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  1. #1
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    What considerations for crank arm length??

    I wanted to change my 53/39 crankset to a compact. I see cranks at 172.5 mm, 175 mm and others. Honestly, I don't know what length is on my bike (I know, measure it, foo!).

    What consideration come into play when deciding crank length? I'm a ~100 mi/wk, average Joe biker with a 19# Scattante Race with Ultegra components and a FSA double crankset. I'm about 6' tall on a 58 cm frame.

    So, what length cranks? Thanks all!

  2. #2
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    Take a ruler and measure the difference between 172.5 & 175mm (about half the width of a pencil). It is so small I don't think I could even tell the difference on a bike. I am 5' 10" and have always used whatever came with the bike - probably 170mm I am guessing.

  3. #3
    Custom User Title Quijibo187's Avatar
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    It's usually stamped near the pedal axle hole on the inside of the crank.

    There's a formula somewhere that helps determine what length you should use, but with that said, I have 3 different lengths that I use.
    "An opinion should be the result of thought, not a substitute for it."
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    http://winnipegcyclist.blogspot.com/

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    Chances are high that your bike came with a 175. It should say on the back side of the arms. In general smaller road bikes will be specced w/ 170, medium ones 172.5, and large's 175.

    Length is pretty subjective. Shorter cranks help you spin better e.g. most track sprinters use 165s, longer cranks help give you torque. Some people like to ride the same length on all of their bikes, some also contend that you shouldn't really jump more than 2.5 mm between bikes (legs won't feel right, perhaps injury or discomfort e.g. going back and forth between 175's to 165's on two bikes you ride regularly).

    FWIW I went from 175s to 172.5s on my road bike which I ride the most. Now my 170s feel short (track bike and fixie) and 175s now feel LONG (mtn), which is crazy when you consider how many millimeters we're talking between those jumps (5= 2x2.5).

    Try riding 172.5s and 175s and see which feels better to you, that's what counts.

    Quote Originally Posted by runway1 View Post
    I wanted to change my 53/39 crankset to a compact. I see cranks at 172.5 mm, 175 mm and others. Honestly, I don't know what length is on my bike (I know, measure it, foo!).

    What consideration come into play when deciding crank length? I'm a ~100 mi/wk, average Joe biker with a 19# Scattante Race with Ultegra components and a FSA double crankset. I'm about 6' tall on a 58 cm frame.

    So, what length cranks? Thanks all!

  5. #5
    Cyclojazzmathiopian
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    If my calculations are correct [probably aren't], the added torque from increasing the lever arm 2.5mm will create a difference of 6.5 W if you have a cadence of 90 on level ground with a velocity of 32kph [roughly 20mph].

    Using
    P=4.5v+.0015v^3
    F=P/v,
    tau=Fr
    dP = 2*pi*cadence(tau_1-tau_2)

    Anyone decently good with physics/engineering want to chime in here? I do math, so this is not my thing.

  6. #6
    Senior Member AnthonyG's Avatar
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    Quote Originally Posted by parabrand View Post
    If my calculations are correct [probably aren't], the added torque from increasing the lever arm 2.5mm will create a difference of 6.5 W if you have a cadence of 90 on level ground with a velocity of 32kph [roughly 20mph].

    Using
    P=4.5v+.0015v^3
    F=P/v,
    tau=Fr
    dP = 2*pi*cadence(tau_1-tau_2)

    Anyone decently good with physics/engineering want to chime in here? I do math, so this is not my thing.
    The problem with making these calculations in relation to cycling is that the answer ONLY applies to the moment in the crank cycle when the cranks are level (9-3 o'clock position). When the cranks are vertical (6-12 o'clock position) the longer cranks make it harder to get over the top so it evens itself out over the cycle of the cranks. The end result is that its awash. Personally I think the short cranks offer the advantage of evening out your power delivery.

    I think the best reason for picking a certain crank length is what's most comfortable for you. Short cranks are easier to spin and make it easier to adopt an aerodynamic position however if they are too short the rider doesn't feel like they are stretching their legs out enough.

    I recommend crank length to be between 19% to 20.5% of inseam. Measure your inseam from crotch to ground in socks and then multiply the answer by 0.20 as a starting point and see what the answer is.

    Anthony

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    What considerations for crank arm length??

