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  1. #1
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    Transferring fit between two bikes - how to pick stem length?

    I'm getting a new frame and want to transfer my current fit over. I'll be using the same bar and saddle, etc but I do need to buy a new stem seeing how this bike has a 1.25" upper stem clamp. I'd prefer to only buy one stem to get the same fit so I don't have to waste money and time fiddling around. Can I do this based on stack and reach? I.e. if my current bike has 100mm long stem but the new bike has 10mm more reach would I get a 90mm stem for the same fit? Assuming the same angle on the stem of course.

  2. #2
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    For frame stack and reach, not really. Reach and stack are dependent on each other, so if they aren't already both identical you aren't going to be able to get a number from them.

    It would actually be easier to use level top tube length, adjusted for any difference in seat tube angle. That will give you a horizontal reach number that can be used to find a horizontal stem length, which you then would have to correct if you are planning on having a stem with a large angle difference from horizontal.
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  3. #3
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    Quote Originally Posted by Kontact View Post
    It would actually be easier to use level top tube length, adjusted for any difference in seat tube angle.
    Isn't this effectively what reach is? Assuming my saddle setback is set up to be the same shouldn't the reach numbers be directly comparable?

  4. #4
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    Quote Originally Posted by alxrmrs View Post
    Isn't this effectively what reach is? Assuming my saddle setback is set up to be the same shouldn't the reach numbers be directly comparable?
    It should be, but it is measured to an arbitrary point on the head tube, so taller head tubes make the reach shorter. If the stack was identical on both bikes than the reach is comparable, if not there will be errors.

    If the stack is close, then the error won't be much either. But the error is about 3-4mm per 1cm of stack difference.
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  5. #5
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    I see what you're saying. The stack is very close of 519mm ('13 SuperSix) and 517mm ('18 Giant TCR disc). I imagine this is where head tube angle comes into play and I have to account for that in the new stem.

  6. #6
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    Quote Originally Posted by alxrmrs View Post
    I see what you're saying. The stack is very close of 519mm ('13 SuperSix) and 517mm ('18 Giant TCR disc). I imagine this is where head tube angle comes into play and I have to account for that in the new stem.
    The difference between any two HT angles is not going to matter - it is only a degree or two over a very short distance. But all head tubes are between 15 and 18° off of vertical, which is why a horizontal measure to an arbitrary point adds up so much.

    But for those two bikes, reach should be accurate enough since it is only 2mm of stack difference, which will only affect reach by .6mm.
    Get a better saddle: www.kontactbike.com

  7. #7
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    Quote Originally Posted by alxrmrs View Post
    I'm getting a new frame and want to transfer my current fit over. I'll be using the same bar and saddle, etc but I do need to buy a new stem seeing how this bike has a 1.25" upper stem clamp. I'd prefer to only buy one stem to get the same fit so I don't have to waste money and time fiddling around. Can I do this based on stack and reach? I.e. if my current bike has 100mm long stem but the new bike has 10mm more reach would I get a 90mm stem for the same fit? Assuming the same angle on the stem of course.
    So an 1.25 steerer tube ?. Didn’t know they made those, thought they were all either 1” or 1-1/8”.

  8. #8
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    Quote Originally Posted by Steve B. View Post
    So an 1.25 steerer tube ?. Didn’t know they made those, thought they were all either 1” or 1-1/8”.
    It's called "Overdrive 2" technology by Giant. Canyon does it too.

  9. #9
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    do it like any bike fit - set your saddle height and set back first. Use a plumb bob off the nose of your saddle to measure how far back it is from the centerline of the crank on your current bike. Measure the saddle height from the center of the crank to the same reference point on the saddle rail on your old bike and transfer both of these to the new bike. Then measure the distance from the nose of the saddle to the back of the handle bar on the old bike and compare it to what you have on the new bike. The difference is going to be how much longer or shorter you need the new stem within a couple mm. It's not perfect because the angle of the stem makes the effective distance a little bit shorter than the measured length of the stem, but unless you are getting a custom length stem this will get you to the right size stem.
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  10. #10
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    Quote Originally Posted by Srode View Post
    do it like any bike fit - set your saddle height and set back first. Use a plumb bob off the nose of your saddle to measure how far back it is from the centerline of the crank on your current bike. Measure the saddle height from the center of the crank to the same reference point on the saddle rail on your old bike and transfer both of these to the new bike. Then measure the distance from the nose of the saddle to the back of the handle bar on the old bike and compare it to what you have on the new bike. The difference is going to be how much longer or shorter you need the new stem within a couple mm. It's not perfect because the angle of the stem makes the effective distance a little bit shorter than the measured length of the stem, but unless you are getting a custom length stem this will get you to the right size stem.
    This is what I would do, and have done, for my rare bike purchases. I generally measure from the back of the saddle, since that is easy to hook a tape over, but the process is the same.

    It does not help if you don't have both bikes handy, and built up mostly. If you want to get the right stem when ordering a frame, for example, then you have to figure the numbers and hope you do it right.


