Tomulrich

I posted pictures of GMs rubber torque tube bushings vs the BMW units. They are different....for people who think they are the same...look closer here are pics......GMs are more$$ because they are a more durable, less flexible material.....they also dont have small "relief cuts in the rubber. GM units seem to be a more heavy duty piece!

I also found the BMW units were rubbing the inside of the torque tube!

I also installed the driveshaft without the Torque tube aroung it to diagnose my severe vibration problem. The dial indicator showed max
.050 deflection......GM max is .118. I think my problem is in the Torque converter or alignment of the converter in the trans...

If you are not familiar with my previous posts click below for reference..
http://forums.corvetteforum.com/show....php?t=1963262
http://forums.corvetteforum.com/show....php?t=1962471

Here are some pics
BMW rubber coupler rubbing inside of torque tube

difference in couplers

white pipe cleaner pointing to area rubbing (smaller diameter or rubber coupler...dont understand how it would rub the smaller diameter portion and not touch the larger diameter region of the coupler

Drive shaft installed with everything except the outer torque tube. Dial indicator checking runout at 4000rpm.........approx .050 which is exceptable

GM harder rubber on right......left is BMW units...they rubbed..some how....also softer rubber...pictures dont lie!!!!!!!!!!!

JUSTIFIED C5

Good info, thanks

0Jeff @ TPE

Did you have that hard plastic ring still installed on the shaft?

Maybe thats why its "Whipping" around. My drive shaft has a had plastic ring right about center. My understanding is, is its designed to keep the shaft from whipping around. It looks like you have eliminated that.

Could THAT be your problem?

Warp Factor

Since that smaller diameter area is between the mounting bolts and unsupported, maybe it's able to "grow" at higher rpms.

lucky131969

You're right, I believe it's called a "snubber". Located mid shaft. It's called out in the service manual as an inspection item when doing an overhaul. I looked it up in the service manual. It's a rev limiter snubber.

From the service manual troubleshooting driveline noise:

A rev limiter (snubber) that is contacting the driveline tube may create a "moaning" type noise and/or vibration that is felt through the shift lever. Slight noise and vibration is to be considered normal during an overspeed condition but should not be present when the overspeed condition has ceased.

Tomulrich

The snubber / ring is only on manual setups. The 3" driveshaft does not flex....it is not needed . There is .050 of flex...which is within limits.

Also the rubber rubing happened when the torque tube was in place...so it was supported. Also the areas that were rubbed off were 100% symetrical...on both ends which would be highly unlikely...if something was out of wack. What ever happened was uniform...like expansion of the rubber.

Also...why werent the high spots on the rubber couplers contacted?? THIS IS THE REAL QUESTION!!!!!
If there was something out of wack the high spot would have hit. I have no idea how the low spots can hit but nt the high spots....it seems physically impossible

Tom

Tomulrich

warp factor...I did think of your expansion theory. I mounted the rubber coupler on a 3000rpm motor and the dial indicator showed no expansion..

I am testing and checking everthing....

Keep the ideas coming

Tom

Warp Factor

Has the driveshaft ever been under a torque load?
Torque might cause these areas to compress and bulge.
Still just taking wild guesses.

Tomulrich

no..on jack stands...bulging wouldnt cause symetric wear marks..across the whole coupler

Tom

lucky131969

Tom,
The snubber is to compensate for an overspeed condition, so you will not see that with the car up on jackstands, nor do I think the lack of a snubber has anything to do with your issue.

Tomulrich

I agree....overspeed from missing a gear in a manual setup .GM does not put one on the autos

Tom

MawneeC5

If only I had this info a year and half sooner When I needed to rebuild mine the BMWs were what was unanimously recommended. I shreaded the front coupler and sent my torque tube through the housing. Granted I was making double stock horsepower...but still.....

Tomulrich

yes...they do not seem to be as durable. You can tell by they type of rubber used..the BMW units seem like they are like tire rubber. The GM units almost resemble polyurethane.

Unfortunately people who are running the BMW unit will have problems.....I know that people selling the BMW units will tell you that they are the same.....but they are not....and you will eventually have problems..........this is why they cost more.

Just read my posts and look at real would results!!!!

Tom

AlwaysInBoost

Tom, I'm surprised more people haven't had issues with the BMW couplers, if they are actually your problem like you suspect. I'm not so sure that it is just because you have a very custom one off setup that might be the exception and not the rule.

I have the GM and BMW couplers at home. And, while physically they aren't 100% identical like you've shown in your pics, the BMW is just as beefy if not more so then the GM unit.

