How do you measure a horizontal top tube length?- Mtbr.com
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  1. #1
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    How do you measure a horizontal top tube length?

    How do manufacturers measure the horizontal top tube length? Is it from the middle of the top of the headtube and follow a horizontal line until the middle or center of the seatpost?

  2. #2
    dru
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    the spot in the middle of the HT exactly in the middle of the TT junction (where the center lines of the two tubes meet) and from there measure back to the seat post horizontally.
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  3. #3
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    Horizontally? If you're comparing what you have to a new frame it pays to measure the old frame from the same points at the new one. That way you're not accidentally comparing Top Tube to Estimated Top Tube.
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    Authorities speculate that speed may have been a factor. They are also holding gravity and inertia for questioning.

  4. #4
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    CP is correct. Measuring from the center of the head tube along the top tube to the center of the seat tube gives you "actual" top tube length. Measuring from the top center of the head tube horizontally to the spot where the seat tube is intersected gives you "effective" top tube length. Effective top tube length is the more useful measurement. However, as noted, when comparing frame geometry it is best to use the method that the frame manufacturer uses for their specs. Some manufacturers will give you both measurements, most will give you effective top tube, and a few only give actual top tube. Effective and actual top tube lengths will be different. How much different depends on the frame design, head tube angle and seat tube angle. So make sure you know which measurement you are being given and compare accordingly.

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  5. #5
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    I'm comparing a local hardtail from my GF to a Trek Lush. The Lush geometry is for the effective head tube.

    I checked at Alubike site, and it says there a number, but my GF bike is a 2003 model and the site only has it for 2012, and i don't know if the frame has been through changes.

    Based on the site, the Alubike top tube in the 14" frame the top tube effective length is the same as the 15.5 lush, which is what I would like, but I prefer to measure and try to see if it's close.

    I'm attaching some pics. I guess that the seatpost point is the right one, and on the headset, I put two points (A and B), which should be the starting point?
    Attached Thumbnails Attached Thumbnails How do you measure a horizontal top tube length?-headtube_start.jpg  

    How do you measure a horizontal top tube length?-seatpost_end.jpg  


  6. #6
    dru
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    You guys are wrong, sorry. Go ask the frame builders forum guys if you don't beleive me. Effective Top Tube (ETT) is what the OP is asking for mistakenly. It is measured exactly as I said. It just so happens to be the exact way you'd measure an old school frame with a horizontal TT. If the OP was asking for the actual sloped TT length I would have told him to meausre from the same place on the head tube. All bike blue prints use the centre lines of each tube for all the measurements, be it TT, HT, DT etc.. ETT came about with the introduction of sloped top tubes, since there would be no good way for assessing frame fit otherwise.

    Here's a drawing from Bike Cad showing exactly how ETT is measured (it is the same dimension that you guys are incorrectly calling estimated or horizontal) The E stands for 'effective', since the top tube on a sloped top tube is not horizontal, but ETT is.

    I've built two frames now, that's how it's done....(I didn't generate the layout of these dimensions, bike cad did. Not everything is shown.)



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  7. #7
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    Quote Originally Posted by dru View Post

    I've built two frames now, that's how it's done....)

    Drew
    Drew? I didn't know God had an actual name.

    Gee, I'll have to send letters of apology to everyone that was on the US National Team from 1988-1992, every fitting I've ever done from 1987-today and re-fit every bike I own. Maybe if I build a second frame I'll get it right.
    Authorities speculate that speed may have been a factor. They are also holding gravity and inertia for questioning.

  8. #8
    dru
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    Caffeine, apologies for my apparent arrogance. That was an over-reaction to my fellow poster's 'help'. While I don't doubt you are speaking the truth as to your experiences, I have never seen a frame blue-print taking TT dimensions from any other location than the one I described. Just because you and the national team do it one way does not make it the 'standard'. I only got into frame building a few years back but have been riding since the 90s and all the geometry charts I've ever seen have it the same as the frame picture I've posted. As for my building two frames, I figured the experts there would have jumped all over me if I had posted dimensions unlike the ones I presented to you. Unlike myself, many of the frame builders have enough cred, don't they?

    Sorry for coming across in a bad way.

    Drew
    occasional cyclist

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    Quote Originally Posted by rzozaya1969 View Post
    I'm comparing a local hardtail from my GF to a Trek Lush. The Lush geometry is for the effective head tube.

