Advertise on Bikeforums.net



User Tag List

Results 1 to 16 of 16
  1. #1
    Senior Member
    Join Date
    Feb 2014
    Location
    Wind Tunnels of Cheyenne
    My Bikes
    Burley Duet [of some unknown year]; And always one less than I think I really need.
    Posts
    160
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Frame design question

    I tried to attach a photo, but I'm computer insufficient...

    I had some pictures of frames explained to me, and I have a frame design question. In the photos, some of the frames have what I would call a more traditional attachment between the headtube, toptube, lateral, and downtube. (TT near the top of the HT, lateral in the middle, and DT on the lower part of the HT. In some of the pictures however, the lateral is at the top of the HT (the DT is still at the bottom), and the TT attaches about 6" or so down the lateral. WHY? All the frames described are supposed to be of the same model, and the person describing the photo didn't seem to be able to see a rational like frame size (both larger and smaller frames had both designs). Is this common, and my newly-to-tandems eyes just hasn't seen it yet? Could the second design (with the TT attached down the lateral) be stiffer? I'll contact Duratec in the morning, but they sometimes take a couple days to respond, and this may cause sleepless nights pondering...

    Sorry, no pics, I haven't learned the secret handshake yet.
    LKW

  2. #2
    Member
    Join Date
    Dec 2013
    My Bikes
    Rodriguez, Ibis
    Posts
    37
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Not sure I'm totally following, but maybe this will help: Stiffener tubes and custom tandems

  3. #3
    Senior Member
    Join Date
    Mar 2013
    Location
    Roseville/Folsom
    My Bikes
    Co-Motion Primera Co-Pilot, Trek Madone 3.1, Trek 7300, Electra Townie 21D
    Posts
    87
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    It is easier to build a frame with the fewest joints and with simple tube connections. Thus most tandems place the lateral into the middle of the head tube. That way there are only 3 tube connections and they are relatively simple fish mouthing shapes to make the tubes meet closely.

    Running the lateral to the top of the head tube provides a better triangulation of the head tube for a stronger connection with less flex. However it does make a much more complicated joint between the top tube and the lateral. It also requires a larger area of thicker butting of both lateral and top tubes to accommodate the larger weld area.

  4. #4
    Senior Member
    Join Date
    Mar 2010
    Location
    City of Brotherly Love
    My Bikes
    Raleigh Companion, Nashbar Touring, Novara DiVano, Trek FX 7.1, Giant Upland
    Posts
    1,065
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    Are you asking about this design?

    81daKdZu7JS._SL1500_-300x137.jpg

    The designers want to keep both top tubes inline so they can't slope the top tube as much as the would on a compact single.

  5. #5
    Senior Member
    Join Date
    Jun 2013
    Posts
    82
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I think standover clearance plays a part as well. With a single bike it's simple; a frame with a shorter top tube has a shorter seat tube and thus the top tube is lower. Also, on a single bike the rider can lean the bike over if clearance is an issue. With a tandem the captain needs to be able to straddle the top tube comfortably with the bike fully upright while the stoker mounts. Also on a tandem, the captain's seat tube length can only be so short because the top tube must connect all the way back to the stoker seat tube.

    Note on this Cannondale how much higher the top tube would be if it connected to the head tube:


    Co-Motion chose to address this issue differently, by allowing the stoker seat tube to be exceptionally short and adding a periscoping seat post:


  6. #6
    Senior Member
    Join Date
    Nov 2011
    Posts
    122
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by mstyer View Post
    I think standover clearance plays a part as well. ... With a tandem the captain needs to be able to straddle the top tube comfortably with the bike fully upright while the stoker mounts.
    +1 on this -- more standover clearance is needed on a tandem than on a single.

  7. #7
    Senior Member
    Join Date
    Feb 2014
    Location
    Wind Tunnels of Cheyenne
    My Bikes
    Burley Duet [of some unknown year]; And always one less than I think I really need.
    Posts
    160
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by geronimo2000 View Post
    Not sure I'm totally following, but maybe this will help: Stiffener tubes and custom tandems
    I've read this one before...wrong end of the stiffener (what I referred t as the lateral tube), but thanks.

  8. #8
    Senior Member
    Join Date
    Feb 2014
    Location
    Wind Tunnels of Cheyenne
    My Bikes
    Burley Duet [of some unknown year]; And always one less than I think I really need.
    Posts
    160
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Bezalel View Post
    Are you asking about this design?

    81daKdZu7JS._SL1500_-300x137.jpg

    The designers want to keep both top tubes inline so they can't slope the top tube as much as the would on a compact single.
    Sorry, blind, so I don't know if your image is what I am talking about, but in the design I am less familiar with, where the lateral [stiffener] attaches to the top of the head tube, the pilot's top tube is angled downward as compared to the stoker's top tube.

  9. #9
    Senior Member
    Join Date
    Feb 2014
    Location
    Wind Tunnels of Cheyenne
    My Bikes
    Burley Duet [of some unknown year]; And always one less than I think I really need.
    Posts
    160
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by vrooom3440 View Post
    It is easier to build a frame with the fewest joints and with simple tube connections. Thus most tandems place the lateral into the middle of the head tube. That way there are only 3 tube connections and they are relatively simple fish mouthing shapes to make the tubes meet closely.

