Excessive drivetrain slop?

[JPuehl]

I recently bought a '01 Cobra and the rear gears where changed to 3.73 before I bought the car.. I notice there seems to be a lot of drivetrain "slop" so I'm wondering if maybe the diff wasn't shimmed properly when the gears were swapped. And by slop I mean that when I'm driving steady speed, let off the throttle, it sort of clunks, same if I get back on the throttle. I recently replaced all the bushings in the IRS hoping it was just tired bushings, but that made no difference.

This past weekend I had the rear in the air so I used some blue painters tape to see how much the tire rotates with the driveshaft "locked" in place. Attached are 3 pix, the first showing the "starting" position of the tire in full stop counterclockwise. The 2nd pic is the movement before the driveshaft starts moving and the 3rd is including the driveshaft/trans movement. There's bare any movement from the trans so I don't feel that is an issue. Can anyone tell me if this is normal or excessive movement/slop?

 

[07GtS197]

Looks fine to me. You have to remember that there has to be play in the driveline. Also since the wheels are after the rear diff and their size compared to the small moving parts, the tires will move a lot in relation to the driveshaft and ring gear. What kind of clutch do you have? A heavy clutch will amplify any slop in the driveline.

 

[JPuehl]

Looks fine to me. You have to remember that there has to be play in the driveline. Also since the wheels are after the rear diff and their size compared to the small moving parts, the tires will move a lot in relation to the driveshaft and ring gear. What kind of clutch do you have? A heavy clutch will amplify any slop in the driveline.

I'm not sure about the clutch and neither is the previous owner. I do know the clutch feels really good for the car having 70k miles on it so I'm sure it was replaced. Part of my concern is that the rotation of the tire was very small when comparing the trans to diff "slop".

 

[07GtS197]

Any amount of movement will get amplified through the drivetrain.

Oh and if there is something messed up say in the diff you’d have some noise coming from there.

 

[JPuehl]

Any amount of movement will get amplified through the drivetrain.

Oh and if there is something messed up say in the diff you’d have some noise coming from there.

Good point and I don't have any whining noises, at least not from the diff, lol.

 

[cobrajeff96]

The clunk you hear might be "Cobra Clunk". Ford actually undersized the front bolts of the subframe in certain years. It was a genuine fuckup of theirs. I think it was a 12mm bolt whereas a 14mm bolt should've been used, or something like that. Lots of people, myself included, upgraded to a 9/16" bolt (14.2mm) which requires using a round file to ream out the holes. This keeps the entire subframe from moving back and forth, which could be what you're experiencing. You don't necessarily need the 9/16" bolts but the 14mm ones should be done regardless IMO.

There's also the front rubber mounts of the differential itself as well as the slop from any of the factory bushings throughout the IRS assembly.

Or it could very well be a tired U-joint in the driveshaft front or rear, or both.

Not to say it couldn't also be excessive differential gear backlash either. Just depends on how much money/time you want to throw at what is essentially a nuissance. Nothing is going to be destroyed if you let it continue the way it is.

 

[JPuehl]

Thanks for the response. About 1 month ago I replaced all the bushings associated with the IRS, including the diff bushings and the upgraded bolt that you reference with the FullTiltBoogie "kit". The slop is the same before and after the update so I know it's not those.

And it's funny you mentioned the smaller bolts causing the "Cobra Clunk" as my car did have the smaller bolts and when reinstalling the IRS with all the new Delrin bushings I have a theory as to why they used the smaller bolt. When I reinstalled the IRS all bolts lined up correctly except the passenger side front bolt. The bolt hole on the inboard side lined up so I was able to get the bolt started, it made it into the bushing but was no where near lining up with out outboard hole in the chassis. My theory is that Ford knew this so they used a smaller bolt rather than correcting the die that made the panels. I did use the correct tools and method to install the bushings and given the bushings are made of Delrin, I know the issue was not an out of alignment bushing. I simply used a rotary file, opened the hole and completed the installation.

The driveshaft is also an upgrade aluminum piece with solid U-joints.

In the end it may just be the way it's supposed to be but is just kinda annoying but not something I'm not used to already. Just want to make sure it's not an actual issue that needs to be dealt with.

 

[lwarrior1016]

I have also found on my IRS that the axle knuckles have a fair amount of slop in them. Mine are relatively new (20k ish miles) and have slack. Could be the 6500 rpm clutch dumps on slicks, but none the less it’s a point of slack. I get similar clunking to yours, and my diff is solid mounted so I know it’s not there.

 

[JPuehl]

I have the aluminum diff front bushings and solid rear diff mount bolted to the Ford performance race diff, all from FullTiltBoogie. The slop did not increase or decrease with all the solid parts. The axles are good to, just wish I knew for sure it's behaving how it should be.

 

[cobrajeff96]

You get the upgraded X-axis joints on the spindles?

If you've upgraded everything and they're not hella old with a lot of miles, you're narrowing it down to the diff gears.

FWIW, my entire setup back there is as solid as it can be and I cannot hear the diff at all. Then again, I had one of the best shops in the country do the gears for me and I'm running CenPeCo gear oil.

 

[JPuehl]

I didn't replace the cross-axis joints because mine are still solid so I didn't see a need to spend the money replacing them. Incorrect gear installation is the only thing left.