Fun with suspension geometry

[mrldcty]

That seems way too high. Are you measuring all this down on the ground sitting on its own weight and not drooped out on jack stands? Are your springs the right way around? Stock length shackles?

I just did a 2" skyjacker lift and didn't need any shims, and actually removed the 2 degree shims I had been told I would need. Also the engine/trans/tcase is designed to have a tilt to it. 0.8 suggests you have a ton of rake (2.5" based on above) and something is off.

Actually, after thinking about this for a minute, the before and after ride height is not a valid comparison because I have new, much taller tires on it. My bad, I should have made that more clear. That was the point of the lift... to allow for taller tires to increase under axle ground clearance. I'm laid up with a sprained ankle right now, but when I get back on my feet, it would be good to put the old wheels/tires back on (they are all still mounted) and measure the ride height and maybe drive it around. The basic geometry of the driveline shouldn't change with different tires, but the new wheels/tires are 21.5 lbs heavier per corner than what was on it... perhaps that extra rotating mass is contributing to the vibrations. Plus the new tires are BFG KM2 mud terrain and the old tires are KO2 all terrain, so the tread is quite a bit chunkier. A drive on the old wheels/tires should help narrow this down.

 

[ryanwk628]

Actually, after thinking about this for a minute, the before and after ride height is not a valid comparison because I have new, much taller tires on it. My bad, I should have made that more clear. That was the point of the lift... to allow for taller tires to increase under axle ground clearance. I'm laid up with a sprained ankle right now, but when I get back on my feet, it would be good to put the old wheels/tires back on (they are all still mounted) and measure the ride height and maybe drive it around. The basic geometry of the driveline shouldn't change with different tires, but the new wheels/tires are 21.5 lbs heavier per corner than what was on it... perhaps that extra rotating mass is contributing to the vibrations. Plus the new tires are BFG KM2 mud terrain and the old tires are KO2 all terrain, so the tread is quite a bit chunkier. A drive on the old wheels/tires should help narrow this down.

Tires could be causing vibrations and explain the height, but assuming all four of your tires are the same diameter something seems off for you to have that much rake front to back. The OME installs I looked at were pretty level. I suspect whatever it is would also be contributing to your pinion angle disparity since, based on my researching lifts on this site, 2.5" lifts usually dont need more than 2.5 deg shims, if any.

 

[mrldcty]

Tires could be causing vibrations and explain the height, but assuming all four of your tires are the same diameter something seems off for you to have that much rake front to back. The OME installs I looked at were pretty level. I suspect whatever it is would also be contributing to your pinion angle disparity since, based on my researching lifts on this site, 2.5" lifts usually dont need more than 2.5 deg shims, if any.

We have a winner! I just hobbled out to the garage and did some more measuring to try to figure out the rake. I missed the factoid that there are two different sized shackles in the lift kit! The ones I have in the front are an inch shorter that then ones I have in the back. Swapping them will level out the truck... is this indeed the missing link?

 

[pbgbottle]

Your geometry looks fine. It doesn’t have to be perfect, I've run stuff several degrees out without issue. 2-3 degrees is not a big deal.
I think you have another issue, transfer case output bearing preload, u joints, rear pinion bearing, I’d start with the U joints.

Yep looks pretty good to me too.

 

[mrldcty]

I appreciate all the great input! As soon as I'm back on my feet (sprained ankle), I'll go swap those shackles and re-measure and re-test. Right now, I would not even be able to push the clutch pedal down!

 

[mrldcty]

OK, with some help from a friend, I got the shackles swapped. Surprisingly, it didn't effect the ride height as much as I thought it would. The front is the same, and the rear dropped by and inch, so still a 1.5" higher in the rear. A test drive revealed that the vibrations are still there, but noticeably reduced, so at least that moved it in the right direction.

EDIT: Adding about 200 lbs to the cargo area dropped the rear end close to an inch and made another incremental improvement in vibration.

