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Crank Length?

Old 05-17-23, 06:00 AM
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I'm about your size (5'10", 30.5" inseam). I mostly ride 175mm cranks. I did switch one bike from 172.5 to 175 (when I needed a new crank, not just because, and I got the longer crank to match my other bikes). I did not change my saddle height.

I didn't notice any difference, except when I looked at my cadence numbers. On the bike computer, with the longer crank arms my cadence was lower by 3-5 rpm from what I expected from the "feel" of my pedal stroke. So, for me, the difference was measurable but not perceptible.
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Old 05-17-23, 07:34 AM
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I have a 34" inseam and have run 170,172.5, and 175 with no discernible difference to me. That is all when I didn't own a car and lived to ride. Racing, touring, commuting, high mileage.
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Old 05-17-23, 08:53 AM
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There are scientific studies on how crank-length affects power, and their results show no difference in power between the length of crank arms on bicycles. But rider comfort does affect power and endurance. Everyone agrees that bicycle frames, stems, seat height etc. all needs to be rider specific, so it is common sense that crank length does too.

I like to compare bicycle cranks to the crankshafts in internal combustion engines. With a gasoline engine power equals torque times rpm, the same goes for a bicycle. If you put 150 pounds on the end of a lever attached to a shaft that rotates such as a bicycle bottom-bracket, you get torque. The longer the lever, the more torque you get, but at what rpm do you have maximum torque? A long heavy leg is going to take more energy to move at 80rpm than a lighter short leg. But because the short light leg is easier to move, it can turn higher rpm for the same energy output, so even though it is comfortable on a shorter crank that makes less torque, the higher rpm makes up for it and produces the same power. The heavy long leg is harder to move quickly and takes more energy to do so, but if it can use a longer crank arm or lever, then it can have higher torque at it's lower rpm and again produce the same power.

I am tall with long legs, so I try to find the longest cranks I can for my bicycles. I have 180mm road-bike cranks and 175 mtb cranks. I tried racing a road bike with 165mm cranks last year and did okay, but later in the season I got on a road bike with 180mm cranks and went 1 mile-per-hour faster than I did with the 165mm cranks. And I was able to go just as fast on the mtb with 175mm cranks as I could on the road bike with drop-bars equipped with the 165mm cranks. When I was younger, the fastest I ever went on a road bike was with 180mm cranks. If I was shorter, especially with shorter legs, I would absolutely be running the 165mm cranks at a higher pedal rpm.

So your level of fitness and training determines what amount of power you can produce, the size of your legs determines what rpm you are comfortable holding for the length of a ride and your efficiency at a certain rpm, and the length of the crank is what will let you produce your power at the rpm you are comfortable at.

I know I am a low-rpm person with my long heavy legs. I am comfortable at about 80rpm and less comfortable when I have to spin higher for long periods, so the long cranks work for me. If I end up with knee problems or reduced flexibility some day I may just have to live with shorter cranks, making less power and going slower, but that does not mean I will not be able to have fun.
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Old 05-17-23, 08:59 AM
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Modern thought in crank length is shorter is better for various reasons so if I was new to this thats the guide Id use.

Im 62 and have a 31 and change cycling inseam depending on shoes and I run 2 175s and one 177. Ive never been able to get used to shorter cranks so I just go with whats familiar.
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Old 05-17-23, 09:06 AM
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Originally Posted by Jrasero
I am 5'5" and used to use 170mm cranks but switched to 165mm. Not sure about opening up the hips, but it does allow me to run less seat post and has less toe tap
Allows you to run less seat post? You mean you lowered saddle?

