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Crank length; does it really matter?

Old 06-22-10, 08:06 PM
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Crank length; does it really matter?

I'm in the process of buying a new set of cranks for my LD rig and I'm wondering if I should go longer. I currently run 170mm but according to most calculations on inseam versus crank length my 85-86cm legs and 181cm height should put me on 172.5mm cranks, so I am wondering if I've been missing out or if I would even notice 2.5mm. Also does that mean I have to lower my seat height 2.5mm to compensate?

When seated I tend to spin more than most people but when climbing I spend a lot of time out of the saddle pushing bigger gears followed by stints of sitting and spinning a lower gear.

What are you guys using? Anyone tried a few different lengths?
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Old 06-23-10, 08:21 AM
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I'm a little shorter than you. I tried 172.5 cranks and really didn't notice the difference. Saddle height should go down, don't know if you would even notice 2.5mm though.
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Old 06-23-10, 08:26 AM
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The longer crank will give you better climbing leverage, but might not spin as well depending on your proportions.
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Old 06-23-10, 09:25 AM
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I've been riding 175's on my df's for decades now. I tried 172.5s for a brief time and was amazed at how short they felt. Gave 180s a try a couple times and they felt fine, but didn't make any noticeable/measurable difference in performance. My recumbent has 170's and those feel fine (172.5s feel too long there - go figure). So I guess I'm in the "it doesn't really matter" camp when it comes to crank length.

According to the "proportional crank length" folks, at 6'3" and 37" inseam (1.9m and 91.4cm respectively), I should be riding something in the 192 - 202 mm range. That'd be interesting to try, but it'll have to wait til I can afford to drop a couple grand on custom cranks for all my bikes. ;-)

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Old 06-23-10, 02:53 PM
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I think crank length is noticeable. 5mm more seems to drop my comfortable cadence by like 3 beats, no biggie. I notice it more on the aerobars or drops because my thighs will start hitting my chest. So think about that.
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Old 06-23-10, 04:14 PM
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Originally Posted by unterhausen
I'm a little shorter than you. I tried 172.5 cranks and really didn't notice the difference. Saddle height should go down, don't know if you would even notice 2.5mm though.
Would it not be a bad idea to adjust your saddle height to fit your crank arms? This could throw off the fit for the rest of the bike.
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Old 06-23-10, 11:14 PM
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crank length matters to me. i run 165's because cadence reduces my chances of mashing, saving knees. for mechanical advantage on hills i use gears, because that's why we have them. as a fun byproduct, i can pedal through fast turns without fear of bouncing off of my pedals. i have no trouble riding with others (ahead of others as well) who have longer, heavier crank arms, especially up the hills.
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Old 06-23-10, 11:23 PM
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i am a little over 6 foot and have no problems maintaining a 90-100rpm cadence with 180mm cranks that are "too long" for me. i can feel that they are longer than my 175s (fixed gear) but it doesnt seem huge.
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Old 06-25-10, 09:15 PM
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Back in the day I raced on everything from 165 to 178. There's a difference, but 'm not sure it's important. If you go back a few decades, you see that crank length has been getting longer and longer. A fellow who would have been on 170s in 1960 would be on 175s today -- and might have passionately argued both, in each different time.

I have come to firmly believe that a 2.5 mm difference -- even if perceptible by an extremely sensitive rider -- is completely unimportant. As I write, my three LD bikes currently hanging in the garage have 165, 170, and 174 cranks. They all work just fine, and other differences between the bikes completely overwhelm any differences in crank length.

(Yes, I know I've mentioned a couple of really weird crank lengths. No typos, just an old pair of 178 mm BMX cranks on a track bike, and a pair of 174 mm cottered steel cranks on a four speed.)
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Old 06-26-10, 03:44 AM
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I firmly believe that crank length matters. Although I have ridden plenty with 170's and they don't cause me problems, I find 165's more comfortable and use them on all my bikes now. I can tell the difference immediately. I put the saddle very slightly farther forward if I'm using 170's, but keep the same amount of extension between the saddle and the bottom bracket shell. My reasoning is that with 165's I keep my saddle slightly lower than the highest I could comfortably ride, because a little more slack at the knee helps me spin faster (especially for long descents on a fixed gear) and the shorter cranks mean that it doesn't cause problems at the top of the stroke. With 170's, I have a little more extension at the bottom, which is okay, and my knee comes a little higher at the top, which is also okay. This worked well enough for a couple of brevets on a borrowed bike earlier this year.
I've ridden a few times with 175mm cranks, the longest being 150 mi on a tandem. 175's are definitely way too long for me, but I was able to manage.
My partner and I have borrowed tandems from friends on occasion, and they typically are too small in back with 170mm cranks, and too large in front with 175mm cranks. We're both the same size, and both use 165mm cranks normally. He's slightly better able to cope with a too-small bike, and I'm slightly better able to cope with a too-large bike, so I've mostly ridden in front. What we've found is that the crank length differential sort of exacerbates the pre-existing differences in our riding styles. Normally, if we're both on geared bikes, he tends to spin a bit faster in a bit lower gear than I do. When I'm riding with cranks 5mm longer than his, I want to pedal a considerably slower cadence in a couple gears higher than he'd like, so we argue about when to shift.
I think there's a range of crank lengths a particular person can successfully ride with, but if you tried out every size in that range, you'd probably find a favorite.

