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Crank Length?

scott dinhoferscott dinhofer
in General Training Discussions

I am generally out there looking for parts to continually upgrade my ultegra equipped P2 to DA in an effort to reduce weight. I weighed in 2lbs over the 50lb magic number on airline shipping this year.

tripped across DA cranks on ST. guy has both the 170 & 175, my bike is equipeed with FSA 172.5

what are the advantages/disadvantages of longer/shorter cranks?

current cranks are 50/34, new one would be 53/39 how does that play in to the crank length? obviously the new ones are "harder" to turn with the larger chain rings? but the shorter crank lenght would then make them "easier"??

Comments

  • kjanracingkjanracing
    You are talkig about two different things entirely-gearing and crank lenght. I think you would want to stay with 50/34 chainrings, mo better/lower gears for climbing. As far as crank lenght maybe go with 170mm. You'll loose a little "leverage", so for equal power, your cadence will be higher. You hip angle at top dead center will be a little more open, so that may be a advantage-or not. Just the opposite for longer cranks. I feel like 175mm cranks are just too long and hard to spin. That said, I wonder if I could tell the lenght just from riding and feel. Is there really much weight difference between the FSA and DA?
  • jdcsbrsk8jdcsbrsk8

    Hi Scott,

    Here are some references for you.  Seems to me 170mm is a good choice.

     

    http://www.powercranks.com/cld.html

    http://www.ncbi.nlm.nih.gov/pubmed/11417428

  • tim cronktim cronk
    Scott, X2 on what Kurt said. Make sure you stay with 50/34 your gonna need those for Mooseman and IMMT! The trend right now is shorter cranks with the benefit being opening the hip angle. When I purchased my Quarq I went to 165 cranks but I'm only 5'6". Heather(also 5'6") and I also just got a pre-buy bike fit from Todd at ttbikefit.com and he put us both on 165cranks . Tim,
  • robin sarnerrobin sarner

    Defnitely stay with the compact cranks.   Coaches would confirm.

    Trend is toward shorter cranks.

  • Coach RichCoach Rich

    + whatever on the compacts.

    This year I went to 165mm cranks on the tri bike, to open up my hip angle as Robin said. I had no issues at all. William Jenks has written an article in the wiki for us about the physics of crank length with regards to power to the rear wheel. Very good.

  • kjanracingkjanracing
    A slight hijack maybe, but Rich, what differences did you notice going to the 165's? I remember riding my track bike on the road with a front brake. It had 165's on it. Felt ok on the flats-easy to spin up. Felt really "underpowered" on climbs...not as much torque. Just wondering about your perceptions. thanks!
  • Coach RichCoach Rich

    Kurt, I had not ridden my tri bike, at all, for nearly 3 years so while I couldn't really tell the difference, I may not be the best example . But get a ruler and lay out the length of 5-7.5mm, the difference between 165mm cranks and 170 or 172.5mm cranks. It's very, very small. I switched from my tri bike, at 165mm, and my road bike at 172.5mm a few times during the summer and felt no difference...other than I was on a road bike

  • William JenksWilliam Jenks
    The change from 172.5 2.5 mm either way isn't large.

    You will want to adjust your saddle, though. You want your feet at the bottom to be at the same place they were before, relative to your butt. So if you shorten your cranks 2.5 mm, you raise your saddle the same amount. This means your knee will come up 5 mm less close to your chest than before at the top.

    As Kurt said, you do lose a small amount of lever arm. (you can calculate the percentage...it's linear) If you go down to 165 mm cranks like Rich did, it means you have to apply about as much more linear force as one gear higher. I have read that people who are more successful with short cranks are the ones who tend to prefer to spin fast, but I am less sure why that is.

    I am also experimenting with the short cranks on my TT bike for next year because I want to have that more open hip angle to try to keep a higher percentage of my FTP relative to what it is on the road bike. We'll see how that goes. No data yet. But just on general principle related to that, I'd go for the slightly shorter cranks (170) over the slightly longer ones (175).
  • TH3 FRBTH3 FRB
    If you are looking to shave weight and add some bling with cranks you might be interested in a set of Lightning Carbon cranks I have - 167.5 and can be either compact or standard but all here would generally agree on compact. These were pricey but bottom line is I can't use them anymore (changed to a Trek BB90 frame) so I need to take what I can get. I can probably be very close to a set of DA cranks.

    http://www.lightningbikes.com/cranks/index.html
  • scott dinhoferscott dinhofer
    Cud be interested, what is asking? Do they come with rings? Will they fit on whatever bb my cervelo p2 has on it?
  • TH3 FRBTH3 FRB

    No rings or spyder.  The spyder is $50 from Lightning then you add whatever rings you want.  You can go either compact or standard.  They have a bunch of different bearing sets available so fitting them on your P2 shouldn't be an issue.

    New they are $650 without rings or spyder.  It's killing me to take such a big hit but they aren't doing me any good sitting in a box - I'll take $250 shipped.

    http://i149.photobucket.com/albums/...202338.jpg

    http://i149.photobucket.com/albums/...202403.jpg

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