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  1. #1
    Join Date
    Sep 2007
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    3,139

    What's your Q Factor?

    I recently took an older SRAM XO carbon crank, swapped the spindle for a wider one, and installed it on my trail bike. The crank arms have a much more significant flare/jog from the BB spindle to the pedal interface and I'm realizing that the Q factor on these cranks are quite wide. I haven't measured it yet but I would guess that it's more in line with my (old) fat bike than with my gravel bike and I noticed my knee hurting on the initial ride on the crank. It has gotten me thinking a little bit about Q factor from bike to bike.

    The little research that I've done suggested that road bike Q factor and mountain bike Q factor varies (150mm for road bike and 170 for mountain). I would guess that different Q factors were created as tires have gotten wider, hubs have gotten wider and even BBs have gotten wider. It seems, though, that Q factor ought to be as much a factor of body mechanics than it is of bike design - i.e. there is likely an ideal Q factor for each person rather than varying Q factors across several bikes in a person's quiver.

    This started me thinking about Q factor across my three bikes - v1 Hightower (currently with the XO crank), Trek Stache (SLX crank with PF92 BB), and a Specialized Diverge (Force 1 crank w/ 386EVO BB). It *seems* that the crank arm interfaces are the same between the XO crank and the Force 1 crank. The Q factor of my gravel bike is much narrower than the Hightower. It would seem that I could achieve a "happy medium" by swapping the XO arms onto the Diverge and the Force 1 arms onto the Hightower.

    I'm wondering where the holes are in my logic and whether any of you are making crank choices to get a more consistent Q factor? How did you determine your ideal Q factor? It seems as though this might be a good idea for my knee(s), even if it means getting new crank arms for my hightower. I'm interested in your input.

    Seth

  2. #2
    Join Date
    Jan 2008
    Location
    BC to CO
    Posts
    3,630
    For reference
    Shimano MTB
    Shimano XTR 9100-1x Q Factor = 162mm (race)
    Shimano XTR 9120-1x Q Factor = 168mm (Trail or same as 2x)
    Shimano XTR 9130-1x Q Factor = 171mm
    Shimano XTR 9125-1X Q Factor = 174mm

    SRAM MTB come in:
    158mm (Race non-boost)
    168mm (non-boost and boost)
    174mm (Eagle DUB)

    ROAD
    Shimano Road
    Shimano Dura Ace 9100 Q Factor = 146mm
    SRAM Road
    Most are Q Factor =145mm

  3. #3
    Join Date
    Jan 2007
    Location
    Upstate
    Posts
    7,752
    I've never understood the fascination with Q factor. I watch guys who's pedal stroke puts their knees above the top tube and conversely see guys who look bow legged. Each of which suggests to me that there's a ton of variability in hip angle. If aero is the purpose (road and TT bikes) I can see the marginal gains of being closest to the BB, but that's bleeding edge stuff IMHO.

  4. #4
    Join Date
    Jan 2006
    Location
    Carbondale
    Posts
    11,313
    I have 0 tolerance for Q.




    oh, wait, this is sprockets and not padded room...

    whoops.
    www.dpsskis.com
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    formerly an ambassador for a few others, but the ski industry is... interesting.
    Fukt: a very small amount of snow.

  5. #5
    Join Date
    Dec 2010
    Location
    whistler
    Posts
    1,164
    I've never thought about it for cranks but an picky with pedals. I hate when my pedals force crank pinching ergo I favor pedals with a wide q factor that put my feet in something closer to a natural width.

  6. #6
    Join Date
    Mar 2008
    Location
    northern BC
    Posts
    25,093
    my FAT bike has a noticably wider BB cuz of the FAT tires but I can't say the differences i the rest of the bikes are all that noticable

    I've ridden shitty exercise bikes in gyms with really wide BB's
    Lee Lau - xxx-er is the laziest Asian canuck I know

  7. #7
    Join Date
    Sep 2007
    Posts
    3,139
    Here are two pictures of the crank arms installed on my Hightower and my Diverge. I suppose I'm not looking for an absolute Q factor number but wondering whether my knees would be better if I could make the Q factor consistent across all bikes. The curve of the arms on the Hightower fit the chainstay(s) pretty well but are just further outboard than they need to be. The diverge is super close to the chainstay(s).

