The Cobra Independent Rear Suspension - Modular Fords

  • The Cobra Independent Rear Suspension

    Cobra owners have been disappointed and also feel let down by Ford regarding the Cobra (IRS) Independent Rear Suspension assembly. The IRS has also been a downright mystery for some owners. There are numerous reasons why some cars wheel hop worse than others and there are several cures that can mask wheel hop to varying degrees. Wheel hop can and will cause serious damage to your IRS assembly. Snapped halfshafts, leaking rear covers and cracked or blown rear covers are the most common of failures. The biggest offender is all of the rubber in the differential mounts, control arm bushings and the subframe bushings. These soft compliant rubber bushings were originally installed in the IRS so they could deliver a nice soft ride that was free of (NVH) Noise Vibration and Harshness, which will simply be known from this point forward as NVH. A secondary reason for wheel hop is rubber compound (durometer) and sidewall stiffness of a hard street tire. The drive tires oscillate between traction and slip when it hops. Additionally, the suspension rubber compresses, then rebounds and repeats. If you remove the "give" found in the OEM suspension rubber or aftermarket poly bushings, it can no longer do that. Some tires, depending on compound and sidewall stiffness, can increase or decrease hop to a certain extent. For instance, a Good-Year Eagle F1 will wheel hop on almost any car especially in cold weather. For the smooth application of power, you need a rigid structure (stiffer chassis and IRS assembly) and tires that have a soft enough compound to grip with a sidewall that will absorb some of the 'shock' of a launch. Chassis flex is also a contributing factor to wheel hop.

    Some background on my experiences

    I’ve been working on cars since the late 60’s. In the early 70’s I was a Ford technician. After a few years in the dealership service department I realized I didn’t want to do that for the rest of my life so I left the business and have been a hobbyist mechanic ever since.

    My first real exposure to an IRS was the 1989 SHO Taurus I bought new. Although this is a front wheel drive car, the rear end in this vehicle rode and cornered like no other car I’d ever previously owned, which by the way were all solid rear axle vehicles. Fast forward twelve years later to when I drove home my brand new, 17 original mile 2000 Cobra R Mustang. I was astounded at the cornering capabilities of this car in its completely stock form. Shortly thereafter I began my fond appreciation for a RWD IRS once I hit the track. It did not take me very long to get up to speed with this most awesome piece of machinery and the faster I went, the faster I wanted to go. One thing that concerned me a lot was the fact that although I was able to pass a vast majority of the Corvettes I encountered on track, there were a few that were faster than me, particularly going through the corners. This bothered me greatly being the die hard Ford guy that I am. After one year of driving schools and open track events with the Cobra R, I had a yearning for a better suspension, in particular, a more efficient IRS. That Winter I decided to eradicate all of the rubber in my entire IRS assembly after reading about Delrin bushing scuttlebutt and the ill effects of control arm deflection I found on the internet throughout 2002 and 2003.

    After searching the internet I found a guy making a Delrin bushing set for the Cobra IRS. After quite a long wait, my kit finally arrived. I disassembled my IRS and started the install of this bushing set. To my horror, I discovered that almost NOTHING fit! Bushings that should have had a press fit were falling into place with more than .010” of clearance and areas that should have had clearance for articulation were press fits as tight as .014”. Panic began to set in. This was late February and I was scheduled to leave for a NASA event at Mid-Ohio in one month.

    I contacted a fellow tracking buddy Ken from New Jersey and he bailed me out, like a true track bud would! I gave him numerous rough sketches of the bushing dimensions I was looking for and he got me hooked up. He did a marvelous job for which I will be forever grateful. I had about a week to put the car back together and bumpsteer it to make it to my M-O event. This was the birth of the eventual Full Tilt Boogie Racing, Mustang Cobra, IRS bushing kit.

