Hello -

I am sure this has been discussed a million times in this forum, but here we go again:

I have been riding with 170mm but planning to buy a compact 172.5mm to get a bit of extra leverage in some cycle-sportives this year.

So the question is: How much of a difference does 2.5mm in a crankarm make? In longer crankarms really better for climbing?

Any advice highly appreciated.

Thanks

I can't feel any difference in a 5mm change, YMMV.

Here we go again.

2.5mm means you can crate 1.5% more moment about the axis with the same force. Not significant. Even if it was, you have to travel a longer distance, so the energy expended is the same. It's like shifting down 0.1 gears.

IMHO, crank length is a matter of fit. If you feel like you are spinning teeny tiny circles, get bigger cranks. (Even then, I doubt I could tell the difference between 170 and 175mm cranks unless I was concentrating really hard.) Otherwise, don't worry about it. It's not going to get you free speed.

I spin a bit faster with shorter arms, 5 rpm from 175 to 172.5, although it took me a long time to notice as if you just switch you will try to keep your old cadence. Some would argue that being a spinner helps. On the other hand, as I don't know your body dimensions I can't really speak about what will happen to you.
All of this may be the reverse obecalp effect. In all honesty, it probably won't matter one bit.