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LastKraftWagen 03-19-14 08:50 AM

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.

Andrew R Stewart 03-19-14 07:47 PM

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.

tuz 03-20-14 07:03 AM

1 Attachment(s)
Is that what you mean?

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?

ksisler 03-31-14 02:25 AM

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.


FBinNY 03-31-14 03:43 PM

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.

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