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Change crank length for $50 per millimeter?

Old 01-14-20, 02:42 PM
  #1  
Moishe
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Change crank length for $50 per millimeter?

I never paid any attention to crank length until last week, when I bought a newly affordable Stages Ultegra left crank power meter for $350, and I noticed that my one-year-old Trek Domain has 175mm cranks, while my nine-year-old Roubaix, now my Zwift bike, has 172.5mm cranks. A few hardly noticeable knee twinges over the 4,000 miles ridden in the past year on the 175mm cranks. I’m 71 inches tall with a longer-than-average inseam of 34.5 inches; cranky mavens say that by height I should have shorter cranks, by leg length longer ones. The shorter cranks kinda sorta feel a little better, but I couldn’t swear to it in open court.

So here is my question: should I just install my new 175mm crank-meter and relax with a couple of Dogfish Head triple IPAs, or should I exchange it for a 172.5mm (or even 170mm) crank-meter, buy a matching right-side Ultegra crank for around $125, and THEN have the beers?
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Old 01-14-20, 05:52 PM
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There are certainly folks who are concerned about a couple of mm difference in crank length. There are also folks who are concerned that they don't have a 15t or 17t or whatever on 11 speed cassettes. However, most people are somewhat adaptable. I have 172.5 on one of the two bikes I ride most often, 175 on the other. I couldn't tell you which was which w/o looking. Look at 2mm on a ruler.... Save the time and drama and continue to enjoy what you have.
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Old 01-14-20, 06:00 PM
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My wife's father had asymmetrical leg lengths subsequent to a fractured pelvis. He was an avid rider and competed in the Mt Equinox hill climb. I'm inclined to agree with Shelby
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Old 01-14-20, 07:53 PM
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I was going to answer your question, but suddenly I am craving a beer. Good luck!
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Old 01-15-20, 03:46 AM
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I have 170mm cranks on all but one bike . That bike came with 175mm cranks and it seems to be not much different . I am a tad over 6 ‘ and an inseam of 35”. I wouldn’t be able to tell even pushing up hill to be honest. Joe
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Old 01-15-20, 04:03 AM
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My cranks range from 170 to 175. The only difference I notice is I tend to mash more with longer cranks, and spin more with shorter cranks. I'd probably prefer 172.5 for all of my bikes, but not enough to spend money to replace cranks that already work with only a minor adjustment. If I haven't ridden the bike with 175 cranks for awhile it takes a few minutes to adapt. Same with the 170 cranks.

I seem to finish long rides or harder workout rides with a bit less leg fatigue with the road bike with 170 cranks. I suspect that's because it's using less range of motion and in my sweet spot. I prefer nearly full leg extension -- not quite toe-down, a la Anquetil, but as close as I can comfortably get. To get that extension with 175 cranks my thighs are more nearly parallel with the ground at the top of the stroke. I find that a bit more tiring on longer rides or harder workouts. But at age 62 with bird legs, I'm a bit more picky than I used to be.
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Old 01-15-20, 09:26 AM
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I am 195 cm (about 77") tall with a ~36" inseam and I am perfectly comfortable riding my bikes with cranks 170, 175, and 180 mm long. I, too, have read about how crank length should be proportional to height or leg length or thigh length, but in the real world you quickly get used to whatever you are riding when faced with tiny differences like that.
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Old 01-15-20, 09:39 AM
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2 different bikes have different geometry that puts your body in different angles. That is the difference you think you notice. 2.5 mm crank length difference, not noticeable.
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Old 01-15-20, 10:22 AM
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tgenec86
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Originally Posted by Moishe View Post
Should I just install my new 175mm crank-meter and relax with a couple of Dogfish Head triple IPAs?
Answer. Yes... This is the perfect solution. While spending money on bikes is a one-time investment and can last many long, enjoyable years, SAVING money on unnecessary bike stuff leaves more money for Dogfish Head Triple IPA's.
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Old 01-15-20, 10:50 AM
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Originally Posted by Moishe View Post
#2 22222]So here is my question: should I just install my new 175mm crank-meter and relax with a couple of Dogfish Head triple IPAs, or should I exchange it for a 172.5mm (or even 170mm) crank-meter, buy a matching right-side Ultegra crank for around $125, and THEN have the beers?
I don't feel that I yet have the experience to add much to the discussion here but I just wanted to say that I dig the the phrasing of your question. However, I will make this suggestion:

