cobraracer46
Active Member
I performed a overhaul to the IRS in my Cobra to fix instability issue under acceleration. It turns out that after I fitted a set of 335/35/17 tires , I started to feel an extreme side to side back end movement under hard acceleration and steadily get worse.. The Back end of the car also exhibited a floaty feel around high speed corners.
Other IRS mods on the car:
MM IRS low profile rear subframe bolts .
A Eaton Tru trac diff with a 4:30 gear set.
http://www.eaton.com/Eaton/Products...utomotiveAftermarket/Differentials/PCT_221471
Eibach Cobra IRS swaybar kit.
After seeing many reports of Cobra owners putting in Delrin , Aluminum bushings and complaining of gear, slam and clunk noises, I went with Poly in the IRS.
Some people say that poly "sucks" or that poly is "cheap" and "crappy." Don't believe it. Numbers showing that Poly is a good bushing material for the Cobra IRS suspension
.. "If you know anything about bushing durometer, control arm deflection, suspension efficiency and vehicle dynamics, you are not going to want poly anywhere in an IRS."
"As most know, the "durometer" of any material is a reflection of its resistance to deformation. Just for the record and for those who would like some numbers to go with the rather nebulous statement above, The "Shore D" durometer rating of Delrin is 83. Depending on its formulation, polyurethane's Shore D durometer can measure as high as 80, but most poly chassis and suspension bushings fall into the 60 - 70 range (about the same as a golf ball). By comparison, rubber compounds top out well below 40. Aluminum "bushings" are really hard mounts, rather than bushings per se. Since aluminum provides absolutely no NVH isolation, you may as well be bolting the two parts directly together (and for all practical purposes, you are)."
After upgrading the IRS with poly, the handling has noticeably improved making painful sacrifices in NVH.
testimonials lifted from a mustang internet forum from Cobra owners who went with Delrin and Aluminum IRS bushings.
"Mine is horrible. REALLY LOUD after solid bushings. unacceptable. sucks."
"Yup Did the same mods as you, and still had the hop and lots of gear noise."
"did all of the above and my clunk rapidly turned from a clunk to a slam. Its so bad now it bounces when I let off the gas, I can feel it slamming back and forth with all the slack in there. I'm about to have a new set of gears installed because I'm afraid its going to break soon with the way it is now."
"I hear so many noises now I'm scared my rear end is going to fall apart. There is even some clunking noises! I know the rear end would be noisy, but the whirring noise is loud as shit and I have frpp 3.73 gears!"
Here are the parts I bought and installed from SN95forums.com supporting vendor Late Model restoration(LRS) during the big IRS suspension overhaul:
From Maximum Motorsprts:
Complete IRS urethane differential mount kit with a removal tool for $56 that will transmit ZERO gear noise to the passenger compartment.
IRS differential mounts, urethane, complete kit, 1999-04 Cobra [MMIRSB-45] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
A complete IRS urethane control arm bushing kit for $65 shipped that will preserve some level of ride quality over broken pavement.
1999-04 Mustang Cobra Prothane Irs Control Arm Bushing Set at LRS - Free Shipping!
Urethane Subframe mounts for $49.95
Urethane IRS Subframe Bushing Kit, 1999-04 [MMIRSU-1] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
$16 for 14 MM sub frame bolts, Metric, not a "pretty-close" English-size. Use metric bolts for installation.
14mm hardware kit for IRS subframe & RLCA mounting, 1999-04 [MMF-4] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
A complete IRS tie rod kit for to replace the original ones that are shot and causing the wheels to wiggle back and fourth.
IRS Adjustable Complete Tie Rod Conversion Kit, 1999-04 Cobra [MMIRSTR-3] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
Sway bar links for $69.95
IRS Swaybar End-Link Set : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
I thought about this one long and hard and since I had the IRS totally apart, I stepped up and installed a $254 Ford racing diff cover as well with the money I save by not buying an IRS kit with Delrin and Aluminum bushings.
1999-04 Mustang Ford Racing 8.8" Irs Rear Differential Girdle, M-4033-G3 at LRS - Free Shipping!.
I spent about $445 for IRS bushings and tie rod kit , plus another $250 for the Ford Racing Cover. Not too bad considering that you can spend over $650 alone just for a home made delrin, Aluminum bushing bushing kit being heavily pushed on various Mustang sites that will transmit gear noise inside the car and degrade ride quality too much. Had I gone with Delrin and Aluminum IRS bushings along with the toe links and the cover, I would have spent well over a grand to fully upgrade the IRS.
Ruin the ride quality, transmit gear noise and pay more? I said no thanks.
A few pics of the install that I did my self
Drilling out the factory rubber subframe bushings and getting ready to use the Maximum Motorsports removal tool.
Fully cleaned lower A arm bushing shell ready for a Prothane IRS control arm bushing.
Cleaning out the bushing shells
I did not have a press at my disposal to swap out the bushings and since the IRS control arm bushing removal tools from Maximum Motorsports, although quality pieces, would not work in my case as they not only remove the factory rubber control arm bushing, but the shell as well. The Prothane IRS control arm bushings go inside of the factory shells so I had to get creative with removing the factory rubber from the IRS control arms.
