Need ideas. 15yr old vibration that just won't go away
 

I bought my 98 back in 2006 with 75k miles on it. Had a slight shudder when the torque convertor stalled [1400rpm ish] when accelerating from a stop. Since then I've:
-cut and balanced driveshaft
-new u joints
-replaced entire rear axle
-rebuilt transmission
-new torque convertor
-replaced all engine and trans mounts
-inspected flexplate for cracks while trans was out
-multiple sets of tires and different set of rims
-harmonic balancer

Nothing has stopped it. It got much worse after the trans swap from flexing the motor mounts. Just finished changing them, and the vibration is back to when I bought it level of annoying. WTF??? What else can I check???
Vibration isn't there once the truck is moving. Its only when starting off from a stop. It isn't there once the rpm's are over 1500 in 1st either. Its almost like running over a rumble strip. I'm lost.

Re: Need ideas. 15yr old vibration that just won't go away
 

One piece or two piece driveshaft?

What does your rear axle u joint working angle measure?

K

Re: Need ideas. 15yr old vibration that just won't go away
 

2pc shaft.
At one point I got frustrated like now and took it to dealer. They put sensors on, drove it, and told me the driveshaft was the issue and too long due to lowering [lowering kit includes spacer bar for carrier bearing to get angles correct] so I had it cut and balanced, changed nothing. Shudder is the same lowered as it was when it was a totally stock truck.

Re: Need ideas. 15yr old vibration that just won't go away
 

Sounds like you've checked almost everything that a smart person would check.

Two silly questions:
1) Is it possible that the noise is coming from tire rub? You mentioned it's lowered and the problem only happens during acceleration. (and a real stretch, but are any parts of the drive shaft rubbing against any parts of the body or frame or exhaust?)

2) Is the exhaust system stock? Plenty of room around it, nothing near the frame or body or tires?

Re: Need ideas. 15yr old vibration that just won't go away
 

I know this is going to be from left field....

Have you checked that all spark leads are good and one is not arcing out when under load...

I remember a couple of cases just like yours where the lead looked good but would arc over to the exhaust when under load...

:chevy::chevy::chevy:

Re: Need ideas. 15yr old vibration that just won't go away
 

I don't think you'll be able to fix it with what you have.

When I was at the Desert Proving Ground I worked on extended cabs with an inherent launch shudder. I was able to fix it by increasing the length of the front shaft (that is, moving the center bearing rearward) by 12-24". Even though I was able to demonstrate a fix the design community did not implement my suggestion because I couldn't explain mathematically why it worked.

I think your only choices are to either create a custom length front shaft or go to a one piece shaft; otherwise it is what it is.

K



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Re: Need ideas. 15yr old vibration that just won't go away
 

Although not cheap ,you can get a program on your phone that will diagnose vibrations...
I would say from a spectator seat that Keith is on it though...
https://m.youtube.com/watch?v=GuI0hH2TuxQ

Re: Need ideas. 15yr old vibration that just won't go away
 

Gm 96-up 4.3 / 5.0 / 5.7 can get a wierd rpm window skip that fells wierd . What brand spark plugs do you run ? When i was turning wrenches back in the day i noticed bosch plugs did this the most . Run factory acdelco and problem gone .

This might be a thing to look in to . Good luck .

Re: Need ideas. 15yr old vibration that just won't go away
 

Very interesting Keith, my ‘98 extended cab longbed has had vibrations from a stop to about 15-20 mph as long as It’s been in the family, (since 2004). I always figured it was axle wrap. Maybe not.

Re: Need ideas. 15yr old vibration that just won't go away
 

This is significant, if my line of thinking is correct.

A couple things you could try, just to see if I'm right:

a) Hang something heavy off the transmission tailshaft. Something big, like an engine block, but still allows you to launch the truck. That will change the resonant frequency of the engine/trans mounting system (which is what you did by refreshing the mounts)*. If it changes the launch shudder, for better or worse, (faster or slower, or change in amplitude) you will at least know if you are barking up the right tree.

b) Make a "junk" one piece shaft. Out of exhaust tubing or junkyard parts or whatever you have laying around. Just strong enough to launch the truck one or two times, to see if the shudder goes away.

c) Play around with the rear axle u joint working angle, aiming for essentially straight during this launch condition (probably not going to work, per Table I attached in the next post; although - this work was done at GCW, if your main complaint is at curb weight then the vehicle might behave differently).

d) Replace the single cardon U joints with CV joints, at both the rear axle and the center bearing (the one at the trans is essentially straight and therefore probably ok). I thought of this just now, only 31 years after it would have been helpful in the resolution of my project. :roll eyes:

If any of these are effective, the next choice would be deciding if you still want to fix it, or if the fixes are painful enough that you'd rather just live with it.

