Find 2 long hi tensile bolts that fit neatly inside the tube ... The tighter they fit ... the better . Cut the thread sections off using a cut off disc in a grinder then round the ends off ... Hammer each one half way into each side of either the loop OR the triangular parts while they are bolted to the frame ... Drill a hole in each tube to do a plug weld so that the bolts are locked in and sticking out like two pins ... Cut the bolt heads off with the cut off disc then round the ends off ... Now knock the loop back in place over the two pins leaving a slight gap at the join and weld around the pipe in stages letting it cool each time ... When you are finished the subframe will be a lot stronger than it was before ...
IF you want to strengthen a stock loop , the only way is to weld two additional bars attached from further back near the loop and angled down to near where the stock lower struts attach to the frame ... anything else is a complete waste of time and effort ... Bars going sideways across the width do jack all except hold the seat in place if it's bolted to them ... The top subframe attachment bolts act like a pivot and the bottom bolts take all the load ...
Check out where the loop cracked off from and you can read the cracks ... if they look rusted or darker from the top down then look shiny at the bottom where they let go ... then that's from hangin' your butt rearwards when landing from jumps etc ... Get your ass off the seat over bumps and when landing from jumps ... start riding the bike more like a jockey ... If the crack starts from the bottom up ... then chances are you've cracked it from loopin' out and hitting the rear of the subframe on the ground ... and in that case ... nothing will be strong enough to handle the force ... except total prevention ... LOL ... If you make the loop too strong ... it will snap off somewhere else such as at the bolt mounts where it's a lot harder to repair ... Hi tensile steel will snap suddenly rather than bend ... if you over weld it or put too much heat spread from the welds , you'll severely weaken it ... That's why the chinese do bad looking welds ... they're doing them as quick as possible to minimise heat spread which if allowed to occur ... lowers the tensility of the base metal ...
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