Crank arm length: 172.5 vs 175. Is there an appreciable difference?

[Fat Cat]

Yes. Exactly. Maintaining the extension is the obvious priority as I understand it from every set the saddle height instruction that I've ever read. Nobody mentions the unloaded portion of the pedal stroke at the top as only a small arc segment actually exhibits the 5mm (2.5mmx2) higher pedal relative to the saddle.

[Nomadmax]

School is IN.

[litcrazy]

As I reflect on my earlier post, I realize I mentioned my height, like many, but not my size 47 shoes and rearward/middish foot cleat position. In my experience, cleat position impacts my experience of crank length quite significantly.

I feel sliding my cleats forward or back by 5mm, quite a bit more than switching between 175mm and 180mm cranks. I imagine shortening the lever not only helps with hot spots and arch fatigue, but also shifts the relationship to the pedaling/power circle. Maybe someone will start marketing different oval chainrings based on cleat positioning preferences.

As I think about this discussion, I do think solving a low bottom bracket height by using a shorter crank is quite similar to moving one's saddle forward to accommodate to long of reach. Surely, some (and perhaps many posters) will comment that both doing these moves, they found a better position. However, I prefer to optimize my position and then have the bike fit that.

I'm happy (admittedly just 100 miles in) that I can now run my favorite crank (180mm square taper Ritchey logic) on my new gravel bike because I designed it with that cranklength in mind. While the lower bottom bracket on my BMC provided a learning moment when I pedal struck so hard I popped the spring out on a Shimano SPD pedal, I'm happier now that my bottom bracket height is optimized to my position. And now that I figured out how to respring a SPD pedal.

If, as predicted above, road bikes move towards super short cranks, I imagine bottom bracker height norms will shift lower making running longer cranks very difficult. Fortunately, I don't mind used items. Especially if they have slack seattubes, a bit short of top tubes, and relatively high bottom brackets.

[cgolvin]

Originally Posted by benb

You always have to multiply the difference by 2.

Originally Posted by Fat Cat

No. Diameter and radius are different measurements.

Indeed, they differ by a factor of 2.

[robertbb]

Originally Posted by Fat Cat

Yes. Exactly. Maintaining the extension is the obvious priority as I understand it from every set the saddle height instruction that I've ever read. Nobody mentions the unloaded portion of the pedal stroke at the top as only a small arc segment actually exhibits the 5mm (2.5mmx2) higher pedal relative to the saddle.

This.

Also, talking about this without discussing cleat position is… incomplete.

As an example, chronic heel droppers (like me) are well served moving cleats backwards towards the midfoot. I noticed immediately a decrease in fatigue, improved recruitment of the posterior chain, more power for free. This invariably necessitates shorter cranks to maintain control.. There’s a lot of fitters out there now extolling the virtues of a rearward cleat position. Indeed some shoe manufacturers like Fizik are responding by changing where they drill their holes.

[Black Dog]

Originally Posted by Fat Cat

2.5 mm equals +/-0.099". That is way less than 1/8" (0.125")

A difference that I openly admit is undetectable to me. But I almost exclusively use 175's since they're cheaper cos they're too long and nobody wants them.

2.5mm is 1/10th of an inch. It’s not that much different than 1/8th. But like what was said the total difference in diameter is double that.