Lifespan of a stitch welded chassis

So, I've been thinking.  This is never a good thing for me to do without parental supervision...

My body swap on the daily is escalating in my head to pretty much a fresh build. With it being a miata, and a high mileage body, i plan to do some stitch welding/seam welding.  Figured that should tighten it back up a bit, if not make it better than new.

Part of this also impacts my idea for the next challenge car.  Seeing how much stitch welding stiffened the amc, i would call it a requirement on the next one. 

In both cases, plans call for extensive street miles, and serious abuse. 

Are there any concerns with cracking, accident survivability, etc in this application?

Tl:dr. What is the downside to stitch welding a street car?

Stitch welded rally cars last longer than non stitch welded cars. 

In my personal experience, it's amazing the difference it makes on high mileage cars. I have a friend with two AE86 Corollas. If you jack up the street car you can't open a door with one corner in the air, or if you jack it up with a door open the latch is off by over 1/2 inch. The rallycross car is gutted so it's a bit lighter, but with just stitch welding, no cage the doors operate the same whether the car is on a jack or not. It's about as apples/apples as I have personal experience with, but the older and floppier a unibody is, the more it helps.

The only disadvantage is that it takes a lot of time and creates places for rust to start.  I know nothing about stitch welding, but I imagine you need to attack it methodically so you don't twist anything.

I've heard it can cause issues with the crash protection of a car. The "crumple zones" no longer crumple as designed. I wouldn't worry about it.

Agreed about being methodical.  Its amazing how much metal moves after welding. 

You bring up a good point about rust. I live in the south, so i rarely think about it. What would be a good way to prevent the majority of the risk?

In the case of a miata, i dont think crumple zone is much concern. Theres only 4 or 6 places that the stitch welding is reccomend. 

The challenge car will be much more extensive, but probably caged as well. Or, at the very least, fixed back/5points/no airbags. So the proper safety gear for no crumple zones.

But would stitch welding create issues when accident repair needs to heppen?

How does a stitch weld begins to rust? Are you people not painting over them? 

I can confirm the comment about a floppy chassis. My ra64 celica has floppy syndrome. Jack a corner tire off the ground as high as you want, all three other tires stay planted. 

But, my Honda Fit, if you jacked a corner, both tires on that side came up. It made it super handy for rotating tires. It’s a very stiff chassis. The second gen fit was something like 4,000% stiffer than the previous gen. Honda installed softer springs to return ride quality. 

Home applied paint is never as good as factory applied paint. In the south it's not as much of a worry, but up here (Ohio), you can tell where a car's been repaired because all the newer welds and body seams will rust far faster than the old ones. I bet it does happen in the south just not as quickly. If it's a fair weather only car you won't notice it as quickly too. But that's the only down side I could think of to stitch welding. 

Trackmouse said:

How does a stitch weld begins to rust? Are you people not painting over them? 

 

I think it’s more from not being able to prep and paint the inside of the welded area, as in inside the rocker. The outside is not nearly as big a deal. Surface rust is fine, it’s rot from the inside that’s a problem. 

joey48442 said:

Trackmouse said:

How does a stitch weld begins to rust? Are you people not painting over them? 

 

I think it’s more from not being able to prep and paint the inside of the welded area, as in inside the rocker. The outside is not nearly as big a deal. Surface rust is fine, it’s rot from the inside that’s a problem. 

That's what I was thinking.  The prep is a b-tch.  Of course you paint over the welds, but inside of boxed members you're going to burn through primer. 

Ideally you would clean the back of the surface to be welded and hit it with weld-through primer.  I know I wouldn't because I'm not that ambitious.

I only mentioned in the context of the lifespan of the chassis.  Anything that is operated hardcore enough to need stitch welding probably has lifespan affecting factors other than rust.  :)

What if you drilled a  few small holes in the inaccessible areas and hosed it with an aerosol rust inhibitor like Fluidfilm?

In reply to Appleseed :

Oooh.  Good idea!

How well does that stuff spray? Like, drill a hole in the top of the rocker, and it gets everything within 6 inches in 360 degrees? Or, brakeclean through a straw, one direction with some splashe effect?

I didn't realize the '86 was so floppy. I drove my Fiat spiders for years and even though they were a roadster, they were quite stiff. you could jack one corner up and the car stayed aligned enough to open the doors.

Granted, they were heavy cars for their age too.

I also agree with welding being a magnet for rust. That's what killed my 318ti, it had been in an accident previous to my owning it and had a nice weld down the middle of the passenger floor, within a couple of years, I barely had a floor left.

Eastwood also makes this internal frame coating stuff. I'm going to use it on the inner structure of the cabover soon. 

Dusterbd13 said:

In reply to Appleseed :

Oooh.  Good idea!

How well does that stuff spray? Like, drill a hole in the top of the rocker, and it gets everything within 6 inches in 360 degrees? Or, brakeclean through a straw, one direction with some splashe effect?

It mists everything within a few feet if you blow it into a hole.  Then the liquid gravitates to the lower spots.  

And im assuming that the liquid is the stuff that we actually want, right?

On the dd miata, rust is a concern.  One of the bit areas that make a difference are the pinch welds in the door frames. The others are the wings in the engine bay, the chassis stubs undef the front fenders, and some reinforcements at the back of the interior.

mazdeuce said:

Eastwood also makes this internal frame coating stuff. I'm going to use it on the inner structure of the cabover soon. 

I know a few people who have used this, but only on California cars, in California. 

As far as application it seems like it's probably the most effective means of coating everything. It is messy though, and will end up dripping.

I do wonder about what it does to/about pre-existing rust. 

So when the previous owner of my Miata had it stripped to the bone, he decided to seam-weld the chassis before starting the rebuild. In this shot, you can see the modifications to the frame rails that are left over from the shell's previous turbo history.

Dusterbd13 said:

In reply to Appleseed :

Oooh.  Good idea!

How well does that stuff spray? Like, drill a hole in the top of the rocker, and it gets everything within 6 inches in 360 degrees? Or, brakeclean through a straw, one direction with some splashe effect?

Fluid film is awesome!  I buy it by the gallon and spray it through a sprayer I bought from them. Comes with a spray wand that sprays all over, so you can poke it into areas and hopefully hit everything.