    When I arrived at M-O in March of 2004 (my first time there in my short career) it was miserably cold and rainy the first day. But on the second day, it was dry and I was able to test my newly configured IRS on a track that I already had some experience on. To my complete and total disbelief, I found my car suddenly going through corners like it was on rails. This was the first time the rear suspension could finally keep up with the front suspension. It drove nothing like it drove previously. I was giddy and actually laughing out loud as I would carve through these turns. It was a complete and total blast. Another thing I discovered was the fact that there were no Z06’s out cornering me anymore, which truly was icing on the cake for me at that point. This was by the way at the stock ride height, with the OEM springs, shocks and totally stock front suspension. The only change to the car was the total eradication of all rubber in the IRS assembly and the addition of an IRS bumpsteer kit.

    Being a road racer, I typically don’t do a lot of hard launches but there are occasions that you leave pit road in a hurry and end up applying a lot of power in a short distance. This previously caused wheel hop with my OEM IRS. I also now noticed as a side benefit, I no longer had any wheel hop with this new bushing kit.

    After glowing about the vast differences in my newly modified IRS to many of my tracking pals and having many of them ride in my car and even some of them drive it, inquiries started to come in with requests for kits. So after some additional engineering and numerous tweaks over the years, the kit has evolved into what it is today. It is the finest, best fitting and performing IRS bushing kit available on the market today. My day job has helped me design this kit to its full potential as I’ve been involved with industrial design and manufacturing since 1973.

    Facts regarding the SN-95 chassis

    The SN-95 chassis covers all Mustangs from 1994 through 2004. This particular chassis is, more or less, a carryover from the 1979 Ford Fairmont. Although it had received platform and suspension changes, it was very old technology at best. It was not the most rigid of platforms either.

    Many people also are puzzled by the fact that although they have reduced some or most of their wheel hop, they wonder why the rear end of their car still feels so out of control when they try to put a lot of power down or traverse a curve at speed. Here are a few things to keep in mind: #1) The Cobra IRS rear suspension is half as efficient as the front suspension because the rear suspension has twice as many control arms AND it also has to deliver propulsion for the vehicle. #2) Rear wheels are mounted to the rear knuckles and the knuckles are mounted to the subframe via control arms with rubber bushings in four places. (Two places on the upper control arm and two places on the lower control arm) When you apply power, this pushes the lower control arms forward and compresses the rubber bushings. The upper control arms are simply along for the ride at this point. Keep in mind the whole subframe assembly is also mounted to the chassis in rubber and who knows what direction the tires are actually pointing at this point. The tires should be pointing forward but clearly they are not if eight control arm bushings are rubber or aftermarket poly. This uncontrollable wandering of the rear tires translates into a vehicle that is hard to control in a particular direction. By removing all of the give in the compliant rubber or aftermarket poly bushings, your rear tires will gain control of their direction and you will feel the rear end of the car plant in a much more controlled manner.

    Shocks and springs also play an important part of this equation. Factory shocks are going to give you minimal help in the effort to reduce wheel hop. If you have a convertible you are going to want to switch your springs to the coupe version at the very minimum, which are slightly stiffer. A lowering sport or race spring would even be better. Aftermarket shocks, particularly adjustable ones are highly recommended as well.

    IRS afterthought

    For the 1999 model year, the SVT engineers at Ford decided putting an IRS into the New Edge SVT Cobra was the thing to do. The only problem was this chassis was not designed for an IRS. They overcame this obstacle by building a subframe to hold the IRS assembly. This move was definitely a compromise. The design itself is not as bad as it seems, but the materials used to meet the NVH levels they were striving to achieve was the straw that broke the camel’s back. It works, but it’s highly compromised. The good news is the entire IRS assembly can be modified and upgraded for less than $1,000.00 if you do the labor yourself. The IRS is still a far superior suspension component over the archaic stick axle and you should allow the IRS to prove itself before you throw in the towel on it.

    IRS differences through the years

    In 1999 the halfshafts on the IRS contained a 28 spline inner shaft. This is determined by the gear carrier (side gears) inside the differential. The 28 spline halfshafts were the weakest halfshafts of all IRS years. In 2001 the halfshafts were beefed up slightly and the inner spline was upgraded from a 28 to a 31 spline. In 2003 the halfshafts were once again upgraded to a heavier unit but still retained the 31 spline inner. In addition, the driveshaft and pinion flange were beefed up as well and the pinion flange on an ’03-’04 is larger and not compatible with 1999-2001 drive shafts so keep that in mind if you’re swapping an ’03-’04 IRS into your ’99-’01. ALL halfshafts from ALL years retain the same 28 spline outer that goes through the hub in the rear knuckle. This means the hubs and knuckles are interchangeable for all IRS years 1999-2004.

