Opinions on Propshaft “Rubber” Couplings!
 

How often in this forum to you hear people wanting the get rid of the “rubber” coupler? There are a lot of people that believe anything made of “rubber” from the OEM is bad or inferior to other after market compounds. Aftermarket companies, in my opinion seem to capitalize on this belief, offering what they call “upgrades” to everything from bushings, mounts, and many other elastomer products. Granted, if you make major changes to output power then you may need to change to something different, but choosing something different without any thought other then it cost a lot and looks pretty may not buy you any improvement, and in some cases will hurt performance and durability.


Compromises
With any changes there are always compromises. There is no fix all perfect product (well, at least that I know of). If you install the very best part money can buy, chances are it will affect some other component in the vehicle causing this other component to operate inferior then before you made the change. The point I'm trying to make here, when you buy something to replace an OEM part, keep in mind other items it might affect. This has nothing to do with couplers but for example, if you switch from a standard crankshaft pulley to an under-drive unit, it will net some increase in power to the rear wheels. That's a good thing. However, to net this power you are compromising vehicle start and operation at very extreme outdoor temps. Not really a big deal for most people. I normally wouldn't be starting my Corvette in – 40 deg temps nor would I be in 120 deg temps. So this compromise seems to be a reasonable one, and changing to a under-drive pulley may makes sense. Again, my point is to understand compromises before making the choice to change something.

Sorry for the above diversion, but I believe it is important to understand compromises, so back to propshaft couplers.

The couplers use in the C5 and C6 Corvettes are design to take minor angler misalignment. In torsion they are relatively stiff. One of the primary functions second to transferring torque, is damping. A lot of people really don't understand how important damping is. A simple example that most understand is what happens if your car has a bad or broken shock absorber. Whichever wheel has this missing shock will bounce non-stop and can make controlling the vehicle nothing short of impossible. The shock absorber's primary function is to provide this very important damping. So, similar to how a shock absorber controls suspension rebound, the propshaft couplers control torsional vibrations caused by the ever changing power transferred from the engine to the transmission.

Not Just Rubber
The couplers are not just a hunk of rubber. A lot of people think so, but they really are not. If you have ever seen one apart, it is nothing short of amazing that they can fit the amount of windings into such a small space. They are interwoven with a cord that looks much like the cording found in a tire. Durability of these couplers are extremely good. Can they fail? Sure, and when they do they can make a real mess. Failed coupler will typically only mess up theirself and damage to other parts is minimal if any at all. A good thing, failures are not common.

Propshaft
We need to at least touch on the subject of the propshaft. All shafts will have resonate points. These resonate points occur in torsion, and bending of the shaft. The frequency that this happens varies with stiffness. Stiffness is controlled by things like the shaft diameter and mass. Some of the possible excitation sources (what excites these resonate points) are shaft imbalance, and torsional inputs.

In any propshaft the design engineer may take different approaches as to where to place the resonate point. The designer can target a resonate point above the operating rpm range or below. Keep in mind this is always a balancing act between cost and functionality. In the case of the stock Corvette the design engineer choose to place the resonate point to a very low frequency. I’m sure you have heard the rattle during a slow launch from a complete stop or when shutting down the engine with the clutch engaged with the transmission in neutral. It really sounds bad. Even though it is mentioned in the owner's manual, a lot of first time Corvette owners will ask about it. The shaft passes through this resonate point at start while loads and speed are down. Changing to something stiffer, like the solid couplers will raise the shaft resonate point. This is a bad idea. Keep in mind that the shaft will still have a resonate point and now that I've made the system stiffer I have two things going against me. First, the resonate point may have been put into a rpm range where the shaft has its highest loads and maybe even more important without damping. This could allow this shaft resonate to peek to a much higher amplitude maybe even to the point of shaft yield. Guess what happens next. For those that don't want to guess, instead of blowing a coupler there is a higher risk of blowing the shaft. That would make for a very bad day!

