckily for this thread, crank arm length is easier to understand. Shorter cranks are probably easier on the knees and are less prone to pedal strikes, but offer less mechanical advantage (leverage). To me a 5mm decrease in crank arm length feels like a two-tooth increase in chainring size, but without the extra distance traveled.
Click to expand...


In usual iMtb fashion, the crank arm length thread ends ups fork offset discussion. You have to love this little corner of the web.
So I do have a question, and most likely am able to infer the answer, but as everyone has an opinion and our very own Herzalot seems to be in the know, I'll put this out there.

I'm looking at a new bike, ( almost bought it an hour ago :eek: ) and the intention is to swap most all the parts over from my current steed to the new one and vice versa. Sooo...the bike I'm looking at comes with a 120mm travel 44mm offset fork. My current fork is 42mm offset, and is now at 140 but I intend to drop it back to 130. So, how do you think the shorter offset and 10mm longer fork would effect handling? On one hand, according to the post above, the shorter offset would increase trail and stability. But 10mm the longer fork appears to me like it would negate the shorter offset and pretty much be a wash.

The other bike I'm looking at is a 140 fork that has a 44mm offset, and the fork I'd use is 140 42mm offset. Would that be very noticeable? hmm..

I'd appreciate any insight all you keyboard pirates may have and your thoughts.

Unfortunately for @DangerDirtyD it's not a tire, so I won't need to measure it, sorry DDD!!
 
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@Danimal, I would get the bike that I truly want more and sort the fork details out later. 2 mm offset not worth considering IMO. 10 mm travel can make a difference but you have top start somewhere, and you can adjust it for 80 bucks next time you service the fork (I think you have a Pike?). Or sooner if you choose.
 
Danimal said:
In usual iMtb fashion, the crank arm length thread ends ups fork offset discussion. You have to love this little corner of the web.
So I do have a question, and most likely am able to infer the answer, but as everyone has an opinion and our very own Herzalot seems to be in the know, I'll put this out there.

I'm looking at a new bike, ( almost bought it an hour ago :eek: ) and the intention is to swap most all the parts over from my current steed to the new one and vice versa. Sooo...the bike I'm looking at comes with a 120mm travel 44mm offset fork. My current fork is 44mm offset, and is now at 140 but I intend to drop it back to 130. So, how do you think the shorter offset and 10mm longer fork would effect handling? On one hand, according to the post above, the shorter offset would increase trail and stability. But 10mm the longer fork appears to me like it would negate the shorter offset and pretty much be a wash.

The other bike I'm looking at is a 140 fork that has a 44mm offset, and the fork I'd use is 140 42mm offset. Would that be very noticeable? hmm..

I'd appreciate any insight all you keyboard pirates may have and your thoughts.

Unfortunately for @DangerDirtyD it's not a tire, so I won't need to measure it, sorry DDD!!
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So are we talking Spur, Ranger, Epic Evo...? Inquiring minds wanna know:rolleyes:...
 
Danimal said:
Epic Evo in my sights. They have a medium in stock waiting to get built, low spec but plastic frame. :) after the swap it'd be like 25/26 lbs. Compared to my current bike that's a huge difference.

But then they have a stumpy too :unsure:....but epic Evo I think could be my jam...;):confused:
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I’d go for the epic evo myself but I’ve probably watched and read one too many reviews. Real world problems, doubt there’s a wrong decision...
 
Danimal said:
Epic Evo in my sights. They have a medium in stock waiting to get built, low spec but plastic frame. :) after the swap it'd be like 25/26 lbs. Compared to my current bike that's a huge difference.

But then they have a stumpy too :unsure:....but epic Evo I think could be my jam...;):confused:
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The Comp with SLX? We sell a lot of those, it’s a nice bike and I wouldn’t worry about 2mm of offset, you will never notice it.
 
Danimal said:
In usual iMtb fashion, the crank arm length thread ends ups fork offset discussion. You have to love this little corner of the web.
So I do have a question, and most likely am able to infer the answer, but as everyone has an opinion and our very own Herzalot seems to be in the know, I'll put this out there.

