175 vs. 172.5 cranks

Here are a few links W/discussions regarding crank arm lengths:

http://www.bicycle-forum.com/tech/Pros_and_Cons_of_Crank_Length_170_vs_175_436180.html

http://www.roble.net/marquis/crank.length

http://www.cyclingforums.com/showthread/t-14581.html

http://www.bikeforums.net/showthread.php?t=8916

Me? I'm 5'11" with a 33" inseam, and use 175's.

Quoted post said:

Originally posted by bowcop
I just ordered a new frame, and after quite a bit of measuing, my local bike shop advised me that I should have 172.5 cranks vs. 175. My last bike hade the 175 but I may have had the wrong ones, but he told me that with my size, I am on the edge and could go with either. My questions is, all things being equal, what are the advantages and disadvantages of the 2 sizes. Thanks.

It's more personal than scientific,espeially if you are between 'sizes'. I ride 170, 7.5 and 75s on different bikes,and an not sure I can tell the difference. Others swear a 2.5mm difference makes or breaks em.

Quoted post said:

Originally posted by bowcop
I just ordered a new frame, and after quite a bit of measuing, my local bike shop advised me that I should have 172.5 cranks vs. 175. My last bike hade the 175 but I may have had the wrong ones, but he told me that with my size, I am on the edge and could go with either. My questions is, all things being equal, what are the advantages and disadvantages of the 2 sizes. Thanks.

The advantage of a longer arm is more torque for a given force, but a larger circle to pedal through....meaning maybe a bit harder to spin at high cadences. But, you're only talking about 2.5mm, or 0.1 inches difference....not much.

I don't think either one is "wrong". Would be hard to tell the difference, as boudreaux said, and either way the work done is the same. I'm 6', with an 86 cm leg length, and rode 175 for the last 12 years...so that's what I put on my new frame.

Quoted post said:

Originally posted by dhk
The advantage of a longer arm is more torque for a given force, but a larger circle to pedal through....meaning maybe a bit harder to spin at high cadences. But, you're only talking about 2.5mm, or 0.1 inches difference....not much.

I don't think either one is "wrong". Would be hard to tell the difference, as boudreaux said, and either way the work done is the same. I'm 6', with an 86 cm leg length, and rode 175 for the last 12 years...so that's what I put on my new frame.

The difference really is twice that because we pedal in circles. The diameter of our pedal stroke is longer by twice the amount of the differences in the crank length between, say, a 170mm v 172.5. Therefore in this case it is 5mm (or 0.5cm or around 1/5"😉.

Some say this difference altho how slight it may seem can affect your cadence, pedal stroke and cause some muscle and ligature problems specially if you have been using a specific crank length over a period of time and then suddenly switched.

Note: Also when switching to a longer crank (assuming u dont change ur cycling shoes and pedals) one should also consider adjusting saddle height.

Quoted post said:

Originally posted by dennis dee
The difference really is twice that because we pedal in circles. The diameter of our pedal stroke is longer by twice the amount of the differences in the crank length between, say, a 170mm v 172.5. Therefore in this case it is 5mm (or 0.5cm or around 1/5"😉.

Yeah, but the lever arm is still only 2.5mm different.

Even if its only 2.5mm. To illustrate: A car's engine has pistons, piston arms and crank. The up and down motion of the piston/arms to drive the crank is called the stroke. Change the stroke of the same engine by using a longer piston arm will effect the engine's efficiency. Universally, a longer stroke (piston travel) will result in a lower maximum RPM redline versus a shorter stroke. But a longer stroke may also yeild higher torque, so on and so forth. If this can happen in an engine, think of how it can also affect us.

Quoted post said:

Originally posted by dennis dee
Even if its only 2.5mm. To illustrate: A car's engine has pistons, piston arms and crank. The up and down motion of the piston/arms to drive the crank is called the stroke. Change the stroke of the same engine by using a longer piston arm will effect the engine's efficiency. Universally, a longer stroke (piston travel) will result in a lower maximum RPM redline versus a shorter stroke. But a longer stroke may also yeild higher torque, so on and so forth. If this can happen in an engine, think of how it can also affect us.

Really?? So?? The 2.5mm difference in the LEVER ARM is still the critical part. And yeah, tanx for the reciprocating engine tutorial. I'm so stupid I really needed it.

Why else would they make it in different sizes in increments as minute as 2.5mm!

I use 177.5 and 180 mm and can feel the difference.

Quoted post said:

Originally posted by dennis dee
Why else would they make it in different sizes in increments as minute as 2.5mm!

Well certainly one length may work better than another.But now you are getting even further from the issue with a reply like that. In the posters case,all the theory and AR formula answers correct to 4 singificant figures may or may not apply to him. Once you are in the ballpark,then it becomes personal and subjective. It's like the never ending question of 'what gearing do I need', or 'how high should my bars be'?? Answer,who the hell knows,but the guy/gal doing the pedaling. Sometimes one has to either go with the general recommendation and be happy or pay the price to experiment. In the posters case the only way he may really know is to pony up for both lengths and try them extensively.

