Welding Tube Joints

[akk]

How do you weld tube joints where three tubes intersect at each end of a frame diagonal? I see these intersections on "professional" built frames. I am talking about a frame that has two long tubes running the length of the car with 90 degree cross tubes spaced down the length of the long tubes forming rectangles. These rectangles are divided into triangles with diagonals neatly fitted in the corners forming a truss.

Are the parts all fitted together and welded only around the exposed edges? This results in the 90 degree cross pieces not being fully welded to the long tubes! This makes a pretty joint, but is it strong?

If the diagonals are made to slide into place after fully welding the cross pieces to the long tubes, a big fill weld is required to fully weld the diagonal at the intersection. This makes a strong joint but not as pretty.

Is there a strength difference? How do the pros do it? Is there another way? 

I would appreciate some one sharing a little wisdom!!?!

Thanks AKK 

[Gwillard]

These are commonly called "cluster joints". As you have seen, they can require some thought before any tubes are even cut. In a perfect world every joint would be 100% welded (if desired) but this is difficult and often impossible to do. A properly designed chassis will have the joints under compression as much as possible. This reduces the liability of incomplete welds and reduces stress on the welds that are in place. While it may seem as though leaving as much as 1/2 of a joint unwelded would result in catastrophic failure, in a properly designed cluster joint this missing weld area has virtually no effect on the overall joint strength and, in some cases, could lead to excess distortion and residual stress if fully welded.
An excellent book on steel tube structures is published by the Experimental Aircraft Association and is titled "EAA Aircraft Building Techniques: Aircraft Welding". It mostly covers gas welding and thin wall 4130 steel, but the general ideas of tubular structure layout, joint design, and tube fitting are the same as when building a race car.

[akk]

Gwillard

Thanks I just ordered the book.

AKK

[jdeleon]

AKK

On some of my builds I telescope two pieces for the diagonal.  This lets me get 100%  weld area and double cross section at the telescoped joint in the middle.

[CNC-Dude]

These are commonly called "cluster joints". As you have seen, they can require some thought before any tubes are even cut. In a perfect world every joint would be 100% welded (if desired) but this is difficult and often impossible to do. A properly designed chassis will have the joints under compression as much as possible. This reduces the liability of incomplete welds and reduces stress on the welds that are in place. While it may seem as though leaving as much as 1/2 of a joint unwelded would result in catastrophic failure, in a properly designed cluster joint this missing weld area has virtually no effect on the overall joint strength and, in some cases, could lead to excess distortion and residual stress if fully welded.
An excellent book on steel tube structures is published by the Experimental Aircraft Association and is titled "EAA Aircraft Building Techniques: Aircraft Welding". It mostly covers gas welding and thin wall 4130 steel, but the general ideas of tubular structure layout, joint design, and tube fitting are the same as when building a race car.

Can you post pics of an example. Thanks.