I ended up Using the Maximum Motorsports IRS subframe bushing tool and a smaller disc from a Lise bearing race driver kit to remove the lower A arm factory rubber bushings along with a little heat from a propane torch. (no need to light the bushings on fire, just get the rubber warm.)
I also made sure to Use plenty of anti size lube on the MM tool threads and not to use any power tools with the MM removal tool since the MM IRS subframe bushing removal tool had to survive and work double duty on the sub frame and lower control arm bushings. Many people by mistake have used an Impact gun with the MM removal tool, only to destroy the threads on the tool before all of the factory rubber sub frame bushings were removed.
Compared to the lower control arm bushings, The factory rubber upper control arms easily come out with just a little bit of drilling, cutting and nothing else.
Maximum Motorsports IRS poly rear diff mount installed.
Maximum motorsports IRS toe links replacing the trashed stockers
Fully upgraded IRS all cleaned up, filled with RED Line synthetic gear lube and ready to be reinstalled. ( 2003 Terminator cobra Half shaft and knuckle assemblies were left off to make the IRS easier to raise into position since I only had a floor jack to work with)
Checking Bump steer with my home made gauge set up. After pulling my hair out, trying to get Zero toe change readings during the bumpsteer procedure, I called Maximum Motorsports and was advised that up to .040'' bump in compression and rebound was acceptable. The best reading I was able to get was around .008" Close enough. I made sure that the IRS only bumped in.
One tip I picked up from another Cobra owner who upgraded a Cobra IRS with poly in the control arms was to mix the Supplied Prothane grease with a bit of Anti size and not to apply grease to the outside of the bushings. You only want the bushings to rotate around the bolts, not the shells so thats what I did.
Properly greasing the poly IRS bushings during installation eliminates the need to occasionally lube them and it eliminates the need to weaken the control arms by drilling into them to install grease fittings.
As for the subframe, I did apply a bit of grease to the outside of the Poly sub frame bushings and with just an aluminum floor jack, the IRS assembly popped back into the car really easy. Since I avoided installing Delrin or aluminum bushings on the IRS subframe there was no need at all to spread the torque boxes or deal with other head aches to reinstall the IRS subframe back in the car.
SO how does the car feel now that the IRS has been upgraded?
The loose fish tail sway sensation I would feel as the car accelerated up to high speed is gone.
Wheel hop is Gone.
Back end feels super solid.
All of these improvements come with no gear noise at all
I was expecting the ride to become a bit more harsh with poly replacing all of the rubber, but I'm in shock that I can't seem to notice any increased harshness.
Other IRS mods on the car:
MM IRS low profile rear subframe bolts .
A Eaton Tru trac diff with a 4:30 gear set.
http://www.eaton.com/Eaton/Products...utomotiveAftermarket/Differentials/PCT_221471
Eibach Cobra IRS swaybar kit.
After seeing many reports of Cobra owners putting in Delrin , Aluminum bushings and complaining of gear, slam and clunk noises, I went with Poly in the IRS.
Some people say that poly "sucks" or that poly is "cheap" and "crappy." Don't believe it. Numbers showing that Poly is a good bushing material for the Cobra IRS suspension
.. "If you know anything about bushing durometer, control arm deflection, suspension efficiency and vehicle dynamics, you are not going to want poly anywhere in an IRS."
"As most know, the "durometer" of any material is a reflection of its resistance to deformation. Just for the record and for those who would like some numbers to go with the rather nebulous statement above, The "Shore D" durometer rating of Delrin is 83. Depending on its formulation, polyurethane's Shore D durometer can measure as high as 80, but most poly chassis and suspension bushings fall into the 60 - 70 range (about the same as a golf ball). By comparison, rubber compounds top out well below 40. Aluminum "bushings" are really hard mounts, rather than bushings per se. Since aluminum provides absolutely no NVH isolation, you may as well be bolting the two parts directly together (and for all practical purposes, you are)."
After upgrading the IRS with poly, the handling has noticeably improved making painful sacrifices in NVH.
testimonials lifted from a mustang internet forum from Cobra owners who went with Delrin and Aluminum IRS bushings.
"Mine is horrible. REALLY LOUD after solid bushings. unacceptable. sucks."
"Yup Did the same mods as you, and still had the hop and lots of gear noise."
"did all of the above and my clunk rapidly turned from a clunk to a slam. Its so bad now it bounces when I let off the gas, I can feel it slamming back and forth with all the slack in there. I'm about to have a new set of gears installed because I'm afraid its going to break soon with the way it is now."
"I hear so many noises now I'm scared my rear end is going to fall apart. There is even some clunking noises! I know the rear end would be noisy, but the whirring noise is loud as shit and I have frpp 3.73 gears!"
Here are the parts I bought and installed from SN95forums.com supporting vendor Late Model restoration(LRS) during the big IRS suspension overhaul:
From Maximum Motorsprts:
Complete IRS urethane differential mount kit with a removal tool for $56 that will transmit ZERO gear noise to the passenger compartment.