I'm looking at a new bike, ( almost bought it an hour ago :eek: ) and the intention is to swap most all the parts over from my current steed to the new one and vice versa. Sooo...the bike I'm looking at comes with a 120mm travel 44mm offset fork. My current fork is 42mm offset, and is now at 140 but I intend to drop it back to 130. So, how do you think the shorter offset and 10mm longer fork would effect handling? On one hand, according to the post above, the shorter offset would increase trail and stability. But 10mm the longer fork appears to me like it would negate the shorter offset and pretty much be a wash.

The other bike I'm looking at is a 140 fork that has a 44mm offset, and the fork I'd use is 140 42mm offset. Would that be very noticeable? hmm..

I'd appreciate any insight all you keyboard pirates may have and your thoughts.

Unfortunately for @DangerDirtyD it's not a tire, so I won't need to measure it, sorry DDD!!
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OK - let's unpack this a little. If you've never ridden the bike with a 120, you won't notice a 130. And you certainly won't notice 2 mm of offset either direction - especially with no comparison. In theory - a longer fork is slacker and will lighten the steering and a shorter offset will stabilize the steering. Sounds like a complete wash. And with no comparison, you won't notice a thing. I am a fan of over-forking a bike anyway.

That said, you are about to betray all that is holy and buy a Specialized. So there's that. :facepalm: :devilish:

Carry on! :confused:
 
And here I thought I was coming to new additions to Crank Arm length... :coffee:

I'll give it my best...
I have tried 170mm cranks and did not like them. I felt the difference and did not like the spin. I wish I could do 180mm cranks but concerned about contributing to pedal strike. I like the leverage 180's give you but I also do not like pole vaulting into trees and breaking pelvis bones. It's an uncommon fear I have adapted over the last couple of years.
 
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mtnbikej said:
I think you have been listening to many scary stories of the boogey man.
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o_OHuh?
"There I was... minding my own business... bee-bopping along around 15 mph at dusk on a local trail when all of a sudden......... gravity ceased to exist! I floated up into the air the bike and I went sideways. Then, a sudden deceleration of motion as the bike went left and I went right around a nice size pine."
 
Mikie said:
o_OHuh?
"There I was... minding my own business... bee-bopping along around 15 mph at dusk on a local trail when all of a sudden......... gravity ceased to exist! I floated up into the air the bike and I went sideways. Then, a sudden deceleration of motion as the bike went left and I went right around a nice size pine."
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Spoken as a man who may have first hand knowledge of said boogeyman. :bang::Roflmao
 
mtbMike said:
Agree...Sidewalk and racers like him are the type that might actually be putting down enough power to get the benefits of 175over 170. Us mere mortals will never notice a power difference.
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A longer crank arm produces more mechanical advantage, meaning it makes the otherwise same setup easier to pedal. But it can be corrected with gearing, so it comes down to what is comfortable for your leg length and tolerance for timing your pedals around obstructions.

I would never be happy with 170. 175 feels pretty spinny already to my leg length. Wife's legs are 5" shorter, she likes her 175mm, had 165 previously.
 
Never run anything other than 175mm. Road or Mtn. On the SS you stand and pedal so much, it really doesn't pose a fit issue, but going shorter would definitely affect the leverage. Without the option to change gears....this would cause a problem.

Even on the super low BB on the Hightower, I haven't felt the need for shorter cranks....yes, pedal strikes happen from time to time. As a masher, I'll take the leverage.


Suz(5'3") has 175's on her SS and 170's on her Tallboy....but the 170's were from her previous Small TB that she had issues with toe overlap on the front tire. The 170's cured that problem. She said she doesn't notice much difference. She also has 170's on her CX bike.
 
mtbMike said:
Agree...Sidewalk and racers like him are the type that might actually be putting down enough power to get the benefits of 175over 170. Us mere mortals will never notice a power difference.
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Incorrect. As I stated above a 170 feels like a two-tooth jump in front chainring, without the extra distance traveled.

I'm a hack, but I can notice the difference between a 175 and a 170 in one climb up Cholla. But I'm also 6'4".
 
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