 

[cruisermatt]

OK, with some help from a friend, I got the shackles swapped. Surprisingly, it didn't effect the ride height as much as I thought it would. The front is the same, and the rear dropped by and inch, so still a 1.5" higher in the rear. A test drive revealed that the vibrations are still there, but noticeably reduced, so at least that moved it in the right direction.

EDIT: Adding about 200 lbs to the cargo area dropped the rear end close to an inch and made another incremental improvement in vibration.

Did you put new U-joints in it yet

 

[mrldcty]

Did you put new U-joints in it yet

Not yet, but they are on the way (Matsubas from Cruiser Outfitters). Also, I fixed my leaking shop press so it will make it easier to swap them once they get here. I'll report back once I have them installed and tested. Doing the fronts too, while I'm at it.

 

[mrldcty]

OK, the rear U joints are in. cruisermatt, you nailed it... almost all of the vibrations are gone. Very drivable now! Many thanks to everyone on this thread... I learned a lot! Now on to working on getting the steering less squirrely and maybe taking a leaf out of the rear so I can take all those weights out of the back. I also just scored an original jerry can and holder, so that will help pull the rear down a bit.

 

[mrldcty]

Update for those interested: I took a leaf out of the rear spring pack, replaced front and rear 2* shims with 4* and got serious about accurately setting the front toe (I have it at 1/8" toe in with the 33's). Another quantum of improvement for the rear vibrations and the truck is now level without additional weight in the back! It handles noticeably better on the road and around corners, tracks great, but steering is still a bit squirrely when on and off the throttle. Also, in 4H/4L, the front vibrates too much on acceleration. Here are the geometry readings after all the adjustments:

Front
DL Angle: 10.9*
Pinion Angle: + 0.7*
TC Angle: + 1.3*

Rear
DL Angle: 12.7*
Pinion Angle: + 3.0*
TC Angle: - 2.0*

The front geometry is really not right, so I think I will swap in 6* shims up front. That should make the TC and Pinion parallel and add another 2* of caster which *should* help with the squirrel. After that, I think I'll update the diffs with rebuilds and lockers. The (original) rear diff has quite a large lash, which could also be the source of the remaining vibrations.

But first... I think I'll drive her while she is still on her feet!!!

 

[1911]

The front geometry is really not right, so I think I will swap in 6* shims up front. That should make the TC and Pinion parallel and add another 2* of caster which *should* help with the squirrel. After that, I think I'll update the diffs with rebuilds and lockers. The (original) rear diff has quite a large lash, which could also be the source of the remaining vibrations.

Usually, you needn't worry about keeping the front u-joint working angles equal but opposite, since it only comes into play in 4WD, and you're usually driving slow in 4WD. So use shims to set the caster, and don't worry about the driveline angles on the front.

On the rear, you want to get the u-joint working angles as close to equal but opposite as possible. Your current angles are within one degree of each other, which should be acceptable.

 

[mrldcty]

Usually, you needn't worry about keeping the front u-joint working angles equal but opposite, since it only comes into play in 4WD, and you're usually driving slow in 4WD. So use shims to set the caster, and don't worry about the driveline angles on the front.

On the rear, you want to get the u-joint working angles as close to equal but opposite as possible. Your current angles are within one degree of each other, which should be acceptable.

Yep, thanks... I'm really happy with the rear geometry. The only time I might want to drive "fast" in FWD is on snowy roads, but we are only talking 30-40 MPH. Since I want to try more caster in front anyway, it's just a bonus that it will help the front geometry. It's been fun so far... the step-wise upgrading has worked well to educate me on how the truck works and all the input from the forum has been super helpful!

 

[JMZ400]

And, BTW, mostly I've been test driving it in 2H, but did put it in 4H and 4L the other day to test that out and I think it actually quieted it down a tad! So, I'm quite sure the vibrations are coming from the rear.