From one of the other threads there was a question about radial loads. I have been asked this a few times primarily in reference to solid couplers and the effects on the pilot bearing. My feelings are that the life of the pilot bearing could be shortened by side loads the normal stock coupler would compensate for. Solid couplers could also side load the snubber, or hub bearings and cause damage.

The last time I checked part number for the couplings are 12456212 for the one that takes the 10mm bolts, 1997 through 2000, and 88894026 for 2001 through 2004 that take the 12mm bolts. You should however confirm this before ordering any parts.

Knowing when a coupling is bad.
http://7uaqsq.blu.livefilestore.com/...lw/coupler.jpg

Update 12/7/09
The list below shows the different propshafts and couplings used on the C5 Corvettes.
http://forums.corvetteforum.com/pict...ictureid=40714

BOLTS
Bolts for mounting the flex couplings use the following bolts.

Part # 12456213 = Torx M10 x 47 mm 12 each
Part # 88894027 = Torx M12 x 52 mm 12 each

Thanks for the info. I'm going with 2 new stock ones for my 99 (55,000 miles) while replacing my clutch.

Jason

I've always questioned the practice of replacing one coupler with a solid one. Seems like it would put more strain on the remaining flexible coupler.

Mark,
You are correct. Here is my response to Jasons to his question about it.

Jason,

Each of the two couplers absorb 50% of the torsional loads put through the propshaft. Remove one of them and install a solid unit and the remaining unit takes 100% of the torsional load. Doing this will reduce the life of the remaining coupler by at least 50% and the chance of shock type failure even higher. The ZR1 Corvette with 638 hp still runs two of the composite couplers.

:cheers:

Great write up. Thanks for the explanation. I have always believed that components are engineered a certain way for a reason.

Some aftermarket companies would have you believe that they know better and their products will improve on the original design of a component. Sometimes this can be true but we have to keep in mind that a lot of these "improved" components are usually not thoroughly tested or the claims are completely unsubstantiated. They just do not have the engineering resources or budget to do the proper testing. We have to choose aftermarket components very carefully. Just my two cents.

Cybermind,

First off thanks for the complement. My hope is that it will help folks with their choices when it comes to making changes to the Corvette.

As far as aftermarket companies, there are an number of very good ones that sponsor this Corvette Forums, and this is where reputation comes into play in my opinion. The good ones will back their products and will bend over backwards to help their customers. One of the great thing with this forum is how users can communicate not only with each other but also with all the supporting venders. I believe a lot of people don't even give it a second thought as to the power of this website. Not to long ago back, before internet venders (alright, really long time ago) could get away with less support and if things didn't work out to well it was no sweat off of their back. They win, end user looses and no one to the wiser.

Compared to years ago I believe most venders have a much better chance of getting things right with the computer design tools that are available now.

Thanks again for your input! :cheers:

Eric D, so you are saying GM doesn't have access to computer programs etc., I think not, they have far better resources but must contend with warranties etc. aftermarket does not have to, i.e. 50-100miles or 5 years. Racing is one thing, street driving another and many forget this and then get unhappy when there new hot cam breaks springs etc. and won't go 50k miles with no problems. can't always have it both ways.:cheers:

Hmm, don't recall anything about GM being mentioned. I believe you mistaken my comment about computer design or maybe I didn't explain it well enough. My point was that the cost of programs for modeling are available to just about any budget and most if not all the better aftermarket suppliers can make use of them. My comment has nothing to do with what goes on at GM or their warranty.

Great thread!

Since when did Gm start offering these couplers for sale?

The two times i did my shaft they did not sell the couplers by themselves.

I used the softer BMW ones and tore them apart and then I went with dual solid from LG motorsports, which gave me an annoying rattle at 650 rpms.

