Some Seam Weld Photos
 

Ugly welds but you can see some of the zones you need to hit. I saw these at an S2000 engine swap thread.

Another one...

BTW why are the factory welds weaker than these?

http://www.lightweightmiata.com/v8/seam/

This site tells it pretty well, basically Mazda just placed all those sheets of metal together and tack welded them, often not even getting all the sheets in each weld.

I don't see how smearing a few boogers in your door jamb is going to tighten everything up

Isn't it already known that stitch welding helps?

real stitch welding, sure, but that looks like it's only warping sheetmetal

+1...

those welds would help, but judging by how they look there was not much penetration of the parent and filler (cold weld)

This is just helping your car rust faster on the back side of the welds that you cant access.

if you want to really want to tighten everything up, just weld in a cage like a boss would

Like I said, ugly. In racing…results are mostly due to the person and not the equipment. Same thing in welding. Seam welding does beef up a unibody. But it has to seem like welding haha. ;-)

Most seam welding is actually pretty forgiving. The glob and grind crowd can succeed here. It can be a decent way to improve your welding skills. Even those ugly welds probably help a great deal and of course they can be cleaned up. There is certainly room for improvement.

You really want to lay a bead of weld to really get some strength. You can basically assume the first and last .25" of a weld aren't really holding anything together. IIRC it is a safe to assume that about 60% weld coverage (weld area vs. total length of area welded) is considered to be a "solid weld".

So when Mazda tightened up the 01, did they improve the welds?
Or was it cheaper for them to do what they did instead? (add the spiderweb bracing under the prop shaft)

Do other car manufs, on more expensive cars, use better welds?

the purpose of seam welding in most cases is getting rid of the the seam "glue" that is used instead of welding on 90% of the unibody. i hope you guys are joking. you guys sound like newbs from those "other" miata forums.

and no, the manufacturer will never get close to the same chassis stiffness adding braces, compared to what a raceshop would get from seam welding.

yes, other maufacturers use more welds or a stiffer chassis design (or tub) to account for the seam filler thats used.

So why don't manufs do similar welding instead of adding weight and cost with braces?

What is this seam filler glue stuff?

All I know is that I did a similar job to my door/window frame seams. Now when I jack up one corner, the door does not rub against the sill like it used to. Could be all in my head, but something is not flexing as it once was.

I don't know about the '01s, but in general NBs have stiffer unibodies than NAs. If you look at the interior floor pan sans carpet you can see part of how they tweaked the deal. A bit more metal or bracing where it would help.

I've heard claims that the early EVOs were factory seam welded. Might be cool to pop off the door opening seals or look elsewhere to check it out. Oh yeah, certain 911s like the GT3 too, supposedly. Drool.

The cost to weld up a chassis like what we are talking about would add a ton of cost and complexity to the design. I imagine that they have to use special robots and jigs on exotic cars to keep them from warping during welding.

BTW…there are different strategies or philosophies on seam welding. Some folks like ending up with one solid weld like you see at the lightweightmiata site, above. Others suggest doing some pattern such as 1 inch welds with some spacing between each weld.

Another aspect is that the sheets of metal can have gaps between them. You can see this on most any Miata by popping off the door opening trim by hand. Like here:
https://www.miataturbo.net/attachmen...ine=1312514817

My thought there is that any gap found after you clean and prep before welding should be clamped shut. Use vice grips or something like that. Others may feel differently and just want to fill the gaps with more weld material.

Because monocoque

The reason for the spacing is to limit crack propagation. If you have one solid weld and a crack starts, it will almost inevitably spread through the entire weld, ruining everything you just did. By making numerous smaller welds, a crack will ruin just one weld. The "pattern" or spaced out method would be the right way to do it.