Actually if you go to shorter cranks then the saddle height should go up when measured to the BB. But if you measure from the top or center of the pedal when furthest away from the saddle, then that distance should pretty much stay the same regardless of what length crank you put on the bike.
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Old 05-17-23, 09:13 AM
  #31  
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I'm 59, 5-9", 31 inseam and have ridden 170s and 172.5s since 1984. I recently acquired a vintage Centurion with 165s and notice the difference a lot. I don't like it, but I can tolerate it.
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Old 05-17-23, 09:24 AM
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Originally Posted by Iride01
Allows you to run less seat post? You mean you lowered saddle?

Actually if you go to shorter cranks then the saddle height should go up when measured to the BB. But if you measure from the top or center of the pedal when furthest away from the saddle, then that distance should pretty much stay the same regardless of what length crank you put on the bike.
yeah mixed that up. Smaller cranks equals greater seat post which can equal to more comfort/flex in the seat post
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Old 05-17-23, 11:24 PM
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I ride 170, 172.5 and 175 on my various bikes and other than adjusting the saddle height a tiny bit, I don't notice any difference among them. I couldn't even tell you which bike has which at this point.
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Old 10-25-23, 09:51 AM
  #34  
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Originally Posted by bruce19
5'9" and 32.5" inseam. I generally ride 172.5 but have ridden 170. I don't feel/see a difference.
Originally Posted by shelbyfv
FWIW, I'm 5'8" and use 175 and 172.5 interchangeably, can't tell a difference.
Originally Posted by robobike316
I have a 34" inseam and have run 170,172.5, and 175 with no discernible difference to me.
Thats because a 2.5mm difference is insignificant to most riders. Ive been riding on 165 and 170 for the most part and recently installed 155s on one bike. With a 29 inseam this felt so perfect, especially in my hips. You will definitely notice a 10mm difference.
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Old 10-25-23, 01:26 PM
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My old knees appreciated dropping down to 165.
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Old 10-25-23, 10:49 PM
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5'10" and 32". All my bikes had 175mm but I recently swapped out my full squish mtn bike to 165mm after a pedal strike sent me face first into a tree, oh and to see if it would help my knees. Felt odd for about 5 mins then I was totally use to it. Don't notice any difference in power and actually climbed a few things I've struggle on all year. I'm not sure if it is helping the knees any since I did 10 miles over the weekend on my hardtail with 175mm fine and 16 today on the squishy with the 165mm and had a bit of knee pain. The extra clearance is the big draw for me and the hardtail will probably also get a 165mm at some point. Road bike I'm probably not going to worry about plus it has a stages PM which just multiplies the replacement cost.
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Old 10-26-23, 05:18 AM
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Originally Posted by spelger
I went from 172.5 to 170... It's not a perceivable diff, to me anyway.
Same, but my dealer recommended it, and I like it.
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Old 10-26-23, 11:34 AM
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I run 170 on my road bikes and 165 on my fixed gear bikes (a little more clearance in corners, easier to spin, easier on my knees).
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Old 10-26-23, 11:52 AM
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Originally Posted by JohnDThompson
I run 170 on my road bikes and 165 on my fixed gear bikes (a little more clearance in corners, easier to spin, easier on my knees).
Do you like a higher bottom bracket for fixed?
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Old 10-26-23, 12:12 PM
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I’ve always run 165’s on my fixed gear. Ideally, I’d like 160’s, or even better, 155’s to match my 29” inseam. But these shorter cranksets are almost impossible to locate.
Funny thing is, e-bike cranksets (no spider) in square taper or ISIS are readily available in all sorts of lengths. Just installed 155’s on my newly acquired e-bike. It originally came with 170’s.
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Old 10-26-23, 12:33 PM
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Custom frame builder Matt Appleman is something of an evangelist for shorter crankarms. He makes the 2XR crankset, which allows for shorter arms (135mm - 175mm).

Here are some of his thoughts from this link: https://www.applemanbicycles.com/res...-crank-length/

While we’re at it, can we ditch the 2.5mm increments? 2.5mm increments in crank length should be banished from the crank length kingdom. A 2.5mm change, it’s something—a start—a pat on the back—a gold star, but definitely not worth swapping out new crankset for. As we’ll see later in this guide, 10mm changes make a significant impact on rider fit and comfort.

