Centurion Pro Tour crank length question
 

I am slowly building up a 77 Centurion Pro Tour and I am looking for a crankset. I think I'm going to stick with a vintage Sugino double or triple, but I'm wondering what length I should go for. The frame is a 57 and my inseam is roughly 31.5. My one bike that is 56 with 73/73 has a 165. My 21" Schwinn Voyageur also has 165. I'm curious does the slightly larger frame necessitate a slightly longer crank arm length? I believe the Centurion is 73/73 with a 54 top tube. Thanks.

Crank length is up to your personal preference. A lot of people will say it has to do with your height. I think it doesn't have so much to do with your height as with your whole musculoskeletal system. If you like 165s, stick with 'em! I switched everything I have to 165s because I found that they were easier on my knees than the 172.5s and 175s that I had been using. And I am 6 foot 2 with pretty long legs and I ride 63cm frames.

I figured out that I like short cranks because I only had knee pain when sitting down and spinning. When standing up and smashing the pedals, it went away even though the load on my knees was higher. I reasoned this was because my knees weren't bending so acutely when standing, so I bought 165 cranks to try out as a test and it worked. Knee pain gone!

Some triathletes say that the super crouched/tucked aero position they use necessitates shorter cranks, because the hips are flexed more by the position so shortening the cranks helps the hip angle open up, which reduces strain and allows a higher cadence. I heard this from a physical therapist specializing in triathletes. So I guess if your top tube is super long, it makes sense to have shorter cranks, and if you are in an upright position you may get away with longer cranks.

So in conclusion, more to do with the rider than the frame.

Frame size has nothing to do with crank length, most would say it's optimized to leg length. "Having said that", most people can ride standard 170mm cranks +/- 5 and not notice too much. If there's some "magic ration" between leg length and crank arms, the variation in leg length is much larger than the available crank lengths.

You'll find that the vast majority of cranks out there are 170mm. If you're used to riding 165, I've got no problem with you sticking to 165, just know your options are more limited.

One of the bike mags from the 80s did some testing with different sizes of riders using both long and short cranks with some surprising results. It's been discussed here somewhere. Some short legs preferred long cranks and vice versa, so whatever floats your boat.

You've got a couple of posts about cranks, but you haven't answered the most important question. What gearing are you looking for? Or might you need recommendations? I also live in Portlandia, so I'm pretty well acquainted with the roads you probably want to ride on, and the hills you'll need to ascend to do so.

Cranks have different BCD's, which will limit the size of the chainrings you can put on it. When you asked on an earlier post what crankset to get, somebody might have suggested an old school Campy Nuovo Record set, which would limit you to a 42t chainring on the small end. Some cranks you just can't find chainrings for, others are plentiful. New or vintage? Budget?

Figure out what gearing you want, and that'll help narrow down the choices. If you aren't sure, post some info about yourself, how you ride, where you want to go, and we can give you some suggestions. You mentioned doing some touring - do you want to camp? That'll require a lot more gear to take with you versus, say credit card touring where you don't need a tent, sleeping bag, and cooking gear to haul around. Almost assuredly this will mean you'll need some low end gearing.

You've got a great start with a great frame, and it looks to be in excellent condition. I'd really like to help you get the most out of it!

All my bikes , except for one have 170 length cranks. The other bike has a 175 length crank and I really like the length of that crank. I have a 35” inseam so my bikes are tall ones . I would swap a couple of my other cranks out when I get the chance.

I dig half step granny on my Schwinn Voyageur which is 30/46/50

SR Apex is what came on ProTours when they went to 15 speed. Good crankset, based on the Stronglight 99 BCD (bolt pattern).

Here's one for sale that's very close to that gearing (50-45-28) I have a 30 I'd swap you for the 28, then all you need is to find a 46t.

Crank length is a function of rider preference: if you have knee issues, a shorter crank reduces the range the knee must flex while pedaling, and will be easier to spin at high cadence. If your riding style is to mash high gears at low cadence, a longer crank will provide slightly more mechanical advantage.

N.B. for a fixed gear bike, a shorter crank increases cornering clearance, so you can take corners faster without pedal strike.

If you're going pretty slow and you hit an obstacle like a rock or a log and you have to muscle your way over it, longer crank arms may help you get over it. That's why they make sense on a MTB. On a road bike they do not have any such advantage and you're better off with short cranks. Unless you like to brag about how big your shoes are, or whatever. There are several old threads on BF where guys whine about not being able to find 185mm cranks or some such nonsense. Be careful what you wish for!
Leonard Zinn, IIRC, is a proponent of insanely long cranks. This makes me question his ideas about other aspects of bicycle design as well. The late Mike Burrows was a proponent of short ones.