Crank arm size advice needed--Please.

[goblue]

I have 172.5 arms on my Dura ace 7800 53/39. I'm buying a compact crankset--Dura Ace 7905 50/34. Considering moving up to 175 arms. I'm 5'11' 185#s. I am 53 years old and do not race. I like fast-ish group rides and weekend metric centuries. The hills in west/northern Michigan where I ride are pretty steep. Any advise would be greatly appreciated! Many thanks!

[Likes2ridefar]

Personally I think it doesn't matter, but that's just me. I have two race bikes. one has 172.5 and the other 175. i've no issues going back and forth and don't really notice a difference after a few pedal strokes.

i'm 6' and long legs. race competitively at a pro/cat 1 level.

[jischr]

2.5 mm is a pair of thickish socks. I have one bike with 170 arms and another with 175 arms (summer vs winter socks). I can't tell a difference, but then I'm not paying attention to it either, i.e., just get on and ride.

Others here will tell you to adjust your saddle or seat post or get a pro-fit done. My body doesn't react to such a small change, or rather I don't notice it reacting, and I don't use a power meter to tell me if I've gained or lost anything.

I doubt you'll notice a difference so get what you want.

[goblue]

I agree...2.5 mms seems inconsequential...Yet why do mfgers offer three arm sizes which are so close in length?

[Pete Serotta]

Performance wise I have not seen it make a difference.

At the same time, I have not heard of a instance when it caused knee pain.

I have a 32inch inseam and have used 170 and 172.5 over the last decade. Now I just use 172.5, No performance reason but that was what Cyclesport (Mike) felt was good.


PETE

[torquer]

Going compact will make a much bigger difference than that small increase in crank-arm length. Ideally, you would like to try each change separately, but cranksets are a pretty pricey area in which to experiment. Too bad nobody offers a test-ride option for various crank options like Competetive Cyclist (I think) does for saddles.
That said, I also went to compact cranks with longer arms (175s) on one bike, and couldn't wait to make a matching switch on the other. Trouble is, I can't tell how much each of those changes contributed, just that the whole package worked for me.

[FlashUNC]

Count me as a lifetime user of 175 cranks, though I'd probably go to 172.5 without noticing a difference.

I did notice the 165 cranks I had on my track bike. That was a pretty big difference, but even then I'd stop noticing it after the first race of the night.

[Louis]

I've told this story before, but it is relevant here:

I'm 6' have an 87 mm inseam . My first bike purchased at an LBS was a 62 (and too big for me) but for some reason it came with a 170 crankset. I didn't know any better and rode it for many years. As I got smarter about bikes I realized that 170 is too short for me. I went up to 172.5 and really didn't notice much difference. Rode that for quite a while, then went up to 175. I immediately noticed a significant difference. It was much easier to "get over" some of the rollers we have around here without changing gears. I considered the experiment a success and have gone to 175 on all my bikes. I've also bought a 177.5 but have not yet had a chance to try it.

Louis

[19wisconsin64]

hi, my vote is for the 172.5....i ride these on my road bike. I'm six feet tall, and this is plenty long. you will be spinning a little more on the hills, so this will be better than the 175s.

just my 2 cents.

enjoy!

[Ralph]

Your cranks need to fit you. Otherwise I don't see how it matters. A couple teeth difference in chainring size makes a bigger percentage difference in leverage than a crank size change. And we all have 8, 9, 10, and 11 cogs in back to change leverage at will. I like what Sheldon Brown said.

http://www.sheldonbrown.com/cranks.html

[dancinkozmo]

..im about the same height as you , normally ride 170 or 172.5 cranks...i found going to 175 gave me achy knees . i read somewhere (zinn?) a bigger pedalling circle places more strain on the knee at the top of the stroke...not sure if thats b.s. or not.
i say, if youve been riding 172.5 for a long time and have had no problems , why risk switching ? if it aint broke , dont fix it :beer:

[Frankwurst]

Stick with whats been working for you. It's always worked for me. :beer:

[RedRider]

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[oldpotatoe]

I have 172.5 arms on my Dura ace 7800 53/39. I'm buying a compact crankset--Dura Ace 7905 50/34. Considering moving up to 175 arms. I'm 5'11' 185#s. I am 53 years old and do not race. I like fast-ish group rides and weekend metric centuries. The hills in west/northern Michigan where I ride are pretty steep. Any advise would be greatly appreciated! Many thanks!

What 'problem' are you trying to solve, what 'question' are you trying to answer with longer crank arms?

'Compact' doesn't mean you 'need' longer crank arms. Your feet have to move more quickly for a given cadence with 175, I'd say stay at 172.5.

[Kontact]

I have also read more than a couple of personal knee strain stories connected to long cranks. If you have the long legs for them, great. But an extra 2.5mm isn't going to make a superstar out of anyone, even while it puts extra wear and tear on the joint.

[RPS]

Your cranks need to fit you. Otherwise I don't see how it matters. A couple teeth difference in chainring size makes a bigger percentage difference in leverage than a crank size change. And we all have 8, 9, 10, and 11 cogs in back to change leverage at will. I like what Sheldon Brown said.

http://www.sheldonbrown.com/cranks.html
Good point. Viewed strictly from a gearing standpoint, like Sheldon's gain ratio, I think the difference between 170 and 172.5 cranks is less than one chainring tooth in difference. It's such a small "leverage" difference that I wouldn't worry about its effect on gearing, particularly on a multi-speed bike.

[dd74]

I mentioned this discussion to my fitter at my LBS, and he said that crank length is very important and should not be changed if you've been custom fitted to a frame. If you've been fitted with a certain length crank, you should stay with that length crank.

[goblue]

Thanks to everone for the advise which I have found to be helpful. I am changing to a compact to make hill climbing easier for me. The bike is a custom fit Serotta Legend SE which is equipped with 172.5. Based on your collective input I will stay with 172.5. Thanks again.