WHY BOTHER CHANGING CRANK LENGTH?

In a word: comfort.

In three words: comfort, stability speed… or… better bike fit.

If you’re pain free and riding your best life… by all means… skip a crank length change. Save money and keep on keepin’ on.

Consider a change in fit and crank length if you:
  • have pain (knee, hip, back) (PS: riding doesn’t need to hurt, even after a long ride)
  • Your riding buddies say that you ride “all over the bike”
  • rocking hips/torso
  • your knee tracks outward at the top of the pedal stroke
  • are of average or short height (some tall folks can benefit too)
  • have a history of discomfort on the bike
  • want more speed
  • want a better fitting bike
  • feel that something isn’t quite right with the world
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Old 10-26-23, 07:39 PM
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I should probably try a change on my bike, as my left knee doesn't seem to track correctly. Can't ride long distances of 50+, or climb hard out of the saddle, though right knee has no problems. Have 172.5mm cranks right now, do they make 167.5?. My trainer bike has 170mm. I will get an MRI soon, as it has pain other than biking also. I'll call it a work in progress. As long as I stay 40 miles and under, it doesn't hurt while riding, but can't ride everyday.
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Old 10-27-23, 02:08 AM
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I traditionally used a 175 and this year switched to a 170. I was experiencing pain in my hip flexors before and that has almost vanished. With some time I am hoping it will clear up entirely.
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Old 10-27-23, 06:24 AM
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Originally Posted by macattack71
Have 172.5mm cranks right now, do they make 167.5?. My trainer bike has 170mm.
165 mm is fairly common.
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Old 10-27-23, 07:11 AM
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6'2'' height & 32'' inseam - using 172.5mm.
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Old 10-27-23, 09:03 AM
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Originally Posted by big john
Do you like a higher bottom bracket for fixed?
Yes, when I built my fixed gear frame, I put the bottom bracket about a centimeter higher than on my road bike, following what is done for track frames. But if you are converting a road frame to fixed gear use, as I did with my other fixed gear ride, you don't have that option. I just take corners a little easier when riding that bike.

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Old 10-27-23, 09:09 AM
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Originally Posted by roadfix
Ive always run 165s on my fixed gear. Ideally, Id like 160s, or even better, 155s to match my 29 inseam. But these shorter cranksets are almost impossible to locate.
BMX cranks are available in many short arm lengths, 140mm or even shorter. And Mark Stonich at BikeSmithDesign has a crank shortening service where he drills and taps a new pedal eye in the arms and cuts off the excess:


https://bikesmithdesign.com/Short_Cranks/index.html
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Old 10-27-23, 10:14 AM
  #48  
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Originally Posted by blkwrxwag
I'm only 5ft 8 so not a direct comparison, but I switched to 165 from 172.5. I'm 57 yrs old and sit at a desk all day. My hips hurt less with the shorter cranks and I would not go back. Remember to adjust your seat higher with shorter cranks.
lower with a shorter crank, or you won't be able to reach the pedalscat the bottom of the stroke.
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Old 10-27-23, 10:17 AM
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I've recently purchased 172.5mm cranks for 2 of my 4 road bikes. I also went from 50/34 back to 52/36 at the same time. But all has been positive. Couldn't even tell the cranks were shorter.
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Old 10-27-23, 10:23 AM
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5.5 X inseam in inches is the usual formula. I have a 29" inseam, 78 y.o., use 170 on my road bikes, 175 on our tandem. Works fine, but works better when I'm in shape with less belly fat. Brevets and long mountain rides have been fine. Main thing that fixes pain on the bike is exercise. If it hurts, work it, that's my motto. Back in my 60s, I had no problem spinning 135 on the road bike. Everything is a little slower now but 105 is not a problem. Practice is what works, creates adaptation.
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