bres dad

What differences would a person notice moving from say a 172.5 to a 170mm crank arm (obviously shorter)? Are there advantages, disadvantages? I'm changing out my crank from a triple to a double (both compact). One crank I'm looking at has 170mm arms. I can get it pretty cheap which is why I'm considering it. Im' not necessarily looking for shorter arms but wondering what differences I would expect or if I should just pass and look elsewhere?

FBinNY 

Most people would never notice a difference unless told. Serious riders, with many miles under their belts, especially folks who spin at a high cadence may notice and will have a preference.

As a rule, taller people can (but don't need to) ride longer cranks, while shorter people will prefer shorter ones. The preference also relates to pedaling style, with spinners tending to prefer shorter cranks, and low cadence "pushers" preferring longer cranks.

Andrew R Stewart 

And to Francis's well done reply I'll add that I known many riders who had, for what ever reason, different arm lengths on the same bike and didn't notice.

I am one of those people who are sensitive to arm length (and "Q" factor). ALL my bikes get 170s. But my wife and her closest riding friend both go between 165 and 170 among their various bikes without complaint. Andy.

Bill Kapaun

I have a limited range of motion in one knee.
I need 165's, else cadence suffers.
175 MM 60 RPM and my foot/leg is thrown off the pedal at the top of the stroke.
170 MM 80 RPM.
165 MM 85 RPM.
160 MM 80 RPM- Just too short for me and definitely felt that way.
Once upon a time I was almost 6'1"
167.5 would probably work, but availability in a mountain style crank for low $???? not likely.

jyl

Mark Cavendish (famous pro road racer) rode most of the 2013 Tour de France with mismatched crankarms before noticing . . . although I imagine some mechanic's day was ruined when Cav did finally realize it.

Bill, look at these cranks that are available in any length you could desire

Product Description | Origin8
Product Description | Origin8

While some of these are intended as single speed cranks, you can mount double chainrings using the appropriate chainring bolts and spacers, and a suitable BB spindle width. I learned this when looking for a decent alloy double road crank in 160mm for my son's junior road bike. The crank took 50/34 T chainrings and shifts fine with a standard road FD.

fietsbob

longer crank arms have you lowering the saddle . (by that much)
I have 170 to 180 cranks on various Bikes , i use them interchangeably.

bres dad

When I bought a new MTB earlier this season as I got used to it I noticed it seemed like my knees were coming up farther than they did on my other bike (then claimed by my wife as her own forcing me to get another, which I like better anyway). I compared the two and sure enough, the old was 172.5 & the new is 175. Wondered if I'd notice the difference going from 172.5 to 170. I've done more research and while opions & experience vary, seems like for sprinting & cadence more people tend to like the 170 especially with shorter legs (I'm 5'10 & 30" inseam). I think I'll try it. Worst case if I utterly hate it, I can flip it and get something different. Price is right too.

Reynolds 

I have 3 bikes with 170 and another with 175, don't notice any differences.

scott967

What I remember is it seems like 170 was the "normal" size. I had a bike built in 89 and put 172.5 on it which was considered "long". At the time I think it was a big deal that Indurain rode 175s and only "Big Mig" could possibly use such long cranks. Nowadays seems like long is in, or at least much more common. Sort of like how 52/42 morphed to 53/39.

scott s.
.

BentLink

I've got both 175 and 172.5 on all bikes but one. I cannot sense any difference at all. My Pug has original MAVIC cranks with 170's, and those feel a a teeny bit different, but I'm quite comfortable. Really, 5mm range is about the difference of sock thickness. I reckon you'll need more variation to see any sensible benefit or problem.

Fred Smedley

I am out of the "Normal" being 5'10" and wearing size 13's liking higher cadence I like 175 or 172.5 as a minimum. I definitely notice a difference between 170-172.5-175. 170's feel like a kiddie bike to me.

e_guevara

I've been on 170 mm cranks for 10 years until I bought a 172.5 mm (it was available and cheap) for the other bike. I alternate between the two bikes regularly and don't notice any difference. I've since replaced the other with 172.5 mm as well for the sake of uniformity on my bikes.

Bill Kapaun

They only come in 5mm increments. I've already done that.
Trying a 2.5mm different crank would entail it having to be VERY inexpensive.

Bill Kapaun

You must have very thick socks!
Keep in mind, when you go to a 5mm longer crank, you lower the seat 5mm.
When at the 12 o' clock position, the pedal is now 10mm (.4") closer to your butt.

SHBR

I prefer 175mm cranks. I notice a difference in leverage off the line at stoplights compared to 170mm cranks.

Although I can totally understand how some people might never notice, leg length and riding style are considerable factors to take into consideration.

noglider 

I was very skeptical that people could notice such small differences in crank lengths, but then I discovered that I'm very sensitive to it. When I try to ride a bike with 175's, it just feels all wrong. This happened long before I even thought to check crank lengths. I even noticed my sister in law's bike feels a little funny. (She lets me ride it when I visit my brother in law, and sister in law and I are the same size.) I looked down, and look, she has 172.5's. I can feel a 2.5mm difference in crank length? Well, I guess so!

krobinson103

I can't tell the difference between 170 and 175. I think I even have some 172.5 cranks somewhere in the fleet but I probably just adjust the saddle to compensate.

noglider 

But hmm. I've been wondering why I can't quite get comfy on my McLean. Well waddya know, it has 167.5mm cranks. I don't know if that is the reason or even a factor, but it's time for experiments.

