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  1. #1
    mtbr member
    Join Date
    Dec 2012

    Endo: How do you measure ETT?

    The M specs on the website list the ETT at 598.
    I'm grabbing a measurement from the middle/top of the head tube to the middle of the seat post parallel to the ground. I come up with less than 570mm. The ride feels cramped and when I'm standing and balanced on the bike, my head is a little forward of the stem, which I'm not used to.

    Can someone take measurements and give your own thoughts/opinions?

    I'm running a 50mm stem, which I have on my old bike, and a straight seatpost.

  2. #2
    mtbr member
    Join Date
    Apr 2008
    Sounds like you may have a small frame. I have seen it happen before in the shop where a bike comes with the incorrect sizing marked. Measure the seat tube and see what you get.

  3. #3
    Reputation: Dusty Bottoms's Avatar
    Join Date
    Jan 2004
    Measuring top tube length: it is generally considered to be a horizontal line from the center axis of the top of the head tube, that goes backwards and interests with the seat tube axis. The trick now is what is the difference between TT length and ETT length? On a frame with traditional construction (i.e. where the seat tube axis pierces the BB shell's axis), there ETT length and the TT length are exactly the same.

    This pic should explain everything:

    On a frame where the seat tube axis does NOT pierce the BB shell axis, the standard TT length then becomes meaningless because the seat tube angle doesn't mean anything. This is a feature on many, many modern frames, not just Knolly frames. Consider the multitude of frames that have "bent" seat tubes (whether actually bent or hydroformed aluminum tubes, or are laid up in carbon this way). While the bottom of the seat tube may match up with the BB shell, the seat tube axis (where the seat post is installed into the seat tube) does NOT pierce the BB shell axis. This is extremely common to ensure that there is enough room for the rear wheel / rear linkage under full compression of the frame. On these kinds of frames, the actual seat tube angle can not be used to make an effective measurement of the top tube length. Additionally, because the actual angle of the seat tube is more slack than the normal range of 72-74 degrees (for an MTB) when the seat is raised, it's hard to get a good indication of where the seat will be a) relative to the BB axis, and b) relative to the head tube.

    Hence, the creation of the ETT - Effective Top Tube length. The idea here is to have a "virtual" seat tube: this is essentially an imaginary axis that is at a prescribed angle (i.e. 73 degrees) and this axis is considered to pierce the BB shell axis. On any well designed frame with a seat tube that doesn't pierce the BB shell axis, the actual seat tube and the virtual seat tube should meet up where the saddle would be in a normal pedaling position. When dropped, the saddle will move slightly forwards, away from the virtual seat tube axis. While it's very obvious on our frames (because the seat tube intersects the down tube visually), the same situation exists on many (in fact, most) modern frames with travel more than about 5" because the tire and rear linkage need somewhere to go when fully compressed.

    The pic above gives a good (but not quite to scale) idea of what happens here. Obviously, I can't speak for other manufacturers, but the situation shown in the pic (with the continuous seat tube) is exactly how Knolly frames are designed. There are obviously a few more tweaks than this in determining frame geometry, but this gives the general idea, and how ETT length and TT length are similar, but not quite the same thing. It also explains how we deal with ensuring that we have enough room behind the seat tube for the rear wheel and linkage, while still allowing a full length seat tube to be used in the frame (especially important for medium and small frames and customers using dropper posts). Assuming that the seat tube angle is reasonable, ETT is the correct measurement to examine when trying to determine the top tube length for fitting frame sizes, as (actual) TT length is meaningless for any frame that doesn't have a seat tube axis that pierces the BB shell axis.
    Director of Sales: Knolly Bikes

  4. #4
    mtbr member
    Join Date
    Dec 2012
    Thanks for the info, Dusty.

    How/why did knolly come up with the 73 degree virtual seat tube angle?

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