Road Cycling - crank arm length

I am not that tall (5'9") and am in the process of getting a new bike. (The frame will be in next week!!! I can't wait!) The guy (owner) at the bike shop is the same height and size as me and on his bike he has a 170 crank arm and told me it really improved his spinning. BTW, he is also a coach for a racing team, and I really trust that he knows what he is talking about. He let me take his bike for a spin the other day and I could feel a difference but I guess I am just skeptical of making such a change. He recommends I put a 170 on my new bike. BTW, I currently have a 172.5

My question is, what are the benefits / drawbacks to switching to a shorter crank arm length? In my mind it seems that I would get less push on my pedal stroke, since you are shortening the diameter of the circle. If this is the case, how is that an improvement on my pedal stroke?

Thanks!

I've seen a lot of contradictory information about crank arm length and I'm sure you'll get a lot here. It does make sense that larger people are going to be more bio-mechanically fit to longer crank arms, and the smaller people to smaller crank arms. But, even very tall and very short people only vary their crank length by small amounts.

It also makes sense that if you spin at a higher rate, you will be more comfortable using shorter crankarms and if you are more of a masher, you might be able to get more out of longer crankarms.

It doesn't make sense, to me at least, that there is any inherent mechanical advantage of one over the other because you can also shift gears to maintain cadence and change your mechanical advantage.

As one data point, I'm 6'1" and I'm most comfortable @ 172.5. I notice that I tend to spin more rapidly and smoothly @ 170 and I feel a little out of round @ 175.

I could never tell the difference between 170 and 172.5.

Just go with 180s. :)

I am not that tall (5'9") and am in the process of getting a new bike. (The frame will be in next week!!! I can't wait!) The guy (owner) at the bike shop is the same height and size as me and on his bike he has a 170 crank arm and told me it really improved his spinning. BTW, he is also a coach for a racing team, and I really trust that he knows what he is talking about. He let me take his bike for a spin the other day and I could feel a difference but I guess I am just skeptical of making such a change. He recommends I put a 170 on my new bike. BTW, I currently have a 172.5

My question is, what are the benefits / drawbacks to switching to a shorter crank arm length? In my mind it seems that I would get less push on my pedal stroke, since you are shortening the diameter of the circle. If this is the case, how is that an improvement on my pedal stroke?

Thanks!

From personal experience, I prefer 170s (I'm 5'8" with a 32" inseam). About 700 miles back (now) I upgraded my Aegis from Ultegra triple 9 spd to double 9 spd Dura Ace. I figured (wrongly) it was a good time to try 172.5s. I put around 600 miles on them and could not get comfy. After 30 miles (consistently) my seat hurt (bad), I had hot spots on my feet (shouldn't have and never had before) and my legs were toast. Something had to give, so I took the Ultegra cranks off of my tandem and installed my Dura Ace rings, put them on the Aegis and walla! I was comfy again!

In your case, if you are comfy with your 172.5s, why change em out? Like in my case it could prove a costly mistake (I'm replacing my 172.5 Dura Ace crankset with an 170 FSA carbon set...hey it's as good excuse as I could come up with and she bought it!). You will probably find the 170s too short and not as efficient as your 172.5s. Although I had immediate relief with my change, with 172.5s I could still climb better, but for me the trade off was far too expensive.

170 sounds about right for ayour size. I'm 5'10 and ride 170, this is the standard size for av-male riders.
The reason that smaller and larger riders use cranks very close to av in length is because nobody makes proportionally sized cranks (140-200mm). Specialities-TA and Thorn (rebadged from something) are the only people suplying a wide range of length.

The latest fashion amongst European pro riders is to fit smaller cranks rather than larger ones.
As far as power goes, cranks dont create power, you do. A rotation of a long crank takes more work than a small one, but you generate more torque, and ride in a higher cog gear, so for each watt you supply, the wheel turns the same.

Hi,
give it a go. I thought about this for a couple of years, put on 165mm cranks this spring; and love them.

longer crank=more torque, shorter crank=smoother pedaling. mountain bikes typically use 175 cranks, track bikes use 165s for the same size rider. I had 172.5 on my road bike for years, but I just went with 170s and it feels great...I'm 5'-8". I think I can spin faster. :rolleyes:

I consider myself more of a gear masher than a spinner but recently I have been trying to raise my cadence a bit and stick with a little smaller gears. I've only been trying it for about 3 weeks. Since trying this I have actually lowered the time I finish my longer morning rides by about 10 to 15 minutes.

When riding the guys 170 I could really feel a difference coming over the top of my pedal stroke. I just am nervous about making such a change. I wonder if I'll regret it later. He told me it really improved his spinning when he switched to the 170. Maybe I'll talk to him and see if I can ride on a 170 for a while and if it doesn't suit me I can switch to a 172.5.

...Maybe I'll talk to him and see if I can ride on a 170 for a while and if it doesn't suit me I can switch to a 172.5.


If that can be arranged, I think it would be of great benefit. Sounds like you'll love the 170s tho.

Years ago I went to a 172.5 from 170 and could not tell a differance to speak of except for a slightly slower cadence. One reason I made the change was that I read that your muscles will not "see" a change of 2.5 mm but any more than that you will work the muscles in a differant fashion. My mountain bike had 175's on it so I went to the 172.5's.

I ride 171s... My favorite (sugino mighty).