agmetal

To clarify the posting title: I'm 5'6"ish, and normally ride 170mm cranksets. I have a build I'm planning with a frame I've been offered for free, and some other parts I've been given by other people, and the crankset I've been offered is 172.5mm. I'm fairly particular about my saddle position, but I'm no racer - I just use the bike as my everyday transportation.

If it makes any difference, this frame I'll be using is substantially taller than I usually ride (58cm vintage Shogun 300 with a horizontal TT, vs. my modern sloping-top tube bikes that range from 48-53cm in their nominal sizes). I can stand over the TT with feet flat on the ground, and the TT is short enough that I can comfortably reach the bars, while the frame height brings them easily to the desired height (unlike a 54ish cm vintage Panasonic frame I have, where the TT is quite long, and I need a tall stem with short reach to ride it)

EDIT: For reference, here's a bike with a frame near identical to the one I'll be using (in size, anyway)...I can ride it set up as shown, but this one has 170mm cranks. I'd probably drop the saddle maybe 1/4" if I was gonna ride it for more than a couple miles:



For comparison, here are my current bikes (note the tall/short stem on the Panasonic on the bottom in the second picture):

johnny99

https://youtu.be/eMAxH_Ud8YE

agmetal

That video focuses mostly on performance - I'm much more interested in noticeable differences in fit, because I have 4 bikes with the same crank length that are set up to fit more-or-less the same, and I'm wondering if I'd notice the difference in length, all else being equal (obviously my saddle height would probably drop slightly)

AnthonyG

What is your inseam without shoes?

If your legs are long enough then you could use either crankset without noticing that much difference, particularly since your not racing. If however 170mm ranks are at your absolute limit then 2.5mm will make a difference. I'm of the view that most people are on bikes with cranks that are too long for them which makes bike riding more uncomfortable than it should be. Then again, how far do you ride?

Anthony

agmetal

Why does "without shoes" matter, if I'm not riding barefoot? I have no issue with the 170s, and I do remember feeling that the crankset on my old Raleigh Sports felt short, and that was closer to 165.

I do at least 12-13 miles a day, at least 5 days a week. Most single-day mileage is 80ish, but I've done over 160 in a weekend before.

AnthonyG

Without shoes is how its measured.

Your the one asking for advice. Its not my issue. If you want good advise then we need good information.

Anthony

agmetal

I wasn't trying to be rude, it just doesn't make sense to me. I don't know offhand, and am not in a convenient place to measure. I typically wear pants with a 30" inseam, but I ride with rather thick-soled shoes (Dr. Marten's 1460s) on wide platform pedals.

bhchdh 

I doubt I would notice a 1/10". There are too many variables; frame geometry, saddle design and material, flat or clip side of pedal, bike shorts or jeans, sandals or work boots etc.

AnthonyG

I recommend that crank length should be between 20 to 21.5% of inseam. Its not a hard and fast rule and there is a range of lengths that suits most people. Doing a quick and rough calculation, guessing an actual inseam of 31" then I calculate that 170mm cranks are at the top of your ideal range. I think you would be better staying with 170mm. Would you feel a huge difference from an extra 2.5mm?

On the shorter rides I doubt that it would make much difference but on the longer rides I suspect that it would start effecting your comfort.

Anthony

jyl

I don't notice going from 175 to 172.5. Don't think I have any bikes with 170. But I'm not sure, I pay no attention to crank length and don't even remember what length is on each bike. 2.5 mm is only 1/10". That's a very small amount. My saddle heights vary by more than that. Heck, the thickness of my short pads probably varies by more than that.

ltxi

^...while I agree on it not being a big deal for me either, top of stroke to bottom is 2/10" and that may be noticeable to some folk. It's about crank rotational diameter, not height to the saddle.

ironwood

I'm about 5'7" and I used to have 170 mm cranks on all my bikes, but I put 165mm cranks on a bike and found that either there is no difference, or that I amm better with the shorter cranks ie. I can spin better, I think. I know someone who claims that he can tell the difference between 170mm and 171mm cranks, but I doubt it. I'm pretty sure I can tell the difference between 165mm and 175mm cranks.

