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8/6 Single Butt Down Tube

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8/6 Single Butt Down Tube

Old 12-31-20, 02:03 AM
  #1  
TiHabanero
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8/6 Single Butt Down Tube

If using a single butt .8/.6 31.7mm tube for a down tube, which end is preferred at the BB shell? Butt length is 190mm with 40mm taper. Using lugs. My thought is to put the .8 end at the head tube to minimize flex at the head tube aiding stability. Is that just poppy-cock?
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Old 12-31-20, 02:05 AM
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unterhausen
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Isn't that a seat tube? What tube is it?
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Old 12-31-20, 02:33 AM
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guy153
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I would put the thick end at the HT as well. Might be poppycock. But I think Brodie said he also puts the longer end at the HT (a double butted tube has a long end if you only cut it in one place, which is quite common to do).

You've got lots more metal down by the BB anyway is what I figure. Also I had a frame fail at the DT/HT junction once (not a frame I built) so according to my anecdata that's a failure point. You've probably got some nasty flex going on up there from the fork flailing around over bumps.
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Old 12-31-20, 11:34 AM
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Yes, it is a "seat " tube, but really what is a tube anyway? Can't get a tube with double butt, so I am stuck with a single but. .6 is mighty thin, but will work as far as I can tell. I hope no stress fracture in the future, but since the frame is for me, no liability there.
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Old 12-31-20, 01:23 PM
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unterhausen
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The structural advantage of a butted tube is overblown. As in, some people will tell you that a 6/4/6 is superior to straight 6, but there is no reason to expect that is true other than weight and compliance. I have no problem with .6 wall, but I'm surprised you can't get a butted tube. on edit: Framebuilder supply tube availability is a little scary right now. Torch and File has some.


I would put the butted end at the head tube. All the bb failures I have seen were seat tube failures. Maybe because people don't take seat tubes seriously.

Last edited by unterhausen; 12-31-20 at 01:26 PM.
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Old 12-31-20, 02:34 PM
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Thanks for the lead, unterhausen. Have not been to that site before. Indeed they do have what I need! Excited to place an order!
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Old 01-01-21, 09:02 AM
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Doug Fattic 
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I asked Kirk Pacenti about where is the best place to put the butted portions of a tube. He has a degree in Engineering I believe (although his isn't as advanced as unterhausen's). My degrees weren't anywhere near any engineering department. What he said was that the "moment of inertia" is about 100mm behind the head tube. I understand that term to mean the place where the tube tries to bend when under pedaling forces. (Those whose education didn't avoid engineering can refine my attempt at a definition). He continued to say that it didn't matter if the down tube into the BB shell was butted or not.

My practice when I am mitering tubes is to 1st mark where the most significant bend is in a tube so I can place the miters so that curve is in the plane of the frame. I roll it on my alignment table to mark the bend and then extend that mark into a line with a piece of aluminum angle the entire length of the tube. Then by several methods I find where the butt starts to taper towards the thinner center. I use a short piece of squared off slip tube (0.058" wall) to help mark a circle around the tube to show where the butts are located. Now I can plan my miters around those markings.

While Kirk said it doesn't matter about where the butt is located on the DT going into the BB (related to bending forces) I suggest for my framebuilding class students to leave 50 mm (if possible) as heat protection. It is easy for an inexperienced brazer to focus their heat a bit too much on the tube while the shell is not hot enough and as a result (if the tube gets red) will bulge out beyond the shell.
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Old 01-01-21, 04:30 PM
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unterhausen
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"Moment of inertia" is how much the tube resists bending. I think he said (or meant) just plain old "moment" -- force times distance, the quantity that bends things.

My rationale for putting the butt at the head tube because that's what buckles when you run into something. And nobody has ever seen a buckled down tube at the bottom bracket, unless something very extreme happened to the bike.
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