MUCH higher power on one bike?
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  1. #1
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    MUCH higher power on one bike?

    I have a CX and a road bike, both set up identically, and the CX bike consistently shows higher power. Both bikes have the same saddle, same relative saddle position to the crank (or at least within 1mm), same power meter brand with the same calibration figure, I use the same pair of shoes, same pedal model, yet the road bike always spits out 50-100kJ less than ~20w avg less on the same loop.

    The only variable would be that the CX bike is heavier/less aero/fatter tires which might explain a portion of the 50-100kJ gap as it takes me a few mins longer, but not the average power. Not only that, but I'm holding that power for those few minutes longer.

    I tried the saddle on the road bike 3mm forward of its relative position on the CX bike (and compensated by going up 1mm) and 3mm rearward of its relative position on the CX bike (and compensated by going down 1mm) and that changed nothing. What else could it be?

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    Could stack/reach between the bikes make THAT much difference when all else is the same, even heart rate and perceived exertion?

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    How difficult would it be to swap cranks - to verify that's not the culprit. Or even better - borrow a set of power meter pedals. Okay, that may be just as unlikely.

  4. #4
    tlg
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    Can you swap the power meters? If you do that and your readings reverse, you know it's an issue with the power meter or bike/fitting.
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    Different power meters or different algorithms to compute estimated power on different devices could be the problem.

  6. #6
    .je
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    What colour is the CX bike? Is it red? Red is the colour of anger.

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    Was the wind speed and direction exactly the same in both instances?
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  8. #8
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    I'm confused by the OP's usage of kJ in this context.

    the road bike always spits out 50-100kJ less than ~20w avg less on the same loop
    does this make sense to anyone?

  9. #9
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    Quote Originally Posted by aclinjury View Post
    I'm confused by the OP's usage of kJ in this context.



    does this make sense to anyone?
    I'm confused by the OP being this obsessive about this stuff.
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  10. #10
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    Quote Originally Posted by .je View Post
    What colour is the CX bike? Is it red? Red is the colour of anger.
    We all know that is the heart of why red bikes are faster. It is the anger.
    To date, philosophers have merely interpreted the world in various ways. The point however is to change it.

  11. #11
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    Quote Originally Posted by aclinjury View Post
    I'm confused by the OP's usage of kJ in this context.



    does this make sense to anyone?
    I’m around 900kJ for my lunch loop on CX, closer to 800 for road.

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    Quote Originally Posted by Lombard View Post
    I'm confused by the OP being this obsessive about this stuff.
    For a cyclist, wondering about a 20-watt difference is FAR from obsessive. Anyone who trains with power would LOVE to gain 20 watts - either through training, a fit change, etc. So, trying to figure out how he's "losing" 20 watts to an unknown factor is perfectly rational.
    Last edited by OldZaskar; 01-30-2019 at 07:12 AM.

  13. #13
    tlg
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    Quote Originally Posted by Lombard View Post
    I'm confused by the OP being this obsessive about this stuff.
    I don't see it being obsessive. He thinks everything between setups is identical, then if so, he's seeing a 10% variance between power meters. That's quite a bit.
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  14. #14
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    What power meters are you using?

    When you say it's the same position relative to the crank - does that mean you've dropped a plumb bob from the nose of the saddle on both bikes and measured the position from that string to the middle of the crank on both bikes and it's the same? And the saddle height is the same also?
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    Fatter tires, heavier bike, less aero could very likely explain the watt difference depending on how much fatter, heavier or less aero the cx bike is. You are basically using more watts to go slower which isn't unreasonable if the cx bike has all the previously mentioned performance degrading features.

    Another thing to keep in mind is your position on the bike. You mention the same seat to pedal measurements but the stack and reach would likely make a much bigger difference as the road bike could have you in a much more aerodynamic position to begin with. If the CX bike has you sitting upright vs a more aerodynamic position on the road bike you would be faster for less power on the road bike even if the weight/tires/aero on the bikes were the same.
    Last edited by taodemon; 01-30-2019 at 10:10 AM.

  16. #16
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    Quote Originally Posted by taodemon View Post
    Fatter tires, heavier bike, less aero could very likely explain the watt difference depending on how much fatter, heavier or less aero the cx bike is. You are basically using more watts to go slower which isn't unreasonable if the cx bike has all the previously mentioned performance degrading features.

    Another thing to keep in mind is your position on the bike. You mention the same seat to pedal measurements but the stack and reach would likely make a much bigger difference as the road bike could have you in a much more aerodynamic position to begin with. If the CX bike has you sitting upright vs a more aerodynamic position on the road bike you would be faster for less power on the road bike even if the weight/tires/aero on the bikes were the same.
    But the thing with power is that you just go slower. Having fatter tires don't matter unless I was setting a speed goal for both bikes but I'm just riding as hard as I reasonably can for an hour in both situations.

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    Quote Originally Posted by Lombard View Post
    I'm confused by the OP being this obsessive about this stuff.
    Yea it's super confusing that I spent $1,500 on two glorified scales and want them to be consistent within less than 10x worse than their claimed variance
    Quote Originally Posted by Srode View Post
    What power meters are you using?