Some LBS's have adjustable cranks for fitting; you might ask if you can set up on the fit bike or trainer and experiment a bit.
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Old 06-26-10, 08:46 AM
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FWIW, I doubt that the tolerances on leg and foot position are so high that you would genuinely notice a 2.5mm difference in crank length.

Nor do I believe that humans are really that sensitive. I'm fairly confident I could alter almost any single parameter of your bike's setup by 2.5mm without you noticing.

And perhaps I'm wrong, but I'm not aware of anyone using an objective measure to actually test the advantages of a different-sized crank, e.g. a blind or double-blind test with different sized cranks. As a result, just knowing "I'm a tall rider, I now have a longer crank, it MUST do X" can heavily influence your subjective judgment.

I'm sure some engineers have good theories, but nothing beats a good ol' repeatable, objective, empirical test.
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Old 06-26-10, 09:22 AM
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Originally Posted by Bacciagalupe
FWIW, I doubt that the tolerances on leg and foot position are so high that you would genuinely notice a 2.5mm difference in crank length.

Nor do I believe that humans are really that sensitive. I'm fairly confident I could alter almost any single parameter of your bike's setup by 2.5mm without you noticing.
If you altered my seat post height by 2.5 mm I would notice it by the pain in my left knee after 20 miles or so. I would imagine longer/shorter cranks would have the same affect as raising/lowering your seat in term of leg extension. My left knee is very particular for some annoying reason. Probably cause a horse kicked it when I was 6. I literally lowered my seat by 2mm and the pain was gone forever.

As for the "advantages" of crank length I definitely agree with you. I don't think there are any. As pwdeegan said, gears can be used to control cadence and mechanical advantage. I'm not quite an engineer yet though Give me a few years
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Old 06-26-10, 05:09 PM
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Originally Posted by kaliayev
Would it not be a bad idea to adjust your saddle height to fit your crank arms? This could throw off the fit for the rest of the bike.
depends on how far you move it. 5mm is nearly 1/4", and I think anyone that has a dialed-in position would definitely feel it. It would blow out my right knee. Moving 1/4" on the seat post will not change any other part of the position a significant amount. I don't think 1/4" on handlebar height is significant, but you could always remove a spacer. 2.5mm is still a fairly significant change in seat height, I don't like to change more than that on my position.

I rode 165mm cranks for a very short time, and I didn't like it. 172.5 felt a little slower than 170mm, but not significantly as I posted above. I'm pretty sure that 175 would be much too long for me and I definitely think I would notice. So it really depends on how much you are changing.
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Old 07-13-10, 12:00 AM
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A quick update, the 172.5mm cranks are on and I did my first 200km brevet with them over the weekend. I'm not sure if I'm imagining it, but I flet like I had more power climbing seated and it didn't seem to effect my ability to spin. No unusual pain anywhere so overall I'm very happy. As always YMMV.

I also run 165's on my fixie and while I like how easy they are to spin and appreciate the extra ground clearance, they always feel like I'm not getting good leverage when pushing up a hill. The 172.5's feel much better in that regard.

As a side note the new gearing 48/34 compact with a 13-26 7-speed cassette also worked well. I don't think I'd be using it for group rides as I would spin out the top end, but great for endurance rides, I may even go to 13-28 or 12-28 for a bit more range. I was most worried that I would be constantly changing at the front but that didn't happen, I was able to stay on the big ring 90% of the time, and with only 7 speeds up back and a friction downtube shifter at the front it was easy to use the whole range on both rings.

Thanks for your input.
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Old 07-13-10, 10:11 PM
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Originally Posted by kaliayev
Would it not be a bad idea to adjust your saddle height to fit your crank arms? This could throw off the fit for the rest of the bike.
I find I need to do this, even with a 2.5 mm difference. It's not just the leg extension at stroke bottom, it's the deeper knee bend at teh top, and the tighter hip closure. When I ride in the drops with 172.5s, I find myself tending bow my legs to avoid belly pressure - yes, I know, I am losing weight, and I do need to.
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Old 07-20-10, 08:49 PM
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By calculation, I should be using 195's, but choosing to err on the side of caution, bought a set of Zinn 190 cranks. When I put them on my road bike, I grounded a pedal in a turn, so moved them to my MTB, and now they work good. I have settled on 180's for my road bikes, and can feel the difference from the 175's I had before. I can pull one higher cog with the 180's with the same perceived effort. YMMV
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Old 07-23-10, 04:36 PM
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I've found the difference shows up big time on MTBs, to a lesser degree on road bikes. I lose a few rpms but gain a little leverage while climbing with longer cranks. If I was a pure spinner, I'd keep the cranks short.
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Old 07-24-10, 01:29 AM
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I'm a tall rider, 1.91 tall with 89cm inseam. I have 175mm cranks on road bike and fixie, and 172.5 on my tourer. I honestly cannot tell the difference. The tourer has a triple chainset, of course, and I guess the different gearing might mask some changes, but...

FWIW I'd guess that very long cranks have disadvantages as well as advantages. More leverage in the hills, but a bigger dead spot at the top of the stroke so more difficult to maintain a perfect pedalling action, and lower cadence. Very much a matter of taste, I guess.
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