    Maybe a winter project will be to measure the Q factors, swap the arms and measure again.

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  8. #8
    Join Date
    Dec 2010
    Location
    whistler
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    Your pedals look to be exaggerating the q difference further. The pedals on the Hightower put you even further out by the looks of it although it could be an illusion from the naked spindle.

  9. #9
    Join Date
    Dec 2010
    Posts
    1,116
    My queef factor is relatively low - just a quiet pfffffft.

  10. #10
    Join Date
    Sep 2007
    Posts
    3,139
    Bumping this back up. I ended up picking up a set of SRAM Red cranks for my gravel bike and have a perfectly good SRAM Force 1 crank in a box. I could sell it, but they have a much straighter crank arm than the XOs that I have currently on my Hightower. They look really similar. Is that crank (Force 1) weaker than the XOs? Am I gonna die?

    I will have to swap the Force 1 arms onto the spindle that I currently have on that bike but I've just gone through that procedure and am familiar with it.

    On the other side of the decision I did a long ride on my HT a few weeks ago with the wide Q factor and my knees felt fine so . . . I might just leave it as is.

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  11. #11
    Join Date
    Aug 2020
    Location
    SLC
    Posts
    735
    The strength of the Force crank is probably fine, the issue is clearance. If the Force crank manages to clear your chainstays, you will still probably be bumping your heels against the chainstays. I miss when SRAM made a 156mm Q-factor XX crank and a 166mm model so you could tailor it to your frame and preferences.

  12. #12
    Join Date
    Feb 2014
    Location
    NorCal coast
    Posts
    1,070
    Forget Q factor, your bike is going to spontaneously combust because you're using Shimano pedals on SRAM cranks.

  13. #13
    Join Date
    Sep 2007
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    3,139
    Quote Originally Posted by Andeh View Post
    Forget Q factor, your bike is going to spontaneously combust because you're using Shimano pedals on SRAM cranks.
    I thought XT was just a little better than XO and not as good as XX?

    Sent from my SM-N986U using Tapatalk

  14. #14
    Join Date
    Feb 2014
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    NorCal coast
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    Quote Originally Posted by sethschmautz View Post
    I thought XT was just a little better than XO and not as good as XX?

    Sent from my SM-N986U using Tapatalk
    It was a joke, at the expense of people like me who get twitchy mixing and matching competing brands.

  15. #15
    Join Date
    Sep 2007
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    Quote Originally Posted by Andeh View Post
    It was a joke, at the expense of people like me who get twitchy mixing and matching competing brands.
    I figured. Mine was as well. :-) It is kind of funny that it *almost* matches SRAM's model assignments though.

  16. #16
    Join Date
    Mar 2006
    Location
    Missoula, MT
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    21,729
    For me, I find the best way to deal with knee pain is stretching and good ankle support from my shoe and good seat fit. No idea what my Q factor is but if it's a little different from the old Candfield Nimble 9 to the new GG, I can't tell.
    Same pedals, same BB shell, different axles though. And different cranks.

    Seriously, stretch the inside of your thighs. Make sure the fit and closure around your ankle is nice and snug. Fiddle with your seat height and angle if you have to. Even your cleat position.

    But streeeeeeetch.

    Oh, and one more thing: are you thin? I'm not, but I have a narrow ass. Usually, seats that come with MTB's or other "popular" ones are too wide for me. Even a 140 is a little wide. Messes with the overall ergonomics and can cause knee pain.
    I should probably get the same seat for both bikes.
    Anyone have 1 of those thin Niner OEM seats floating around? A couple Silverado chromos? I seem to like narrow and flat and long nose. Should I try an Aeffect? [/Hijack]
    No longer stuck.

    Quote Originally Posted by stuckathuntermtn View Post
    Just an uneducated guess.

  17. #17
    Join Date
    Mar 2006
    Location
    Missoula, MT
    Posts
    21,729
    Streeeeeeeeeeeeeeetch
    No longer stuck.

    Quote Originally Posted by stuckathuntermtn View Post
    Just an uneducated guess.

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