The IRS Tech Article

Snake Eyes

Active Member
Established Member
Apr 25, 2002
Scottsdale, AZ
I. Preface
Here are my thoughts on the IRS. This is based on all information I have been able to collect on the 1999 - 2004 Mustang Cobra Independent Rear Suspension (or IRS for short). I know this is a lengthy read but it is well worth the time as it is intended to answer all technical questions about improving the 99-04 cobra IRS so please be patient and read carefully. I have taken great time to write this article to answer any questions you may have. If there is anything not covered or any changes, suggestions or questions feel free to contact me.

II. Introduction
The IRS must be looked at as a suspension system and addressed as such. There is no one part that will cure wheel hop. Let me say that again, there is no one part that will stop the hop. I get tons of questions about what part will fix their problem. Sorry but there is no magical part that will cure your wheel hop but I will attempt to tell you why it is happening and how to correct it permanently in all situations at any power level.
An origional sketch of the 99/01 Cobra IRS
A picture of the 99/01/03/04 Cobra IRS

III. Problem Description
A. Why the Cobra IRS hops
The problem with the IRS is Ford’s NVH (Noise Vibration and Harshness) engineers took the suspension as designed by SVT and determined that it was not luxurious enough for their special edition sports car and thusly recommended very soft bushings, springs and shocks to provide a more comfortable ride. What that left us with is an excellently designed performance suspension that was compromised for the sake of making the car ride smoother. What those engineers forgot is that this car was not designed to be a Rolls Royce. It is the last true muscle car and is driven by performance enthusiasts. And with access to aftermarket parts we can maintain that same ride quality but attain amazing performance as well with little to no trade off.

B. Hop and why it occurs
Suspension hop occurs when the rear tires are in a limited traction situation and the tires are allowed to move in unwanted directions. This means any time the car is trying to apply more power to the ground than tires, suspension and road conditions will allow. You can think of this just like when a drag racer smokes the tires, or a car spin the tires on the street. The IRS hops because when the tires begin grip the pavement and place load on the suspension the soft rubber bushings in the IRS allow the suspension components to move relative to each other in directions other than desired. This causes the tires gain and loose grip causing the suspension to oscillate between loaded and an unloaded state. This rapid uncontrolled movement is what you experience as wheel hop.

IV. Problem Solution
A. Overview
In order to gain and maintain traction you must keep the rear tires planted to the ground. Therefore good tires are a must. Obviously those tires must to be properly inflated to perform correctly. You also need to keep those tires on the ground so a good set of shocks and springs are very important. Coil-overs are best but stiff standard (non-coil over) springs will help but at the cost of ride harshness. Adjustable shocks are great for those who like to open track or drag, and they can be dialed in to combat wheel hop. Additionally in a coil-over arrangement they can be adjusted to match your spring rate. Needless to say subframe connectors are an absolute must. No suspension can be expected to perform properly if the chassis is not rigid and dependable. And possibly worst of all the IRS comes with soft rubber bushings in its attachment points, control arms and differential mounts. Even the 03/04s still use rubber though stiffer than 99/01s. Let’s look at this component by component.

B. Subframe Connectors
Subframe connectors should be the first modification done to a mustang. They improve every aspect of your cars performance. Griggs and Maximum Motorsports sell the best. MM's full length subframe connectors are the standard and most people will not need anything stronger. For those wanting the strongest frame possible go with Griggs Complete frame kit, also recommended for high horse power convertibles that don’t want to run a roll cage. Those wanting the ultimate look no further than Griggs World Challenge Frame Kit (note they are visible in the car but are the strongest mustang frame kit in the world).
Maximum Motorsports Full Length Subframe Connectors
Griggs Complete Frame Kit
Griggs World Challenge Frame Kit

C. Shocks
Shocks can be a matter of taste. Any of the top quality manufactures (Koni, Bilstein, Tokico, etc.) will offer quality shocks that will be better than the stock 99/01 offerings and a set of used 03/04 Bilsteins are an inexpensive but quality unit. You will want to choose your shock to match your struts in the front along with your springs and whether or not you plan to run them in a coil-over arraignment. I recommend going with a coil-over conversion for reasoning see the Q&A section. I have a Griggs coil-over kit with Koni adjustable shocks myself, and I recommend them over MMs IRS coil-over kits because Griggs installs spherical joints into the lower eyelets of their shocks where as MM uses polyurethane bushings which over time may fail. If you plan on using OE 03/04 cobra Bilsteins in a coil over application you will need MM's Upper Shock Mount Kit.

D. Springs
As stated before shocks and springs should be matched and you should determine if you wish to run coil-overs or stick to standard springs. If you choose to stick to standard springs you should make sure the spring you choose has a spring rate of 600lb/in or higher to avoid wheel hop. For the extreme budget minded a set of 03 coupe springs with one half coil cut will provide good results. If you are buying new consult with the shop you are buying from. Again I wouldn’t go anywhere but Griggs Racing or Maximum Motorsports.