Peter White makes a recommendation vaguely in favor of long cranks. I don't have the URL handy. Maybe the trend of "longer is better" is a good one. On the other hand, if you ask @rhm, you'll get another viewpoint.

reddog3

I just recently read an article on crank length, with supporting data, which stated max power output is achieved with a crank length of 140.

I tried searching it again, can't find it. But came across this interesting article- Crank Length: Coming Full Circle - Cycling Utah | Cycling Utah

Might raise some eyebrows, but there are benefits to shorter cranks. Depends on your usage though. If you're just tooling the neighborhood on your hybrid crank length is not important.

rhm

Yeah, okay, I'll bite.

Longer cranks give you more leverage, which is what you need for powering over obstacles at low cadence. So on a mountain bike, like, when you're actually riding on a mountain, it often happens that you hit a root or something you weren't quite expecting, it knocks you off course, and now you're obligated to getting over a rock you had planned to miss. This happens at like 5 or 8 mph. You need long cranks to get over obstacles like that.

If you're not riding on mountain, you don't need them. On roads they offer you no advantage. On the contrary, longer cranks require your legs to move more, but the additional motion is at the extremes of the range of motion where you don't have that much strength anyway. Moving your foot around a longer radius takes longer, requiring you to use a higher gear, in which it is more difficult to accelerate quickly. So in general, shorter cranks are better on road bikes.

It doesn't really matter how tall you are. Tall people and short people and very short people all walk up the same flight of stairs. While I'm sure shorter people would benefit from shorter cranks, the industry doesn't offer many cranks in short options (165 is not, in my opinion, short; it's only the shortest available).

I'm 6' tall and have ridden extensively with 140, 145, 152, 160, 165, 170, and 171 mm cranks. I also tried 127's, 172.5, and 175's though not as much. I got used to all of them, and found nothing uncomfortable or inefficient about even the really short ones. I rode thousands of miles on the bike with 140's. The only problem was that when you put short cranks on a bike made for long ones, you have to raise the seat; the crank ends up unnecessarily high off the ground, which gives you no advantage while riding, and a disadvantage when you try to put your foot on the ground. I eventually settled on 165 mm cranks because they're widely available and that size suits the old bikes I like to ride.

I think I read that, too, a while back; and like you i can't find it now. If you do find it, I'd appreciate if you let me know. Thanks!

Bill Kapaun

Interesting about the "leverage" comments.
Don't these people know why they have all those gears?

rhm

Did you click the link to the Cycling Utah article that Reddog3 posted? It went into the connection between crank arm length and gearing.

Leverage only becomes an issue at very low cadence, when I presume you're already in your lowest gear. This came home to me a few years ago when we got a March snowstorm during the day, and on my evening commute I found myself hammering through 3" of wet snow on my folding bike with 16" wheels and 152 mm cranks. My lowest gear is only faster than walking as long as I can maintain a decent cadence, and if my cadence drops below maybe 60 rpm, I might as well be walking. I hammered through it, much to the detriment of my crank; ruined the tapers on the drive side. A longer crank would have been easier on me and, I suspect, easier on the machinery as well. Could be the bolt just wasn't tight enough.

FBinNY 

The following is opinion, so give it the weight it deserves.

I keep hearing the "leverage" argument regarding longer cranks, and think it's nonsense, for two reasons.

1- we're talking of crank length changes of 1.5-3% or so, which is much less than the typical 10% step in gearing, so even buying the leverage argument, we need to keep some perspective.

2- since bikes are geared, we can achieve ANY leverage wanted through the gearing and there's no need to try to buy another 2% through crank length.

The issue of crank length relates to leg length, and the height of the knee at the top and bottom of the stroke. The effects of a long crank on someone with short legs is immediate, as they find their knees rising too high even with the saddle as high as possible. OTOH this is of little importance to folks with longer legs, but a longer crank may feel better by giving them more knee an thigh movement which optimizes knee angle at the center of the power stroke (pedal horizontal).

Since we all set saddle height based on the bottom of the stroke, crank length allows us to improve (hopefully) the leg angles at the top and mid stroke positions. Again, it's a small change, and casual riders won't notice, but long time riders with good cadence may notice a change because the foot isn't turning the circle that their muscles remember.

So use the crank length that best suits your leg geometry and pedaling pattern, and get the leverage from the gearing.

Bill Kapaun

If your cadence drops below 60 RPM, you need a lower gear.
It simply boils down to the distance the pedals move per revolution vs the distance the bike moves.
A longer crank means the pedals move a greater distance in one revolution, thus a lower gear.
Leverage wise, a 10% longer crank with a 10% higher gear results in equality.
Thus you use the crank length that best physically works for your leg length/physical condition.
Trust me, when you get old, the knees don't bend so readily.
Chances are, using too long of a crank when you are young will accelerate the onset of "bad knees".

rhm

I take it as a given that a longer lever has more leverage. A marginal increase in crank length must give a marginal increase in leverage. Have I misunderstood what leverage is?

When I described the extremely limited circumstances where such leverage is advantageous in cycling, my implicit point was: in most cycling situations, leverage is irrelevant.

Yes, I meant that to be implicit in the story I told.