# The difference between 170, 172.5 175 crankset lengths?



## NYC_CAAD (May 4, 2011)

I have been planning on buying a set of SRAM red 22 50/34 exogram crankset BB30. Can someone please explain the different lengths and why. (170, 172.5 175) I have no idea.

169 LBS
5'10
80 mile rides on weekends.

Thanks..


----------



## Wookiebiker (Sep 5, 2005)

There are a lot of different thoughts on crank arm length ... and all of them depend on your basic philosophy and how sensitive you are to fit changes.

My personal thought is you choose your crank length based on leg length, and more importantly femur length. The longer the femur, the longer the crank ... the shorter the femur the shorter the crank.

The reason for this ... Longer cranks with shorter legs will make for a poor hip angle. On top of that having shorter femurs a shorter crank will bring the pedals back toward the saddle, which can help compensate for Seat Tube Angle. The opposite applies for longer cranks with longer legs/femurs.

For others ... they will tell you to ride what ever came with the bike.

Others might tell you that you can spin shorter cranks easier than longer cranks, but the longer cranks will allow for more torque ... which is partially true.

In the end ... find what works for you and go with it.

Personally ... I have short femurs and am 5'11" tall with a 32.5" inseam ... I ride 170mm cranks on all my bikes.


----------



## Mapei (Feb 3, 2004)

For me, it's a matter of how high my knees go on the upstroke. Though none of the lengths are outright uncomfortable, I can never get my knees quite flexed enough with the shortest crank length, and the longest length flexes my knees a little too much. 172.5 is just right. BTW, I'm not a big guy. My pants inseam is a modest 30 inches. I'm about 5'7". 

A lot of people will tell you that the crank length differences are tiny and inconsequential. But after riding over forty years, and having had (and still having bikes with) the three common crank lengths, I'm convinced the differences matter. It also must be asked why the heck the component makers offer varying crank arm lengths if it didn't make a difference? BTW, my wife's old 165 length cranks felt silly.


----------



## Flbikejunkie (Mar 18, 2012)

I am 5'9" and prefer the 172.5. The 175 can have some toe overlap with the back of the front tire at slow speeds riding through narrow areas such as the driveway/ parking lots


----------



## Salsa_Lover (Jul 6, 2008)

I am 5'11" shorter legs than torso, I ride a 172.5

Some people say the 2.5 mm make no difference others say it matters a lot.

For me I have tried the 3 sizes and simply the 172.5 feel better


----------



## spdntrxi (Jul 25, 2013)

I'm 5'8" with 30.5 inseam and have all lengths available to me (ZED2 cranks).. 172.5 just feel better to me


----------



## joeyb1000 (Feb 15, 2011)

I'm 5' 10" and ride 170mm. Many people use 172.5mm. Either one would be fine.


----------



## eflayer2 (Feb 15, 2002)

depends. I rode 170 on my 3 bikes for a few years and all was fine. Found a good deal on a 172.5 and thought lets try that. It felt different and at first I thought I had more power. But after 500 miles it seemed I was getting knee pain on a regular basis. Switched it back to 170. Won't ride anything else again. I just works for me. Oh I am 5'11 with pretty normal proportions.


----------



## NYC_CAAD (May 4, 2011)

Wow! Great replies guys.. 

Just checked my cannondale (size 54). The cranks installed are 172.5 with shimano 105 pedals. Seems that i have the same problem as "eflayer2" -----> knee pain on a regular basis. And have been trying to figure out "why" the pain. I usually ride with hex keys for testing my handlebars and length on the saddle hoping that will help..


----------



## wim (Feb 28, 2005)

NYC_CAAD said:


> The cranks installed are 172.5 with shimano 105 pedals. Seems that i have the same problem as "eflayer2" -----> knee pain on a regular basis.


But before you blame the crank length for that, keep in mind that the difference between a 170 mm crank and a 172.5 mm crank is a measly 2.5 mm—the height of a stack of two dimes. If your saddle height is even close to reasonable now, the likelihood of getting knee problems from that 2.5 mm crank difference is not very high.


----------



## tom_h (May 6, 2008)

NYC_CAAD said:


> ... Just checked my cannondale (size 54). The cranks installed are 172.5 with shimano 105 pedals. Seems that i have the same problem as "eflayer2" -----> knee pain on a regular basis. And have been trying to figure out "why" the pain. I usually ride with hex keys for testing my handlebars and length on the saddle hoping that will help..