E. Bushings
The rubber bushings in the IRS must be replaced. In my experience solid bushings are by far the best option. Polyurethane is a poor material for any moving part with a few notable exceptions. It is best to buy your bushings as a packaged kit as it is cheaper and you should replace all of them. The best kit to buy is AC427Cobra’s kit. It is complete, well engineered and performs exceptionally. Alternatively Maximum Motorsports offers a similar kit but at a higher cost and in my opinion with less refinement and engineering.
My Review of AC427Cobra’s Complete IRS Bushing Package
MM’s IRS Grip Box

1. Sway bar Bushings
While you are at it you should replace your rubber sway bar mount bushings with Maximum Motorsports urethane bushings. Note these are included in MM’s IRS Grip Box.
IRS Urethane Swabar Bushings

E. Braces and Other Fixes
1. Differential Ear Brace
1999 and 2001 cobras were not equipped with a brace that became standard on the 03/04 cobras. This brace adds two lower attachment points for the front of the differential pumpkin, making it more secure and lest prone to rotating under heavy acceleration. So all 99/01 owners will need to purchase an IRS heavy duty cross member (aka the differential ear brace or differential pinion support brace). I recommend the stock 03 brace as the aftermarket replicas are more expensive and offer no gain. The cheapest place I have found to purchase is Brown Brothers Performance Parts.
03 Cobra Cross Member

2. 12mm Bolt problem
Many 2003 and 2004 Cobras were erroneously fitted with 12mm bolts rather than 14mm bolts in the front mounting point of the IRS. This causes the IRS’s forward mount point to shift under load as the bolt is smaller than the sleeve in the bushing. There are several options to replace this bolt. First is to purchase the necessary hardware when buying your IRS bushing kit. AC427Cobra offers them as an add-on to the kit and Maximum Motorsports sells them separately (Bolt, Nut). The second option it to purchase them from your local Ford dealer. The part numbers you will need are W704939-S426 Bolt M14x109 H and W520215-S427 Nut M14 HEX FL.

3. Tire Clearance
Because of the Rear cantilever bracket clearance can be a problem when running wide rear tires. The rear subframe mounting bolts can be replaced with a button head design which allows bigger tires to be used without spacers or grinding on the OE bolt. Maximum Motorsports also offers such a bolt.
MM’s Low Profile IRS Subframe Bolts

For those who need additional wheel clearance with 10.5 inch wide replica style wheels (AFS, Replica Alloys, etc.) I recommend a set of billet wheel spacer from Lightning Force Performance. They will give you just enough clearance without pushing the wheel beyond the fender lip. I personally run 335/30/18 Viper spec Michelin Pilots on 18x10.5 AFS 03 cobra wheels with this spacer and a button head bolt with no clearance problems.
LFP’s 3/8” Hubcentric Wheel Spacer Kit

4. IRS Subframe Bracket Brace
The rear IRS subframe mounting point attaches to the car using a cantilevered bracket that is prone to flexing under load because the outer half of this bracket is completely unsupported. This flexing cause’s unwanted alignment changes and contributes to the IRS feeling like it is moving under the car. Billetflow's offers a subframe bracket brace that reinforces the rear mounts and eliminates flexing thus improving corning stability. Steeda offers a copy cat product for less money but Billetflow’s kit is stronger and better fitting.
Billetflow’s IRS Subframe Bracket Kit
Steeda’s IRS Subframe Bracket Kit

5. Pumpkin Cover Brace
To prevent the IRS differential cover from breaking when you launch you will want to pick up an IRS differential cover brace from Billetflow. This brace has held up for 800+ HP cars launching on slicks at the track and is cheap insurance against failure and damage.
Billetflow’s Differential Cover Brace

6. Shock Tower Brace
A shock tower brace improves rigidity and prevents chassis flex over the IRS (recommended if you run coil-overs). It is also advisable to weld the brace rather than bolt it in to add strength to the shock towers. If you are running coil overs with a stiff shock increased loads will cause additional stress on the mounts. A brace can be purchased from Hans RaceCraft. Kenny Brown used to make a brace that I liked more but they are now out of business but there are a few places you can still find one.
Hans Racecraft’s Rear Shock Tower Brace
Kenny Brown’s Rear Shock Tower Brace

F. Lowering and Associated Problems
1. Bumpsteer Issues
For cobras that have been lowered Maximum Motorsports makes adjustable IRS tie-rod ends that allow you to move the tie rod end back in to correct alignment. This is the equivalent of a bump steer kit for the IRS. Note there are 2 kits available in addition PHP’s toe link bars perform this function also see the section below for details. Also note that the Tie Rod Kit is included in MM’s IRS Grip box listed above.
MM’s Rear Tie Rod & Bumpsteer Kits

