trying to diagnose this problem 01 cobra rear IRS

[mike01svt]

I have been trying to find out what this is for about 6 months now.. and finally gave up and took it in to a mechanic..

the symptoms are when you are at a stop and let the clutch out there is a bad clunk in the rear.

no whirring noises at all when driving coming from the driveshaft.

most of the time when shifiting if you dont baby the clutch and shift fast it does the same clunk.. like there is "slack" in the driveshaft..

if you slowly let the clutch out from a stop, you can feel it "take up the slack" and then take off fast and there will be no clunk...

01 cobra IRS with the PHP IRS brace and bushings

 

[bouncer18]

there is always going to be slack in an irs.... you got some play in your cv-axles which you are probably hearing.. mine use to do that too.. it was annoying

the only thing that it could be that i can thik of is a bad u joint?

 

[mike01svt]

well i assume there was always gonna be some slack... but this feels "not normal"

It is in the shop now.. only thing i have changed is the php irs brace, removed that huge thing that looked like a brick that would sit under the pumpkin, and added the irs bushings...

 

[Snake Eyes]

That is known as the clunk. It is extremely common. Almost every IRS cobra has it. To what degree depends on the car but here is what causes it and how to cure it. First off the entire IRS has unbelievably soft rubber bushings that allow every part to move around. And second for some reason Ford did not know that you need tighter tolerances for the gears in an independent rear with this much power. A solid rear you can have sloppy gears and you wont notice but with an IRS you will. So if you want to fix the problem permanently replace all your bushings and get your gears set properly. Check out The IRS Tech Article (link) for more information.

 

[mike01svt]

That is known as the clunk. It is extremely common. Almost every IRS cobra has it. To what degree depends on the car but here is what causes it and how to cure it. First off the entire IRS has unbelievably soft rubber bushings that allow every part to move around. And second for some reason Ford did not know that you need tighter tolerances for the gears in an independent rear with this much power. A solid rear you can have sloppy gears and you wont notice but with an IRS you will. So if you want to fix the problem permanently replace all your bushings and get your gears set properly. Check out The IRS Tech Article (link) for more information.

well on this other forum, they suggested to change the 12mm front IRS bolts to 14mm... since the 12mm was incorrect...

what has been changed was, dogbone removed, PHP brace installed, steeda bushings installed.. bolts remaind stock.

 

[Snake Eyes]

Those bolts are for the sub frame. Read the article trust me. It will explain everything you need to know in plain english.

 

[mike01svt]

Those bolts are for the sub frame. Read the article trust me. It will explain everything you need to know in plain english.

will do.

 

[BreBar21]

FWIW, my IRS is as built as an IRS can be and I still have a little clunk.

 

[mike01svt]

my issue is that i think i broke my IRS somehow.. when i got my cobra 2900 miles.. clunk was non-existant... then i added the blower and mods ect... i first noticed the clunk after the PHP brace was installed and dogbone removed... bushings remained stock and i drove it for a long while like that.. REALLY BAD CLUNK.. then i looked under and saw the pumpkin was SOOO loose i could move it with my hand... so i had the bushings installed and clunk was greatly reduced.. but still very noticeable..


thats why i think I CAUSED something to mess up.. bu i have not idea what it coulda been

 

[Snake Eyes]

FWIW, my IRS is as built as an IRS can be and I still have a little clunk.

It shouldn't. What have you done to it?

my issue is that i think i broke my IRS somehow.. when i got my cobra 2900 miles.. clunk was non-existant... then i added the blower and mods ect... i first noticed the clunk after the PHP brace was installed and dogbone removed... bushings remained stock and i drove it for a long while like that.. REALLY BAD CLUNK.. then i looked under and saw the pumpkin was SOOO loose i could move it with my hand... so i had the bushings installed and clunk was greatly reduced.. but still very noticeable..


thats why i think I CAUSED something to mess up.. bu i have not idea what it coulda been

Well when the car is new everything is going to be ok but as time goes by you put stress on the IRS and the bushings deteriorate it will get worse and worse. Since you have a blower you are putting even more stress on your rear end, further accelerating the wear and tear. If you replace all that rubber with delrin, aluminum, and polly as I describe in the tech article you will keep everything from moving thus eliminating the slop you have right now. The clunk you hear is parts moving that should NOT be able to move.

 

[mike01svt]

It shouldn't. What have you done to it?