    The 1999 and 2000 Cobra R models had knuckle cross axis joints that were pressed in and did not have a flange and retaining ring holding them in. These were all recalled and updated with the flange and retaining ring style cross axis joints.


    Almost every owner and former owner of an SN-95 chassis vehicle will tell you that a “full length subframe connector” is extremely important for these cars. Honestly it should be the first mod on these vehicles, even before a CAI or a tune. They are absolutely critical with convertibles. If you have a convertible you are going to want to seriously consider a subframe ‘system’ that has spider or matrix bracing that also ties into a jacking rail on the rocker panel. The FLSFC is the first step in eliminating wheel hop. It stiffens the platform which makes for a good foundation.

    The front torque brace on the ‘99 SVT Cobra, the ‘00 Cobra R and the ’01 Cobra did not incorporate attachment to the front differential mounts. This is to help stabilize the front of the differential housing. The ’03 and ’04 Terminators incorporated attachment of this front pinion brace to the front differential mounts. Some people refer to this as a “hard launch brace”. This is something you want on your IRS assembly to help stabilize the differential housing, particularly with high horsepower cars.

    If you are a drag racer or just a guy that likes to drop the hammer once in a while to impress your friends, you definitely need a “rear cover brace” on your differential at the absolute minimum. The upgraded Ford Racing cover mentioned in the next section is the cure all for the differential and if you’re a road racer, you’re probably better off with the New Ford Racing cover as well.

    The aftermarket brace for the rear cantilevered subframe mounting point typically called a “Mathis Brace” will not shore up the rear mount for you very much if it’s just bolted to the inner wheelhouse sheet metal. However, if you fabricate a heavy angled bracket and weld the inner mount to your trunk floor by the subframe rail, you will then have something of substance. I would only recommend this to convertible owners that are striving for the complete and total elimination of wheel hop. This should not be necessary for coupe owners.

    Differential Covers

    The OEM differential cover is thin and weak which is why aftermarket rear cover braces appeared on the market. The OEM cover was originally made thin to save on material, manufacturing it for the OEM vehicles. Because of this, many people have cracked or broken the OEM covers and a few aftermarket covers were developed. The Fore cover is a nice piece but it’s extremely expensive and they originally had some breathing issues with it causing fluid to come out of the breather. The LPW cover is much more affordable but there have been some clearance issues with it. Almost everyone has had to grind the “LPW” letters off of it in order for it to clear the OEM rear differential mounting bracket. There may be some fitment issues with these aftermarket covers when combining them with a rear diff lowering mount. The Ford Racing cover is the latest version and I believe that it is the best option. The Ford Racing cover has tapped holes that are ready for a differential cooler for open track and road racing applications. The suction port doubles as a drain as well. One caution when using the Ford Racing cover with a rear differential lowering mount, there will be some clearance issues with the inner toe link mounts on the subframe combining those parts. Full Tilt Boogie Racing now offers a machined Ford Racing cover that will bolt in place with our Delrin isolated rear diff lowering mount without any clearance issues. Rear diff cover braces will not fit with aftermarket covers, nor are they needed.

    Upgraded halfshafts

    If you want my opinion on Level 5 halfshafts, I will tell you that they cause FAR more issues than they are supposed to correct. If you’re thinking about buying them or you bought them, you’re wasting your money. Level 5 halfshafts are NOT stronger than the OEM halfshafts. My advice is run with the Ford ’03 upgraded version halfshaft and if you have removed ALL of the rubber in your IRS and you still break them, you might possibly want to look into a 9” SRA. The IRS does have limitations and if you like to do a lot of drag racing with 5k launches and you have 800 or more RWHP you may want to consider a 9” SRA. More halfshafts have been broken due to wheel hop and the flopping of IRS components mounted in OEM rubber or aftermarket poly than halfshafts that have been broken because of sheer power.

    Masking wheel hop

    Many people have masked some or most of the wheel hop issues in their car by using several approaches. One approach has been a simple tire change to a softer (lower tread wear rating) ET street tire or a drag radial. Some have used air bags, some have used shocks and springs or combinations of all the above. Although eliminating wheel hop is an improvement and a step in the right direction, there are still lingering issues that can and will cause serious damage to your car that need to be addressed.

    The differential came from the factory mounted in rubber bushings on the front and rear mounts. These mounts have an immense amount of give to them and wear out exceedingly fast. Most Cobra owners refer to the movement in these mounts as the “Cobra Clunk”. Without a rear cover brace, if you do not address these mounts, you will at the minimum develop a leak, eventually crack a rear cover, blow a rear cover completely out or possibly snap a halfshaft due to the lack of stability in the IRS assembly.


    Back in the early days before the IRS was completely understood, people threw ‘pills’ at their IRS. These pills were Blue pills, red pills and black pills. A ‘pill’ was a somewhat popular recipe of parts for curing wheel hop. Some of these things they threw at the IRS helped and worked. They became quite popular back in the day. These fixes showed some success and at that time people wanted answers and cures. However, I’m not a very big ‘pill’ fan. I’m glad they helped people and I’m glad that some of the people were reasonably happy with the fixes. But there are some things in the ‘pill’ recipes that I simply disagree with, and I will leave it at that.

    IRS Maintenance

    One item that is frequently overlooked regarding the maintenance of the IRS is regular checks of the torque on the rear axle halfshaft retaining nuts. If these nuts work loose, the bearings lose their pre-load. If the knuckle bearings lose their pre-load they self destruct in short order. If the bearing is run long enough in that condition that will eventually destroy the hub. The torque on the 36mm nuts that attach the halfshaft to the rear hub on the knuckle should be checked on a regular basis. If your car never goes to a race track, three or four times a year should be sufficient. Torque these 36mm nuts to 240-250 ft. lbs. If you’re going to a race track, torque before EVERY event. If you find this nut keeps coming loose, it should be replaced with a new nut. If a new nut works loose, the spline in the hub is worn out and the hub should be replaced. If you’re replacing the hub, replace the bearing at the same time as long as you’re going that deep into the repair. Be sure to use the Ford double tapered roller bearing and not a cheap aftermarket ball bearing. This is not a place to save $10 or $20, trust me.

    Fixing the IRS

    If you want to fix the inherent problems with the IRS, this will require the removal of the IRS assembly from the chassis. This process entails the removal of all rubber in the entire IRS assembly. The only places you are allowed to have ANY rubber are the subframe mounts for the sway bar, that’s it. All remaining rubber must be removed.

    This job is a considerable undertaking. It requires some car wrenching skills and it’s nice to have an assistant helping. An assistant will make the job go much easier because two heads are always better than one. The speed of the job depends a LOT on the people doing the work and the amount of tools/equipment at your disposal. If you have a couple of knowledgeable people that are good wrenches, a lift, air tools, a trans jack and a well equipped shop, you could knock this job out in five to seven hours working at a steady pace and longer if you’re going to be taking your time.

    Your car came from the factory with a 12mm bolt holding the front of the subframe into the chassis. They had to do this on the assembly line because they were unable to get a 14mm bolt in there fast enough to keep up with car assembly. A 9/16” Grade 8 bolt or a 14mm bolt will fit in this location. Sometimes they are a bit of a challenge to get in but they will help eradicate the dreaded “Cobra clunk”.

    Be aware that the replacement aluminum front differential mounts can be a tad on the noisy side. This is the only area where NVH will be significantly elevated. The subframe mounts, rear differential mount and the control arm bushings will hardly raise NVH levels enough to be noticed. Aftermarket exhaust that is loud will help mask the noise from the front diff mounts. Some people have used the sound deadening matting available at car stereo shops to minimize the sound. This is self adhesive matting you put down under the rear seat on the floor pan right above the differential. It's called Dynamat and it's available at car audio stores and also available at Eastwood.

    Solid Rear Axle

    Many people who do not understand the IRS very well think an SRA is a cure all. The main reason for this is because the solid axle has been around for centuries and it’s not complicated. My opinion is if you want to put an archaic stick axle in a car the SVT engineers thought would be better served with an IRS, by all means it’s your right to do so. And as long as I don’t have to drive it, I’m ok with it. It’s your car, do with it what you want. But I would advise HIGHLY against it. There are several reasons a Cobra costs more than a Mustang GT and the upgraded suspension with the IRS is one of them. Many of the people that put an SRA in their Cobra could have easily used their IRS had they understood it better. Most people drive their cars on the street 99% of the time and the IRS will deliver superior ride and superior handling for that 99% of the time. The only way I would recommend a stick axle in a Cobra would be if you had a dedicated drag race car that you hauled to the strip on a trailer. The IRS is a FAR superior suspension component than the archaic stick axle. Besides, a Terminator for example, already has a poor F/R weight distribution percentage of 57/43 and putting a SRA in the car makes this worse. There is a reason why every high performance sports car from every single manufacturer comes from the factory with an IRS today. How many true high performance sports cars have SRA's in them? Well none actually! Unfortunately for Cobra owners, they have a sour taste in their mouth regarding the IRS. The IRS was delivered from the factory in a very poor state. You really need to remove ALL of the rubber (and or any aftermarket poly) from the entire assembly including the subframe to raise the efficiency level of the IRS.


    Many people don’t understand what bumpsteer is and what sort of affect it has on a car’s handling. Bumpsteer is when your wheels steer themselves as they travel up and down in their suspension range. The undesirable steering is caused by bumps in the road or track interacting with improper angle of your IRS toe links. When the rear tires on your car move up and down in their normal suspension travel, the rear toe setting changes. This is not efficient and it unnecessarily scrubs speed and wears tires. It also gives you the uneasy feeling you don’t have control of your car.

    Most auto manufacturers design their cars to push or understeer when taken to the limit. A push is infinitely safer than oversteer. It warns the driver much sooner and is easier to recover from. But a push is not fast, nor is it efficient. You can optimize the IRS by installing a rear bumpsteer kit and bumpsteering the suspension which will increase cornering efficiency. This is of particular importance if you've lowered your car with aftermarket lowering springs or coil-overs. A lowered car that is not bumpsteered will be a downright ill handling car!

    Another one of the benefits to bumpsteering the IRS is getting an upgraded toe link. The OEM toe links are marginal at best. After adding additional horsepower they become a weak point.

    Going to the race track

    If you plan on taking your IRS equipped car to a race track, you’re going to want to consider a cooling system. I’m not talking about the drag strip, I’m talking about road racing. If it’s a driving school, or an open track event you will eventually want to consider this. For your first few events you will most likely be ok without cooling. But once you get the hang of pushing the car near its limit, you will increase the need for differential cooling. The stock Ford posi-traction unit uses clutches and they generate a LOT of heat especially with a supercharged high horsepower engine. A clutchless posi-traction unit like a Torsen T2-R will help keep diff temps down. But lower gears like 4:10’s will increase diff temps on track.


    I hope this article provided some insight for you regarding the Cobra IRS. At Full Tilt Boogie Racing we offer an entire website dedicated specifically to the Cobra IRS.

    We have a frequently asked questions page to answer almost every question that’s ever been asked regarding the IRS.

    We have a videos page where we’ve compiled videos of rear brake jobs, knuckle and halfshaft removal, a complete series of videos of our IRS bushing kit install, videos explaining bumpsteer and how to adjust it accompanied by a few other videos.

    We have a page dedicated to identifying the location of all of the bushings contained in the IRS assembly.

    Another page dedicated to assisting in the installation of the bushing kit with photos and tips.

    A listing of our dealers and installation facilities.

    We have a page dedicated to setting the pinion/driveline angle.

    A testimonials page where happy customers have sent us messages or have posted on the internet regarding their experiences with our kit.

    A brake information page that lists the weight of our rotor and brake products as compared to the OEM rotors.

    Our order page with all of our products can be found here.

    And last but not least, we have a contact email in case you have questions regarding what parts will be best for your application or in case you’ve already bought parts from us and have installation questions. We normally are available 24/7-365 and your inquiries will be addressed in a timely manner, unless I’m gone racing without internet access.

    Bruce and Steve
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