More likely to be:
- saddle height (5mm changes can be significant).
- fixed rotation cleats not having any rotational "float"
- leg length differences requiring spacer shims or cants.


----------



## dkilburn (Aug 1, 2009)

Maybe you could think about getting a good bike fitting?


----------



## Kerry Irons (Feb 25, 2002)

NYC_CAAD said:


> I have been planning on buying a set of SRAM red 22 50/34 exogram crankset BB30. Can someone please explain the different lengths and why. (170, 172.5 175) I have no idea.
> 
> 169 LBS
> 5'10
> ...


The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks tend to mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard.

There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.
=============================
Fred Matheny Summary: There have been studies of crankarm length, but the results aren't consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual.

So, longer crankarms aren't a panacea for time trialing. In fact, there are dangers associated with them. The added length makes your knees bend more at the top of pedal strokes and extend more at the bottom -- both of which can lead to biomechanical injuries if you jump from 170 mm to, say, 180 mm.

Also, longer cranks reduce cadence -- and a brisk cadence is the key to good time trialing.

All this said, many time trialists use crankarms 2.5 mm longer than those on their normal road bike. Because 2.5 mm (one-tenth of an inch) isn't much, it rarely causes an injury. But the jury is still out on whether that bit of extra length actually improves performance.
=============================
Jim Martin tests as reported in VeloNews: 16 bike racers of various heights doing maximal sprint power tests of under four seconds duration on cranks of 120, 145, 170, 195, and 220mm showed no statistical difference between crank lengths. Seat height to the pedal was maintained throughout, but fore-aft saddle position and handlebar height were not readjusted with crank length changes, despite variations with crank length of pedal-to-knee relationship and saddle-to-bar drop. This also led to Martin’s assertion that he could see no point to positioning the knee over the pedal spindle.

Further Martin tests showed no statistical relationship between metabolic cost and either pedaling rate (RPM) or crank length, using nine trained cyclists riding 145, 170 and 195mm cranks who pedaled at 30-, 60-, and 90 percent of their lactate threshold at 40, 60, 80 and 100 RPM. On the contrary, power output and pedal speed (pedaling rate times crank length), accounted for over 98 percent of the variation in metabolic cost.

In another test, Martin had 10 racers perform a 30-second maximal sprint on 120mm and 220mm cranks at 135RPM for the 120mm and 109RPM for the 220mm. he found that, while the rate of fatigue was less for longer cranks, the fatigue per revolution was identical. This led him to suggest that track sprinters, rather than spinning at high RPM, should select the gear at or just below the one at which they produce maximum power output. The higher gear, as fatigue per revolution would be constant, would get the rider to the finish sooner, as fatigue would take more time to set in.


----------



## AvantDale (Dec 26, 2008)

So basically just go with what feels good to you.


----------



## CliveDS (Oct 7, 2009)

This is something that has intriuged me for some time, I asked a friend of mine who is in the know and had him write something for my blog. 

See here: Glory Cycles Product Reviews: Which Crank Length is Right for You?

Bottom line: Shorter list better if you are trying to make a choice but it's no dealbreaker either way.


----------



## eflayer2 (Feb 15, 2002)

there is much written in the recumbent community about using shorter cranks. on a bent you can't stand up so all pedaling is done with your back supported by the seat. longer cranks in that situation force your knee to sometimes bend beyond the comfort zone. many many many bent riders ride with cranks as short as 155; even to the point of shortening normal cranks and retapping a hole for the pedals. i do believe a couple of mm's can make a big difference to those with sensitive joints or any sort of knee issues. i have no scientific evidence, just know my i am more comfortable with shorter cranks than many.

consider the number of variables in the human body ie like how the heck do all those parts work together? and maybe, just maybe a crank length experiment will make a difference in how YOUR parts all work together.


----------



## c_h_i_n_a_m_a_n (Mar 3, 2012)

Crank lengths? Is one of those components that you are not able to easily test out before deciding. You could just go out on ebay to get a few different lengths of the Force crankset and try them out? That way you are really trying them out for yourself.

I think your physical measurements do matter. Your riding style (cadence) matters as well and I think this will depend on your muscle make-up.

But it is also a factor that seem to not matter so much to most manufacturers out there as they seem to group a large proportion of us into the same group.

I just went with what a couple of online sites recommended, which was 2.5cm shorter than what came on my MTB. I am not able to tell the difference, and the geometry on both my bikes was very different and so any difference will not be accurate.