TA has a sizing chart that showed my optimum size crank as 167.5. They and SunXCD are the only ones making that length, I believe.

Can you staddle a 58cm frame? If so, you must have very long legs for somenoe your height.

Carbonfiberboy 

I have 170mm cranks on all my bikes, except for 175mm captain's cranks on our tandem. I notice mostly that it's harder to pedal when really low, but it's no big deal. I'm so used to swapping back and forth now that I hardly notice it at all anymore. It certainly doesn't cause a problem. I've done 15 hour saddle-time rides with both crank lengths, always tiring but no issues. My inseam is so short that I theoretically should be riding 165s, but bikes don't come stock that way, oh well.

bakes1

We are talking about around 1/10th of an inch here correct?

wphamilton

Yeh. I think worst case you'd heal-toe it a couple of mm more, if you kept your ankles that steady to begin with.

stringsonbikes

If it is free or a good deal try it and see if you notice. I think some people are more sensitive than others. I am the same height as you and have 3 bikes with 170mm and 2 with 175mm cranks. To be honest I dont really notice a difference. I have done centuries on both and put in consecutive long days. I am also very much a spinner if you are a masher it may make a bigger difference to your knees.

TiHabanero

The difference is 2.5mm. That's it. Will your body experience the difference. Doubtful. I have many bikes and have 3 different length cranks on them, and honestly, I perceive absolutely no difference at all between them. They all feel the same as far as torque and spin go. There just is not any real significant difference in the feel. None.

ironwood

The reason I put 165mm cranks on one of my bikes was that I converted it to 650B and the botom bracket was lowered and I figured shorter cranks would give me better clearance and reduce the chance of pedal strike. As I mentioned above, I either don't notice the difference, or I prefer the shorter cranks. Or maybe it is just that I prefer the bike.

Shorter cranks also might lessen the amount of toe overlap with the front wheel or fender; for some cyclists this is an issue.

springs

It seems to me that the only place you'd notice is on very steep climbs where you're in your lowest gear and struggling to turn the cranks. Then a longer crank might be beneficial.

TiHabanero

However will you be able to really perceive the difference? I dare say not, and can honestly say my personal experience is that I never have felt a difference.

springs

Well, I've never had the chance to compare two difference crank lengths with all other variables being equal, but since the change is down in the 1-2% range I agree it would be hard to perceive.

MWF78RIDER

My road bike has 170 mm crank arms; the mountain bike has 175 mm. Frankly, I cannot tell any difference.

Ali89

I think you will notice. In order to reach the bottom of the pedal stroke properly you will need to move the saddle 2.5mm down from your usual height. This drop means that at the top of the stroke the knee will have to move 5mm higher than they're used to and I think you will notice that. It can cause knee problems particularly if your flexibility is not great.

Bikerdave222

I have used 170 and 175 and I can tell a difference. I think I can tell a difference with 172.5 even. With the smaller crank I can spin better.

ESTrainSmart

You will notice a difference in your fit and the way you ride, even at low power. Your natural cadence will drop which will cause you to apply more pressure per pedal stroke. This isn't a bad thing as long as you don't have knee tracking issues or pedal with quad dominance. If you do have a slight amount of either, flexibility and mobility exercises will be more crucial to preventing overuse injuries. With the longer crank, you'll need to have a more forward saddle position to prevent the braking effect from taking into effect at the bottom of the pedal stroke.

Since you don't plan on racing, I highly recommend going back to the 170mm because it doesn't require as much conditioning to turn it comfortably. You'll also be at a higher cadence which won't fatigue the muscles of the hip and leg as easily.

I wrote a post discussing the differences of crank length. I think you'll find a lot of the information helpful.

Eat Sleep Train Smart - Personal Training & Coaching Official Blog: Crank Arm Length: Everything You Need To Know