Framebuilders - Which tube is most vulnerable during full load / overload touring

Looking at the old fashion bike, a lot of heavy duty bikes had double top frame.
I am planning on building 1.5 length frame with couplings.
Which tubes from regular tube kit should be replaced by thicker & stronger one ?

Double top tube is kinda a fad. Evidence of which being that one sees even quality frame builder making them with curved tubes. Depending on how it is done, it can reduce the size, and increase the quality of the triangle, but as to whether that is good for something, better than a single small triangle alone, a different tube placement, gussets, oversize tubes, etc... Is another question.

If it was me it would be a totally unique tube set, the seat tube would not change. You mention the top tubes, but as you say, that is just a thing they did in the past, sometimes to carry advertising, and has nothing to do with long tubes, really. The tube beef is going to depend on ride stuff other than the frame length, like rider size and ability. Basically the main triangle can be fairly similar to a regular frame, but the stays have to be done specially. When you build a frame, for the same riding style, the seat to crank angle (the seatube can vary depending) is fixed, and handlebar reach is fixed. These are biomechanical. You can vary the head tube angle, just so long as reach is not adversely affected. The problem is that the rear parts are made to a limited length. Consider:

1) Sakkit and Brandt approach is to simply use an untrimmed chainstay. This will get you well beyond regular touring bike, but the bike will not be freakish.

2) You can use a construction like a fork, you could probably use a fork. The "steering tube" is welded to the BB. Over length seat stays can be simply extended with additional tapered, or straight tubing. Or wishboned.

3) You can use early MTB construction/Bike Friday construction (Bontrager?) where two sizes of bent tubing are slide fit together, this has a ton of construction advantages, and structural advantages. I don't know who this is going to be built by, but S&S don't just sell their couplers to anyone, so in addition to studying BF tech on stays, you can familiarize yourself with alternative coupler approaches.

4) You can consider how modern Ti, and MTB bikes are built in general. They use straight gage tubing for the stays. These days they sometimes throw in fancy bends, though while it looks cool, it probably does not improve the bike much. But you will see lots of cheap ways of getting your stays done. In the early days of MTBs this was the race winning approach used by many factories.

I would probably not add anything to stiffen the rear triangle until I rode it. Shimmy is not really the test since it has many non-frame causes. I would ride the bike, and then add bracing if and as required. When you are making a really out there bike, particularly with panniers, and lots of BOs, I think it pays to rig it in the white, and play with it, and sort stuff out. Then paint.

The pics are more Arvon bikes. The tandem has a lot of couplers and can be assembled as seen, with a child size rear, or the front section can be assembled to the stays to make a single. I think the blue bike was made for a very tall guy, which explains some of the geometry.

Not sure what your main question means, but the vunerable part that occurs to me is the DT to HT joint. And the solution there is gussets. MTBs used this approach, and it is barely noticeable. Particularly when you add all the bags. Correct gusset placement is on both sides at the points of tangency, not as one sees on choppers on the CL. I'm guessing 58 thou.

What is a 1.5 length frame and what is the regular kit?

I think the most vulnerable points are the head joints and steer-tube crown; they are not triangulated. So a stronger DT, steer tube and blades are good. For lateral rigidity a stronger TT and chainstays, or twin laterals from dropouts to HT.

A second parallel top tube doesn't really help. In the best cases it reinforces the HT-DT point, but the other end weakens the seat tube. When the TT lands in the middle of the HT I'm not sure it reinforces anything. The proper way is with a tube from the lower head to seat cluster, or from the upper head to BB. And for the fork you can add tubes from the drops to the steerer above the HT.

What is a 1.5 length frame and what is the regular kit?.

:):)
1.5 frame is my "imaginary" pet project I am going to build
basically it is "smaller" brother of a tandem bike


If you really want to know more
go to the
http://www.bikeforums.net/showthread.php/846744-Single-touring-using-tandem-WHY-NOT

What is a 1.5 length frame and what is the regular kit?.

:):)
1.5 frame is my "imaginary" pet project I am going to build
basically it is "smaller" brother of a tandem bike


If you really want to know more
go to the
http://www.bikeforums.net/showthread.php/846744-Single-touring-using-tandem-WHY-NOT

My guess is where you weld the headset of the second bike into the cut out seat tube of the first bike is going to be your weak point.

1.5 is essentially a Surley Big Dummy or its competitors. There isn't much to worry about really. You probably want to make sure that the frame is triangulated because a minimalist rear "triangle" could fold fairly easily.

Since they locked the other thread... This would not be a good prospect for building with lugs. Doubtless some of the joints could be assembled that way, but any time you divert from fairly standard formats you will have to make the lugs to fit together the odd angles and tube intersections you are proposing, and the use of lugs will make even less sense than it normally does. Though it can be done...

Saw this:
http://lovelybike.blogspot.com/2012/09/long-tail-tales.html
especially the bit about a bike hauling a suitcase and the folding long tail bike for easy of flying and thought of your ideas.

I hope that Yuba has a really stout seat tube because step through frames like that have been known to fold, even without an extra long rear triangle