2. Toe-Link Bars
For high horsepower cars the rear toe link bars have been reported to flex. Paul’s High Performance and Maximum Motorsports offer a set of toe-link bars that replace the stock bars. The PHP bars are by far the beefiest of the bunch looking like a set of Honda half shafts. The Maximum Motorsports bars can be purchased separately or with the tie rod ends mentioned in the section above where as the PHP bars are only sold as a complete kit and eliminate the need for any of MM’s tie rod parts.
PHP’s IRS Toe Link Bars
MM’s Rear Tie Rod & Bumpsteer Kits

3. Sway bar Issues
Also when you lower the ride height of the IRS you change the position the sway bar rests at. Returning it to the original position will further improve the responsiveness and corning ability of the rear suspension at the lower ride height. Both AC427Cobra and Maximum Motorsports offer adjustable end links.
AC427Cobra’s Complete IRS Bushing Package
MM’s IRS Sway Bar End Link Set

4. Alignment Issues
Finally it is necessary to have your IRS aligned, see the alignment section below. If possible try to find a shop that can adjust your bump steer and corner weight your car if it is equipped with coil-overs.

G. Elevated Power Levels and Associated Problems
1. Half-Shafts
Now that you can plant your rear tires with confidence you may snap you half shafts though admittedly you were at greater risk when the rear end hopped. Regardless you may wish to purchase some stronger half shafts. Raxles makes half shafts for the 99/01 guys. A set of used 03 half shafts are a cheap upgrade for 01 owners who are near stock power levels. Remember they will not fit the 99s because 99s came with a 28 spline differential and half shafts where as 01s and 03s use 31 splines. Though you can upgrade your differential to a 31 spline unit and run those half shafts as well. If you want the strongest available the Driveshaft shop makes near indestructible half-shafts though they are not needed on the vast majority of cars.
The Drive Shaft Shop’s Half Shaft Selection

2. Spindle Bearings
The stock spindle bearings have been known to wear prematurely under heavy track use and you may want to consider spherical spindle bearings. They are offered by Maximum Motorsports, Downunder Performance and Sean Hyland Motorsports. I do not run any of these products as of yet because my stock bearings are still in excelent condition so I can not offer a recomendation on over another.
MM IRS Cross-Axis Joint Replacement Kit
Downunder Performance
SHM’s Rear Upright Spherical Bearings

2. Differential Oil Cooler
A differential oil cooler may be advisable for those who frequently run open track events. I would make one yourself seeing as the kits I have found are overpriced. The parts list for the 2000 Cobra R diff cooler can be found here.
Parts List for 2000 Cobra R Differential Oil Cooler

V. Conclusion
If you follow this list of upgrades you will have an IRS that plants like a solid, handles like a Porsche, rides like a cloud (compared to a similar level 8.8) and is as close to bullet proof as possible.

VI. Q & A
Q. Why shouldn’t I just swap in a solid axle rather than build my IRS?
If you choose to swap your IRS for a solid rear axel that is your decision but I believe you are making the wrong decision and Chuck Schwynoch of Maximum Motorsports agrees with me.
NASA Pro Racing – Interview with Chuck Schwynoch, CEO
Maximum Motorsports, Inc.
June 28, 2006
By Andy Bowman

Q. The #91 MM Mustang is using an IRS rear suspension, what drove you to this decision, how has the effort been going so far?

A. We had a growing number of customers who called us and said that they were not happy with their IRS, and asked us if they should swap in the proven MM Torque-arm/Panhard Bar rear suspension. We couldn't give them a truthful answer until we had done our own testing to directly compare the Ford IRS to a well-sorted Torque-arm/Panhard bar system. Our AI race car had won races, and set track records, so it was the perfect candidate for comparison. We swapped the rear end over to the IRS, with all of the parts we had developed over the previous year on a street-driven 2003 Cobra. We left the front of the car exactly as it was with the Torque arm suspension. We maintained the exact same rear track width, and even swapped over the same brakes, rotors, calipers, and pads, from the solid axle. We then hit the track for testing. By the end of the first day of testing we were 3 seconds under the AI track record at Buttonwillow. With the IRS we had to learn what alignment and bumpsteer settings worked best. That's what testing told us. Without track testing, we really could not have given our customers an honest answer to their questions about the IRS. Even if our testing had shown the IRS to be grossly inferior to a solid axle, we at least would then be able to help our customers make an informed decision about modifying their car. As it turned out, the IRS is superior to a solid axle fitted with a Torque-arm and Panhard bar, in most aspects.
And he is speaking of a Torque-arm/Panhard bar built solid rear vs. a built IRS. A stock four link solid rear is a joke by comparison. If they race it competitively in a very demanding series and they are a suspension/race shop and they say it is faster and better in most ways I'd tend to believe them. Add that to the fact that every world class sports car has an independent suspension. I don't think SVT was trying to install an inferior suspension in the top line car.