    I wanted to change my 53/39 crankset to a compact. I see cranks at 172.5 mm, 175 mm and others. Honestly, I don't know what length is on my bike (I know, measure it, foo!).

    What consideration come into play when deciding crank length? I'm a ~100 mi/wk, average Joe biker with a 19# Scattante Race with Ultegra components and a FSA double crankset. I'm about 6' tall on a 58 cm frame.

    So, what length cranks? Thanks all!

  8. #8
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    Delete this - double post, sorry.

  9. #9
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    The crank length is stamped on the back of the crank arms - no need to measure.

    - Crank length is decided by your inseam.
    - Find a crank length calculator online
    - Plug in your measurements
    - ....
    - profit

  10. #10
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    Quote Originally Posted by AnthonyG View Post
    The problem with making these calculations in relation to cycling is that the answer ONLY applies to the moment in the crank cycle when the cranks are level (9I don't care -3 o'clock position). When the cranks are vertical (6-12 o'clock position) the longer cranks make it harder to get over the top so it evens itself out over the cycle of the cranks. The end result is that its awash. Personally I think the short cranks offer the advantage of evening out your power delivery.
    From an engineering point of view, that's right on. Also, being a human in the loop, you would have to assume the force applied is constant from one length to another. That may not be the case.

    Bottom line; I don't care about that. Being a mechanical engineer for 22 years now, I realize that this length difference is not only neglible but near impossible to quantify, so forget it.

    With that, I think I answered my own question - 2.5 mm (~.1")....who cares!

  11. #11
    Dough Mestique
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    Leg length and personal preference determine crank length for roadies. It's one of those things people fret too much about. My guess is your bike is running a 175. That's probably just fine.

    BL


    www.lanterne-rouge-bikeworks.com

    "Next time, I will not make the same mistake twice!"

  12. #12
    Senior Member border reiver's Avatar
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    if you start googling this subject you will find a huge amount of (sometimes contradictory) information about the importance of crank length. Basically there are 2 prevailing schools of thought: (1) any crank length in the normally spec'd range (e.g. 165 - 175) will make so little difference to any rider within a wide height range that it mostly comes down to what you are used to and what you are comfortable with. (2) crank length is important (and much downplayed by manufacturers because they want to stick with a few standard lengths), and should be proportional to leg length or femur length, and there are various formulas out there for determining which crank length you should use.

    A third idea is the one that is most intriguing to me, and has to do with open and closed hip angle. For example, if I go from 170mm cranks to 175mm cranks then there are two prevailing affects, first, I have to lower my saddle by 5mm relative to my bottom bracket to maintain the same seat "height;" and second, at the top of each pedal stroke my knee is an additional 5mm closer to my chest, the combined affect of which impacts my open/closed hip angle. Why this may or may not matter is discussed in a recent article that explains it better than I could:

    http://www.slowtwitch.com/Tech/Crank...r_tri_727.html

  13. #13
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    Quote Originally Posted by runway1 View Post
    Honestly, I don't know what length is on my bike (I know, measure it, foo!).
    Stamped on the inside of the arm by the pedal.

    Quote Originally Posted by runway1 View Post
    What consideration come into play when deciding crank length?
    Mostly inseam but other things you can look at are height, flexibility, hip angle, femur length, ect...

  14. #14
    Whatever wheelsucker1's Avatar
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    The "standard" OEM is 172.5 mm.

    Very broad advice: If you're between 29 and 32" inseam 172.5 is fine, between 31-33" 175 is better but either will work and you won't know the difference after a few miles.

  15. #15
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    I have a 927mm (36.5") cycling inseam. Does Campy, Shimano, or Sram make a 180mm crankset?

    S-

  16. #16
    SmackTalk'rExtraordinaire
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    FWIW
    Lance 5' 10" rides a 175
    Contador listed as 5 10" (certainly a bogus height) also rides 175

    I think I read on analyticcycling that longer cranks have a very marginal benefit as long as it does NOT impede your leg as it reaches towards your chest when in the tuck/aero position.
    "One should respect public opinion insofar as is necessary to avoid starvation and keep out of prison, but anything that goes beyond this is voluntary submission to an unnecessary tyranny." - Bertrand Russell

  17. #17
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    Crank length is only of real importance for very short or very tall riders who are almost certainly riding the wrong size.
    For anyone inbetween they are riding approx the right size.
    You can fine tune the crank length to favour a slower, larger diameter pedal turn or a faster, smaller diameter turn according to your style. This has no effect on power output and the difference in torque is offset by using different gear ratios. Same power out in results in same speed on the road.