IRS differential mounts, urethane, complete kit, 1999-04 Cobra [MMIRSB-45] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
A complete IRS urethane control arm bushing kit for $65 shipped that will preserve some level of ride quality over broken pavement.
1999-04 Mustang Cobra Prothane Irs Control Arm Bushing Set at LRS - Free Shipping!
Urethane Subframe mounts for $49.95
Urethane IRS Subframe Bushing Kit, 1999-04 [MMIRSU-1] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
$16 for 14 MM sub frame bolts, Metric, not a "pretty-close" English-size. Use metric bolts for installation.
14mm hardware kit for IRS subframe & RLCA mounting, 1999-04 [MMF-4] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
A complete IRS tie rod kit for to replace the original ones that are shot and causing the wheels to wiggle back and fourth.
IRS Adjustable Complete Tie Rod Conversion Kit, 1999-04 Cobra [MMIRSTR-3] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
Sway bar links for $69.95
IRS Swaybar End-Link Set : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!
I thought about this one long and hard and since I had the IRS totally apart, I stepped up and installed a $254 Ford racing diff cover as well with the money I save by not buying an IRS kit with Delrin and Aluminum bushings.
1999-04 Mustang Ford Racing 8.8" Irs Rear Differential Girdle, M-4033-G3 at LRS - Free Shipping!.
I spent about $445 for IRS bushings and tie rod kit , plus another $250 for the Ford Racing Cover. Not too bad considering that you can spend over $650 alone just for a home made delrin, Aluminum bushing bushing kit being heavily pushed on various Mustang sites that will transmit gear noise inside the car and degrade ride quality too much. Had I gone with Delrin and Aluminum IRS bushings along with the toe links and the cover, I would have spent well over a grand to fully upgrade the IRS.
Ruin the ride quality, transmit gear noise and pay more? I said no thanks.
A few pics of the install that I did my self
Drilling out the factory rubber subframe bushings and getting ready to use the Maximum Motorsports removal tool.
Fully cleaned lower A arm bushing shell ready for a Prothane IRS control arm bushing.
Cleaning out the bushing shells
I did not have a press at my disposal to swap out the bushings and since the IRS control arm bushing removal tools from Maximum Motorsports, although quality pieces, would not work in my case as they not only remove the factory rubber control arm bushing, but the shell as well. The Prothane IRS control arm bushings go inside of the factory shells so I had to get creative with removing the factory rubber from the IRS control arms.
I ended up Using the Maximum Motorsports IRS subframe bushing tool and a smaller disc from a Lise bearing race driver kit to remove the lower A arm factory rubber bushings along with a little heat from a propane torch. (no need to light the bushings on fire, just get the rubber warm.)
I also made sure to Use plenty of anti size lube on the MM tool threads and not to use any power tools with the MM removal tool since the MM IRS subframe bushing removal tool had to survive and work double duty on the sub frame and lower control arm bushings. Many people by mistake have used an Impact gun with the MM removal tool, only to destroy the threads on the tool before all of the factory rubber sub frame bushings were removed.
Compared to the lower control arm bushings, The factory rubber upper control arms easily come out with just a little bit of drilling, cutting and nothing else.
Maximum Motorsports IRS poly rear diff mount installed.
Maximum motorsports IRS toe links replacing the trashed stockers
Fully upgraded IRS all cleaned up, filled with RED Line synthetic gear lube and ready to be reinstalled. ( 2003 Terminator cobra Half shaft and knuckle assemblies were left off to make the IRS easier to raise into position since I only had a floor jack to work with)
Checking Bump steer with my home made gauge set up. After pulling my hair out, trying to get Zero toe change readings during the bumpsteer procedure, I called Maximum Motorsports and was advised that up to .040'' bump in compression and rebound was acceptable. The best reading I was able to get was around .008" Close enough. I made sure that the IRS only bumped in.
One tip I picked up from another Cobra owner who upgraded a Cobra IRS with poly in the control arms was to mix the Supplied Prothane grease with a bit of Anti size and not to apply grease to the outside of the bushings. You only want the bushings to rotate around the bolts, not the shells so thats what I did.
Properly greasing the poly IRS bushings during installation eliminates the need to occasionally lube them and it eliminates the need to weaken the control arms by drilling into them to install grease fittings.
As for the subframe, I did apply a bit of grease to the outside of the Poly sub frame bushings and with just an aluminum floor jack, the IRS assembly popped back into the car really easy. Since I avoided installing Delrin or aluminum bushings on the IRS subframe there was no need at all to spread the torque boxes or deal with other head aches to reinstall the IRS subframe back in the car.
SO how does the car feel now that the IRS has been upgraded?
The loose fish tail sway sensation I would feel as the car accelerated up to high speed is gone.
Wheel hop is Gone.
Back end feels super solid.
All of these improvements come with no gear noise at all
I was expecting the ride to become a bit more harsh with poly replacing all of the rubber, but I'm in shock that I can't seem to notice any increased harshness.