    I checked at Alubike site, and it says there a number, but my GF bike is a 2003 model and the site only has it for 2012, and i don't know if the frame has been through changes.

    Based on the site, the Alubike top tube in the 14" frame the top tube effective length is the same as the 15.5 lush, which is what I would like, but I prefer to measure and try to see if it's close.

    I'm attaching some pics. I guess that the seatpost point is the right one, and on the headset, I put two points (A and B), which should be the starting point?
    Does not really matter if you use "A", "B", or the center of the HT/TT junction. Since you are measuring horizontally the effective TT length will be nearly identical.
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  10. #10
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    Quote Originally Posted by shiggy View Post
    Does not really matter if you use "A", "B", or the center of the HT/TT junction. Since you are measuring horizontally the effective TT length will be nearly identical.
    Just have to account for varying HT/ST angles
    Honestly... ahh I give up

  11. #11
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    Quote Originally Posted by highdelll View Post
    Just have to account for varying HT/ST angles
    Maybe, but unless it's a big difference in the angles, I'm starting to think that it won't make a huge or significative difference, maybe just one or two mm.

    Thanks to everybody...

  12. #12
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    Quote Originally Posted by dru View Post
    Caffeine, apologies for my apparent arrogance. That was an over-reaction ... Sorry for coming across in a bad way.

    Drew
    No problem, we're good. Let's get this guy on a bike.

    Below is a document Park made available a while back. I've been using a similar form for years. So long as the measuring is done from the same points every time, it is easily transferable from bike to bike so a rider can have multiple bikes be the same.
    Attached Thumbnails Attached Thumbnails How do you measure a horizontal top tube length?-mtb-positioning-chart.jpg  

    Authorities speculate that speed may have been a factor. They are also holding gravity and inertia for questioning.

  13. #13
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    Quote Originally Posted by dru View Post
    Caffeine, apologies for my apparent arrogance. That was an over-reaction to my fellow poster's 'help'. While I don't doubt you are speaking the truth as to your experiences, I have never seen a frame blue-print taking TT dimensions from any other location than the one I described. Just because you and the national team do it one way does not make it the 'standard'. I only got into frame building a few years back but have been riding since the 90s and all the geometry charts I've ever seen have it the same as the frame picture I've posted. As for my building two frames, I figured the experts there would have jumped all over me if I had posted dimensions unlike the ones I presented to you. Unlike myself, many of the frame builders have enough cred, don't they?

    Sorry for coming across in a bad way.

    Drew
    It appears that manufacturers measure this both ways.
    Here is a geo chart from Salsa:

    And one from Surly
    15mm is a second-best solution to a problem that was already solved.

  14. #14
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    Looked around a little more, and while most companies do not specify, the first three that had diagrams (Kona, Pivot, and C-Dale) all measure from the center of the TOP of the head tube, NOT where it meets the top tube:
    Kona:


    Pivot:


    C-Dale:
    15mm is a second-best solution to a problem that was already solved.

  15. #15
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    Yeah, the brands' charts are only helpful if they have the relevant info. I'm hoping he's able to measure the frames in person so he can put the ends of the tape on the same points on both bikes.
    Authorities speculate that speed may have been a factor. They are also holding gravity and inertia for questioning.

  16. #16
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    Good info.

  17. #17
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    Quote Originally Posted by kapusta View Post
    Looked around a little more, and while most companies do not specify, the first three that had diagrams (Kona, Pivot, and C-Dale) all measure from the center of the TOP of the head tube, NOT where it meets the top tube:
    Kona:


    Pivot:


    C-Dale:
    It sucks that. Some companies give a chart, other just the numbers.

    The bike we're searching for is a Trek and, from the diagram, it does look that it's measured from the headtube.

    We're going to buy the bike in a week or so, but the bike should arrive at the LBS pretty soon (I hope), so I want to go with my GF to measure it. Unfortunately, I don't know which stem it comes with, the salesman told us it comes with a 90mm for the small, which I feel is pretty large, but that can be solved with a shorter stem.

    I want my GF to test the bike before buying. From my measures, and if I go with from where I think that Trek measures the bike and comparing to her current bike, both look pretty similar, and I'm having her testing as many bikes as possible for her size, but unfortunately it's hard to do here in Mexico.

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