/diy/ - Do It Yourself

I have a basic soldering iron like pic related but Radio Shack brand (got it a few years before they went tits up). What is the max thickness wire this should be able to do? I believe it's 50w.
I've tried soldering 18ga wire with it and it just doesn't seem to get the wires hot enough, but the surrounding insulation does heat up. Also tried doing it to bike brake cables and nothing. The tip (chisel tip) also won't tin even after I sanded it down to clean it. I have thin leaded solder.

Is it the iron or did I somehow fuck up installing the tip or something?