sweetspot

I am going to switch from 2x Shimano 105 to 1x GRX and I will be changing the crankset in the process. I am 178 cm height with 85 cm inseam and my current crank arm length is 172,5 mm. I am wondering if I should stay with that length or try something shorter like 170 mm? I have read that now it is a trend to use shorter crankset arms because it will be better for my knees and will allow me to spin faster. What do you think?

unterhausen

I think either would be fine. When I tried 172.5 arms the first time they felt sluggish. Second time I didn't know they were 172 until I had been happily riding them for a year and the person I bought the cranks from pointed it out. Now I ride 175 interchangeably with my preferred length of 170. I really can't tell the difference.

My gravel bike has relatively low high gear and 175mm cranks, so in the Wednesday night race, I have had to spin at a high rate to keep up. I don't find it any harder to spin than on bikes with shorter cranks. The gearing is the thing that protects your knees and lets you spin, not the crank arm length.

mstateglfr 

If you like the 105 crank arms and the reason you are changing the crankset for a reason other than fit, then keep the length.

I am 6'5 and have 170mm cranks on my gravel bike. Later this month I am going to swap them out for a 172.5mm crank because I want to try out different gearing. I got both cranksets because of availability/cost since I have never really cared if they are 170, 172.5, or 175. My main road bike has 175mm. 2 other road bikes are 170mm. My touring bike currently has a 180mm crank because I had it laying around and wanted to see if I liked it or disliked it more than all the other lengths.
Ends up I couldnt care less to all the lengths. 180mm, 175mm, 172.5mm, 170mm - couldnt care less. I turn my legs and they go around in a circle on all of them. If the circle is 340mm in diameter, my legs are fine. If the circle is 360 in diameter, my legs are fine.

I am sure the length of crank arm affects people more when the leg length is shorter since its a greater difference relative to leg length.

Anyway, point of this is- why change if you like the current length? And you may change only to just not find it matters to you.

chas58

I'm your size. For me, it is more RPM dependent.
For my tandem or mountain bike (where I need the torque and leverage) I'm at 175
For my track bike, where I need HP (& high rpm) I'm at 165.

What do you like to do - spin, or mash the pedals?