Advertise on Bikeforums.net



User Tag List

Results 1 to 5 of 5
  1. #1
    Senior Member
    Join Date
    Feb 2014
    Location
    Wind Tunnels of Cheyenne
    My Bikes
    Burley Duet [of some unknown year] (the guinea pig); 2001 Ventana ECDM (the project); And always one less than I think I really need.
    Posts
    334
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Tandem headtube attachment design question

    I can't attach a photo, and I am blind so I had someone describe the following, so I hope it makes sense and someone can make sense of my question.

    I'm looking at a custom tandem frame, and the builders have sent along some pics of their completed works, and for the same frame model there appears to be two different designs for how the toptube, lateral [stiffener] and downtube attach to the head tube. Hoping someone has that simple "this is the difference" answer.

    In the first design (which I've come across all the time), the toptube attaches to the top of the headtube and the downtube to the bottom, with the lateral somewhere in the middle (usually with all three tubes more or less "touching").

    In the design I've never seen before, the downtube is still attached at the bottom of the headtube, but the lateral [stiffener] is attached tp the top of the headtube, with the toptube sloping downward and attaching to the lateral approximately 6" before the headtube joint.

    The question is "WHY?"

    Some opinions are in regards to standover height (usually more is required for tandems than comparibly sized singles), but both designes are pictured in both larger and smaller frames...If it were standover then why not (in the case of the larger frames especially) just make the captain's part of the frame smaller, or lower where the toptubes meet the seattubes down?

    Another opinion is front end stiffness, and it seems the second design would be more stiff on a vertical axis with the lateral/downtube/headtube triangle being "larger," but would this also reduce rotational stiffness around the horizontal axis?

    Any opinions would be appreciated. Sorry, no pics...I haven't learned the bikeforums secret handshake yet.
    LKW

  2. #2
    Andrew R Stewart Andrew R Stewart's Avatar
    Join Date
    Feb 2012
    Location
    Rochester, NY
    My Bikes
    Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Raleigh Pro, Trek Cycle Cross, Mongoose tomac ATB, GT Bravado ATB, JCP Folder
    Posts
    3,724
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    LKW- Outside of fashion the difference in potential stand over height is the usual reason to have the TT join the lateral behind (and what will be below) the top of the head tube. One might make an argument of easier tube mitering and prebrazing set up but i think it's a debatable point. Another possible reason is to change the front end's strength/stiffness. Again I think this reason is not a biggie. Andy.

  3. #3
    tuz
    tuz is offline
    Senior Member
    Join Date
    Dec 2006
    Location
    Toronto/Montréal
    My Bikes
    Homemade mixte, track, commuter and road, Ryffranck road
    Posts
    1,160
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Is that what you mean?

    sears-tandem.jpg

    My first guess for that design would be standover height. But if they make it in different sizes then perhaps it is to accommodate a longer headtube which would allow for a higher handlebar height?
    homebuilt commuter, mixte, road and track (+ Ryffranck road)
    bla bla blog

  4. #4
    Senior Member
    Join Date
    Sep 2012
    Posts
    1,535
    Mentioned
    8 Post(s)
    Tagged
    0 Thread(s)
    LKW; I would posit that there were probably several root causes. I'll suggest a few;

    -> Need for small frames with upright captain's position
    -> Need for easy fitting of those couplers to yield take apart frames
    -> Maybe someone did the math and found it works well enough
    -> Maybe it saves 100 grams
    -> Maybe to ease the mitering on the tubes or the number of angle settings
    -> Just to be different?

    Haven't seen one yet that had much appeal for what that matters.

    /k

  5. #5
    Senior Member
    Join Date
    Apr 2009
    Location
    New Rochelle, NY
    My Bikes
    too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
    Posts
    22,424
    Mentioned
    66 Post(s)
    Tagged
    1 Thread(s)
    A key consideration in how three tubes are attached at the head is the length of the head itself. On taller frames there's plenty of room at the head for the tubes to be brought up to it individually.

    But smaller frames have limited space, so alternatives are considered. That's also true if there's an effort to lower the standover height.

    I don't think it's a question of better or worse, but what is best suited to a particular purpose.
    FB
    Chain-L site

    An ounce of diagnosis is worth a pound of cure.

    “Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

    “One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

    WARNING, I'm from New York. Thin skinned people should maintain safe distance.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •