Saturday, January 20, 2018
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IRS Subframe Cradle Drop

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The following instructions are a guide to removing approximately 2 inches from the 99-04 Mustang Cobra IRS subframe assembly. This allows for more ground clearance and better suspension geometry in the vintage Mustang chassis. The following is only a guideline and should only be undertaken by certified welders.

Lowering the IRS cradle requires removing 2 inches from the rear mounting arms and rotating and relocating the front mount points to match. This is best done in a fixture that holds the cradle in the right angle. After the cradle is cut you can simply install some 2 inch risers and weld the parts back together.

Side shot showing the cradle bracket. The rack is designed so I can raise everything up two inches and keep everything at the correct angles.
Rear of the cradle. Note the higher angle of the rear of the cradle - it's approx 3 1/2 inches higher in the back.

Use the front pinion snubbers to hold the front of the cradle in place

I used a 2x3 and some 1/2 inch spacer to equalize the rear of the cradle. Also not shown is a bracket I used to secure the frame to the rack.
There is a hole on each side of the cradle. I use a long bolt and use these two holes to help align the cradle.
Make sure you get the inside/outside correct or it won't fit. The cradle is not welded in the middle of these parts.
First cut on the rear is approx 2 inches down from the bottom of the bushing. I use 2 inch tape to mark
The front rotation point isn't critical, but I am about 1 3/4 inch from the reinforcement bracket. There are a lot of weird indents on this piece, and this point is the least problematic

After the first cut, place another piece of 2 inch tape to mark the second cut needed
Two inch section removed from the back part of the cradle
Rotation cut on the front. Note I made my jig where the cradle itself acts as a cutting guide.
Cradle removed. I install a set of 2 inch squre tube spacers to all sides to move the cradle into position for welding.
Hard to see in the photo, but once you raise the cradle, there is about a 1/2 inch difference in the rotation of the front mounts.
Mark right in front of the ABS hold down hole and remove approx 3/8 inch from the cradle
Elbow ready to be re-welded
Be careful , I had this happen before. If you get the cut wrong you can come up with a big gap on the rear driver’s side, as you are cutting on the arc of the pipe. This will have to be heated up and bent over, and I'll still have to add some metal to get it to mate up.
The passenger side comes out much better. Heat it up, bend it over and weld it up. As long as your fixture holds the four mount points correctly and holds the cradle in the right spot you shouldn't have a problem.
A finished cradle TIG welded and ready to go.
Shots of the finished welds. Since this is a structural piece, we burnish the weld low and finish off with high strength fill
wire and double TIG weld.