Well when the car is new everything is going to be ok but as time goes by you put stress on the IRS and the bushings deteriorate it will get worse and worse. Since you have a blower you are putting even more stress on your rear end, further accelerating the wear and tear. If you replace all that rubber with delrin, aluminum, and polly as I describe in the tech article you will keep everything from moving thus eliminating the slop you have right now. The clunk you hear is parts moving that should NOT be able to move.

well... i assume that replacing all the bushings would fix the clunking problem if the bushings ARE the problem..

I guess it's just a coincidence that when I added the PHP brace w/o the IRS bushings it was terrible and soo loose i could move it with my hand. then when i added the IRS bushings it reduced it even more, but it's still there.

I always thought it was the u-joints going out... I am gonna look into ordering the Subframe bushings as well..

 

[BreBar21]

It shouldn't. What have you done to it?

Everything but coilovers...delrin in the arms, aluminum for the chunk, poly in the subframe. I plan to go and check all the diff bushings to make sure that's not the issue.

 

[mike01svt]

Everything but coilovers...delrin in the arms, aluminum for the chunk, poly in the subframe. I plan to go and check all the diff bushings to make sure that's not the issue.

whats the chunk ??

 

[RoyWoods]

whats the chunk ??

I think that was a typo. My guess is he meant to say "clunk," and was referring to his MM aluminum differential bushings.

I have found that you can greatly affect the amount of clunk you have by how your pinion angle is set up. If you get it way off, you're going to experience a noticeable increase in the amount of clunk you get. IMO, you don't need to get the MM aluminum differential bushings, if your only goal is to eliminate clunk. Get them for all of their positive benefits. (As a side note, I do have the MM aluminum differential bushings and think they are great).

I'd check your pinion angle and see what it's at. I'd also double check your rear differential bushing and make sure it's in there like it should be. You might also want to consider switching to the MM poly differential bushing kit or the aluminum kit. Just make sure that all 3 bushings are of the same material (be it stock, poly, or aluminum).

 

[mike01svt]

update:


since the car is in brownsville and I am in San Antonio, I had my dad take it in...


he drove the car before and felt the clunk...

test i told the mech to do...

car on and in gear... press the brake and let the clutch in and out over and over again..


they could see the whole rear diff moving.. they found that some bolts were loose enough to tighten with their hands.. this was all while my father watched... they then tightened them and took it for a drive.. they said the clunk was gone..


my dad then test drove the car with the radio off, windows down, and said he felt nothing like before... this could have solved the problem... the mechs said it was the bushings that attached to the actual diff... without seeing it for myself I have no idea what bushings to order to get the rubber replaced..

got poly already in on the diff mount (PHP) so that only leaves about a dozen other possibilities... heading to brownsville to see and test for myself next weekend... this was a reason I was selling the car...

got fed up with the little quirks it had.. so if this fixes it, i may just keep it.

 

[bouncer18]

solution... put a solid axle in it...

if a clunk is gonna cause you to sell the car then i hate to see what happens when you launch it and snap a half shaft....

 

[mike01svt]

solution... put a solid axle in it...

if a clunk is gonna cause you to sell the car then i hate to see what happens when you launch it and snap a half shaft....

well the clunk was soooo annoying cuz i couldnt find the damn thing!!! LOL

i can replace a halfshaft in a weekend.. but this clunk has been annoying me for over 6 months.. and now that it's gone, i can proceed to finish up other things possibly..

forged engine... or new clutch

the clunk was really pissing me off...

 

[BreBar21]

whats the chunk ??

The differential.

 

[BreBar21]

I think that was a typo. My guess is he meant to say "clunk," and was referring to his MM aluminum differential bushings.

I have found that you can greatly affect the amount of clunk you have by how your pinion angle is set up. If you get it way off, you're going to experience a noticeable increase in the amount of clunk you get. IMO, you don't need to get the MM aluminum differential bushings, if your only goal is to eliminate clunk. Get them for all of their positive benefits. (As a side note, I do have the MM aluminum differential bushings and think they are great).

I'd check your pinion angle and see what it's at. I'd also double check your rear differential bushing and make sure it's in there like it should be. You might also want to consider switching to the MM poly differential bushing kit or the aluminum kit. Just make sure that all 3 bushings are of the same material (be it stock, poly, or aluminum).

My pinion angle is slightly off and I think that might be the issue. I haven't had an opportunity to crawl under there and adjust it. Once I do, I'll let everyone know if that cures it.

 

[Snake Eyes]

Pinion angle affects "the vibe", driveline rigidity affects "the clunk".