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The Mystery of the Cobra Independent Rear Suspension

Discussion in 'Front Page Articles' started by ac427cobra, Jun 16, 2009.

  1. ac427cobra

    ac427cobra FULLTILTBOOGIERACING.COM Staff Member Super Moderator

    Oct 20, 2002
    In the race shop

    The Mystery of the Cobra Independent Rear Suspension

    Many Cobra owners have been disappointed and feel let down by Ford regarding the Cobra IRS assembly. It’s been a downright mystery for some. There are numerous reasons why the car wheel hops 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 mounts. These soft compliant rubber bushings were installed for a nice soft ride that was free of Noise Vibration and Harshness, which will simply be known from this point forward as NVH. A secondary reason for wheel hop is sidewall stiffness of a hard street tire. It oscillates between traction and slip when it hops. Additionally, the suspension rubber compresses, then rebounds and repeats. If you remove the "give" (suspension rubber) it can no longer do that. Some tires can increase and or decrease hop to a certain extent. For instance an Eagle F1 will wheel hop on almost any car. For the smooth application of power, you need a rigid structure (stiffer chassis and IRS assembly) and tires that will absorb some of the 'shock' of a launch. Chassis flex is also a contributor 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 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 cornered like no other car I’d ever previously owned. Fast forward twelve years, 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 bothered me 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 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 Mid-Ohio in a month’s time. I contacted a fellow tracking bud Ken from New Jersey and he bailed me out, large style!

    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 together and bumpsteer it to make it to my M-O event.

    When I arrived at M-O (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. It drove nothing like it used to drive. 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 and totally stock front suspension.

    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. 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, inquiries started to come in with requests for kits. So after some additional engineering and a few tweaks over the years, the kit has evolved into what it is today. It is the finest, best fitting and performing kit available on the market. My day job has helped me design this kit to its full potential. I’ve been involved with 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 eliminated 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. Here are a few things to keep in mind: your 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 along for the ride. Keep in mind the whole subframe assembly is also mounted to the chassis in rubber and who knows what direction the tires are actually traveling at this point. The tires should be pointing forward but clearly they are not. 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 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, which are slightly stiffer.

    IRS, an 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 to be 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 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 updated 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. ALL halfshafts from ALL years retain the same 28 spline outer that goes through the hub in the rear knuckle.

    The early 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 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 very important for these cars. It’s absolutely critical with convertibles. 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 another area to help stabilize the differential. The ’03 and ’04 Terminator incorporated attachment of this front pinion brace to the front differential mounts. Some people refer to this as a “hard launch brace”.

    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. If you’re a road racer that rarely pulls a hole shot in your car, we refer to those rear cover braces as ‘ballast’ and we really have no need or use for them whatsoever. Our Cobras are heavy enough as they are!

    The aftermarket brace for the rear cantilevered mounting point typically called a “Mathis Brace” will not do a lot for you just bolted to the inner wheelhouse sheet metal. However, if you make an angled bracket and weld the inner mount to your trunk floor, you will then have something of substance.

    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. At least 95% of the people buying them are wasting their money, perhaps more? 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 an upgraded halfshaft. I firmly believe more halfshafts have been broken due to the flopping of IRS components mounted in rubber, than 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 ET street tire or a drag radial. Some have used air bags, some have used shocks and some have used springs. 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. 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 severe angle you are asking the halfshaft to operate at when the differential winds up.

    The Pills​

    Back in the early days before the IRS was completely understood, people threw pills at their IRS. Blue pills, red pills, black pills and maybe even purple and green pills as well (not sure on the purple and green actually). Some of these things they threw at the car helped and worked. They became quite popular. 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 a majority of the people were reasonably happy with the fixes. But there are some things in the ‘pill’ recipes that I simply disagree with. I will leave it at that.

    IRS Maintenance​

    One item that is frequently overlooked regarding the IRS are regular checks of the torque on the rear axle halfshaft nuts. These are the 36mm nuts that attach the halfshaft to the rear hub on the knuckle. The torque on these nuts should be checked on a regular basis. If your car never goes to a race track, 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.

    Fixing the IRS​

    It’s come time to fix the problem. This requires removal of the IRS assembly from the chassis. Step one is the removal of all rubber in the entire IRS assembly. The only places you are allowed to have ANY rubber are the sway bar mounts. 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. An assistant will make the job go much easier. Two heads are always better than one. Speed depends a LOT on the people doing the job 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 four or five hours.

    Your car came from the factory with a 12mm bolt holding the front 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 should go 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 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's loud, will help mask the noise from the front diff mounts. Some have used the sound deadening matting available at car stereo shops to minimize the sound. This is a self adhesive matting you put down 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:

    Eastwood Company: Search Results for dynamat

    Solid Rear Axle​

    Many people who do not understand the IRS very well think an SRA is a cure all. 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. 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. The only way I would recommend a stick axle would be if you had a dedicated drag car that you trailered to the drag strip. The IRS is a FAR superior suspension component than the archaic stick axle. There is a reason why every sports car from every major manufacturer comes from the factory with an IRS. How many true 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.


    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 tires on your car move up and down in their normal suspension travel, your 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 car builders 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 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 re-bumpsteered will be a downright evil 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 power 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 a hang of pushing the car near it’s limit you will increase the need for cooling. The stock Ford posi-traction uses clutches and they generate a LOT of heat especially with a supercharged high horsepower engine.

    Miscellaneous Images​





    I hope this article provided some insight to the mysterious IRS for you.

    For more information on the complete kit with photos of the components along with installation tips and photos, follow the link here:


    - AC427Cobra

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