Originally Posted by
yashinon
"My bike is mostly all stock except for the cranks, which I customized to ultra-short 153 mm cranks with a lower-geared chain rings
Best decision I ever made, as I am a very short person (5'2) and these cranks are at least 5x more comfortable and usable than my old 170s, no kidding. Although I do wish I had bigger gears (the stock chain rings had slightly higher gearing than what I have now)."
The '14 Trek supposedly comes with a Shimano Acera M391 crank. However the images of the crank "arms" that I see online are different from the arms that came with my bike. My "arms" seem rather long, being 5'6" does this prevent me from pedaling faster, reduce cadence, etc?
Attached is a photo.
I can't tell just by the photo--Shimano Acera M391 cranks comes in either 170 or 175 mm. You could either look up your bike's specs or look at the inside arm of your crank as the length is usually inscribed there. Your cranks look a lot like my old ones (2015 FX 7.4 WSD), by the way.
As to whether your cranks are preventing you from pedaling optimally, only you can really tell. The majority of people use crank lengths of 170-175 mm no matter what their height is with seemingly no problems (plus it's not like they really think about their cranks or have anything to compare with). I did have a dutch bike with shorter 165 mm cranks which I noticed was easier for me to spin compared to my FX so I looked into the possibility of changing my cranks. As to what is the supposed optimal crank length, the lazy way to calculate it is approximately 10% of height, or the more accurate method of 21-21.7% of cycling inseam.
Based on that, I estimate your optimal crank length is at or around 165 mm, which is lucky for you in case you decide to change cranks as there are many available options at that length. Finding other lengths aside from 165, 170, 172.5, 175, or 180 is like searching for a green unicorn. Changing cranks is not that simple and is quite costly (you might need to change the entire crankset with chainrings) so I do not recommend changing cranks unless you really feel held back by your current ones. Also, 5 mm is not that big a difference and you might not even notice it. When I changed my cranks, the difference was 17 mm so it was noticeable. My cadence increased, but it was already quite high even before I changed cranks.