Power MUCH lower on one bike?
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  1. #1
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    Power MUCH lower on one bike?

    My CX/road bike saddles are in the same position fore/aft. I verified this 3 different ways (plumb bob off knees, plumb bob off saddle nose, and putting bike in a doorway and hooking pedal on it and measuring to the same spot on the wall) and BB to saddle rails yet power is WAY lower on the road bike. I'm talking like 70 watts lower on the same loop with same heart rate and PE (I know PE is kind of useless but still).

    Both bikes have the same model saddle, use the exact same shoes/cleats, same pedal model, same brand power meter, etc so the BB to rail measurement should be the same, right? On the road bike I FEEL too low, get frontal knee pain after, etc so I'm probably too low, but why does that height work on the CX bike? I raised the road saddle a full 1cm higher and power is still lower with slight pain. I was under the impression road/CX should be set up the same way.


    Is there a factor I'm missing?

  2. #2
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    Here's what I would do:

    1) Put your plumb and rulers away

    2)Adjust the saddle until it feels right

    3) Ride it.
    "L'enfer, c'est les autres"

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    Quote Originally Posted by thisisthebeave View Post
    Is there a factor I'm missing?
    Two actually:
    1. You forgot you already asked this question.
    2. You haven't answered our question... Can you swap the power meters?

  5. #5
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    Quote Originally Posted by OldZaskar View Post
    Two actually:
    1. You forgot you already asked this question.
    2. You haven't answered our question... Can you swap the power meters?
    I forgot I posted that other thread. Anyway, one power meter is GXP and the other is BB90 so maybe with a different BB I could but even if the power meter was wrong/defective, how do you explain the saddle needing to be 1cm higher on another bike to avoid knee pain?

  6. #6
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    Or is it a function of seat tube angle?

    Is it going to measure further to the same relative position on the bike with the slacker STA? But should the saddle being in the same fore/aft negate that?
    Last edited by thisisthebeave; 05-30-2019 at 06:02 PM.

  7. #7
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    Quote Originally Posted by thisisthebeave View Post
    Or is it a function of seat tube angle?

    Is it going to measure further to the same relative position on the bike with the slacker STA?
    Is your saddle in the same place as the other bike? You said it is, right? The power meter and your body don't care what the seat tube angle is, only where the saddle is. You could have an 80* STA and a 75mm setback post...wouldn't matter if the saddle is where it should be for your fit.
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  8. #8
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    measure the triangle, bb straight up to level with seat rails, from that point to the rear of the seat, and from the rear of the seat to the bb.

    Compare the figures, they will be different.
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  9. #9
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    Quote Originally Posted by cxwrench View Post
    Is your saddle in the same place as the other bike? You said it is, right? The power meter and your body don't care what the seat tube angle is, only where the saddle is. You could have an 80* STA and a 75mm setback post...wouldn't matter if the saddle is where it should be for your fit.
    Yea, as far as I can tell they're in the same spot.

    I dropped a plumb bob off the nose of the saddle with the cranks at 9 oclock and measured the line to the edge of the pedal spindle. I dropped a plumb bob off the edge of my knee and both were in the same spot. And I put the bike in a doorway, looked the pedal on the door frame, and lined up a mark on the wall with the nose of the saddle. All 3 methods show that the saddle is in the same spot in terms of fore/aft. Then I measure the same spot on the rails to the center of the BB and when they're the same, I get knee pain. If I raise the road bike 1 full cm, pain goes away but power is still low. I'm compensating fore/aft with these changes.

  10. #10
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    Well if it isn't the saddle, measure the bars.
    If it's not the bars it's you PM's.
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  11. #11
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    Quote Originally Posted by duriel View Post
    Well if it isn't the saddle, measure the bars.
    If it's not the bars it's you PM's.
    I'm thinking it's this. Which 2 powermeters are you using? How long since they've been calibrated?
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  12. #12
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    Quote Originally Posted by duriel View Post
    Well if it isn't the saddle, measure the bars.
    If it's not the bars it's you PM's.
    If your bars are not in the same place, you are tilting your pelvis more/less/differently (drop) and stretching to reach/compress (reach) which is affecting the former (pelvis relative to BB, knee relative to BB). You're so focused on the seat in your original post but there's no mention of the bars, crank length, or pedal platform height for that matter. Details. Details
    "That pretty much sums it up. I'm 43 and my max is ~178-180. If that HR chart was mine or Froome's, we'd be on the verge of death. But for you it probably looks like a normal workout." -TLG

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    Quote Originally Posted by 9W9W View Post
    If your bars are not in the same place, you are tilting your pelvis more/less/differently (drop) and stretching to reach/compress (reach) which is affecting the former (pelvis relative to BB, knee relative to BB). You're so focused on the seat in your original post but there's no mention of the bars, crank length, or pedal platform height for that matter. Details. Details
    I use the same model pedals and saddle for both bikes and wear the same pair of shoes. Both bikes have 175mm cranks. Stack height on one bike is 637, the other is 656. The 637 bike has bars in the highest position and the 656 bike is one 5mm spacer down from the highest. The reach on the bikes is 428 and 386. The 428 bike uses a 10mm shorter stem and the bars themselves have less reach so there's a difference there but not as big as you'd think. Anything else?