Q. Isn’t building the IRS more expensive than a built solid axel?
No. For instance the Maximum Grip Box from Maximum Motorsports for the SRA costs MORE than the Maximum Grip Box for the IRS. So there goes that argument (as seen here). Not to mention that ac427cobra's bushing kit costs a fraction of what MM's do...

Q. Are the Install costs for a built IRS more than a torque arm/panhard rod SRA?
I can tell you that it costs more to weld parts on to a car and setup a new suspension than to replace bushings and bolt on a few parts for the IRS.

Q. Even with a built IRS as described above wont the IRS still break?
The only parts that can fail in the IRS once properly built as described above are the Diff (same as in a SRA) and the half shfts instead of the axles in a SRA. If you have a properly articulating suspension you will not break 03 half shafts unless you are running extreme power at the strip (and most aren't). If you are then yes you may want to look in to DSS shafts but 03 shafts are just as strong as stock SRA axles. Just because some uninformed people when out to the strip and broke some shafts before knowing that modifications were needed to prevent part breakage doesn't mean it is an inferior suspension. That is like saying the SRA is crap because some people ripped their torque boxes up. They didn't know that they needed to weld some bracing not the suspensions fault.

Q. The solid rear axel seems to be fine right out of the box why shouldn’t I just run that?
The SRA is a poorly designed suspension and that is well documented...
Taken from the Griggs Racing website
An even larger concern is found in the rear suspension. Ford uses a 4-link design, but with the upper two control arms angled heavily outward. This means the lower and upper rear control arms are not parallel, so as the suspension moves the upper arms are twisted in their bushings. During performance driving this quickly leads to a near total binding of the rear suspension, called roll bind. With the axle bound, it acts like a giant anti-sway bar, causing the rear roll stiffness to skyrocket and the overloaded rear tires to loose traction and spin. This is why the rear end snaps into uncontrollable fishtailing when cornering, and it is also why the rear tires break loose at the drag strip once the body starts rising from the initial power hit.
And Griggs is a strong propionate of the Torque Arm, Panhard Bar SRA. I know Bruce Griggs personally and he and I have had discussions about the topic a few different times. He doesn't like the built IRS in comparison to a built SRA but one thing we completely agree on is that the Fox and SN95 SRAs are totally unacceptable as delivered from the factory. I run his front suspension and it is amazing, I also run a few of his rear parts but for the most part the rear of my car is a mix of parts all working together to create super car like handling and performance. Based on what can be saved on parts and labor costs, a built IRS is a cheaper, less labor intensive, and superior suspension in all cases to a built SRA (except dedicated drag race cars).

Q. Won’t the delrin bushing squeak eventually?
With grease fittings installed you should never have squeaking. All you have to is have a technician shoot some grease in to the joints every few oil changes and your done. I have had delrin bushings in my Griggs front control arms for years now and all I have to do is add some grease about every 2 years or so you can probably expect similar results in the IRS but your mileage may vary.

Q. Do I need level 5 half shafts?"
Most likely not, but it depends. The 1999 half shafts are really not sufficient for mildly to highly modded cars drag racing with slicks or DR's even after you remove all of the rubber preventing wheel hop. I say run them till you break them but you most likely will. Your best bet at that point would be to purchase better shafts from Raxles or upgrade to a 31 spline differential and install '03 half shafts. 01 owners face the same situation but would not need to upgrade the differential to run 03 half shafts. If you then break one of the '03 Ford half shafts, and you have already removed all of the rubber from your IRS assembly, you need to look at DSS shafts.

Q. What if I already broke a 03 half shaft?
If you have then it may be time to upgrade, but only if you've removed all of the rubber in your IRS assembly and still broke a half shaft. More half shafts have been broken due to wheel hop directly caused by the components mounted in rubber, than broken because of sheer power. Once all rubber is removed from the IRS assembly and you break a half shaft, then by all means you're a candidate for DSS shafts.

Q. What is bump steer?
Bumpsteer is the term for the situation when the toe angle of a wheel changes as the suspension moves up and down, such as when driving over bumps, or with body roll during cornering. This happens when the arc that the spindle travels during bump and droop is not the same as the arc of the outer tie-rod end. If the toe changes more than a very small amount, the rear wheels begin steering the car.

Q: "How important is bump steering the IRS?"
Bump steering the IRS is not required but highly recommended and you will notice a difference.

Q: "Why do I hear a clunk in my drivetrain?"
There are a few likely reasons for the clunk. Your transmission and gears can combine for quite a bit of play, but that usually is not the main source of the "clunk". A few different things can contribute to the Cobra "clunk". The most likely are the bushings the differential is mounted with. The rubber OEM differential bushings are soft and spongy. Defiantly not the kind of material I would select to mount a component that is expected to translate all the driveshaft’s rotational force to the half shafts. These soft bushings allow the differential to move and that clunk you hear is most likely that movement. The situation is even worse for 99/01 owners because the bushings for those years were even softer and Ford had not yet developed the differential support brace. See section E1 for further details.

