Like a banana

[baz-r]

Originally Posted by Wynand

Even when steel is tack welded to a solid steel base will have some distortion when cut loose, light steel moreso.
Here is what happens to steel when welded, regardless of type of welding such as Mig, Tig, stick etc.
Weld metal is deposited at a high temperature, above the melting point of material. For steel, this is around 2,500°F (1,370°C). As the weld cools to room temperature, it shrinks, but is restrained from doing so by the adjacent cold base metal, resulting in high-residual tensile stress. The weld is now like a stretched rubber band, with the workpiece holding the ends. This is the reason that the base metal moves, or springs back, when the clamps holding the workpiece are removed, distorting the part.

The only way to control this to minimize distortion is to keep welds as small as possible (read cool) and stagger welds to prevent heat buildup.
Also important not to weld on one side only as this will pull the frame into a banana shape. Take a piece of off cut plate, tube, angle iron etc and run a few welds on the same side and see what happens

there are other ways but none i can think are practical for home build on a roadster
like preheating, postheating in an oven to the point of destress
there is all sorts of stress and tension in the complex frame of the chassis when its fully welded. you try and cut a tube out and you will soon find out
if you had a problem area in your build of your chassis where the welded joint was problematic if you had a oxy/gas torch it could be heated