Crankarm - 170mm vs 172.5mm

[Crankin]

I've said this before, but when I bought my current bike, the LBS owner tried to switch me from 165s to 170s. I said OK, I would try the 170s. Within one ride, I knew I hated them. I felt like I was struggling to turn the cranks and pushing much harder than I ever did, no matter what gear I was in (the bikes had the same gearing). In theory, the LBS owner said the longer cranks would "unleash the power he knew I had in me," which I suspect was just a crock. I am a spinner; I always have been. I didn't start out trying to be this way, or train any special way, but it's what I feel comfortable with. In fact, I can barely stand on the bike and only do very rarely.
On a group ride a couple of weeks ago someone asked me how I trained to get such a high cadence. I replied that it's just the natural way I ride (and I don't think an average cadence of 80-85) is that high.

[aicabsolut]

It's really personal preference.

My legs are very sensitive to any changes in fit, down to mere millimeters. I had a really hard time finding the right saddle position when I got my first bike. After much agonizing over fit, the LBS noticed that the cranks were 175mm. My bike's specs were for 172.5mm. So, since the bike was built wrong from the factory, they swapped out the cranks. Any knee strain or discomfort I had been having was gone.

I know plenty of people who absolutely do not notice a 2.5mm difference between cranks.

Aside from comfort issues, longer cranks will help you apply more torque to the cranks (one reason some use longer cranks for TTs).

[tzvia]

Crank length is, as Crankin pointed out, as much a function of how one rides, their modus operandi, as it is due to leg length or femur size or whatever. I too am a spinner, and typically crank my little legs on my 165s at 90~95rpm and it feels quite normal on the road bike and I rarely stand or mash. I used to have 162.5 arms on a previous bike and I felt a difference going to 165. Small but not imperceptible to me, and if felt right. Maybe if I were a masher, a torquer, I would want 167.5 or 170 like I use on the MTB. Remember, it's 2.5mm further out in all directions- your leg will come up higher, go down lower, and move forward and backwards 2.5mm further out from center.

[nscrbug]

Crank length is, as Crankin pointed out, as much a function of how one rides, their modus operandi, as it is due to leg length or femur size or whatever. I too am a spinner, and typically crank my little legs on my 165s at 90~95rpm and it feels quite normal on the road bike and I rarely stand or mash. I used to have 162.5 arms on a previous bike and I felt a difference going to 165. Small but not imperceptible to me, and if felt right. Maybe if I were a masher, a torquer, I would want 167.5 or 170 like I use on the MTB. Remember, it's 2.5mm further out in all directions- your leg will come up higher, go down lower, and move forward and backwards 2.5mm further out from center.

Hmmm...that's an interesting point you make there. I'm wondering if a change in crank size would also warrant a change in height or fore/aft of my saddle? Any thoughts on this?

[OakLeaf]

Hmmm...that's an interesting point you make there. I'm wondering if a change in crank size would also warrant a change in height or fore/aft of my saddle? Any thoughts on this?

Fore/aft unlikely, height yes. If you set your saddle height based on your knee angle at the bottom of the pedal stroke, it will have to come down by 2.5 mm if you're switching to a crank that's 2.5 mm longer. Conversely, if you set your saddle height based on your knee angle at the top of the pedal stroke (as my knees require), then it will have to come up by 2.5 mm to preserve the same angle.

[smilingcat]

the 2.5mm doesn't sound much but when the pedal is at the bottom, its 2.5mm lower and at the top its 2.5mm higher for a total difference of 5mm. Some people will notice some will not.

For 165mm versus 172.5mm, that's a big difference most people will notice. If you notice that your seat position height and fore/aft moves by even a few mm makes a difference, then you will probably notice the difference between 172.5mm or 170mm spin on a bicycle with both crank arm length and go with the one you feel comfortable. If your inseam is short, you may prefer 165mm. Torque you have to generate is higher on 165mm but it would be easier if your knee isn't bent all the way. My performance was much better with 165mm instead of standard 172.5mm.

[g19]

The human body is not able to note every change that it's undergoing so there could very well be noticeable difference, but the human body just isn't fine tuned for every change to note.

Maybe this is just one of those things where the body looses ...

There are tons of opinions on this and too many times is a scientific one and power related.

The other side is that the longer you go the more difficult it gets to keep spinning high or it becomes less natural. This is one thing that is most times overlooked. And there is the issue for the pressure on the knees.

So bottom-line, wil you notice it, probably not, will it have an effect on riding-style or knees, it could be.