Best Way To "Butt Join" Square Tube With Round Tube?

[Ahmdo Molah]

There was once a Lightfoot Cycles in Montana that made some awesome recumbent bikes & trikes.

Their seats were mounted on square profile tube rails.

Lightfoot Bushmaster Seat Rail Weldment(cropped side view).jpg

Lightfoot Bushmaster Seat Rail Weldment(cropped front view).jpg

I know that the upper round top tube is TIG welded to the short round vertical support tube. But I'm not certain how the square profile tube is joined with the round top tube, as the paint(powder coat) covers up the joint and smooths it out.

It's hard to tell by photos if the round top tube actually gets inserted into a larger square profile tube or if the joint is a "butt joint".

I would think that the strongest joint would be possible by;
1. Insertion of the round tube(if possible) into the square tube
2. TIG tac weld the exterior contacts (i.e. 12/3/6/9 o'clock sections)
3. Fill with bronze braze the corner gaps as far into the joint as necessary.

But I wouldn't be surprised if Lightfoot's method was;
1. Insertion of the round tube into the square tube
2. TIG tac contact points, then fill(externally) with steel filler rod the corner gaps.

OR

simply a "butt joint" with TIG tac contact points, then filled with steel filler rod the corner gaps.


How would you join such square profile tube with a round tube for best results?

[steve garro]

I'd cap it & cope to the flat, personally

- Garro.

[false_aesthetic]

What Steve said

[Ahmdo Molah]

I'd cap it & cope to the flat, personally

- Garro.

D O H !!!

LOL

Thanks! That would indeed be a lot simpler & cleaner to TIG weld and also allow stress to be distributed "evenly" along the ends of both square & round tubes.

That is likely how Lightfoot did it too.

Now I can setup my 3D CAD model of a Lightfoot Ranger* with a sound weldment for simulation analysis.
*https://web.archive.org/web/20110728...nt_bicycle.php
*https://lightfootcycles.com/blogs/al...-mountain-bike

Cheers~!