    Running the lateral to the top of the head tube provides a better triangulation of the head tube for a stronger connection with less flex. However it does make a much more complicated joint between the top tube and the lateral. It also requires a larger area of thicker butting of both lateral and top tubes to accommodate the larger weld area.
    I can see the design with the lateral [stiffener] attached to the top of the headtube being stiffer on the vertical axis, but wouldn't two attachment points (lateral and down tube) be less stiff rotationally?

  10. #10
    Senior Member waynesulak's Avatar
    Join Date
    Jan 2008
    Location
    Ft Worth, TX
    My Bikes
    650B tandem converted from Santana Arriva, Santana Noventa, Boulder Bicycle 700C, Gunnar Sport, Trek TX700,
    Posts
    1,694
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by WheelsNT View Post
    +1 on this -- more standover clearance is needed on a tandem than on a single.
    Tandems do need good standover for the captain and that is one reason to use this design. The other reason is to allow for fewer sizes to fit more riders. Make a short standover and the frame fits a wide range of customers on a very limited number of frame sizes to be manufactured by standardized production facilities. Each additional size manufactured in means a whole lot of new slightly different sized tubes and angle cuts on tubes at the joints.

    This is a very important cost factor where the total volume of tandems sold is fairly small compared to singles. In addition to the obvious cosmetic difference, the trade offs are in things like water bottles not fitting in traditional locations without side loading cages or maybe not fitting at all and cramped cable runs that add friction to brake and derailleur cables.

  11. #11
    Senior Member
    Join Date
    Feb 2014
    Location
    Wind Tunnels of Cheyenne
    My Bikes
    Burley Duet [of some unknown year]; And always one less than I think I really need.
    Posts
    160
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by mstyer View Post
    I think standover clearance plays a part as well. With a single bike it's simple; a frame with a shorter top tube has a shorter seat tube and thus the top tube is lower. Also, on a single bike the rider can lean the bike over if clearance is an issue. With a tandem the captain needs to be able to straddle the top tube comfortably with the bike fully upright while the stoker mounts. Also on a tandem, the captain's seat tube length can only be so short because the top tube must connect all the way back to the stoker seat tube.

    Note on this Cannondale how much higher the top tube would be if it connected to the head tube:


    Co-Motion chose to address this issue differently, by allowing the stoker seat tube to be exceptionally short and adding a periscoping seat post:

    Maybe, but the way the photos were described to me, both designs were found on both larger and smaller frames. If standover were the primary issue, then why not build the large frame for a pilot who needs a little more standover a little smaller?

  12. #12
    Senior Member
    Join Date
    Feb 2014
    Location
    Wind Tunnels of Cheyenne
    My Bikes
    Burley Duet [of some unknown year]; And always one less than I think I really need.
    Posts
    160
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by WheelsNT View Post
    +1 on this -- more standover clearance is needed on a tandem than on a single.
    Seems like if this were the primary reason, it could be more easily achieved by attaching the top tubes lower down on the seat tubes as opposed to recalculating and readjusting all the jigs to move the lateral [stiffener] up the headtube and attach the toptube somewhere down the length of the lateral.

  13. #13
    Senior Member
    Join Date
    Nov 2011
    Posts
    122
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by LastKraftWagen View Post
    If standover were the primary issue, then why not build the large frame for a pilot who needs a little more standover a little smaller?
    Making the frame smaller also lowers the bars.

  14. #14
    Member
    Join Date
    Dec 2013
    Location
    Illinois
    My Bikes
    2005 Fuji Roubaix Pro, 1986 Raleigh Olympian, Bike Friday Family Tandem
    Posts
    37
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I think more frame builders are in fact doing that. You can't discount aesthetics and tradition in these kind of design decisions. Mountain bikes were the first to depart from the orthodoxy of horizontal top tubes, and road bikes eventually followed. Now we're seeing the same thing gradually happen in tandems. Historically, bikes were sized by their seat tube, but the cycling community has come to appreciate that the top tube length is probably the most important contributor to a good fit. On modern bikes, stems are easy to swap out, and long seat posts are available, so why not make a frame a little stiffer, lighter, and easier to stand over by shortening the seat tube? Joining the top and lateral tubes before the head tube I think is a way to avoid a very complex joint at the head tube on shorter head tubes, and it probably saves a little weight as well.

  15. #15
    Senior Member
    Join Date
    Mar 2013
    Location
    Roseville/Folsom
    My Bikes
    Co-Motion Primera Co-Pilot, Trek Madone 3.1, Trek 7300, Electra Townie 21D
    Posts
    87
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by LastKraftWagen View Post
    I can see the design with the lateral [stiffener] attached to the top of the headtube being stiffer on the vertical axis, but wouldn't two attachment points (lateral and down tube) be less stiff rotationally?
    Perhaps minimally since the lateral on a conventional setup attaches near the center of the head tube (and thus at the rotational center so to speak) it provides less strength. One has to consider the loads on the bike frame and the weight carrying is the much larger load here than twisting loads. The structure of the entire frame is rather handicapped dealing with twisting loads being close to a flat plane.

  16. #16
    Senior Member
    Join Date
    Feb 2014
    Location
    Wind Tunnels of Cheyenne
    My Bikes
    Burley Duet [of some unknown year]; And always one less than I think I really need.
    Posts
    160
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Update: The official word from the builder is based on the difference in height between riders... With a pilot much taller than a stoker, the toptube drops into the lateral and when they are closer in height the toptube meets the steer tube.

Posting Permissions

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