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Bad Vibrations!!!

J

jlower1499

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Franklin, TN
Hello I am hoping someone can help me to pin point a problem on the K5 I just bought. It has ~6 or 8" lift with blocks and has a noticably bad vibration when accelerating and slowing down. If at anytime I put it in Nutreal, there is no vibration. Therefore, I would assume I could eliminate the tires and bearings. It has locking hubs, which are unlocked, so I would also assume that it is not in the front end either. I checked the motor mounts and the motor doesnt move (At ALL) when reving up. The U-joints are new and the only things I can come up with are the angles of the drive shaft relative to the output shaft of the transfer case and rear diff. Or possibly the rear torsion bars as there arent any there. I would assume that if it were the torsion bars, or lack there of, I would be able to reduce this vibration dramatically by slowly accelerating, but thats not really the case. I'm afraid of driving this truck and dont know what will happen if I continue to. In addition the welch plug at the slip yoke just started leaking. As far as I can tell it is is 700r4 tranny and a NP 208 transfer case. Please help.
 
Sounds like a rear u-joint angle problem. With that much lift there is no real "easy" solution. What you should start with here, get your post count up to the required minimum. Then you can take some pics of the driveshaft angles and post them up here. Take good pics as close to horizontal as possible. Get a pic of the t-case end and the axle end. Maybe one from the side of the vehicle to give an idea of how high the truck is too. The more pics you can take that you think will help, the better off you'll be in getting help.

With that said, typically a 6" or taller lift requires a custom driveshaft(s) to be made. I just did a 4" lift and had to have a rear CV style shaft made.
 
Thank you for the advance. I'll try to work on the post count thing. Not sure about this whole blogging business. Still a little new to this.
 
I seem to remember that others have had vibrations from bad transfer case mounts as well........ It might be worth checking????

RIZ
 
Thanks. I'll check them. FYI..I can confirm that there are the 1" spacers between the from and the transfer case support bracket.
 
Transfer case mounts seem to be fine and the 1"inch spacers are in place.

I used a digital level to figure out the angles of the drive shaft between the output shaft of the drive shaft relative to the rear differineetial and the output shaft of the transfer case. The angle between the output shaft of the T/C and the drive shaft is 12.4 degrees and the angle between the rear diff and the D/S is 3.7 degrees. Does anyone know what the maximum angle should be for a slip yoke with 2 U-Joints?
 
I used a digital level to figure out the angles of the drive shaft between the output shaft of the drive shaft relative to the rear differineetial and the output shaft of the transfer case. The angle between the output shaft of the T/C and the drive shaft is 12.4 degrees and the angle between the rear diff and the D/S is 3.7 degrees. Does anyone know what the maximum angle should be for a slip yoke with 2 U-Joints?
 
Well there's your problem. If you have a standard driveshaft (single U-joint at each end) then those two angles should be the same. Stated another way, the T-case output should be parallel to the pinion. This makes the acceleration of the driveshaft throughout its revolution cancel between the two ends (well, almost). The bad news is that both angles will end up at 14 degees or more, which will never be vibration free. But it may be tolerable.

If you have a C/V shaft, then the rear U-joint angle should be 0-1 degrees. This is really the only sure-fire fix for vibrations on a short-wheelbase lifted truck.
 
How tall are the blocks and what angle is cast into them? The angles may end up much closer if the blocks are turned around. But if the blocks are tall, axle wrap may cause vibrations anyway (U-joint angle changes between cruise and acceleration).
 
2 u-joints meaning one at the t-case, one at the axle? If so, the angles should match. Lets say, for argument sake, the t-case output shaft comes out at 0* and the d-shaft points down to the axle at 12.4*. The rear axle pinion should also come out at 0* so that the d-shaft points up at 12.4*. These 2 angles will cancel each other out while the shaft spins.
 
The block measure 4.5" tall in the front and ~5" tall in the back. The angle is 5 Degrees. Axle wrap is certainly a concern, but wouldnt that be a minor problem under slow acceleration? Thank you for your help!!!
 
Yes, the driveshaft has one U-Joint at each end of it. The angles I measured where between the shaft and the t/c output shaft (12.4*), and also between the shaft and the rear diff (3.7*) The angle of the shaft is 20* relative to the ground (0*). I am considering swithcing to a double cardan. Is this the only option, or can I make this equipment work? Thank you for your help!!!
 
Technically, yes, you could make it work by shimming the axle pinion down. But, even if you get rid most of the vibes, you are exceeding the working angle of the u-joints making them more liable to breakage.

Like Blue said, the right way to fix is a CV style shaft. I just had one made for mine, CV and slip yoke at the front.
gallery_195_54_592381.jpg

standard joint at the back.
gallery_195_54_229252.jpg


Mine is mostly good right now. Still have to shim the axle pinion up a bit.
 
Brian,

Thank you for your input, knowledge, and personal exp. It sounds like that is my next step and basically my only option. I do have another question..In the picks you sent and also you stated you have more shimming to do, but should the angle of the rear diff be o relative to the drive shaft? In other words should the rear diff point to the output shaft of the T/C? Also what will I kill by running this truck the way it is for a couple of weeks (street only) as this is my new daily driver. Thanks again!
 
With the shaft like I have, the pinion yoke should point to the t-case within 1* of difference, typically 1* down. That way, under power, it evens out to near 0*. And under cruise it's still within 1*. What the drive line shop that built my shaft told me was that it was OK within even 1.5*. And the working angles of a typical u-joint should not exceed 10*.

Whether or not your current set up will work is up to you. I don't have it here in front of me to see or to drive to make any sort of opinion on that.
 
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