An alternative reason could be the front subframe mounting bolts. On some 2003 and 2004 cobras Ford accidently installed 12mm bolts rather than 14mm bolts that allow the subframe to shift during acceleration and breaking and cornering. Larger bolts will remedy this situation see section E12.

Q. Wont the suspension be more noisy with all those solid bushings?
Yes and No. I didn't notice any additional vibration or harshness because any bumps or road irregularities are transmitted to the suspension rather than the car. The solid bushings in most locations should not add any NVH to your car but the solid bushings used to mount the differential will increase your cabin noise but nothing terrible and certainly not a deal breaker IMO. But as I said in my review the noise increase is equivalent to installing an aftermarket shifter without installing a shifter gasket or any noise canceling material. You just hear a little more gear whine from the differential after it is solidly mounted. If you are worried about it buy some Dynamat and you will be fine.

Q. I’ve heard that drag radials will cure my wheel hop, is that true?
They will help but that is far from a solution those that say buy some drag radials and you will be fine are wrong. Maybe if you drive like a grandmother then yes you don’t have wheel hop anymore but your recipe won’t work for the rest of us.

Q. Why does my IRS hop more now than it used to?
The suspension performs better when it is new because the rubber has yet to soften up and begin to deteriorate. This means they allow even more undesirable suspension movement.

Q. What is the difference between the 03/04 IRS and the 99/01 IRS?
From 99 the 01's had 31 spline diff and half shafts which was no real improvement as the spline count was not the problem. The 99 & 01 shafts taper down at the CV boot and create a weak point. The 03 Shafts are a constant diameter and are by comparison much stronger. The 03's also had stiffer bushings along with better shocks, stiffer springs, the addition of a forward differential pinion brace, and lower inner toe control mounting points. These improvements made it handle a little better, improved durability and reduced bump steer. Everything else is the exact same and you can achieve all that and more with aftermarket parts discussed above.

Q. Do I need to do coil-overs, 98% street car?
Bottom line, No. No one needs them but I strongly recommend them. If for no other reason than you can have a high wheel rate with a lower spring rate. What that all means is you can install a spring that is half as stiff in the front and three fourths as stiff in the rear in a coil overs configuration as opposed to a standard configuration and still have better performance. By increasing the spring’s mechanical advantage you increase its effectiveness and reduce NVH. Not to mention reduced weight, ride height adjustability and you can swap springs much easier if you want to change spring rates.

Q. What should I do to the front end of my car to make it handle better?
The front end of a mustang is fairly simple. A basic outline is as follows.
Budget = shocks, springs, caster camber plates, bump steer kit, x2 ball joints and aluminum rack bushings.
Mid-Range = coil-overs, caster camber plates, bump steer kit, x2 ball joints and aluminum rack bushings.
Hi-End = Tubular K member & A arms, coil-overs, caster camber plates, bump steer kit and aluminum rack bushings.
Now your best bet is to call up Griggs Racing or Maximum Motorsports and discuss your goals with them and they will gladly assist you.

Q. What settings should I align my car to?
To answer this question you should understand what each setting is and what it does. I would you recommend you read this article first. Below are my alignment settings. Now realize that they are very aggressive but I did that for a reason.

_______Alignment Settings______
Front Camber
Front Caster
Front Toe

Rear Camber____-0.75°____-0.75°
Rear Toe

First off you want to get as much front caster as possible so have the technician set that at the limit of the adjustment range (if you have a bump steer kit). You may want to keep them equal if you drive on very flat roads. Here in Nebraska most roads have a very pronounced crown to aid in drainage of rain and snow so I have half a degree of cross caster to keep the car from pulling toward the passenger side. If you live in a place like Arizona where there is no precipitation and the roads are virtually flat you probably want to keep them even. Now my front camber setting is also quite aggressive. Unless you have an aftermarket suspension (K member, A arms, bump steer kit, etc.) you will probably want to keep yours around 0.1° negative. Now front toe depends on how much caster you run because the more caster you are able to get the less toe you will need to keep the car going straight without having to constantly correct the wheel whenever you encounter an irregularity on the road. So if you are able to set a high positive caster like I am you will be able to reduce your toe and consequently reduce your tire wear. If not then I would stick with the factory setting of .5° toe-in. (Notice the measurements for my toe settings are in inches not degrees.) As for the rear 0.75 degrees negative camber will put a little more weight on the inside edge of the tire but not a lot and based on what I have seen on a few IRS camber curves at stock ride height the camber goes further negative through droop and bump but when the car is lowered the camber goes positive as you droop just about until you would have been as stock ride height then begins going negative again. So that is the reasoning for me adding in a little negative camber to combat the positive camber gain through initial droop. So depending on your ride height and tire wear you may want to adjust this setting for your cars' characteristics. Basically if you are wearing the inside edge of the rears less negative rear camber is in order and conversely if the opposite is true more negative camber in the rear. Rear toe in is also largely dependent on what modifications you have done. Hopefully you have done a bump steer kit in the rear and if so your toe stays constant through the range of motion. I added a little rear toe in. This is because on a rear drive vehicle (with a positive offset wheel) the thrust of the rear wheels tends to pull them toward toe out and ideally you want zero toe in the rear so if you have just a little rear toe in the power of the car will pull the wheels in to a zero toe alignment and plus you would not want a toe out situation in the rear or the car would be all over the place under power.

