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175 vs. 172.5 cranks(14 posts)

175 vs. 172.5 cranksjvr-oclv
Feb 4, 2002 6:21 AM
Am I really going to notice a difference between 175.00 and 172.5mm cranks? I have a chance to get a good deal on a set of Dura ace cracks(172.5) but current cranks(Ulterga) are 175.
re: 175 vs. 172.5 cranksweiwentg
Feb 4, 2002 7:34 AM
probably not. I went from 170 to 175 without problems, and i'm 5'5". not a huge difference. they say, though, that the shorter cranks will help you spin faster. not sure if that's the case.
re: 175 vs. 172.5 cranksKEN2
Feb 4, 2002 9:22 AM
How tall are you/what's your inseam? Could be that you'll prefer 172.5 although the difference is pretty subtle to feel. In general you don't need 175 on a road bike unless you're well over 6' tall, depending of course on actual inseam.
re: 175 vs. 172.5 cranksjvr-oclv
Feb 4, 2002 9:53 AM
I am 6'3" with inseam of 38". 2.5mm = .098 in. Not much diff. between the too.
re: 175 vs. 172.5 cranksnipfest
Feb 4, 2002 9:56 AM
Velonews had an interesting article concerning power output vs. crank arm length some months back. They might still have the article on their website. Basically, their "study" didn't find any great advantages for longer or shorter lengths.
re: 175 vs. 172.5 cranksgrzy
Feb 4, 2002 2:35 PM
It will be like having slightly stiffer gearing 2.5/175 = 1.4% which is about 1/4 of the difference between a 39/27 vs. 39/28 gearing change. You'll have a bit more clearence in pedaling through corners if this matters to you. Many of us will run a 175 on a MTB and a 172.5 on a road bike and won't really notice the difference. Unless you're legs are really long (or short) it shouldn't matter much. I'd be more concerned in the age of the DA cranks.
re: 175 vs. 172.5 cranksIan
Feb 4, 2002 6:22 PM
Someone on this board once said, "Stack two credit cards on top of one another, that is 2.5 mm." I don't remember who said it, but I think those are wise words.

I've had others argue with me, but I maintain that if I built up a bunch of bikes with 172.5 mm and 175 mm cranks and had riders try them out, they would only be able to guess the correct crank length 50% of the time. And that is just because they would have a 50% chance of guessing correctly.

Anyway, through all that, what I am saying is go for the deal on the D/A.
FWIWgrzy
Feb 4, 2002 6:34 PM
I just happen to have a set of clipers right here. Two credit cards is 1.55 mm, but if you stack them up on the embossed characters it's much closer at 2.37 mm and wear makes a difference. Anyways - your point remains valid - 2.5 mm isn't very much and the symbolsim of having to pay a high price (hence the credit cards) for a small change isn't lost on me.
FWIWIan
Feb 5, 2002 8:09 PM
Yes, I checked the measurement also. Not quite as precise as you, I just used a normal ruler to make sure it was close. And the embossed numbers do need to be there, that is why I didn't say a drivers license.

And, as much as I would like to take credit for being creative, the symbolism was not intentional.
175 mm gives more leverage on hillsPaul
Feb 5, 2002 4:02 AM
You have to look at the circumference which is really 5mm more (175 vice 172.5). I have both, and really don't notice the differnce. Other people tell me a shorter crank is better in the sprints, makes sense.

i would re-adjust my seat height going to a longer crank.
Fuzzy Mathgrzy
Feb 6, 2002 5:10 PM
Back to geometery class for you.

Circumfrence is the total arc length of a circle (i.e. 360 degrees) and is:

C=pi x 2 x r

Thus the difference in *circumfrence* between 172.5 mm and 175 mm crank arms is 3.1415 x 2 x 2.5 mm = 15.71 mm. The circumfrence mearly describes the path length that the pedal spindle must travel in one revolution. The ratio between the two circumfrences is equal to the ratio between the radaii since there is a direct linear relationship and constant terms.

In any event the circumfrence is NOT the key - it's the force times the lever arm, also known as torque T = F x r which determines the "leverage on the hills". A confusing thing in the gearing analysis is that people will use the number of teeth on chainrings and cog sets since there is a direct relationship to radius/diameter and it's easier to quickly count teeth than it is to get the effective radius of a chain/sprocket combo.

Your poiint remains valid that a 175 crank arm will give more leverage in the hills, but then so would lower gearing. Ultimately you need to understand the biomechanics of the rider - you probably wouldn't advise a 5'0" woman to run 175 (or larger) crank arms if she had trouble getting up a hill.
Fuzzy Math_Marty_
Feb 7, 2002 5:30 PM
I recently went from a 170mm to a 175mm i do notice a difference on the hills, its much easier to spin a tad bigger gear.. and the flats.. i cant say theres a difference... i still spin roughly the same cadence in the same gear... and guess what? roughly at the same speed.. who would have thaught.. hehe nab em whilst u can!
Dohgrzy
Feb 8, 2002 10:45 AM
Changing your crank arm length doesn't change your cadence - one revolution of the crank, no matter what the length, will still give you the same amount of rotation on the rear wheel if the gearing is the same. Your speed will be *exactly* the same, but you should've known this. What varies is the force (i.e. torque) applied which has the effect of a different gear. It's all first year Physics stuff.
re: 175 vs. 172.5 cranksWoof the dog
Mar 10, 2002 10:35 PM
Every milimeter counts when you pedal into a turn. Maybe!

Woof, the racing dog.