K




*the resonant frequency, which is what the spring/mass system "wants" to vibrate at, is described by the formula attached below. In changing the mounts you affected "k", the spring rate of the lump mass engine/trans system. Increasing the mass, "m", by adding a weight will decrease the resonant frequency, perhaps to a range where it is no longer offensive. That's why you will sometimes see weights hanging off of rear leaf springs, or transmissions, on production vehicles (like Astro/Safari vans, for example).

Re: Need ideas. 15yr old vibration that just won't go away
 

Found a bit more detail:

Re: Need ideas. 15yr old vibration that just won't go away
 

Wow Keith, thats amazing info.

As for some questions posed:
Cap, rotor, and wires have been changed in the trucks life with me without changing the vibration. I have always used R44LTS plugs after I got the truck. I'm positive its not an engine running issue only because I cannot replicate the issue regardless of rpm or load once the truck is rolling, and I was getting 22mpg for a long time with the issue still there.
I also don't think its tire rub, as it did this when stock.
Exhaust is mostly stock except muffler replaced and turn down on the muffler exit so less exhaust system in there these days.

I highly appreciate all the ideas, thank you!

Re: Need ideas. 15yr old vibration that just won't go away
 

How hard are you accelerating when this happens? Could it be wheel hop? How old are the shock absorbers? And are they configured for anti-wheel hop? I.e., one shock angled toward the front and one angled toward the rear?

Or could it be a limited slip rear differential slipping and grapping intermittently? New fluid with proper additive for the posi if it has one?

Re: Need ideas. 15yr old vibration that just won't go away
 

BTW, I like Ken's (EDIT: OOPS! I mean Keith's) data and methodology. I'm having flashbacks to a Vibration in Design class I had maybe 30 years ago. It was actually kind of a fun class. And later I used the ideas to size rubber-element dyno driveshafts a few times. Great stuff.

Re: Need ideas. 15yr old vibration that just won't go away
 

Thank you. It was/is fun. I did that full time for about 4 years. My technician (RIP Bill) was awesome, so he should get a lot of credit here. General Motors has the finest tools (and teachers) available, plus - you get to use high falutin' words and impress your friends.




So - having given this further thought - I would describe Tom's situation this way:

Ever "twanged" a ruler off the side of a desk?

That's what his powertrain is doing. His engine/trans assembly is vibrating vertically on the mounts (at about 13 Hz, to be precise), based on some to-be-determined input.

For every noise or vibration you have to have three things:
…..a) an input
…..b) a noise transmission path
…..c) a response

If you ring a bell you have:
…..a) a hammer (the input)
.….b) the air around you (the transmission path)
…..c) the vibration of the bell dome (the response)

Take away any of those and...no more noise:
…..a) Don't hit the bell = no noise
…..b) get rid of the air = no noise
…..c) clamp your hands over the bell to dampen the vibrations = no noise

Obviously - some solutions are more practical than others. For example, taking away the air would result in some collateral damage that we would not want to experience, I presume.

So - back to Tom's truck:

The power cube wants to vibrate at 13 Hz (response). It's designed right in, to every truck of this configuration, just as though the engineers meant to do it on purpose (they did, btw).

It's just looking for a reason to light off and the driveline is more than happy to provide that excitation force (input) by kicking the trans tailshaft twice every time the drive shaft completes one revolution (which correlates to the accel/decel generated by the single cardan u joints - that's what "2nd order of driveshaft RPM" means in the order tracked data above). Single cardan U joint disturbances are "load dependent" - so the amount of force applied makes a difference. Ease off on the throttle - it gets better. Add a load to the pickup bed, or hook up a trailer - it gets worse.

The vibration transmission path, in this case, is through the frame/mount structure and up into the seat bracket, into the seat cushion and then into Tom's seat.

