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Old 04-09-2007, 08:47 AM   #1
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Box your factory rear control arms

** Originally postedby Jim Langley and Chris Neighbors on http://www.fordmuscle.com **

Aftermarket rear control arms are a popular upgrade for performance oriented late-model Mustangs. The stock control arms are a stamped design that is prone to flex and distortion, leading to poor rear wheel traction and wheelhop. Unfortunately aftermarket control arms can be expensive, and in many designs are overkill for the daily driver / weekend warrior. Fortunately there is an affordable solution for the do-it-yourself enthusiast -box your stock control arms. Boxing refers to welding plates onto the open side of the "U" shaped control arms. This results in a dramatically stronger structure which will not distort or twist under heavy torque. You can increase stability even further by installing a set of polyurethane bushings.

Our modified control arms with Energy Suspension bushings perform
as good as expensive aftermarket control arms for less than half the price!
(Energy Suspension rear control arm bushing kit for Mustangs, part no.
4.3115G, approx. $75)

Since we had a spare set of upper and lower control arms and Energy Suspension bushings laying around, we decided to box them up and throw them under Project Green Machine II, my '92 LX daily driver. The typical procedure for boxing your control arms would be to remove them from the car and trace the open side on a sheet of steel (3/32" thick is fine). Then, using a plasma cutter, air nibbler or similar tool, cut the shapes out. (On the lower control arms (LCA) you also need to notch the plates to allow the sway bar to mount properly. ) Finally, you weld the plates to the arms (or pay a muffler shop as in our case), grind off any excess metal, paint, and reinstall.

We had a local muffler shop weld up the plates to our control arms.
That may be the worst looking bead we've seen, but heck it's strong and no
one will see them under the car. Note the cutout for the sway bar in the
lower arm.

The plate for the upper arm should not extend all the way to the end of the
arm, otherwise it will interfere with the axle housing. Leave about 1.5" from
the center of the bolt hole.

Boxed, painted, and fitted with new bushings. Our arms look just as good as
most of the aftermarket stuff!

The LCA plates have been modified form the original design with the cut out for the sway bar now being more of a kidney shape than rectangular. It is possible to weld them on upside down so make sure the offset side of the cut out is towards the inner side of the arms. We know that because we had one right and one wrong! A little grinding with our die-grinder and carbide bit took care of the problem. If the cuts are wrong the sway bar will be hard to put in and the bolt holes will not want to line up. The welding we had done at a local muffler shop for $40. The welds are not pretty but are certainly strong.

Bushing Removal
Now the hard part had arrived - removing the old bushings. Actually if you have a press the job is easy. Alternatively a machine shop would probably charge you $30-50 to press them out. We wanted to try and remove them ourselves and keep the expense as low as possible. The upper control arm bushing is easily removed by first using a heavy mallet and socket to pound out the metal sleeve. Then using a 1/4" drill bit we pummeled the rubber bushing until it was weak and full of holes. Finally hammering it with the mallet forced it out. The same technique can be used on the round bushing on the lower control arm. However the large oval bushings on the LCA are a different story. The oval bushings are actually bonded to the arm with a glue that is stronger than the bushing itself. The bushing or sleeve would not budge when using the sledge hammer. We also tried drilling and then pounding without success. Finally we decided to try an industrial heat gun to try and melt the adhesive and break the bond to the arm. Well, the first bushing actually ignited so we just let it burn to see if it would completely burn out. After about 20 minutes the bushing was toast and only a sticky residue was left. The residue was removed with a cordless drill and wire paint stripper. We highly recommend that you have your bushings pressed out by a machine shop as burning them probably violates several EPA laws. But we also have to admit that the fire technique is by far the easiest way to remove bonded rubber bushings.

A hydraulic press would be ideal to remove the bushings, however we
used a drill, mallet, and heat gun to get the job done. A little propane torch
would work just as well to burn out the bushings.

Once the old bushings were out we washed the arms and prepared for painting. A coat of primer and gloss black paint ensures that the new arms won't rust away after the first winter of abuse. After the paint was dry we greased the polyurethane bushings with synthetic lithium grease and pushed them in by hand. The hammer was needed on one stubborn oval bushing. The only bushings we did not replace were the ones in the differential housing where the upper control arms mount. They are tough to remove and ours were in good shape so we left them alone.

We installed the newly modified arms and went for a road test. The boxed arms really improved the handling of GMII in the twisties. The rearend "wiggle" that used to be apparent on hard turns was gone. Handling is now more predictable and the tires "bite" harder when you apply the throttle. The road noise and ride stiffness increased a small amount, however that is expected with polyurethane bushings.

The last criteria would be how they responded at the strip. The previous best 60 ft. on stock control arms was 2.26 seconds. The new best with boxed control arms is 2.194 seconds, and improvement of nearly one tenth! The same radial tires and launch technique were used at the same track in similar weather conditions.

Needless to say we're very pleased with the improvements. The cost of approximately $120 (bushings and welding labor mainly) is less than half of a new set that would not be any stronger. Better handling that will benefit at both the dragstrip and autocross combined with safer street manners is a true win/win/win scenario!
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Old 04-11-2007, 09:15 PM   #2
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Re: Box your factory rear control arms

Looks like alot of work for what its worth ... you can buy upper and lower combos for like 200 bucks well worth it for all that i think
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Old 04-11-2007, 11:03 PM   #3
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Re: Box your factory rear control arms

Ya seems like a lot of work when you can get new LCA's for pretty cheap, but never would I change the uppers on a fox, unless you want to have to reinforce your torque boxes or rip them right out
9.83@143MPH @ 14psi on 94 Husky

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Old 04-11-2007, 11:06 PM   #4
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Re: Box your factory rear control arms

Not a lot of work if you have access to a welder and some scrap steel laying around (I'm sure people could con Martin into doing a little welding work). A couple hours worth of work and you're laughing. If you're on a tight budget this will do you for quite a while....... as long as you reinforce your torque boxes like Matt mentions.

Also, the Ford factory uppers are pretty good control arms.... for a car that's primarily a street car that only sees occassional track use, I wouldn't bother spending $150(US) (or whatever the price might be) on a set of aftermarkets - buy a shifter instead for only a few bucks more and keep the factory uppers.

Last edited by SilentNoise; 04-11-2007 at 11:13 PM.
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Old 04-11-2007, 11:23 PM   #5
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Re: Box your factory rear control arms

the welding is the easy part, it is pulling out the old bushings and putting in the new ones that will be a real pain, but I guess if you are on a tight budget, it would work alright. As for the uppers though, you could go ahead and box them if you really think it would be necessary (probably not though) but poly bushings in the uppers will rip your torque boxes out, they should only be rubber otherwise they bind.
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Old 04-12-2007, 06:43 AM   #6
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Re: Box your factory rear control arms

Originally Posted by MadMatt View Post
but poly bushings in the uppers will rip your torque boxes out, they should only be rubber otherwise they bind.
Agree with you there 100%
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