Foxbody IRS rear brackets (my how-to) - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 4 Old 05-24-2018, 09:23 AM Thread Starter
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Foxbody IRS rear brackets (my how-to)

I'm cross posting this, so some of y'all may see this in my build thread/instagram/SVTp thread.

I took it upon myself to find a better way to build the IRS brackets for a foxbody. Some of us are aware of the dilemma, but it seems slightly gleaned over due to the perpendicular bolts taking most of the attention. See below for 99-04 cobra rear IRS upper brackets bolted to a foxbody's stock quad shock mount holes.



After measuring the hell out of everything, I've determined the rear quad shock bracket hole is exactly 3/4" lower on a foxbody over an SN95. As you can see in the picture, this isn't exactly a "bolt-on" affair for fox cars, and will require some work. Everyone seems to focus on the vertical bolts that go into the trunk, and for good reason. But ultimately, there is a better way to properly build rear IRS brackets to fit foxes and SN95 chassis cars without having to even touch the frame.

I started with 1/4 inch steel plate. I had some left over from an old Chevelle frame, but any 1/4 mild steel works. I drew out a template and started cutting.



This what I came out with



Not all brackets are the same. I had to cut a groove into the others sides steel plate to properly fit against the IRS brackets. But I then jigged it up



Once it was welded, I took an angle grinder to it on the inside and out to clean the welds up and ensure a flat fit against the frame.

It's difficult to see/take photos of, but I marked the center location of the hole with two lines on the frame prior to drilling. I then mounted my brackets, got them square and even, marked the location, ensured it was 3/4" lower than the center of the hole above it, and drilled a 1/2" hole. Factory quad shock holes are threaded in M12-1.75 (thanks Stuart) so a 1/2" hole suffices. If you're lucky and nail the location, it bolts on immediately.



Naturally these aren't finished, painted, or completed. But with both hole locations being present, I didn't have to weld next to a tank full of gas, cut my frame, make the brackets fox pattern only, or do anything sketchy. These will be able to come off and bolt to any 79-04 car I own now or in the future.

Next update I will have the brackets taper like factory to get rid of the excess hanging below the frame, and my solution for the vertical bolts. This is just how I chose to do it, and I feel it can't hurt to add some more steel/stitch welding to a spot welded bracket anyway.

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post #2 of 4 Old 05-24-2018, 09:44 AM
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Well done. I'm watching this in case it might help if I decide to give the IRS a shot for my 92 Lincoln. I don't have the quad shock mounts, so I still don't know what it will take to mount the IRS properly. Only my UCA mounts match the Fox Mustang, my lower mounts are forward a bit(same as the Turbo Coupe/Cougar etc), it has longer LCA's than a Fox.


Don
92 Mark VII LSC SE - soon OBDII and 4R70, GTC kit and 347, custom paint.
98 Explorer Limited project to add 4WD, XP8 parts, KB blower, paint, 337.
99 Limited mail vehicle, SOHC 4WD, later 306/4R70W swap. 1998 Mountaineer spare. 72 Ranchero 351 Clevor project.
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post #3 of 4 Old 05-24-2018, 09:50 AM Thread Starter
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Quote:
Originally Posted by CDW6212R View Post
Well done. I'm watching this in case it might help if I decide to give the IRS a shot for my 92 Lincoln. I don't have the quad shock mounts, so I still don't know what it will take to mount the IRS properly. Only my UCA mounts match the Fox Mustang, my lower mounts are forward a bit(same as the Turbo Coupe/Cougar etc), it has longer LCA's than a Fox.
In your case, I would look at finding an SN chassis car and cutting the Quad shock bracket threaded holes out of the frame. Basically make a large flat plate to insert into your frame and burn in. As for the front, you could sleeve the front tubing assuming the angle is the same. Or if you are really feeling froggy, you could hard mount the whole assembly.
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post #4 of 4 Old 05-24-2018, 10:15 AM
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Good thoughts, thanks. I've browsed for a used IRS a few times, finding the right one close enough etc, is the hardest part.

Don
92 Mark VII LSC SE - soon OBDII and 4R70, GTC kit and 347, custom paint.
98 Explorer Limited project to add 4WD, XP8 parts, KB blower, paint, 337.
99 Limited mail vehicle, SOHC 4WD, later 306/4R70W swap. 1998 Mountaineer spare. 72 Ranchero 351 Clevor project.
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