So our options are:
…..1) Do something to the vibration transmission path - if you were to disconnect the powertrain from the chassis the vibration would not be felt by the driver. You can actually do this on a chassis roll dynamometer, and it works, but obviously this is not a good solution for real life.
…..2) Affect the response - change the mounts so they are super stiff or super soft. Change the mass of the system by adding weight. These would also both have some affect, as described by the formula attached above - we just don't know if it would move the disturbance enough to be out of the problem range.
…..3) Don't hit the bell. Stop it. Just.....don't. Sounds easy enough. Our options for affecting the input are related to doing "something" with that driveline:

……...Add CV or double cardan joints
……….Single cardan U joint operating angles (maybe)
……...Move the center bearing rearward (maybe)
……….One piece shaft (likely)
……….No shaft
……….Rope drive/chain drive
……….Front wheel drive
……….Shorten the wheelbase 18 to 24 inches
……….Don't accelerate from a dead stop
……...Limit the amount of power available at launch through engine calibrations
……...etc

Here again - all of these are potential solutions but some are more practical than others.

K

Re: Need ideas. 15yr old vibration that just won't go away
 

I like the no shaft idea. Maybe mount engine and trans in bed so they connect directly to the axle?:lol::mm:

Re: Need ideas. 15yr old vibration that just won't go away
 

Rope Drive, baby!

Re: Need ideas. 15yr old vibration that just won't go away
 

This is a long shot question but I didn't see it mentioned.

You have a 2 piece driveshaft. Have you ruled out the driveshaft carrier bearing?
Will it shudder under load with the brakes locked - no driveline rotation?
When it starts to shudder would you say the shudder matches engine RPM or driveline RPM? Knowing which one should at least tell you if the problem is behind or in front of the tail shaft.

Re: Need ideas. 15yr old vibration that just won't go away
 

Keith....His truck is a 2wd C1500, Does that alter your input on the situation?

Re: Need ideas. 15yr old vibration that just won't go away
 

It changes the resonant frequency of the engine/trans assembly on the mounts (moves it up in range), but - no. The scenario and explanation would be the same.

K

Re: Need ideas. 15yr old vibration that just won't go away
 

Question, why didn't GM use a 1pc driveshaft from the factory on these trucks?

Re: Need ideas. 15yr old vibration that just won't go away
 

That'd be a really long shaft.

One piece vs two piece has to do with the driveshaft "critical speed", the point at which it begins to swing like a jump rope.

It is affected by length, outside diameter, wall thickness, material, mass and rpm. Highest shaft rpm is affected by vehicle top speed, rear axle ratio and tire diameter. Length is affected by wheelbase and driveline configuration (trans length, t case, etc).

The driveline is also affected by any unusual or unpredicted system resonances, which can crack a Tcase or some other failure mode even at a speed lower than the critical rpm.

From an OE perspective the driveshaft choice (one piece vs two piece) can be manipulated by controlling any one or more of those variables. Vehicle top speed is limited by several different components, including tire speed rating or driveshaft critical speed. Sometimes the driveline is the designated "fuse" (as directed by the program team) and speed is controlled through the use of fuel or ignition cutoff or some other ECM parameter, in order to maintain a one piece system as a cost save or warranty avoidance.

You can also move to a one piece if you are willing to get extreme on the shaft itself: larger diameter, special lightweight materials (aluminum, carbon fiber, "metal matrix"). That's where the piece cost discussion enters in.

You or I (as individuals) might decide to do a one piece shaft figuring we are not going to be driving all that fast.

So, in this particular case, someone (or a group of someones) decided to play it safe and just go with a two piece design, probably because it had always been that way, not realizing they were going to get into some other problems.

K

Re: Need ideas. 15yr old vibration that just won't go away
 

Tom - I'm pretty sure this would fix your problem:

Re: Need ideas. 15yr old vibration that just won't go away
 

So, my truck starting making a metal noise and had vibrations at all times as speed went up and it kept getting worse. Found the front u-joint had no more needle bearings in one cup, they were all ground to dust and gone. Put a new u-joint in and no more vibration! At speed at least. Dam vibration from a stop is still there lol

Re: Need ideas. 15yr old vibration that just won't go away
 

Not for nuthin, I chased my tail on my EC S10 for a couple months like this. Stock carrier and double cardans, nice and smooth. Factory carrier finally died, and no biggie, I swapped it. Vibration like crazy. Destroyed the OE gettin it off, so no comparing. The first cheapo died pretty quick, so I got the super bad boy one. Same problem. Got a whole different single cardan shaft, better but not great. Finally got out the math and did some consultation. Rearend angle and trans angle were dead nuts the same. So, I said to heck with it and cut the Carrier mount loose and dropped it a good bit and made the first shaft basically straight and square out of the trans, and a complementary angle to the rearend. Smoothed right out. I believe for whatever reason, the aftermarket Carrier bearing is significantly taller. I don't know, but it worked, and I have my truck back. Good luck in your quest.