Clunk in back end --Need serious analysis (not random guesses)
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
More and more convinced the c-beam is making contact somewhere.
Drifting
Joined: Aug 2009
Posts: 1,978
Likes: 244
Torque on rear axle nuts at the hub? Miles since last time Teflon washers were replaced?
Though I would think both of these things could/would make noise in reverse as well, I wouldn’t rule them out. Probably missed it in the thread, but I’m sure you’ve checked all u joints already.
Though I would think both of these things could/would make noise in reverse as well, I wouldn’t rule them out. Probably missed it in the thread, but I’m sure you’ve checked all u joints already.
Le Mans Master
Joined: Aug 2017
Posts: 6,944
Likes: 1,490
From: New Jersey
Just out of curiosity... does it appear to slightly "torque steer" when getting on it too? I probably couldn't hear a thud or noise on mine when it does this but I do get an ever so slight shift/pull on spirited turns or straight line pulls... Not guessing, just asking for more observations as it can hone in more what may be causing it....
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
Torque on rear axle nuts at the hub? Miles since last time Teflon washers were replaced?
Though I would think both of these things could/would make noise in reverse as well, I wouldn’t rule them out. Probably missed it in the thread, but I’m sure you’ve checked all u joints already.
Though I would think both of these things could/would make noise in reverse as well, I wouldn’t rule them out. Probably missed it in the thread, but I’m sure you’ve checked all u joints already.
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
Just out of curiosity... does it appear to slightly "torque steer" when getting on it too? I probably couldn't hear a thud or noise on mine when it does this but I do get an ever so slight shift/pull on spirited turns or straight line pulls... Not guessing, just asking for more observations as it can hone in more what may be causing it....
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
Team,
Following up on this issue I've had for a while. After extensive conversations with you all and even reaching out to ZFDoc...nothing!
I decided to get under the car yesterday to ensure the batwing is tight enough and look for rub points. Nothing obvious!
While lowering the car, I laid eyes on the bump stops and their distance to the top of the knuckles. I put some grease on the top of the knuckles and took the car for some aggressive driving. Sure enough, the grease had transferred to the bump stops.
Bottom line: After all I've done to this car because of the "thud", it turns out that the most likely problem is the suspension bottoming out.
I don't have regrets about freshening up the suspension on a nearly 30-year-old car.
Ric
Following up on this issue I've had for a while. After extensive conversations with you all and even reaching out to ZFDoc...nothing!
I decided to get under the car yesterday to ensure the batwing is tight enough and look for rub points. Nothing obvious!
While lowering the car, I laid eyes on the bump stops and their distance to the top of the knuckles. I put some grease on the top of the knuckles and took the car for some aggressive driving. Sure enough, the grease had transferred to the bump stops.
Bottom line: After all I've done to this car because of the "thud", it turns out that the most likely problem is the suspension bottoming out.
I don't have regrets about freshening up the suspension on a nearly 30-year-old car.
Ric
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Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
No, not really. The suspension is squatting to the point of hitting the bump stop. Any car would do exactly the same when it bottoms out. It doesn't help the rear end is lowered on this car.
My point here is that people who may have a solid thud in the rear suspension may want to consider the bump stop as the culprit.
Ric
My point here is that people who may have a solid thud in the rear suspension may want to consider the bump stop as the culprit.
Ric
Racer
Joined: Sep 2001
Posts: 365
Likes: 93
From: Chesterfield VA
Well its all going to be a guess until you pull it apart. The center pin bore of the carrier is famous for wearing out. So when you are stopped and there is slack in the driveline, when you accelerate the pin hits the bore. Its loud and you can feel it in the seat when it happens. An auto is worse at hiding than a manual trans and in a auto it is really bad going from Drive to reverse or back again. There is no fix for it except to replace the carrier.
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
Well its all going to be a guess until you pull it apart. The center pin bore of the carrier is famous for wearing out. So when you are stopped and there is slack in the driveline, when you accelerate the pin hits the bore. Its loud and you can feel it in the seat when it happens. An auto is worse at hiding than a manual trans and in a auto it is really bad going from Drive to reverse or back again. There is no fix for it except to replace the carrier.
I'm certain enough I was simply bottoming out that I have moved on from this issue.
Racer
Joined: Sep 2001
Posts: 365
Likes: 93
From: Chesterfield VA
Ok then you didn't describe the issue very well. A vette will not bottom simply by letting the clutch out and moving or going to Drive from Park. This is also not really a busted differential, you could drive 50k miles on it and not know it an issue except for sound. We saw a ton of this during rebuild where this was bad and some worse than others. Glad you found your issue though.
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
Team,
I'm back again for this same issue...plus another one.
Issue #1: The clunk/thud or whatever it may be, it is still there. It happened again, this time backing up into my garage. Curious, I took the car back out and did several forward AND reverse starts by bringing the RPMs up to about 2K and slipping the clutch out. Doing this in reverse, I get a double "bang" or thud in quick succession. Forwards, I still get the thud as well. SO, while the car may squat to the point of bottoming out going forward, it would NOT do this going in reverse.
