I recently came across these posts:
Installation on Elantra 2009 - Suspension Subframe Crossmember AND Installing on Elantra 2009 -Cross-Member (55410-2H100).

Needless to say, while I've seen some pretty negative feedback as far as replacing rusted rear cross-member subframes on Elantras, I am staying optimistic!I have a 2003 Hyundai Elantra with only 103,000 miles. Unfortunately, as it is a Michigan vehicle, and was not kept in a garage by the previous owner (my mother-in-law) - meaning it has a ton of rust on the body, frame, undercarriage, etc. All of a sudden one day, the rear suspension cross-member K-subframe just BROKE in half (it's hanging on by some rust)! While I don't have pictures of my vehicle's rear cross-member subframe, I've included image diagrams of what it looks like, along with the other components in the rear suspension assembly. Since it does not have a rear axel, this cross-member k-subframe is the closest thing the vehicle has to keep it from swaying or fishtailing from side-to-side...much less hold up the integrity of the rear end and keep it from collapsing!

While the cross-member k-subframe has rusted in half, the rear trailing arms and rear suspension control arms are also very rusted. I'm sure it could use new ones.

I have found several replacement rear suspension cross-member subframes on eBay (some include trailing arms), as well as my local junkyard (which is a lesser price). I am wondering if I purchase the rear cross-member subframe, and purchase new trailing arms, bolts, bushings, and anything else that is completely rusted (sway links, etc.), then can I repair it even if the current cross-member subframe's bolts are rusted to the frame?

Can the existing bolts be cut off from the current frame if I have replacements? Additionally, is there any way a steel support plate can be added in between where the subframe bolts onto the frame andthe frame itself, as to ensure the integrity of the frame and prevent the frame from disintegrating around the new bolts? I don't know much about welding...so I'm not sure if some new steel plates can be welded to the heavily rusted frame, to increase the structural integrity.

The vehicle only has 103,000 miles on it, so I don't want to scrap the vehicle just yet. The engine runs great. The rear suspension - broken subframe and rusted (but still intact) trailing arms - is the only major issue with the vehicle, besides a stuck driver's side window and some rust on the body exterior. I doubt I will find an '03 Elantra that is in excellent shape and just needs a motor - at least for a decent price.

At this time, I can't really afford another vehicle. I have about $500 to my name, which I was hoping to use to repair my Elantra.

Shows how trailing arms and cross-member subframe fit into rear suspension

 

Have you made any progress on the Elantra? I am literally in the exact same position right now as you I have a welding machine and the entire service manual for the car I have a serious wiring issue I want to fix it all up and modify it for the system I have in if. I’m also looking into remapping and tuning but I can’t really find squat. 🤦🏼‍♂️

 

I have decided to tackle this issue started with it yesterday and ended with me fuming and calling it a quits. Going in for round 2 I have 3 of the 4 crossmember mounting bolts out the 4th broke out of the frame leaving a massive cavity I’m going to have to weld back together and I haven’t been able to get the rear suspension bolts out the the arm on either side as well as the 2 from the center bracket. More then likely going to have to cut them out and run to Gilmans hardware store and find some replacement parts.