    When you say it's the same position relative to the crank - does that mean you've dropped a plumb bob from the nose of the saddle on both bikes and measured the position from that string to the middle of the crank on both bikes and it's the same? And the saddle height is the same also?
    Stages and yes I dropped a plumb bob off the nose

  18. #18
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    Quote Originally Posted by thisisthebeave View Post
    But the thing with power is that you just go slower. Having fatter tires don't matter unless I was setting a speed goal for both bikes but I'm just riding as hard as I reasonably can for an hour in both situations.
    taodemon mentions stack and height differences. The position on the cross bike could be more efficient than the position on the road bike. You're doing energetic tempo on both, riding "as hard as" you "reasonably can?"

    Add to positioning, relative frame flex. Measurements are a function of stress off the crank, right? A light weight road bike could be imperceptibly "flippier" than the cross bike. The frame could dissipate more energy than the stouter cross frame, thereby lessen the stress on the sensor and give a lower reading for the same perceived efforts.

  19. #19
    tlg
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    So you never answered.... can you swap the power meters between bikes?


    Have you tried them on a trainer? That would eliminate any aero/weight/elevation variations. Run each bike for 5min holding the same speed. Compare the power results.
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  20. #20
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    Quote Originally Posted by taodemon View Post
    Fatter tires, heavier bike, less aero could very likely explain the watt difference depending on how much fatter, heavier or less aero the cx bike is. You are basically using more watts to go slower which isn't unreasonable if the cx bike has all the previously mentioned performance degrading features.

    Another thing to keep in mind is your position on the bike. You mention the same seat to pedal measurements but the stack and reach would likely make a much bigger difference as the road bike could have you in a much more aerodynamic position to begin with. If the CX bike has you sitting upright vs a more aerodynamic position on the road bike you would be faster for less power on the road bike even if the weight/tires/aero on the bikes were the same.

    This is what I was thinking, too; it takes more effort to push a heavier bike than a lighter bike and therefore more watts, but you don't necessarily go faster or slower, just different efforts for the same speed.

  21. #21
    tlg
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    Quote Originally Posted by Methodical View Post
    This is what I was thinking, too; it takes more effort to push a heavier bike than a lighter bike and therefore more watts
    Not on a flat road. A 5lb heavier bike will reduce your speed ~0.04mph. Nowhere in the realm of 20watts.

    And on a 10% climb, a 5lb heavier bike requires 11watts more.
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  22. #22
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    Quote Originally Posted by tlg View Post
    Not on a flat road. A 5lb heavier bike will reduce your speed ~0.04mph. Nowhere in the realm of 20watts.

    And on a 10% climb, a 5lb heavier bike requires 11watts more.
    So the theory that a heavier bike requires more watts is correct, then, whether is 2 watts, 10 watts or whatever. It's part of the problem. Now, along with some of the other stuff, it can add up to what the OP is seeing.

    Note: I will assume you used some sort of calculation to come up with your numbers above, right.

  23. #23
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    Quote Originally Posted by thisisthebeave View Post
    But the thing with power is that you just go slower. Having fatter tires don't matter unless I was setting a speed goal for both bikes but I'm just riding as hard as I reasonably can for an hour in both situations.
    The fatter tires could be making it so you have to exert more watts for less speed so yes they will matter even in this to some degree. How is your heart rate for both bikes on the same loop. If it is equal then there might be something off. If you heart is slightly higher on the CX bike loops with the higher watts then you are likely going harder, even if your overall speed results are slower.

    Another way to test this would be to put both bikes on a smart trainer and see how close the power meter readings are in relation to the power the smart trainer is reading. Depending on what you have access to this might be easier than swapping the power meters which would still be the most effective way of getting to the bottom of things.

  24. #24
    tlg
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    Quote Originally Posted by Methodical View Post
    So the theory that a heavier bike requires more watts is correct, then, whether is 2 watts, 10 watts or whatever. It's part of the problem.
    It's not a theory. It's physics. Yes, about 1 watt. In the scope of this conversation, it's not "MUCH higher power" of 20 watts

    Note: I will assume you used some sort of calculation to come up with your numbers above, right.
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  25. #25
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    Quote Originally Posted by taodemon View Post
    The fatter tires could be making it so you have to exert more watts for less speed so yes they will matter even in this to some degree. How is your heart rate for both bikes on the same loop. If it is equal then there might be something off. If you heart is slightly higher on the CX bike loops with the higher watts then you are likely going harder, even if your overall speed results are slower.

    Another way to test this would be to put both bikes on a smart trainer and see how close the power meter readings are in relation to the power the smart trainer is reading. Depending on what you have access to this might be easier than swapping the power meters which would still be the most effective way of getting to the bottom of things.
    Question. Curious, can't the OP upload the rides to Strava and let it calculate power to see if Strava sees a difference just to see. I know the PM should be more accurate, but for comparison sake. Just out of curiosity.

    Note: I'm not very familiar with Strava though, so I don't really know how it all works.

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