1) Crack open a Dogfish Head triple IPA and enjoy while reading the responses to your question,
2) Don't discount the small niggles that you feel you are experiencing,
3a) If the $125 is an absolute budget buster for you --> crack open Dogfish Head triple IPA
#2 , relax and call it a day,
3b) If the $125 is not of consequence to you; then by all means spend it, install crank, have peace of mind, enjoy IPA#2 . . . and finally,
3c) If budget is really is of no concern to you as your accountant handles the affairs of your trust fund anyway, then pay for a professional bike fitment and take the advice given regarding crank length for you . . . and then of course, kick back and enjoy a smooth Dogfish Head triple IPA while being confident in the knowledge that you you have done everything to ensure that your future rides will be pain and worry free.
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Old 01-15-20, 11:12 AM
  #11  
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A slightly different take here. If you have knee tracking issues such as chrondomalacia patellae (CP), you may find your knees are much more sensitive to crank length. I triggered my life of CPdoing several things wrong but I had been doing all but the crank length change with no issue. (Riding in the cold with not enough on my legs, not stretching my hamstrings and upping my mileage and intensity after a huge conditioning setback - major TBI. But my knees didn't complain until I took my race bike out and rode 100 miles to get ready for that weekend's first race of the season. And that race bike had 175s. My fix gear, 168s.)

A few years ago, I picked up a sweet Campy crankset and built a bike around it. 172.5s. My knees never liked the switch between all the rest of my bikes and that one. After one year, I put 175s back on. Lesson learned.

Ben
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Old 01-15-20, 11:17 AM
  #12  
Moishe
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Okay, I drank the beer. The big mistake I made was to Google "crank length knee pain," which was just asking for trouble. What returned was a lot of alarmist advice about the crippling effects of riding with cranks that might be a tiny bit too long. This just fed into my Cyclist Hypochondria. Thanks to you all for setting me straight. And by the way, if you've never had Dogfish Head IPA, give it a try. It's widely available, and if you can't get to Stowe, Vermont for some Heady Topper, it's about the best there is.
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Old 01-15-20, 11:22 AM
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I have bikes with 172.5 and 170mm cranks. I can't tell the difference.
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Old 01-15-20, 07:49 PM
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I will notice the longer cranks when I try to ride at higher cadence.

I also notice a better , more comfortable, bicycle fit with shorter cranks. I am 5'8" though.

I will agree that people can adapt, however if you have 4 bicycles at 170mm cranks and one at 172.5 you'll feel it.
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Old 01-15-20, 08:15 PM
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Two bikes, 175 and 180, can't tell the difference.
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Old 01-15-20, 09:39 PM
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This is a very active forum, I bet you could find somebody to trade with in the for sale section
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Old 01-15-20, 10:56 PM
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I notice the difference in crank length as soon as I start pedaling. Then, once I'm rolling the shorter ones (170's) are easier to spin fast. 175'a give more torque, but I won't be able to spin them as fast so it's a wash. But they do seem to give me a little more knee pain.

I once got a fixed gear bike with 165's and the pedal circle just felt too small. I had to switch it out for 170's which seems to be the sweet spot for 5' 7" me with a 31.5" inseam.

If you think you'll be happier and/or more efficient with the swap, do it. You're gonna drink beer either way.
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Old 01-16-20, 07:46 AM
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With “hardly noticeable knee twinges over 4000 miles” I wouldn’t be concerned unless they are frequent. But after 4000 miles I would believe you would have noticed major problems
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Old 01-16-20, 08:02 AM
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I ride from 160 (fixie cranks on my drop bars 3 sp) to 185 on my BMX race bike. I think the 185 is a little long and am switching to 180s, but neither hurts my knee.
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Old 01-16-20, 08:19 AM
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I grew up with 170 cranks, and I'm 5'6" with PBH of 81-ish cm. About 15 years ago I came into a first-generation 53/39 Chorus double chainset with 172.5s, for $25.00! Another $30 and I had the correct BB. At first I noticed it was a little more effort to pedal at cadence, but I don't notice any such problem now. And that chainset has VERY low Q! I still prioritize 170 if I have the choice because most charts/equations say 170 is most likely best for me, but no longer notice which length I'm riding.

At 66 yo my knees feel pretty good. Been lowering the saddle incrementally to reduce pelvic rocking, which is good but it reduces hip closure angle, so the knees can hit the belly. But not a problem so far!
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Old 01-16-20, 11:32 AM
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I had a bike fit done yesterday, and crank arm length came up. Since the fitter is a physical therapist, and racer in his own right, he has an opinion, that he backed with a study done, where riders swapped between abnormally longer and shorter cranks, and ultimately, power remained the same, as the riders adapted to the extremes. That being said, due to my ankle in an immobilizing brace, he suggested staying with shorter arms, though we fit the bike yesterday with my existing 172.5 arm crankset. He did suggest swapping to a compact, to match my steel bike...

He also raised my seat without me looking, to prove I wasn't rocking my hips like I thought when I originally lowered it. He proved I wasn't, with the seat covered in sensors...
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