Vibrations after Rear Dobinson LT Install?

[Greydog]

Hi all, I’m hoping someone has some good solutions here!?

I’m getting 2 distinct vibrations following a Dobinson rear LT install (springs, shocks, upper & lower adj. Plinks):

45 mph: the truck develops a vibration typically only on less smooth pavement, mostly goes away when you immediately transition to smooth pavement. It’s worsened when applying light-moderate throttle. Also reduces quickly off throttle. This started immediately after the suspension install. It can develop into a light bucking of the truck in the right situation.

- Are the upper/lower link rubber bushings or the IMS shocks really that soft?
- this did not occur with the prior Fox 2.0 shocks or SPC lower links.
- Has anyone replaced the links rubber bushings with the poly that comes with the kit, and how did it feel?

70 mph: this one develops gradually over 60 mph more like a sound or resonance, but really hits at 70 under throttle. Heavier throttle makes it much worse.

- Thinking this may be driveline, but the transfer case and pinion angles are about 0.8 degrees from each other.
- my shop says the U joints are good at 65k miles.
- I seem to feel a but of play inside the transfer case output, but hard to tell.
- when I pound on the drive shaft, it’s very solid down low at the pinion, but has some noise/feel as I work up to the TC.

Thanks for any feedback or experiences you can share!

- Mark

 

[gordonp]

Hi, [MENTION=19164]Greydog[/MENTION]:

I offer sympathy :-O and a couple of quick ideas for you to consider:

I went with Icon 2" rear-lift springs (and Bilstein stuff up front). I found I had a sort of "growl" that I associate with pinion-angle mismatch, after performing the lift.

First thing that quieted everything for the first few months: grease! I diligently greased all three rear-driveshaft zerks (red synthetic stuff for the U-joint crosses, Valvoline 3% moly for the slip-yoke).

But the real fix: I replaced the rear upper control arms with adjustable ones. In search of a bit more offroad flexibility, I selected Opt Offroad:
Rear Adjustable Control Arms (03-24 4Runner, FJ, GX)






– Opt Offroad

I used a magnetic angle-finder and found I had 2-deg mismatch with my 2" lift. I turned the adjusters in a 1.5 turns (shorten them, with my 2" lift) and achieved 0-deg mismatch. Cycling the suspension during normal driving (bumps, load-variations) will ensure the needle-rollers cycle around through the grease, and do not remain static.

For these rear upper control arms, I changed the Zerks to right-angle, just so I could actually get my grease-gun ontol them. Also, the joints were initially really, really tight, and resisted greasing ... but persistence and use gradually allowed me to get some grease into the Johnny Joints.

In actual use: I got rid of my growl. YAY! I also find the rear suspension is more limber, and flexes nicely. The combination of OEM stock-rubber in the lower control arms seems to absorb road-inputs nicely, while upper polyurethane seems to not add noise, but firmly maintain my angle-settings.

Summary: I did zero out 2-deg angle mismatch after 2" lift. Lots of grease in the rear driveshaft zerks seems to also squelch vibrations.

HTH!

BTW - with my 2" lift, I also relocated the Panhard axle-mount upward by 2". This shouldn't have any impact on vibration, but it ensured handling was the best it could be: neutral angle under "normal" conditions, with raised roll-center for improved handling during crosswind, bumps, etc. To consider.

 

[Greydog]

[MENTION=664029]gordonp[/MENTION], thanks for the feedback! I’ll recheck the pinion angle again, thought it would be good at 0.8 but perhaps I need to get it closer to 0.

I live in the Northeast, do you think those Johnny Joints on the arms would hold up? I’m hesitant to go that route.

Thanks, Mark