: Rear End Clunk ??



jdodman
07-16-04, 08:22 AM
I did a search and could not find a thread on this, but I know I have read about it prior to getting my V.

I have developed a loud clunk like noise in the rear when moving through 1st to second gear. It is especially loud in heavy traffic. Certainly I can smooth it out with more exagerated clutch/gas applications, but I cannot decide if this is new or it was always there..

Anyone else pick up on this ??

benjet
07-16-04, 11:22 AM
If it happens at low speed/low accel then we get to welcome you to the driveline backlash club.

http://www.cadillacforums.com/forums/showthread.php?t=10546&highlight=backlash

GNSCOTT
07-16-04, 12:30 PM
Is there a standard GM vehicle that doesn't have backlash? My buddies Mustangs all had it too. Don't think i've driven a standard vehicle (car or truck) that didn't have it.

StealthV
07-16-04, 01:09 PM
Backlash is a required part of geartrain design. Near impossible to elminate completely unless each mating part is perfectly matched. The noise really becomes pronounced on those select few cars where the tolerance stackups aren't in their favor.

I've been driving clunking T-56 transmission cars for over eight years and it is something you learn to eliminate over time through shift/clutch/throttle technique. In day to day driving, my V doesn't clunk since I've learned over the years on how to drive around it. Have heard the clunk on a rare occassion but it was from me being lazy screwing up clutching, etc.

Be conscious of your clutch engagement while depressing and releasing the pedal. Becoming a little smoother in your technique will eliminate 99% of what you hear.

jdodman
07-16-04, 05:50 PM
As always, thanks for the response. I always thought this one was going to be attributed to user error. I love the manual trans but also tend to get lazy when driving in heavy traffic, which is (of course) when it becomes the most pronounced and prevalent.

Practice...Practice..

baf_ctsv
07-16-04, 06:28 PM
I have never had a car that clunked like this car. My hunch is this will be designed out of the drive train when all of the bugs are gone !!

StealthV
07-16-04, 06:29 PM
It's been in every T-56 car I've ever been in - F-bodies, Vettes, Vipers, Mustangs.... for the past 10 years. Seriously doubt it is something "they" are working on changing.

6104696
07-16-04, 11:05 PM
Go to the cadillacfaq.com site and there is an excellent explanation of the clunk and it's cause. I agree 100% with that explanation.

I consider myself somewhat an expert on the clunk, as it resulted in my rejecting some (3) early V's. I ended up buying a V, clunk and all, after I confirmed a few things, as discussed below.

1) Most who own or have driven the V's seem to agree that there is drivetrain backlash that results in a low speed clunk or clang on 1-2 shifts. It is on most if not all 2004 V's.

2) Some of us, me included, agree that it is unusually pronounced in this car. The caddy service manager and sales manager both agreed that something was "wrong" when they heard it on an early V in Feb. I have been driving mustangs, camaros, etc, for many years and many miles, and never heard a car that clangs so consistently. Mustangs only clunk when you screw up on the clutch disengagement...and you know it when you do. I have driven a Z06 recently and it does not make the noise. M5 doesn't do it, and S4 doesn't do it, unless you try to induce it. Never driven a Viper or a DB9, who use the same trans. However, the issue seems to be the rearend, not the T56. On that first V, the dealer replaced the trans. and I think then the rear diff.

3) After a second V came in with the same noise, an engineer flew to the dealer here in VA from Lansing.....tested and tweaked, and declared the noise "normal backlash associated with a high performance drivetrain."

My personal opinion: whatever. It's normal for the car; it is an undesirable and probably uneccessary lack of refinement in a car that is intended to compete with much more refined drivetrain. Think of it as a byproduct of the $25K savings over the 2003 M5's. Don't worry about it; string out your revs a little in first gear to make the smooth shift to second. Your 4 year/50K warranty should cover you for any short term problem, and I am sure that one of the fine gentlement on this board will let us know as soon as a TSB comes out, if it ever does.

Can anybody confirm or deny whether the 2005's have the same clunk?

Anyway, think of the clunk as an added "feature"....

StealthV
07-16-04, 11:42 PM
My 2005 will clunk but only if the clutch is disengaged abruptly under load. My Z28 did the same thing. Every other T-56 I've every been in has done it under the same conditions.

The T-56 is a great transmission, it just lets you know its there. Don't push the clutch in under high load conditions; which you never need to do anyhow unless you're in an emergency situation.

This is not a new problem, but new people exposed to a high performance car with a T-56. There is no design flaw, nothing wrong, no fix coming. Drive the car and enjoy it already. Don't like it? Buy a Camry.

Search any forum of a T-56 car for clunk, driveline noise, etc....