------- Notes ------
Front Caster
Front caster can only be adjusted with aftermarket caster camber plates. I recommend Griggs, Maximum Motorsports and Steeda (the new 4 bolt design only).

Rear Camber
When the wheel is off the ground, and the upper knuckle is loose, the weight of it tends to rotate the wheel towards positive camber of course. Adjusting the eccentric to the max negative camber by rotating the bolt, only goes so far... the upper arm end is slotted on both sides to allow the bolt to adjust, and to get max camber out of it, it needs to be pushed back in the slot further on both sides of the bolt (The bolt will torque down at an angle if you let it, as the eccentric is on the opposite side).

Q. I am getting irregular tire wear what’s the deal?
It is most likely a factor of a few things. Your alignment settings have a lot to do with it (see above) and your tires must be properly inflated. Even if it looks fully inflated on the outside edge remember there is more weight on the inside edge because of negative camber so adding a few PSI may help. Your driving habits play an important part and unless you have modified your IRS as described above the compliance of your all the parts moving you never even keep your rear wheels in alignment under power and when she squats the camber goes even more negative. Finally if you are a real corner carver the stock diff leaves a lot to be desired in the twisties and causes some additional tire wear.

Q. I want to upgrade my sway bar where do I go?
Virtualy no one makes sway bar upgrades because the factory bar is very good. If you want to upgrade your handling the sway bar is the last place you need to mess with (unless you are talking about upgrading the bushings or end links).

=============== IRS Drop Instructions ================
1. Get car up on 4 jack stands with the tires about 8-10" off the ground
in the rear and 4-6" in the front. A little angle is important to keep
the trans from leaking when you pull out the driveshaft.

2. Remove catback. Soap or spray lube on the hangers ease insertion/removal.

3. Remove Driveshaft use 12pt 12mm socket. Pull it out of the trans.

4. Remove tires 13/16" socket.

5. Remove lower shock bolt. 18mm socket, disconnect bottom only, leave them hanging.

6. Remove ABS sensors from differential and pull/pry the fasteners off
the IRS subframe. Get it completely off the IRS. They detach under the
rear seats. You can pull up through the floor if you pull the big
rubber grommet up.

7. Remove Emergency brake cables. There are little c-clips mounting them
to the calipers. DO NOT remove the spring, way harder to get it back
together. Just push on the little arm holding the cable end till you
can work it out. I use vice grips to compress it. Make sure e-brake is
off, of course. Also have a helper pull all the slack to the side you
are working on.

8. Remove brake calipers, two 15mm bolts. Put jack underneath bottom of
spindle, just support it. Mark the position of the eccentric washer
where the UCA mounts to the top of the spindle. Remove upper
spindle/UCA bolt 18mm. Pull back on top of spindle and slip the brake line
out from under the UCA. Remove brake line bracket, 7mm bolt, which mounts to UCA. Replace the upper spindle bolt momentarily.

9. Remove rear IRS subframe bolts, 18mm. Support IRS at the center of the main
rear beam, directly rear of the pinion mount bracket (back of the
diff). Take both bolts out once it is supported. Use some 4x4's or
jack stands to support it a few inches under its normal position (you
don't want to lower it to ground till the front is loose.) You just
want to lower it enough so the springs will come out.

10. With the rear supported on 4x4's or jack stands, put the jack under
the front differential brace. Remove the front IRS subframe bolts.
Lower the front down. It may stick, both the ones I have done needed a
few smacks with a mallet. Once loose lower to the 4x4's then reposition
the jack, lift a bit, remove 4x4's and lower to ground or pull out from
under the car on top of the jack.

11. Reinstall in reverse order. Be sure to get it up close again on
4x4's, too much angle and the front bushings will not go in.
*CRITICAL that you install the FRONT first because those bolts have to go through fixed holes. The REAR bushing nuts (which are movable) and can be positioned to line up with the hole in the bushing. After the fronts are installed (and still a bit loose) swing up the rear, look through the hole, line up the nut behind it and insert the bolt.
Remember to swap out the stock 12mm front IRS bolts for the new 14mm bolts from

=============== Differential Removal ===============
1. Get car up on 4 jack stands with the tires about 8-10" off the ground
in the rear and 4-6" in the front. A little angle is important to keep
the trans from leaking when you pull out the driveshaft.

