fabrik8

I forgot I was a member here, and I came across this thread completely accidentally through Google.

So first and foremost: Where do some of you people come up with this stuff?

Ever seen a WRC car hit a rock, ditch, tree, etc? They're all stitch welded. They crumple, quite a lot, and they have full cages and braces and everything.

Stitch welding only makes the sheet metal seams not shift, but sheet metal is really easily overloaded and buckles very well. It's going to absorb impact no matter what, and the crumple zones will do their job no matter what. So when you get into an accident, you'll still have very, very buckled sheet metal. You'll just have stiffer seams where the metal attaches, because the sheet metal can't move as much with relation to the other pieces of sheet metal. This doesn't affect accident performance at all. Like whoever said earlier, you're not breaking welds in an accident, you're buckling (crumpling) sheet metal. If anything, stiffer stitch welded seams should transfer force more effectively into the sheet metal, turning force into buckling instead of shifting the seams around. Basically, you can have incredibly stiff seams, and the sheet metal will still buckle because it's much weaker than the seam itself.

Crumple zones are box sections that are preformed to deform in a very controlled way under impact, and changing to a stitch weld isn't going to affect that deformation more than a little (if at all). Again, the sheet metal in the crumple zones will buckle way before the seam type becomes a factor.

Assuming that stitch welding will make the whole chassis into an infinitely stiff, solid block of steel ready to turn you into a cloud of red mist on impact is a very wrong assumption. Sheet metal doesn't work that way.

Imagine welding two pieces of thin sheet metal together at a seam, and punching/riveting two more pieces together. All the pieces are going to buckle somewhere in the middle (not the joint) when you overload them, completely independent of the type of joint used to attach them together. However, until you reach the yield point of the sheet metal (and it's the pretty much the same point regardless of the joint), the welded seam will be much stiffer because it won't allow the pieces to move with respect to each other.

Don't put the slightest amount of paranoia into stitch welding; if you're worried about safety you need to learn a lot more about materials and basic chassis concepts.

There are only two drawbacks to stitch welding: Time, and repair. It takes a long time to weld an entire car. If you need to repair a panel that has been stitch welded on, it will take a lot longer to remove and replace. Other than those two things, there are no downsides.

Stitch welding the whole car should add less than 10 pounds of weight, you're not adding anything but tiny filler rod or MIG wire unless you add other reinforcing braces, plates, gussets, etc.

Use a good paintable seam sealer when you're done, it's easy to find (body repair supply shops) and easy to use, and will eliminate corrosion and noise problems.

And please, do an actual stitch welding job, with inch (or two) long stitches and then skip someplace else to keep the heat distortion down, then come back to where you were.. You can prep the entire car before you even think about welding, and then spend a few days welding without doing anything else. If you properly stitch weld, you can do a few welds, move to another seam, do a few there, move somewhere else, etc. You don't need to weld, prep, weld, prep, weld, prep because you can just move where you're welding instead. This is a better method because you're progressively increasing stiffness all over a little at a time, instead of locally stiffening one area at a time.

Put the car on something very solid, and make sure the height from the ground is as even as possible. Shim it if you have to so twist preload is minimum during welding. If you keep the car on the suspension and wheels, the chassis is going to twist depending on where in the car you are and how much the suspension sags under your weight.. Not good, but probably not a big deal. It's easy enough to put it on jackstands or something like that, so better safe than sorry.

Don't do the little bitty round spot weld every 2" thing, it doesn't do much overall compared to actual stitch welding.. It might be fine for a street car, but you've already got everything prepped, and you're already welding, so why not do a higher performing weld type that will give a stiffer chassis?
You should end up with a continuous or near-continuous weld seam when you're done, not a dotted line of 700 little round tack welds. Those little tack welds still allow the sheet metal to rotate around the weld, and if you overload it in some off-plane direction it will wrinkle between the welds. Remember, the whole unibody is assembled with spot welds (from a resistance welder) and if you do a lot more spot/tack welding, you're just making many more of the same welds, not better types of welds.

Remember: Making spot/tack welds along all of the seams is not stitch welding, and it doesn't work as well. And you're not going to die a horrible death because your chassis is stitch welded.