F-Bodies
http://web.camaross.com/forums/showthread.php?s=&threadid=14252&highlight=transmission+clunk

Z06
http://www.z06vette.com/forums/showthread.php?t=49528&highlight=driveline+clunk

GTO
http://www.ls1gto.com/forums/showthread.php?t=2783&highlight=clunk

Mustang Cobra
http://www.svtregistry.com/cobra/2003/faq.html#_Toc35092457

http://forums.stangnet.com/showthread.php?threadid=328655&pagenumber=2

globed70
07-16-04, 11:56 PM
:yeah:

6104696
07-17-04, 09:04 AM
I think that I need to amend my note since apparently not all V's have this clunk. The clunks described in the threads listed in Stealths note are different clunks than what I have heard in all the V's that I've driven and I think a different clunk than what jdodman is describing. It is a clunk that is heard in any low speed low torque shift from 1-2. If you run the motor up above 2800 or so before shifting then you don't hear it.

The clunk when suddenly disengaging the clutch in a high load condition is different, and that I can induce in the caddy, mustangs, etc. This is different.

Clunks when you push in the clutch in a stationary car are of course normal.
I agree that there may not be a fix coming, but the clunks that I have heard are indicative of something that should be easily correctable. I am over it, but I will be watching to see if it is a problem which gets acknowledged and addressed.

Take a look at http://www.cadillacfaq.com/faq/answers/dlinenoise.html

baf_ctsv
07-17-04, 10:19 AM
I would hope that GM feels that this is not normal for a performance luxury car. If they are to compete on the Global scale they will need to design out these types of problems. Do 5 series BMWs clunk? I do not believe so. I just hope GM has engineers or marketing people read these forums!!

StealthV
07-17-04, 10:33 AM
The FAQ desciption of the noise is what I was trying to convey as well, seems like we're all referencing the same noise. The FAQ describes it well and also states that GM says it's normal.

On those V's that didn't make the clunk, I'd wager I could get them to do it if I operated the clutch with the intention of making the noise. Especially after they had a few hundred miles on them after the driveline has gone through its initial break-in.

My '96 Camaro T-56 was whisper quiet when it was new right off the truck. A few weeks later I began to notice the clunk. At first was similarily alarmed, discovered it was normal and then learned how to drive around it. My V has gone through the same evolution - quiet new and now with 2000 miles, I could clunk it at will if I wanted to but of course I don't. That'd be silly.

From my gearhead view of the world, this much-ado-about-nothing clunk topic is just like the people complaining about the lumpy, engine-shaking idle assuming that since a BMW doesn't do it, there has to be something wrong with the V. The V's engine better shake at an idle and if it doesn't, then you've got something wrong. :banghead:

globed70
07-17-04, 03:25 PM
The presumption is that one should consider the refinement of BMW with regards to the V. Well, then how the heck is GM going to deliver 400hp at $50k if they don't use existing components without re-engineering?

Z06CADY
07-18-04, 01:49 AM
I just loaned my son my pick up and drove home in his 2002 WS6 Trans Am six speed. What a difference (read the V is sooooo much better). But the thing is that the TA does not have the drive shaft "clunk" that my V has. You can shift it just like in the good old days and make it smooth, and that was good.
What ever the differance in the drive shaft is, that is the way to go!

Robert Newman
07-18-04, 10:14 PM
The clunk is not a result of gear backlash. It is caused by the low torsional stiffness of the drive shaft rubber isolators. This is a design problem that Cadillac at this time will not resove. The dirve shaft has a rubber torsional coupling. The low torsional flex of this coupling added to the normal torsional flex of the aluminum drive shaft and drive axles winds up like a rubber band during the applied high torque of low and second gear. When you release the clutch abruptly these wound up parts suddenly release resulting in a clunk. The clunk is actually the noise of the pinion teeth impacting the ring gear teeth and the noise is transmitted to the rear wheels. I had my wife drive the car slowly and abruptly release the cluth as I walked along side the rear wheels and the noise was very clear from the wheels. Then we put the CTS-V on a lift with the parking brake applied. I stayed in the car and let the cluth out enough to put torque on the drive shaft. Next I released the clutch abruptly can created the clunk. Five of the Cadillac dealer service people watched the drive shaft under the car, and all confirmed that I am correct. They do not have a solution. I contacted the Southeast regional service manager. He told me the noise is considered normal. This is disapointing.

I am interested in your comments to my comments above. Please complain to Cadillac.

GRN :bouncy:

Rich H
07-18-04, 11:42 PM
The clunk is not a result of gear backlash. It is caused by the low torsional stiffness of the drive shaft rubber isolators. This is a design problem that Cadillac at this time will not resove.

Until someone can prove that this design flaw affects durability of the drive train Cadillac is unlikely to do anything about, IMO. I have read about a number of rear-end failures on this board - many of them were attributed to the cumulative affects of wheel hop associated with hard launches.

