For me, it depends on riding style. Started racing years ago with 170mm. Next bike and one after that have 172.5 and seem more comfortable for my masher style. Mountain and touring bikes have 175mm. Ride all of them mostly in the big ring (53, 49, 50 respectively). I realize that using a small gear for climbing is more efficient (I do so in actual races), but I like the muscle mass from pushing higher gears and it keeps me on the rivet during training. Almost as good as intervals.

Sloar: You're clearly not a short person given the size of that Trek you provided and what it's seat height infers for your leg length. Still, you've chosen a better length in the 170's as you say you're a spinner. For what it's worth, I've found that staying on the saddle as much as possible rather than standing while climbing mattered quite a lot for my knees. Climbing out of the saddle puts a lot of strain on knee joints; it's far better to exploit all of your wide gear range and stay on the seat while spinning. That's how you get to keep on riding when you're an old guy.

What a great thread - lots of good comments here. I have always used 175s, until I bought a recumbent a few years ago. That bike needed shorter cranks - pushing out infront of you is different from pushing down. Anyway I put 165s on that bike and it rode great - spinning up to 95 rpm was easy. I've since sold the reumbent and on my road bikes now I have 175 cranks or 172.5 - I don't notice much difference unless I'm trying to spin above 85 rpm. It seems the higher the cadence the easier it is with somewhat shorter cranks.

"Climbing out of the saddle puts a lot of strain on knee joints ..." Not necessarily bad. I have chrondomalcia patellae and my knees like coming out of the saddle. For one thing, when I throw the bike back and forth as part of the dance, the range of knee bend is less. They like that a lot. A real force and not a lot of change of bend angle isn't that harmful for my knees. (I do suffer from being a born mountain goat. Spinning high RPM and cranking going slow up hills will never be fun for me. But I can do the dance at whatever speed my body dictates,)

I have 4 road bikes (including one cross/gravel bike) and a mountain bike. I'd have to look for which crank is on which bike, but I know that I have 170, 172.5 and 175. I can't say I can tell the difference when I'm riding. I know some people are much more in tune to these things, but I'm not. I used to think that all the cranks just had to be 172.5 because, well, just because. But at some point I found a crank I really liked for the buildin 170 and another in 175

The bikes are set up as identically as possible for the various measurements that impact the relationship with the crank, like saddle height and fore-aft vis a vis the pedal spindle.

+1 for me. My joint hypermobility means that my knees risk dislocating slightly at extreme angles, and standing up on the pedals lowers the angle I'm requiring them to bend. My back can straighten too, if I've been tucked down in an aerodynamic position for too long with my back slightly arched. If I've been spinning or cranking hard, seated, for a long time, I use a climb out of the saddle as a way to give my knees and back a break.

It's interesting how simplistic cycling doctrine "sit and spin, it's faster and easier on your knees" breaks down when you start looking at the diversity in cyclists. Sometimes I wonder if we can't blame this on Lance, too. I remember hearing comments from cycling's armchair quarterbacks that Jan Ullrich really mashed high gears and it was bad technique. Some went so far as to suggest that a cyclist who used a low cadence like this was doomed to second place. I've talked to some of those people recently, and they deny ever having said those things. Some of us are just Jan Ullrich, and that's not a bad thing anymore! Or if you're a little older, how about Laurent Fignon? He lost by a rounding error in '89 spite of not using any aero gear. My dad, an inveterate spinner, prattles on about how Lemond won because his technique was better. Plus ça change, eh?

no science just what i have experienced. I ride 175. had a torpado with 170 and rode it a lot until the spider cracked it was ok but always felt a a little off. Got a 84 team miyata with 165, rode it very little....just felt way to short, which was sad as the cranks were like 7201 durace....pretty rare Have 177.5 on the bike now (as i got a beautiful set of 7400 durace.) not a lot of rides but the 2.5 over 175 hasn't seemed to be an issue ...time will tell (anywone want to swap 175 for 177.5 durace ?)

Eisnic: Agree completely about staying in the saddle when older or having knee issues. I have tailored all my bikes to this exact style....use gearing rather than standing on the cranks. I have noticed distinct improvement in my somewhat severe knee arthritis (stiffness & pain). Another rarely mentioned benefit of biking (esp spinning) for seniors is that it stimulates the lymphatic system (this relies on major muscle movement since it does not have an active pump like the heart)......which is a major detox system (our lifestyle accumulates a lot of toxins over the years). A final issue for seniors is balance....I have modified all my bikes by fabricating rear wheelbase extensions (including 3 road bikes) using pieces of aluminum plate and sliding dropouts along with laidback seat posts to lower the saddle to ground height so that I have a "flat foot" stopped position....no leaning, easier to mount, very stable. I ride all local pavement so no pedal strike concerns.

I'm 5'1" with a 28.5" inseam and find that 145mm cranks work best for me. I've ridden 170, 165, 152 and 145mm. 145mm lets me spin comfortably and apply power to the pedals sooner in the revolution since my knee isn't so bent at the top.

Cool. What cranks are you using, and where did you find them?

you guys are gonna love me because the bike I ride most has 180mm cranks--- dura-ace that I like quite a lot! most my other bikes have 175 or 172.5(I am tall) but i have no problem with the 180 and love the ride so never have thought of changing it. I do ride up hills quite a bit with is so maybe that makes me like it better - its also not geared for "spinning".

I also have over-large muscled legs that have always been better for sprinting and strength than endurance and reps- so it may just be a case of pickles for peter.

I think I do have a 175 7400 dura-ace crankset- i don't mind 177.5 . its at work and I don't really recall the exact condition as I love the 7400 crankset, my favorite, so I have had several and I can't remember which set this is in terms of scuffs etc. now with COVID i rarely go, but I go once a week and can check for you if you are serious. I think they are 175... might be 172.5... but I think all the ones I have had were 175

Ride what feels good. We adapt to how we train, and long term, efficiency is probably genetically determined.

They’re made by Origin8 and they’re triple cranks. They also make them in short sizes for compact double chainrings as well. Found them on eBay but there are still a few sellers out there that stock them. They seem to be harder to find these days.

I have 172.5 on road bike and also pushing age a bit. Last year I focused on spinning and I tended to suffer from burning pain behind the right leg knee cap, on the outside of it. Few weeks easing off cured it. So spinning may not be cure all for older knees, it depends what your knees are prone to suffer from. I have no problem with out of saddle climbing.

Focusing on spinning tends to give more useful aerobic exercise (as a rule I tend to ride harder gears and trying to wean myself off it), I noticed that at some point when spinning, my spin would lose rhythm or it feels like that. I theorize it is due to cranks being on the longer side 172.5 for my 6'8 (and a half or almost 6'9).

I found same cranks with chain rings but shorter @170 on eBay, am tempted to buy to try out but get swayed off it by those who tell me, I won't notice the difference. I guess they are right, those few mms might be noticed by those who ride huge mileage and are in top shape, live on their bike.

I have a 35” inseam so long legs for 6’ . I have always had 170’s . I bought an old ItalVega and restored it . It came with 175’s on a 25” frame I left them on the bike as they really made no noticeable difference to me and they are nice Campy Super Record cranks.