2. Remove catback. Soap or spray lube on the hangers ease insertion/removal.

3. Remove Driveshaft use 12pt 12mm socket. Pull it out of the trans.

4. Remove tires 13/16" socket.

5. Remove lower shock bolt. 18mm socket, disconnect bottom only, leave them hanging.

6. Remove Emergency brake cables. There are little c-clips mounting them to the calipers. Make sure e-brake is
off, of course. Also have a helper pull all the slack to the side you are working on. DO NOT remove the spring, way harder to get it back together. Just push on the little arm holding the cable end till you can work it out. I use vice grips to compress it.

7. Disconnect ABS sensors from diff T40 torx or 1/2" wrench.

8. Remove brake calipers, two 15mm bolts. Use some coat hanger wire or zip ties and hang them from the IRS mount bracket or anywhere out of the way.

9. Disconnect Toe Link Bar from spindle. Pull out cotter pin. Remove castle nut 18mm. DO NOT HIT the taper bolt with a hammer; you will crush it because of the cotter pin hole. You can either use a tie rod puller (AutoZone 25202 3.5") or the pry bar way. Spray taper bolt with penetrating oil, use long screwdriver or prybar and apply downward force on the cast end of the bar while striking the rearmost point on the spindle. Pry downward and hit it hard, it will come out.

10. Remove half shaft/spindle assy.
Put jack underneath bottom of spindle where it meets LCA, just support it. Mark the position of the eccentric washer on the Camber Bolt where the UCA mounts to the top of the spindle. You will realign these marks during reassembly to maintain current suspension setting. Forgetting to do this will mess up your tires and ride feel.
Remove upper spindle/UCA bolt 18mm.
Lower jack all the way down allowing the springs to stretch out. Remove jack.
Remove lower spindle/LCA bolt 18mm.
Pull half shaft out about 1/2"
Use flat screwdriver/prybar between diff and inner hub to make sure it is coming out of the diff. Pull half shaft all the way out.

Repeat for the other side.

11. Remove two 18mm bolts from diff cover-to-rear bushing. Leaves the rear diff bushing bracket hanging momentarily while we proceed to get the diff out.

12. Remove Front Differential bushing bolts 15mm

13. Remove nuts off front (only) LCA pivot bolts 24mm. Leave the bolts in place though. This allows the front differential support brace to be removed.

14. Remove Diff. Support rear of diff with a jack, remove front support brace and diff will move forward and down and out of the car. It is about 70lbs so be careful. Do not tip it or you will spill "fish guts" the nastiest smelling oil there is.

15. Remove Rear Differential bushing bracket bolt. 15mm and lots of extensions from the drivers side. Very tough to get to initially. Easier if you also remove drivers side toe link bar from its inner mounting point 13mm bolt on bottom, 15mm nut above.

Good time to reseal the diff with Permatex Ultra Grey ($4 Autozone) or Ford TA31 Diesel Sealant ($15 dealer only)

=============== Torque Specs ===============
Subframe-to-body bolts 76 lb-ft
Subframe-to-rear bracket bolts 76 lb-ft
Subframe rear bracket-to-body bolts 59 lb-ft
Shock absorber-to-lower arm and bushing bolts 98 lb-ft
Shock absorber-to-body nuts 30 lb-ft
Upper arm and bushing-to-subframe nuts 66 lb-ft
Upper arm bushing-to-knuckle nut 66 lb-ft
Lower arm and bushing-to-subframe bolts 184 lb-ft
Lower arm and bushing-to-knuckle nut 85 lb-ft
Toe link-to-subframe nut 35 lb-ft
Toe link-to-knuckle nut 35 lb-ft
Stabilizer bar bracket bolt 41 lb-ft
Stabilizer bar link nuts 35 lb-ft
Rear axle diff rear insulator-to-axle housing bolts 76 lb-ft
Rear brake disc dust sheild-to-knuckle bolts 89 lb-in
Brake line-to-rear brake caliper bolt 30 lb-ft
Parking brake cable bracket-to-lower arm bushing bolt 11 lb-ft
ABS sensor bolt 17 lb-ft
Axle shaft-to-hub retainer 240 lb-ft
Driveshaft to pinion flange 83 lb-ft
Wheel nuts 95 lb-ft

Front shock absorber upper nut - 74 ft/lbs
Front shock absorber-to-spindle nuts - 148 ft/lbs
Shock absorber upper mount-to-body nuts - 30 ft/lbs
Shock absorber upper mount-to-body bolt - 30 ft/lbs
Wheel hub and bearing retainer nut - 258 ft/lbs
Stabilizer bar bracket nuts - 52 ft/lbs
Stabilizer bar link nuts - 14 ft/lbs
Anti-lock brake sensor bolt - 53 in/lbs
Anti-lock brake sensor wire bracket nut - 21 ft/lbs
Ball joint-to-front wheel spindle nut - 129 ft/lbs
Front suspension lower arm-to-body nuts - 148 ft/lbs
Tie-rod-to-spindle nuts - 41 ft/lbs
Steering gear-to-crossmember nuts - 52 ft/lbs
Wheel nuts - 95 ft/lbs

