Like a banana

It does happen, being sensible I mean but very rare as I find it very disturbing

[Wynand]

Originally Posted by ayjay

I've been reading too many of those posts that warn against distortion as the welds cool -- having nightmares ,waking up screaming!

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

[holdenchris]

hello all, been a fabricator for too many years now my advice is spend as much time getting fit and gap right,smaller gap,smaller weld,less distortion
hope this helps chris

[CTWV50]

So it's not just me then!

[jerkins]

If you hold down the back of my chassis the front is about 3 - 5 mm off the ground. I worried a bit, then someone suggested that I check the straightness of a production car...
...I felt a lot better after doing that!

I wouldn't worry about a bit of up-and-down bending from front to back, but I would be concerned if there was a significant twist, or if it was bent to one side.

[robo]

Thats why a lot of people have opted for yellow bodywork. Landrovers chassis can be 20mm twisted from the factory and thats apparently ok!

[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

[MarkB]

Bananas and custard is quite nice but awkward to drive as your feet slip of the bits of banana and if you get custard in your eyes then well it's game over really. maybe safer to have a yoghurt and use a spoon

[CTWV50]

So I've decided like many that if I tacked the whole frame together I'd end up with a badly distorted frame when it came to fully welding it, with my limited welding skills, experience, potential for the table sagging under the weight and build space, I've decided to get the basic bottom rail frame fully welded and work out the distortion in it before continuing any further, I've learnt a lot doing this and now have an appreciation of weld procedure and heat management.

One thing I have found is that if you do get distortion that needs sorting, a good method is grind back the weld on the high side of the frame put a small V groove over the old weld and reweld. The heat generated doing this pulls the frame back in to shape and I now how a frame sitting pretty much level with minimal distortion. I'm a bit of a perfectionist and struggle to leave things if they aren't right but I'm happy with it.

A few more welds and a bit of tidying up and I should be ready to continue!

[jerkins]

I think that managing distortion is a major skill in the craft of welding!

I was a total novice when I started. I was told by more experienced folk to tack everything up, check straightness and symmetry, then alternate my welds across the car. In other words, weld a joint on one side, then do the matching joint on the other side. This isn't a hard and fast rule - if I was welding up a T-joint in square tube I would do one side of the upright, then do its opposite side, so that it ended up at 90 degrees (ish), THEN I'd go to the other side of the car.

Basically though, don't go up one side and back down the other!