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I always wonder how people weld floors on and claim to not have distortion. Pumping any amount of heat into a joint absolutely will pull the material toward the weld. All you can do is put as little heat into the assembly as possible, and alternate sides... or rivet the panels on. Welding distortion was a big reason why the floor isn't welded in my chassis.

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Try going to the 1/4" hole and see what happens. Sounds like too much localized heat and with only filling a smaller hole you have less time heat is applied.

The only plug welds I've done has been much thicker material and much bigger holes. I never had any sort of distortion.

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I can explain it to you,
but I can't understand it for you.

what do you expect if you are trying to weld a sheet of steel the size of a barn door to the chassis, one end is buckled before you even get there to weld it because it's one big sheet.

this was one of the reasons i welded my floor in sections, and having made that descision, it was obviouse that i should insert it into each bay and weld round the perifery so that it was flush with the frame, as a bunus, the floor could be made of varying thickness material, 16 ga. where i would stand or put my feet and 20 ga. under the seats, the 16 ga. works like a stiffener to the tubework of the chassis.

in the whole floor i only have one panel that has distorted in the passenger foot well and i put that down to me rushing the welding process.

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Yeah, I guess I'll just have to live with it. I had considered doing sections and setting it withing the tubes like you John, but I decided against it on the grounds that it seemed like more work.

Carguy, I had started out with 1/4" holes and had the issue right from the start. In fact, with the 1/4" hole it was difficult to keep the arc on the tube as it would just right to the sheet and cause it to heat up/melt/distort before I got anything on the tube. I experimented with enlarging the holes to 5/16" and 3/8" and it made it a bit easier, though the 3/8" do take too much filler I think.

It sounds as though you welded around the outside first then did the plug welds. Too late now but the best way to do it is start in the center and work you way out. You would do one then go to the opposite side, do the other then back to the other side etc. It takes some time but can be done. That way the metal has somewhere to travel. When they are all completed then weld around the perimeter.

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Graham,
www.earleymotorsports.com

Earley, I started in the very centre of the sheet with the plug welds, thinking exactly the same way as you. The issue I had initially was not the sheet expanding laterally and causing bowing over the whole sheet, but rather a local deformation right around the plug. It would pull up away from the tube as it started to heat up which resulted in a sheet that wasn't flush against the tubes. It's only after I got sufficiently frustrated with that and gave up that I went and did the perimeter and interior welds, but at that point the sheet was starting to look like a teenager's face anyway.