=============== Step by Step Transmission Removal ===============
1. Disconnect Battery
2. Get car up on jack stands, tall ones, need 12" under the tires airspace
3. Remove Shifter handle, bezel/plate and then shifter.
4. Remove mid pipe
5. Remove driveshaft (use a trans plug or towel to keep the tail shaft from leaking, or drain it).
6. Remove Trans mount
7. Unplug all electrical connections from the trans
8. Remove clutch inspection cover
9. Detach clutch cable
10. Remove lower 4 trans bolts
11. Support with trans jack
12. Remove remaining upper trans bolts
13. Slide trans back and lower trans jack down

=============== Useful links ===============
AC427Cobra’s Informational Videos:
FTBR Videos

Postban's IRS build up article along with IRS drop procedure and torque specs can be found here:

A good mustang suspension article:

A very good post about mounting larger wheels and tires on an IRS cobra.

A nice article about driveline alignment (no more 80 MPH vibe).

Any Comments, Suggestions or Questions PM Me
Last edited:


Born in the G-Code
Established Member
Feb 2, 2003
Excellent write up. The only thing I would add is about the binding issue with the Delrin Nick sells. I have them in my car and had to tear the IRS apart a second time to fix it. The fix is very simple. Also, if possible, rent the MM tool for removing the stock CA bushings. MM does not sell it, but a few people have them floating around. I have one I have had a few people use. It makes removal WAY easier.

Also, some people have fitment issues with Nick's Aluminum diff bushings and have to reduce those as well.

Here is the thread with all the information on how to fix Nick's Delrin bushings,


Snake Eyes

Active Member
Established Member
Apr 25, 2002
Scottsdale, AZ
This article has been posted here and on other forums a few times but I figured if we have the 99/01 faq and the supercharger faq I might as well post this and have it made a sticky article for the forum. It will be updated as nessissary as well.

BreBar21: That is a good point, at present modular fords is down. But I will add that info.


Well-Known Member
Established Member
Aug 26, 2001
Milford, MI
Why doesn't this article doesn't mention anything about the KB modified IRS cradle? Not only do they offer it with modified pickup points for better geometry but you can also eliminate the cradle to frame bracket which both eliminates weight AND a lot of play in the whole system.


We added the bracket on the bottom.

Why does it say that the KB arms are not recommended for street use? The only thing you have to do is to notch the frame rail. The weight approx 1/2 of the stock arms and offer a MUCH better double shear mounting point if you are using coil overs.

I also include heim joint sway bar end links.

We also race the IRS without the front "anti hop" brace and it seems that using the harder bushings in the IRS to cradle points on the differential accomplishes the same thing and saves you a couple #'s in the process.

You can also find a LOT of info on setting up the IRS under the Cobra R forum.
Last edited:

Snake Eyes

Active Member
Established Member
Apr 25, 2002
Scottsdale, AZ
Thanks Jeb I'll update it. I've been waiting for Kenny Brown to update his site so I can post some links about his parts the I reference in the article but they apparently don't have any interest in that...


New Member
Established Member
Sep 9, 2002
EN ,ahamO
- Aj - said:
I'm pretty sure KB is out of business now...

Kenny brown's front aluminum bushings are not that good. They did not take into account pinion angle change and the force applied on the housing ears.


Well-Known Member
Established Member
Aug 26, 2001
Milford, MI
Stitch said:
Kenny brown's front aluminum bushings are not that good. They did not take into account pinion angle change and the force applied on the housing ears.

Actually their front bushings are completely adjustable so I'm not sure what you are talking about.


New Member
Established Member
Sep 9, 2002
EN ,ahamO
93SVTCobra said:
Actually their front bushings are completely adjustable so I'm not sure what you are talking about.

Then you've never used them to adjust pinion angle on an IRS car.

As rotation of the pinion rises, the mounting points must be angled appropriately, flat washers are the same thickness on both sides therefore and uneven load is placed against the ears of the pumpkin. MM's kit are tapered to allow for this angle change. In other words, don't by the KB kit.

Snake Eyes

Active Member
Established Member
Apr 25, 2002
Scottsdale, AZ
To balance the driveshaft angle. If you dont keep the two angles = or close to, you will end up with vibe regardless of how well your driveshaft is ballanced.


New Member
Established Member
Sep 9, 2002
EN ,ahamO
93SVTCobra said:
And why would the driveshaft angle change?

If you install gears, the pinion flange runout can change and the whole pinion angle can change as a whole. My pinion was pointing down 2* which gave me a difference angle between my pinion and my crankshaft of 4*. Ford specs are for the difference to be less then 1*.

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