Now if we could link wheel hop to the sudden release of torsional force caused by the flexing driveshaft (if your theory is correct) we might have an argument. BMR thinks that the flexing of the driveshaft helps initiate wheel hop and they are planning to produce a solid driveshaft to replace the stock unit. I haven't seen it yet.

Z06CADY
07-19-04, 12:46 AM
Robert Newmen,
So if the drive shaft mount is made more solid by replacing the rubber mounts with something else, not metal, this might make the shift more old school like?

StealthV
07-19-04, 02:01 AM
There's probably a good reason it is rubber and not solid. NVH is going to go up with a more solid driveshaft system. Just like putting aluminum bushings in the shifter - ok in a track car, not ok for the daily commute.

benjet
07-19-04, 11:58 AM
I have read about a number of rear-end failures on this board - many of them were attributed to the cumulative affects of wheel hop associated with hard launches.


Not to thread jack but -

Uh, who's (on this board) rear end failed due to hop/launching? While I agree I've read about several failures, I don't recall any of them having anything to do with hop/hard launching.

wildwhl
07-19-04, 01:01 PM
Ben -

The only failures I've read about due to hard launches were not on this board - but over at LS1.com there have been a couple. Broken half shafts I believe. I can confirm that my rearend did not fail due to driving habits. The whine was there since very shortly after I bought the car (probably there initially, just didn't notice it until I turned the radio off) and looking at the differential after it was yanked out of the car I would guess it was either a clearance/assembly problem, or simply that the diff gears were of the incorrect hardness.

I will confirm, though, that wheel hop will eventually break things. Look elsewhere on the net and you'll find evidence of this.

benjet
07-19-04, 01:31 PM
The only failures I've read about due to hard launches were not on this board - but over at LS1.com there have been a couple. Broken half shafts I believe. I can confirm that my rearend did not fail due to driving habits.

Right - I was picking nits with his "many" comment, I do agree that 1 or 2 (maybe a few even) but MANY of the total failures....I don't think so.

Mine whines and did NOT when new (I drove it quite a bit with stereo off), my dealer claims this is normal. Will try to get them to change and sample the diff lube next time.

-Ben

wildwhl
07-19-04, 01:51 PM
Right - I was picking nits with his "many" comment, I do agree that 1 or 2 (maybe a few even) but MANY of the total failures....I don't think so.

Mine whines and did NOT when new (I drove it quite a bit with stereo off), my dealer claims this is normal. Will try to get them to change and sample the diff lube next time.

-Ben

Agreed, there haven't been many and even the total number of differential failures I've read about only add up to a very small fraction of a percentage of total vehicle sales. Really shouldn't be an ongoing issue, I hope, for others to worry about.

However, just in case, best to keep information flowing here for those that may be unaware of the potential problem. Those of you that hear noise I would not recommend to just dismiss as normal hypoid gear noise - but instead have the dealer drain your fluid and check it out. In my case no oil analysis was necessary - the naked eye told the story well enough. We all have warranties for items such as this - best to just be safe now while your warranty is still intact.

Rich H
07-19-04, 11:07 PM
Not to thread jack but -

Uh, who's (on this board) rear end failed due to hop/launching? While I agree I've read about several failures, I don't recall any of them having anything to do with hop/hard launching.

Let me restate my case: "many of them COULD be attributed to the cumulative affects of wheel hop associated with hard launches." Unless someone has dissassembled the rear differential after SEVERAL hard runs with wheel hop they are in no position to refute it. The point is that the damage is cumulative such that severe stress applied to a component over a period of time will cause that component to fail. You don't need a doctorate in Mechanical Engineering to figure that out.

There is information on several boards about CTS V rear end failure and I have read of several cases where hard launches at the track on on the street have destroyed the rear differential or broke half shafts. That's evidence enough for me to predict that severe wheel hop can damage either or both. Personally, I prefer not treat my car with this type of abuse so I doubt I'll ever have to worry about it.

Rich H
07-24-04, 12:03 AM
Another one bites the dust.

Broken half shaft (http://www.ls1tech.com/forums/showthread.php?t=194192)

StealthV
07-24-04, 01:11 AM
With the immense torque the LS6 delivers (I know, it is underpowered :deadhorse) the 4,000 RPM side-stepping the clutch launches that GrooveCity performed will break the half-shaft of any factory street car.

No surprise GrooveCity broke it, he beat the **** out of his car for 3,000 miles.

Rich H
07-24-04, 09:14 PM
Not to beat a dead horse - but, I think the wheel hop may have had something to do with it - before he installed the BMR kit. Hopefully the rear differential is still intact and will remain so for a few thousand more miles.

Some people need to test the limits of machinery so that others (like most of us) can benefit from product development. Normally this is done on the race track - but not always. Maybe if enough of these half shafts break, Cadillac will get the message.