rocks in a can sound
10-09-2011, 03:24 PM
#1
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rocks in a can sound
I've been told by several forum members that the "rocks in a can sound" is normal on manual C5's when starting off in first........but no one has been able to tell me "definitively" what actually causes it. My clutch works perfectly fine but was wondering what causes the sound and what the repair would be if there is any to get rid ofthe sound.
thanks in advance for all your help.
Bob
thanks in advance for all your help.
Bob
10-09-2011, 07:23 PM
#3
Drifting
I have never heard any sound from my clutch. Look at the pictures in Post #2.
http://forums.corvetteforum.com/auto...d-america.html
http://forums.corvetteforum.com/auto...d-america.html
10-09-2011, 07:31 PM
#4
Drifting
This is a known Corvette thing. They even talk about it on the Z06 video you got when you bought one. I don't remember 100% why they said the noise happened, but I seem to recall it had something to do with the drive shaft configuration. But who knows, my memory may be off.
10-09-2011, 11:24 PM
#5
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I wondered the same thing. Eric D posted this a while ago, read the section on the propshaft.
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.
Update 12/7/09
The list below shows the different propshafts and couplings used on the C5 Corvettes.
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
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.
Update 12/7/09
The list below shows the different propshafts and couplings used on the C5 Corvettes.
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
10-10-2011, 12:05 PM
#6
Burning Brakes
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I complained straight to the factory when I purchased my 2004 CE about the "rocks in a can" or "marbles in a can" FWIW I was told that it has to do with the dampening on the engine and drive train. A larger engine damper would reduce performance and this was a compromise they made. My grandfather who worked for Chrysler in their drivetrain area told me it was more liekley due to tolerances in the the alignment of the crankshaft/flywheel output to the clutch/drive shaft.
