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The names of the tubes are well established for standard single-rider diamond frames, but seemingly less so for tandems.

For tandems, there's a bit more variety in frame design, but the one I've just bought - for example - has 3 lengthwise tubes plus the downtube: a sloped toptube, a horizontal tube between the 2 bottom brackets ("bottom tube"?) and a third tube in between them (I've no idea what to call that). all of these meet the head tube together.

Tandem, but what are the tubes called

The tube in the middle is common but not always used the same way - Dawes, for example, have the real toptube meet this diagonal tube behind the headtube, so the uppermost tube isn't straight in the front triangle. Others have a horizontal front toptube and very sloped rear toptube, allowing stepping through the frame.

Is there a standard set of tube names for tandems?

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    This answer has a diagram, but does not contain any references or much in the way of discussion or alternative terminologies: bicycles.stackexchange.com/a/74186/452
    – Will Vousden
    Commented Sep 5, 2022 at 9:10
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    Meanwhile, this page suggests that what's termed the "lateral tube" in the above link can also be called the "stiffener tube".
    – Will Vousden
    Commented Sep 5, 2022 at 9:44
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    @WillVousden I prefer the 2nd - "lateral" seems too much like it should mean a tube going across rather than along. OTOH "stiffener" is a good description of the purpose, less so if we want to refer a piece of metal. It establishes "keel tube" though, which I hadn't come across
    – Chris H
    Commented Sep 5, 2022 at 10:06
  • I wonder if "lateral" is more to do with the twin-tubes of a mixte, which are often thinner than a single tube. My post at bicycles.stackexchange.com/a/74187/19705 was a bit vague originally, and noone's managed to find a clarification. Could be the small spreader-tube is the source of "lateral" ?
    – Criggie ♦
    Commented Mar 9, 2023 at 2:42
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    @Criggie I think I've seen a tandem with twin tubes similar to that, but I'm not sure if it had a spreader tube. That little tube certainly does run laterally. I guess a nearly-longitudinal tube could be named for providing lateral stiffening, and abbreviated
    – Chris H
    Commented Mar 9, 2023 at 6:56

1 Answer 1

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Saying that there is a standard is a stretch. The tubes that are common to tandem and single could be called "standard."

  • Head tube
  • Down tube
  • Seat stay
  • Chain stay

Then things start to vary.

Here is one diagram of tube terminology, referred to in its article as a "direct internal design". Other types in the article are the "double diamond", "Marathon" and "ST design".

Direct internal design diagram

The frame in the original question is a lot like the one in the diagram. The biggest difference is that the top tubes are more down angled into the seat tubes.

Here's another diagram

Annotated diagram of a Trek tandem

The second drawing breaks up the front and back top tube into "rear top tube" and "front top tube". Comparing both drawings, these are the parts unique to a tandem:

  • Captain seat tube or front seat tube
  • Stoker seat tube or rear seat tube
  • Captain top tube or front top tube
  • Stoker top tube or rear top tube
  • Keel tube or boom/bottom tube

Both drawings have the lateral tube. We could break up the lateral tube into captain/front and stoker/back lateral tube.

If you were talking with someone who knew that the front spot on the tandem is the captain and the rear seat is the stoker, then I think those terms—along with the tube name—would communicate the location of the tube. Front and back would probably be the most widely understood terms.

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