Also, keep in mind that in our cars, since the trans is behind the TT, the drive shaft isn't affected by the torque multiplacation of the trans like on a BWM where the trans is in front of the driveshaft. for that reason alone I believe the BMW couplers are beefy enough even for my measly 700/600 power level. I have the BMW couplers in my 01Z and have no issues after much driveline abuse.

Tomulrich

these BWW couplers come out of the 318.....not very powerful!

I understand what you are saying......and dont think the couplers were the problem...but they didnt perform without problems...the TT driveshaft is like all the others in my application...

Tom

0Jeff @ TPE

I understand you may be having problems, but to make a blanket statement that BMW couplers are NOT as good due to 1 experiemental TT/Transmission setup is not to awefully fair.

The BMW couplers have been in use for a few years now. I would be willing to bet, that there are hundreds of couplers that are in cars all over (to include my own)

There are anywhere between 500rwhp-950rwhp cars running BMW couplers. Why aren't we seeing these types of failures? Why is is that the only one we've seen it on, is the one setup that has been modified to the hilt? Could it be that the BMW couplers just dont work in YOUR application?

Just a thought.

Tomulrich

In my opinion they are not as strong. I have had 3 engineers at Pratt and Whitney (design and test jet engines / one of them balances 6 foot turbine shafts that run 60,000rpm) run some tests on them. Hardness of the rubber is close to 1/2 of what the GM units are. We also ran som flexability tests on the couplers to check for the possiblity expansion. It took 2x the pressure to move the couple out of plane .250". This is directly related to the hardness of the rubber.

These 2 tests alone show that the potential of movement / flex is greater with the BMW pieces. This in turn could lead to premature failure.

As far as my setup...the driveshaft / couplers / shafts ..etc , etc are all the same as what everyone else is using. These were my findings. I didnt just slap things together and make a statement when I had a problem..

I am very mechanically enclined and usually test items when I have a problem.......anyone else on this forum ever jig up a fixture to run the driveshaft open so that measurement can be taken throughout the RPM range? Anyone else ever had the couplers check by engineers?

I know my setup is different in other aspects.......if people want to save a few bucks to run the BMW couplers......thats fine! But I think everyone is entilted to hear and see what I found. The forum is to post information that can help people avoid mistakes in the future....

I wish I had known about this before I purchased the BMW units...

I understand there are businesses selling these....I am not here to hurt businesses...............but people have a right to know what my findings are!
I can tell you that I have been screwed over and over again and wasted alot of money by businesses telling me wrong information just to get a sale.....with no testing or info to back it!

Guys .....I am posting what I find......keep the posts coming.......USEFUL posts are appreciated......

I am very frustrated after spending close to 15,000 on this conversion and am asking for people to post info that may be helpful.....any take away any info they feel may be helpful

Thanks Tom

0Jeff @ TPE

I dont sell the couplers, but I DO use them. If they are found to be insufficient in normal applications, i'll never run one again.

But until this thread, i've never seen a failure (outside of regular failures)

Whats the PN on the GM piece?

SpeedyD

OK, I have a theory for you. Here's what we know:
1) under no load and basically a low RPM you are getting a bad vibration.
2) The coupler must be expanding to make the low area rub
3) it's been tested that at the RPM you are expierencing the rubbing it wouldn't "normally" flex

Here's some answers, I think:
1) If it's that low of an RPM with no load I have a hard time believeing just the rubber rubbing a little bit would cause that kind of vibration.
2) I beleive the coupler is expanding, but it's not from the load at the rear on it, it's from not being aligned. I'm going to guess, and I could be totally wrong, that that coupler is being forced to bend like a U joint. This wold cause the compression and the expansion. It would also make it expanding at all the low points as the high points are can't compress becasue of the steel inserts. I don't beleive it's the rubber rubbing that is causing the vibration. It's at that "joint" it goes from being able to flex, the low points, to stiffening up, the metal inserts, back to being able to flex, stiff, flex, you get the idea, and that is casuing the vibration. I think this is the only thing that answers all the questions.
Wow, this all sounded good till I just thought of the fact that the coupler was rubbing all the way around the torque tube.

Next idea is that your shaft leading out of your tranny is bent, thus causing the angle in the coupler and making it rub all the way around the torque tube.

One question, is it the tranny side that is rubbing?

Hope some part of this helps. Seems very weirs the low part of teh rubber is rubbing, and it's rubbing all the way around. Something is causing a pretty big load on that rubber.

Good Luck!
Dan

Brandon619

Hey Tom buy some solid ones that will fix your coupler flex. I put a solid one on the rear and the BMW up front .