    Quote Originally Posted by alxrmrs View Post
    I'm getting a new frame and want to transfer my current fit over.
    What's the old frame and the new frame? Brand, model, year, and size? If you give that, and maybe a geometry chart, I bet you someone here will give you their take on the specific numbers.

    Not me, but someone.
    .
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  11. #11
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    Quote Originally Posted by Srode View Post
    do it like any bike fit - set your saddle height and set back first. Use a plumb bob off the nose of your saddle to measure how far back it is from the centerline of the crank on your current bike. Measure the saddle height from the center of the crank to the same reference point on the saddle rail on your old bike and transfer both of these to the new bike. Then measure the distance from the nose of the saddle to the back of the handle bar on the old bike and compare it to what you have on the new bike. The difference is going to be how much longer or shorter you need the new stem within a couple mm. It's not perfect because the angle of the stem makes the effective distance a little bit shorter than the measured length of the stem, but unless you are getting a custom length stem this will get you to the right size stem.
    You'd need to copy saddle to bar drop before measuring saddle to bars distance.

  12. #12
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    Simple answer. Laser level.

    Get one of the laser levels, or borrow one, that does horizontal and vertical lines at the same time.

    Stand the bike upright, not leaning, against something, use a rubber band to secure the handlebar drop end if needed to keep it from rolling away.

    Put the laser on something so that the height is exactly lined up with the center of the bottom bracket.

    Use a T-Square level put against both of the tires on the same point, on the ground, to line up the laser so that it's pointing at the bike exactly perpendicular to the wheels.

    Make sure the cross hairs are exactly in the center of the bottom bracket.

    Once setup, you can use a tape measure to measure setback from the vertical line. You can measure height from the tip and the back to get both height and angle. You can follow it up with an angle finder on a flat surface put on the saddle to double check the angle.

    Once you have everything recorded, move the bike away and put the second bike on the laser setup.

    This method works for me within a millimeter in all directions for all parts of the bike fit. As long as the cross hairs stay in the center of the bottom bracket and you can read a tape measure, this is the best at home method in terms of accuracy.

    Measuring from the wall or floor or anything else really is sloppy work and leads to sloppy bike fits.
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  13. #13
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    Quote Originally Posted by Jay Strongbow View Post
    You'd need to copy saddle to bar drop before measuring saddle to bars distance.
    Yep, forgot that, assuming you can get the same drop - depends on the frames. (first cup of coffee error).
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  14. #14
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    This..

    Quote Originally Posted by MMsRepBike View Post
    Simple answer. Laser level.

    Get one of the laser levels, or borrow one, that does horizontal and vertical lines at the same time.

    Stand the bike upright, not leaning, against something, use a rubber band to secure the handlebar drop end if needed to keep it from rolling away.

    Put the laser on something so that the height is exactly lined up with the center of the bottom bracket.

    Use a T-Square level put against both of the tires on the same point, on the ground, to line up the laser so that it's pointing at the bike exactly perpendicular to the wheels.

    Make sure the cross hairs are exactly in the center of the bottom bracket.

    Once setup, you can use a tape measure to measure setback from the vertical line. You can measure height from the tip and the back to get both height and angle. You can follow it up with an angle finder on a flat surface put on the saddle to double check the angle.

    Once you have everything recorded, move the bike away and put the second bike on the laser setup.

    This method works for me within a millimeter in all directions for all parts of the bike fit. As long as the cross hairs stay in the center of the bottom bracket and you can read a tape measure, this is the best at home method in terms of accuracy.

    Measuring from the wall or floor or anything else really is sloppy work and leads to sloppy bike fits.

  15. #15
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    A regular level works fine, as does using surface you know is level and a plumb bob.
    Get a better saddle: www.kontactbike.com

  16. #16
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    Sign up for this website (it's easy and free). Allows comparison between bikes including using different length stems, stem degree (+/-) as well as spacers. Just be certain the Geo listed for your bike is correct as I have seen a few that were off just a bit.

    https://app.velogicfit.com/login


    Quote Originally Posted by alxrmrs View Post
    I'm getting a new frame and want to transfer my current fit over. I'll be using the same bar and saddle, etc but I do need to buy a new stem seeing how this bike has a 1.25" upper stem clamp. I'd prefer to only buy one stem to get the same fit so I don't have to waste money and time fiddling around. Can I do this based on stack and reach? I.e. if my current bike has 100mm long stem but the new bike has 10mm more reach would I get a 90mm stem for the same fit? Assuming the same angle on the stem of course.

  17. #17
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    Quote Originally Posted by KMan View Post
    Sign up for this website (it's easy and free). Allows comparison between bikes including using different length stems, stem degree (+/-) as well as spacers. Just be certain the Geo listed for your bike is correct as I have seen a few that were off just a bit.

    https://app.velogicfit.com/login
    I can't seem to find a sign up page.

  18. #18
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    Quote Originally Posted by alxrmrs View Post
    I can't seem to find a sign up page.

    sorry, try this link
    https://www.velogicfit.com/

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