So if I am reading this right, the reason I am repl my pilot bearing now after 18 mos, and repl bent prop shaft could be the al coupler I put in the rear 18 mos ago? I am not sure why my prop shaft bent. I do not drag race, do burnouts or abuse my car except on road courses. I road race my car so I thought the al coupler was an improvement. I definitely do not want to do this again, not so much the money ($13 pilot bearing and $460 prop shaft), but it is a lot of work pulling all that stuff. I am ready to reinstall Monday, so should I put my rubber coupling back in while it is out? Thanks for the interesting post.

Just added a part number listing of C5 propshafts and couplings. :cheers:

Good question. One thing is certain...the aluminum is not compliant in any way, while the rubber bushing is(by comparison).

I posted on this some months ago when I did my clutch. My couplers were cracked, so I ordered new ones from Corvette Central (BMW replacements). Upon receiving, I immediately noticed a difference in the quality and durometer of the two couplers. One was very soft, and the other was consistent in quality and durometer of the GM couplers I was replacing. I questioned Corvette Central on the disparity, to which they offered no explanation(although they took the return for a refund). The softer coupler came in a plain CC bag, while the good coupler came in a BMW bag(with a BMW part number). I went to my local BMW dealership, and obtained another good coupler consistent with the same quality as the GM units. Based on how soft the coupler was, I would have never installed it on my car, so I'm not surprised you trashed yours.............

I trashed mine after about 45k miles and 500+RWHP. I replaced them with the BMW couplers and they're holding up fine. I was PM'ing with someone on another forum who kept tearing up his transmission running 2 solid couplers and poly motormounts and tranny mount..Nothing was absorbing any shock and it cost him 3 tranny's and one rear with only 425RWHP.

there have been 100's sold, I've installed a few into mildly mod'd C5's

great thread for a shop to step in providing real world findings on 500hp plus cars

What are the BMW part numbers ?

I had a twin disc installed and I think they put in a solid coupler. I now have a terrible rattle at 650 RPM's in neutral with the clutch pedal out. When i push in the clutch pedal, the rattle goes away. I can also make it go away if I lightly push the shifter stick against one of the gears with the clutch pedal up. I can also make it go away by bringing the RPMs up to about 900. I was thinking it was my clutch but now i think it might be the propshaft coupler. What do you all think?

Thats not the coupler making the noise... its the twin disc clutch. :yesnod:

How much HP will the stock couplers take. I'm running a TT system so my power comes on relatively smoothly compared to SC cars. I don't know how much HP I'll have after the new build but I suspect somewhere between 700-900 probably near the middle. I'd prefer for multiple reasons to continue running the stock couplers.

A second question should I need to replace a coupler is are the 11mm couplers that much better than the older versions? Or is that 13mm bolts and inserts? My car is a 2000 so I'd have to tap new holes which doesn't sound like too big a job but more than a little PITA.

This is not just a case of couplers with larger bolt holes, i.e. 10mm vs 12mm. There are two, completely different, torque tube/driveshaft setups and judging by your year model, you have the earlier version, i.e. what is commonly called the "10mm" version vs. the "12mm" version. The "10" and the "12" numbers are basically just a shorthand used when discussing the two different torque tube/driveshaft assemblies and refer to the two different sized bolts used to mount the rubber couplers inside.

The earlier version torque tube assembly (the so-called "10mm" version) has a physically smaller driveshaft inside, 58mm in diameter IIRC. The later version, using larger couplers with 12mm bolt holes (beginning in '01) has a 63mm (IIRC) diameter driveshaft.

The good news is, if you are building a setup with high horsepower, you will want to get a later version torque tube assembly as it bolts right up in place of the older, earlier version with no mods, cutting, or drama. The "innards" will not interchange, so you need to replace the whole assembly. Locate a good 2002 - 2004 6 speed torque tube (for 6 speed cars) and check/replace the couplers, or bearings inside, as needed. If you don't have, or can't locate a part number, the easy visual way to tell them apart is that the rear bell-housings on the later versions are welded to the torque tube as opposed to the earlier versions which are bolted on.