  18. #18
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    I'm one of maybe 4 or 5 BFers with a strong opinion on crank length. Look for posts by me related to crank arm length. I believe that I'm in the minority - the other strong crank arm length opinion BFers disagree with me.

    I think crankarm length can be used effectively to help riders with leg speed (or a very smooth pedal stroke) or compensate for those that don't.

    I have short legs, 29-30" inseam, 67 cm from BB to saddle top (175s - for 170s add 0.5 cm since I raise the seat).

    I used to ride 167.5s for a long time (10+ years), when I had a lot (relatively speaking) of fitness. I had a good pedal stroke, spun well, and usually maintained 110-112 rpm in steady hard efforts (climbing, tests, etc). My max rpm was 286 rpm, way below Scott Berryman's 300+ rpm, but it was pretty good for me. This is an example of using crankarm length to compliment good leg speed.

    Then I got older, slower, heavier, and probably more powerful. I find now that I pedal much slower, sprint at lower cadences, etc. My max rpm is below 250 rpm, which I consider to be relatively poor. I now use 175s.

    During an interim period of 5-6 years between the 167.5s and 175s, I rode on 170s.

    I tried using 170s last year, after a five? years of 175s, and didn't have much success. With significantly less training I am better on 175s in 2009. More field sprint wins, feel better at the end of races (usually). Due to significantly lower fitness, I haven't finished a few races due to the insane pace, which, ultimately, requires a minimum amount of fitness to sustain.

    I have two friends who ride 60 cm frames, or thereabouts. Both ride 180s. Neither race anymore, but both were good Cat 3s.

    SRAM makes 180s in all levels.

    If I were 6' tall I'd go minimum 175s, and try 180s, especially if I didn't train that much (100 miles weekly is maybe 2x more than me, but 1/2 of what many "fit" riders do). I'm convinced that a lower fitness rider benefits from longer cranks due to the better leverage.

    +5mm longer cranks will help in the power roller hills, where you can step up your gear significantly. Instead of shifting into the small ring for one particular hill (Bethel) I can roll up it in the big ring. With 170s there was definitely more stress on my legs, and I had to spin to keep my legs from loading up.

    With practice you can have very good speed too. BMX racers usually use 180s and they have one tiny gear. Watch a clip of BMX racers, they fly. They use the longer cranks for the holeshot, but have to maintain good speed else they'll get passed. My leadout man used 180s and was beating me in sprints just before he quit racing (he's one of the two guys I mention above), and this was on the Gimbles 120 sprint where I tended to do pretty well.

    cdr

  19. #19
    Senior Member daxr's Avatar
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    The oldschool thinking on it from back in the 80's was that you could benefit by tailoring your crank length to your event.

    Such as, for a time trial go 2.5 mm longer and add a tooth to the chainring, so you'd be applying the same effort to ride faster. The longer length would make accelerations slower, but for a steady speed type event its a benefit.

    Same kind of thinking for crits - go 2.5 mm shorter for faster spin-ups, and take advantage of the handling benefits of faster pedal speeds and responsiveness out of turns and in the pack.

    And for road racing, it wasn't unusual to hear recs for longer cranks in the mountains, which added leverage to push bigger gears. I read once that Merckx used 180's sometimes.

    In any case, I think the differences are pretty minimal and have always just ridden 175's. (6'1").
    "... the age of Happy Motoring is over. Many Americans have already bought their last car -- they just don't know it yet."

    James Howard Kunstler, 2008.

  20. #20
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    I have a 34" inseam and run 172.5s on the road bike and 175s on the TT bike and rain bike and can definitely feel the difference. I'm 99% sure it is not a placebo effect.

  21. #21
    Refrigerator Raider Hater fordmanvt's Avatar
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    Quote Originally Posted by BustaQuad View Post
    I have a 34" inseam and run 172.5s on the road bike and 175s on the TT bike and rain bike and can definitely feel the difference. I'm 99% sure it is not a placebo effect.
    Being that the 2.5mm difference is radius, the diameter difference is 5mm.

    I would expect that shorter cranks would lead to higher seat position, and that overall crank length would be determined by the knee height at the top of stroke.

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