As many here have said, it's personal and no amount of theory is going to answer the question conclusively. But my two cents would be as follows: if you like spinning at a high cadence, go with the shorter crank, whereas if you like to crunch bigger gears at a relatively low cadence, go with the longer one. If, as the LBS guys have said, you are "between sizes", you can't really go far wrong in either case.

I changed crank lengths when I purchased a new frame. I am 175cm tall (5'9"😉 but I have long legs for my height with an inseam of 86cm (~34"😉.
So when I changed bike i decided t go from 172.5mm to 175mm. and I find tha the longer length suits me better. I don't spin a great deal but it hasn't stopped me from being able to do so. It is particular to the preferences of the individual. I tend to prefer TTs and Road races (though I still ride a lot of crits because that's all we got round here) and I think the longer cranks tend to suit more.

Quoted post said:

Originally posted by boudreaux
Well certainly one length may work better than another.But now you are getting even further from the issue with a reply like that. In the posters case,all the theory and AR formula answers correct to 4 singificant figures may or may not apply to him. Once you are in the ballpark,then it becomes personal and subjective. It's like the never ending question of 'what gearing do I need', or 'how high should my bars be'?? Answer,who the hell knows,but the guy/gal doing the pedaling. Sometimes one has to either go with the general recommendation and be happy or pay the price to experiment. In the posters case the only way he may really know is to pony up for both lengths and try them extensively.

I dont believe that there is no "science" to it. Its not a case of crank manufacturers produce these lengths to suit your individuality. A cycling coach i once spoke to recommends that a male cyclist whose height is 5'2" to 5'5" and an inseam of 'x' (i now forget) should not use a crank more than 170mm. Cyclist whose height is less than that = 165mm or less and so on and so forth.

I also once read an article about a software used by frame manufacturers and those in sports medicine called BIO-RACER that tackles different sizing related issues for optimum bike fitting. It not only provides the graphical info for optimum bike config but also optimum stem length, saddle height and, yes, crank length for a number of measurements, variables and preferences provided to it.

Quoted post said:

Originally posted by dennis dee
. A cycling coach i once spoke to recommends that a male cyclist whose height is 5'2" to 5'5" and an inseam of 'x' (i now forget) should not use a crank more than 170mm. Cyclist whose height is less than that = 165mm or less and so on and so forth.

Soooooooooo? it's a RECOMMENDATON! And, FYI,thre are 167.5 cranks.

Quoted post said:

Originally posted by boudreaux
Soooooooooo? it's a RECOMMENDATON! And, FYI,thre are 167.5 cranks.

Goody you can name them! What a smart guy!

160mm
162.5mm
165mm
167.5mm
170mm
172.5mm
175mm
177.5mm
180mm

Quoted post said:

Originally posted by bowcop
I just ordered a new frame, and after quite a bit of measuing, my local bike shop advised me that I should have 172.5 cranks vs. 175. My last bike hade the 175 but I may have had the wrong ones, but he told me that with my size, I am on the edge and could go with either. My questions is, all things being equal, what are the advantages and disadvantages of the 2 sizes. Thanks.

It may be a marketing ploy on the part of your LBS.

The standard issue on group sets sold with bikes is 172.5mm. Here I speak for Australia and it probably applies elsewhere.

I use 175mm cranks and on one occasion I purchased a new bike that was in the LBS's inventory. The LBS had to specifically order a 175mm crank and was left with a 172.5mm crank to get rid of through a future sale.

Quoted post said:

Originally posted by bowcop
... my local bike shop advised me that I should have 172.5 cranks vs. 175. My last bike hade the 175 but I may have had the wrong ones, but he told me that with my size, I am on the edge and could go with either...

That 'edge' is actually a hazy guideline.

I don't know your measurements or anything else about your specifics, but what I can tell you is that it is my personal opinion that you are probably better off going a little longer than shorter where this is concerned, especially if your LBS is using a conservative formula for making the calculation.

One tip: a few years ago I went from 170 to 175, then went out hammering, and subsequently stuffed up one my knees. The physio guy said it was bursitis, but I dunno. Anyway, I put the 170s back on and it took A YEAR for my knee to get back to normal.

I did prefer the 175s -- I definitely felt as though I had more torque, but with 170s I feel like I can accelerate a bit quicker, and you can pedal around corners without your pedal hitting the road😄 (actually a serious consideration if you're doing criteriums).

Anyway, what was the question ? Oh yeah....I would notice a difference of 2.5mm regarding feel, but I wouldn't notice a power difference.

If I were you I'd stick to the 175s if that's what you're used to. Also, you can convince yourself that you're getting more leverage.

Quoted post said:

Originally posted by dennis dee
Goody you can name them! What a smart guy!

160mm
162.5mm
165mm
167.5mm
170mm
172.5mm
175mm
177.5mm
180mm

Go ahead,why should I do the homework for the 'real' smart guy.