Crank length question

[Andy Jennings]

Just changed over to a 19" Specialized S Works MTB frame which is bigger than my previous frame and includes 170 cranks.

Should I be considering 172.5 cranks and what difference does that make to me as the rider in practical terms? (I am trying to work things like will it change the ease of pedaling in a given gear? any effect on cadence? Riding position? or anything else.)

Sorry for the basic question but I am having trouble working it out.

 

[alfeng]

Longer cranks will give you MORE leverage (slightly easier pedaling) if-or-when you feel you need it on climbs ...

How tall are you?

I would think that for someone riding a Large-or-Extra-Large frame that a 175mm crank would be the minimum length to be using ... and possibly, a crank with 180mm arms ...

Longer cranks will result in slower cadence (it's mostly just a matter of arithmetic... a bigger circumference vs. a smaller circumference ... but, your riding position will have an effect, too, if you cannot achieve as comfortable a riding position)
​

Ultimately, crankarm length depends on the YOU and your riding style.​

 

[Froze]

Alf is spot on, it depends on the type of rider you are, if you prefer higher cadences, or have knee issues the shorter crank will help with both of those. It's kind of like walking up a flight of stairs, does it seem to hurt more taking every other step or taking every step? A lot of pro teams are trending toward shorter cranks for various reasons.

 

[dabac]

There are formulas based on leg length, and how much that is above/below the knee that you can use to find a good starting point.
Some people, particularly among road riders, consider crank length very important.
Others, not so much.
Have a friend who left the "important" category when he discovered his commuter bike had one 170 and one 175 mm crank, which he hadn't noticed while riding.
I think there's a study done by Leonard Zinn also showing that the immediate importance of crank length is often overrated.
It's generally agreed though that it's easier to maintain a high(er) cadence on shorter cranks.
Longer cranks can be harder on the knees.
One thing knees don't like much is repeated high loads on high bend angles. Something which longer cranks contributes to more than short cranks.
MTBers tend to use longer cranks than road bikers.
Longer cranks do provide more leverage which can be helpful if you need to muscle through a short but difficult section of track w/o going down a gear.
If you were to swap to longer cranks on an existing bike, don't forget to reset the saddle height as the pedal at its lowest will be lower than before.

 

[Andy Jennings]

Many thanks for all the replies.

I had hoped that the longer cranks would help with the knees, but as it's the other way around, not so much now. lol

I understand the info in the replies and it basically confirmed my thoughts, but in a clearer way.

The knee pain has been getting better and is limited to an ache in the knees now, rather than pain, during high stress and after the ride. Not a big deal but with existing knee problems from past training and running I wanted to get the best I can.

The other thing, of course, is that a lower saddle height and longer cranks will bring my knees higher to my torso when riding, if I am reading this right. That said I am quite badly overweight and my gut could create problems with the higher knee position.

I think this may be an adjustment left on the back burner for a while until my tendons strengthen up and my weight is at a more suitable number. lol

Thanks again for the help. Much appreciated.

 

[MikeWMass]

Back in the day, pretty much everyone rode 170's. Now there is much more variation. I have 2 bikes with 170's and one with 175's. If I didn't know they were different, I am not sure I would notice. However, one of the bikes with 170's is a tourer with a low bottom bracket, and I have grounded the pedal when pedaling on fast corners; that would be even more likely with longer cranks.