I think no one here can fully convince you unless you try it out for yourself. I wanted to get the 3 different lengths through ebay to try it out for myself but that would mean getting cheaper cranksets and then having to sell them on, but I just wanted to ride my 'new' bike. I thought that for my recreational use, it was just an experiment which would not reveal effective information to me.


----------



## MR_GRUMPY (Aug 21, 2002)

I'm 6'2" with short legs. When I switched to 175's, years ago, it took me a month to get my spin back. I now run out of spin around 150 rpm.
.
.


----------



## bitgeek (Apr 2, 2010)

I'm a dirty old mountain biker from way back and always ran 175mm cranks. I'm about 5'9" with a 32" inseam. I've been spending a lot more time on my road bike the last several years though and didn't realize how low my cadence was (high 70's) until I put on a computer that gave me that data. After focusing on increasing my cadence I didn't have much success getting above the low 80's. I had a discussion with a friend who is a wrench about this and their feeling was that longer cranks on mountain bikes made sense as lower cadence was the norm. However, you want a higher cadence on the road and in that case shorter cranks make more sense. I took their advice and went to 172.5's and I've seen my cadence come up into the mid 90's this year. It's taken some getting used to but I don't have a lot of the knee fatigue I was used to and realized while reading this thread that I don't remember the last time my knees actually hurt. I'll stick with the shorter cranks as they seem to be working for me. Your mileage may vary.


----------



## Edhunt (Sep 1, 2012)

"...while the rate of fatigue was less for longer cranks, the fatigue per revolution was identical."

I have 170mm's on my Giant (triple) and Trek (double), 175mm on my Fuji (compact) and 172.5mm on my SS. 

I am a masher (mid 80's is all I can sustain for extended periods of time) but at 5'8" I find the 175mm is a little much for my knees, but I can power up hills better and find myself shifting much less. With the 170mm's I have to spin much more up hills and find myself getting fatigued about 25% sooner. I also have to do a lot more shifting to accelerate quickly. 

I know all things are not equal, but I am wondering how much of the difference is the crank length between the 175mm (compact) and the 170mm (double and triple), and how much is the 50 vs 53 big ring.


----------



## tom_h (May 6, 2008)

Edhunt said:


> .. I am wondering how much of the difference is the crank length between the 175mm (compact) and the 170mm (double and triple), and how much is the 50 vs 53 big ring.


It's straightforward, really.

If you consider the 'mechanical advantage',

170 vs 175 cranks = 3%

50 vs 53 ring = 6%.

There are small biomechanical differences -- the chainring doesnt affect leg extension -- but most people most of the time will notice the chainrings above all else.


----------



## Edhunt (Sep 1, 2012)

I don't know where your numbers for the cranks come from, but by your math the difference I am feeling is +- 9% due to running 175mm on a 50 vs 170mm on a 53. 

To me it "feels" a little more than that, but I believe that would be consistent with my experience.


----------



## tom_h (May 6, 2008)

Edhunt said:


> I don't know where your numbers for the cranks come from, but by your math the difference I am feeling is +- 9% due to running 175mm on a 50 vs 170mm on a 53.
> 
> To me it "feels" a little more than that, but I believe that would be consistent with my experience.


Numbers for crank is just the ratio of crankarm lengths. 2.9% actually, I just rounded.
Torque = lever arm x force perpendicular to lever arm, or just vector dot product if you prefer. [Oops, I meant vector CROSS product! ]

Your chosen combinations of crank length & ring size will be additive, and the mech advantage works does indeed work out to: 
(53/50)*(175/170) = 9%

BTW, track bikes intended for steeply banked velodromes will typically have cranks that are 5-7mm shorter than a comparably sized road bike frame. 
I "may" sort-of notice it for a couple laps, after that the body adapts & it's unnoticeable. 

I'm currently on 172.2mm road cranks, next time I buy a crank I'm going shorter (170) just to get a little extra cornering clearance, when I pedal thru turns.


----------



## Edhunt (Sep 1, 2012)

tom_h said:


> Torque = lever arm x force perpendicular to lever arm, or just vector dot product if you prefer.


Ah yes, high school physics. It's been a while...

Interesting about track bikes. When I switch back and forth between my bikes it takes me a few miles to adapt, but I still notice the difference when power is needed. 

I was thinking about splitting the difference and putting 172.5mm on at least one of the 170mm bikes, but I will just have to think about it. It only really matters at about 3/4 of the way up a longish (I live near the Appalachians) climb, but when it hurts, it hurts. Maybe I just need to HTFU.


----------