  14. #14
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    ... SO your definitely NOT in the same 'position'!
    Calibrate your PM's.
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  15. #15
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    Quote Originally Posted by duriel View Post
    ... SO your definitely NOT in the same 'position'!
    Calibrate your PM's.
    They're calibrated and have a very similar calibration figure (905 and 907). Would the position of my torso change power THAT much? I thought the position of my legs/knees versus the BB would matter more.

  16. #16
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    Quote Originally Posted by thisisthebeave View Post
    They're calibrated and have a very similar calibration figure (905 and 907). Would the position of my torso change power THAT much? I thought the position of my legs/knees versus the BB would matter more.
    Hip angle will have an effect.
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  17. #17
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    Quote Originally Posted by cxwrench View Post
    Hip angle will have an effect.
    So what do I do, lower the bars on my road bike to try to match it to my CX?

    I don't have a power meter on my MTB but I set the height and fore/aft the same way and I have no knee pain on that bike but even 1cm higher on the road bike I still have some frontal knee pain. I guess I could try going up even more, or tweak the fore/aft a little bit.

  18. #18
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    Quote Originally Posted by thisisthebeave View Post
    So what do I do, lower the bars on my road bike to try to match it to my CX?

    I don't have a power meter on my MTB but I set the height and fore/aft the same way and I have no knee pain on that bike but even 1cm higher on the road bike I still have some frontal knee pain. I guess I could try going up even more, or tweak the fore/aft a little bit.
    Make the bike you produce less power on EXACTLY like the one you make more power on. I don't know why this is so hard to figure out.
    Saddle height, setback, reach, drop. Same saddle. Hip angle is why so many triathletes are going to 165mm crank arms...they can get their front end low while not closing off the hip angle as much as with longer cranks.
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  19. #19
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    Quote Originally Posted by cxwrench View Post
    Make the bike you produce less power on EXACTLY like the one you make more power on. I don't know why this is so hard to figure out.
    Saddle height, setback, reach, drop. Same saddle. Hip angle is why so many triathletes are going to 165mm crank arms...they can get their front end low while not closing off the hip angle as much as with longer cranks.
    I've already done this subject to the limitations of different frame geometry. They have the same saddle, setback, etc. I'm blown away that a full CM higher on the road bike means low power and still slight knee pain when setting up my mountain bike with the same height/setback (it also uses the same saddle model and pedal model) was all it took to get it dialed in.

  20. #20
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    Quote Originally Posted by thisisthebeave View Post
    I've already done this subject to the limitations of different frame geometry. They have the same saddle, setback, etc. I'm blown away that a full CM higher on the road bike means low power and still slight knee pain when setting up my mountain bike with the same height/setback (it also uses the same saddle model and pedal model) was all it took to get it dialed in.
    "subject to the limitations of different frame geometry"? So you're saying the positions are not the same? Moving the saddle fore/aft, raising/lowering the bars and saddle, keeping saddle angle and bar angles the same: all of this should allow you to get the bikes the exact same unless you have hit the mechanical limits of one of these things. The only variable that you can't control is BB height, and that should have very minimal effect except to increase/decrease aero drag.

    And just so you are clear on this, having two power meters guarantees two different answers. A person with two watches can rarely be sure what time it is.

  21. #21
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    Quote Originally Posted by Kerry Irons View Post
    "subject to the limitations of different frame geometry"? So you're saying the positions are not the same? Moving the saddle fore/aft, raising/lowering the bars and saddle, keeping saddle angle and bar angles the same: all of this should allow you to get the bikes the exact same unless you have hit the mechanical limits of one of these things. The only variable that you can't control is BB height, and that should have very minimal effect except to increase/decrease aero drag.

    And just so you are clear on this, having two power meters guarantees two different answers. A person with two watches can rarely be sure what time it is.
    I don't even care about the power variance as much as why does the same height and setback give me knee pain on one bike than the other when everything that should matter (shoes/cleats/saddle/pedals/setback/height) are all the same.

  22. #22
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    i ride the same bike most of the time. Some days I get knee pains, some days I don't.
    If you answer me that, you will have your answer.
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  23. #23
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