Alright, with a bit more info to play with:
Disclaimer: I’m not going to claim everything here is 100% accurate. I’m not a professional welder, I’m just trying to share what I’ve learned working in a metal shop. I’ve been through exactly what you’re going through, and am trying to help. For example, i approached my first unicycle frame thinking it’d take 2 weeks to machine. It took 6 months. I spent 2 months just revising the drawings before the guy teaching me even let me touch my metal. So with that said:
This has nothing to do with you, but flux cored wire sucks. Period. It has uses, namely when you have two flat piece of 1/4" mild steel that want to be attached. It is an excercise in futility to use flux cored wire on structural steel such as bike tubing.
If you insist on not buying a regulator and renting a tank of argon, here’s a hint: flux cored wire makes slag. You can’t lay another pass or do more welding on top of a previous weld until you chip off the slag. WEAR EYE PROTECTION FOR THIS.
On another note, MIG is not the ideal proccess for this stuff. If I were you I would either A: invest in a good TIG welding rig (meaning Hobart, Miller, or Lincoln, not some generic ebay crap), or B: invest in a good (meaning not Harbor Freight) set of oxy-actylene regulators and torches. Rent some tanks, and learn oxy-actylene welding.
MIG isn’t really precise enough for welds like this. If we had to make those at work, we would whip out the TIG welder (well, not literally). If the tig welder were broken, we’d get the oxy-acetylene rig. If the oxy-acetylene rig were broken, we would send the customer somewhere else. Note, the shop at work has 4 MIG welders.
If you want to do good work making unicycle parts, you really need to become proficient with an O/A welder and a TIG welder.
By saying “your settings are too high” you show that you aren’t yet familiar with you MIG welder. What do you mean by “settings”? There’s wire feed, amperage, wire guage, arc length, method of deposition (spray or globular), polarity, speed of welding, and wire extension to be worried about here. This mess gets even worse when you use a sheilding gas. Let me repeat, you need someone who does this junk professionally to teach you. Can you enroll at a local community college? Do something like that, because it’s dangerous and foolish of you to just pick up a MIG welding gun and start laying down pools of molten metal. Again, I’m not trying to stomp on your hopes of making something useful, but you have gotten way ahead of yourself.
EDIT: As for the axle washers, well, this exposes another thing you need to become proficient with, although less so for welding. You need to have an idea of what measurements translate to, and what they mean to your design. You can’t just arbitrarily change the design like you did, and expect to have something useful come out. By lowering the position of the plates the way you did, you made them too weak to be useful.