Issue #2: I'm getting this high frequency vibration between 65 and 75 MPH. Pressing the clutch at those speeds REALLY aggravates the vibrations until speed falls below 65. Same happens if I put the transmission in neutral at those speeds...no matter clutch in or out...same VERY noticeable vibration. I flipped the driveshaft 180* thinking I had previously installed it out of phase, but that didn't help. The driveshaft balancing weight at the rear is still there.
- Are these two issues related?
- Is the diff carrier on its way out as suggested by J.Abbott?
- Did replacing both u-joints throw the balance off on the driveshaft?
I'm frustrated to say the least.
I'm back again for this same issue...plus another one.
Issue #1: The clunk/thud or whatever it may be, it is still there. It happened again, this time backing up into my garage. Curious, I took the car back out and did several forward AND reverse starts by bringing the RPMs up to about 2K and slipping the clutch out. Doing this in reverse, I get a double "bang" or thud in quick succession. Forwards, I still get the thud as well. SO, while the car may squat to the point of bottoming out going forward, it would NOT do this going in reverse.
Issue #2: I'm getting this high frequency vibration between 65 and 75 MPH. Pressing the clutch at those speeds REALLY aggravates the vibrations until speed falls below 65. Same happens if I put the transmission in neutral at those speeds...no matter clutch in or out...same VERY noticeable vibration. I flipped the driveshaft 180* thinking I had previously installed it out of phase, but that didn't help. The driveshaft balancing weight at the rear is still there.
- Are these two issues related?
- Is the diff carrier on its way out as suggested by J.Abbott?
- Did replacing both u-joints throw the balance off on the driveshaft?
I'm frustrated to say the least.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
Since car is lowered half shaft working angle can be an issue setting up vibration along with rear knuckles hitting bump stops. Try tightening rear spring bolts and raise back of car a 1/2 or so. Count the turns so you can reset rear body height. Noises are nearly always trial and error unless their is obvious contact witness marks.
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
I took a short video from under the differential. I understand there a certain amount of slack in the drivetrain, but I wonder if this is too much.
No play at the pinion yoke or u joints... All seems solid.
No play at the pinion yoke or u joints... All seems solid.
Melting Slicks
Joined: Nov 2016
Posts: 2,256
Likes: 3,084
From: So Cal
2023 C4 of the Year Finalist - Unmodified
I've watched the vid 10 times. I can't tell anything other than there is a 'clunk' that seems abnormal to me.
The way I diagnose what may be "loose" is to compare what is moving (that shouldn't be moving) to things that aren't moving. In that vid, everything is moving the same amount.
I assUme that the camera was placed on the ground under the car, then the car pushed fore and aft, correct? Was the hand brake set (there is no rotation of the axle shaft)? If all = YES, then I'd be looking at the dog-bone bushings. Cheers.
The way I diagnose what may be "loose" is to compare what is moving (that shouldn't be moving) to things that aren't moving. In that vid, everything is moving the same amount.
I assUme that the camera was placed on the ground under the car, then the car pushed fore and aft, correct? Was the hand brake set (there is no rotation of the axle shaft)? If all = YES, then I'd be looking at the dog-bone bushings. Cheers.
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
I've watched the vid 10 times. I can't tell anything other than there is a 'clunk' that seems abnormal to me.
The way I diagnose what may be "loose" is to compare what is moving (that shouldn't be moving) to things that aren't moving. In that vid, everything is moving the same amount.
I assUme that the camera was placed on the ground under the car, then the car pushed fore and aft, correct? Was the hand brake set (there is no rotation of the axle shaft)? If all = YES, then I'd be looking at the dog-bone bushings. Cheers.
The way I diagnose what may be "loose" is to compare what is moving (that shouldn't be moving) to things that aren't moving. In that vid, everything is moving the same amount.
I assUme that the camera was placed on the ground under the car, then the car pushed fore and aft, correct? Was the hand brake set (there is no rotation of the axle shaft)? If all = YES, then I'd be looking at the dog-bone bushings. Cheers.
A bit more context on the video:
- Rear up in the air with wheels on
- Parking brake disengaged
- Tranny in 1st gear
- Camera on the floor facing up, just to the right of the diff
I didn't use the parking brake and instead used 1st gear so I could rotate the wheels.
I would agree with you, there seems to be an abnormal amount of clunk in the diff, regardless of the normal slack in the drivetrain.
Could this be the reason for the banging I'm hearing going forward and reverse?
Could this be the cause for this newfound vibration that feels like the driveshaft is out of balance?
I really appreciate any and all guidance.
Ric
Thread Starter
Burning Brakes
Joined: Jun 2022
Posts: 845
Likes: 212
From: Florida
By the way, the dog bone bushings were replaced with SuperPro last year. The have just a few thousand miles on them.
I've taken a prybar to the whole suspension and nothing is moving and/or loose.
It's beginning to feel like J. Abbott was right.
I've taken a prybar to the whole suspension and nothing is moving and/or loose.
It's beginning to feel like J. Abbott was right.