I am 5'6, 130lbs. My bike is 52cm. I found a good deal for my tripple crankset 9 spd, but its only for 172.5 mm. Can i use this or i rather stick with 170mm? Will it change any of my performance if any at all?

You will sit lower on the bike, you will lose the ease of spinning a smaller crank will give you. You will have readjust the way you sit on the bike (more towards the rear).
For your height maybe you should try it first on someone's bike before you consider doing so. I'm 5'8" and I did convert from a 170 to a 172.5 and do like it, but I do notice the lost of a little ease of spinning, yet on the hills I gain a more satisfactory comfort for climbing with the larger crank. So it's a give and take thing.

Actually, if you make the right adjustments you will sit slighly forward and higher than with your previous crank.

The idea of moving higher and forward (but same total distance to the BB as previous seat position.) is to reduce the amount of knee bending that would normnally happen with a longer crank. Moving forward also opens the hip angle which normally closes with a longer crank.

P.S. The only way to get a higher seat without increasing distance to the BB is to move it forward also. (higher seat increases distance to the pedals, but forward seat decreases distance to the pedals...so they cancel out but you get a higher "knee saving" seat in process). Of course after you do this you may or may not have to make a slight stem/handlebar change depending on how much pressure is on your hands

The important measurement to consider is your inside leg measurement. See, https://www.cranklength.info/ and check the crank length calculator. If 170/172.5 is about right for you sa small difference wont be detrimental. If you realy should have a shorter crank and your going longer then this wont be benificial.

Regards, AnthonyG

Yeah the femur length matters because the femur/tibia ratio affects how much the knee bends at the top of the stroke and the hip angle.



But you still need to understand guys with "short" 28" inseams are getting away with using "long" 170mm cranks becasue the small size frames use crazy 76 degree seat tube angles, etc which moves the seat massively forward and consequently lets them place their seat high enough so they don't have knee problems.

With longer cranks you will need to reach an extra 2.5mm to the pedal at BDC so the seat has to come down not up. Your knee will rise 2.5mm more at TDC making it more difficult to maintain a low crouch. Moving forward will help with leg reach but will compromise the amount of power you can deliver and you may need a longer stem to compensate.
My advice FWIW; stick with 170.

Yes....The seat will need to come down if you don't properly compensate (because the pedal is now lower at the bottom of the stroke). If you understand anything about how moving a seat forward will decrease distance to pedals then you will understand how you can now raise the saddle.

If the longer crank forces the knee to bend more at the top and the hip angle to become more narrow then you will lose power.

I don't know why you think moving the seat forward will make you lose power (especially with longer crank)? It will change the positioning on the bike though (definitely)

P.S. Every short legged person riding a small bike already moves the seat forward (they may not know this) becasue their frame does this for them through the crazy steep seat tube angle. Steep seat tube angles position seat forward.

For me (36" inseam), a measly 1 degree change in seat tube angle equals a whopping 1.5 cm change in the fore/aft of the seat. Most short legged folks are riding bikes with as much a 4 degrees steeper seat tube angles than I ride. That's a huge relative difference and explains how they get away with using "proportionally long cranks"

This makes no sense. Remember that we are moving seat forward, but raising it too. How does opening the hip angle and reducing knee flex at the top of the stroke reduce power?

It's 2.5 mm. That's the thickness of a stick of gum. It will make almost no difference at all.

2.5mm is bugger all, and hardly worth it.

These crank length sites are very interersting, but they are not the be-all and and-all of the discussion, and they pretty much say that themselves. Lennard Zinn is probably the most adamant that his proportional ideas are correct for everyone, but the other most often quoted sites (https://www.cranklength.info/ and https://www.nettally.com/palmk/crankset.html) present their ideas as just theories, although they do back their stuff up with biomechanical research.

this is a quote from the Kirby Palm site:

"While I have the utmost confidence in my crank length formula, I do not claim any special expertise in the general setup of a bicycle for a rider. In fact, I had been postponing creating this page waiting for someone who knows better to set one up."

So, before we start quoting these guys as providing us with any hard and fast "rules", just remember that no REAL HARD evidence has yet surfaced that suggests these formulas are anything more than very good ideas backed up by some reasonable data.

As I said on the other thread, this link is probably the most balanced that I've found:

https://www.bsn.com/Cycling/cranks.html

[Following is a quote from the link above]

"Going to 180mm (or longer) cranks, should one be able to effectively use them, requires raising the bottom bracket, possibly a steeper seat-tube and longer top-tube and/or stem--- which is why this works best in time-trials using a variant of the Position Americain. The downsides on handling of shifting weight to the front combined with less bottom bracket drop (what the author has called a "cross between a tri and a cross bike") could well, in road competition, offset any depreciation in efficiency of using shorter cranks."

They make good points here and it backs up what I been saying all along. (longer crank needs steeper seat tube angle w/longer stem and top tube [like a small riders bike])

Well, I never thought that half of a centimeter would make that big of a deal. Heck, my left leg is probably 1cm shorter than my right leg!

Most of this talk applies to more extreme cases (not yours)

2.5mm is a drop in the bucket.

Isn't 2.5mm a quarter of a centimeter?

Yeah, we'd have to say that bit of the article is wrong

Sorry, sorry! my terrible math skill.
Now you know why I was the only Asian kid flunked the Math test!

I thought that article you dug up at bsn.com made some good points about the steeper seat tube angle and longer top tube/stem.

I went through this same thought process around a year ago. The crankset that I wanted was only immediately available in 175mm rather than the 172.5mm that I was used to. I've ridden the bike with the longer crank for a full year now and as soon as I figure it out I'll tell you about any performance difference that I can sense.

More leverage will help you climbing.
But it means less clearance if you like to pedal through corners.

But MerckzMad is right.

Yeah 2.5mm amounts to about 1.5% increase in length.