Fun with suspension geometry (1 Viewer)

[mrldcty]

Here is my story:

Bought my first cruiser (1980 FJ40, all original with 49K miles!) a couple of months ago. Her name is Ruby! I'm trying to keep the stock feel, but make it more capable and make sure it is reliable and drives well on and off road. I started with rebuilding the brakes and the knuckles (which were leaking) and installing the EMU HD lift kit so I could put on taller wheels/tires to increase the clearance a bit. Did a bunch of other stuff while I was at it, as recommended by Nick at Torfab (thanks Nick!!).

After the lift, I have had a lot of throttle sensitive vibration in the rear, which is apparently characteristic of driveline issues, especially when the driveline angle is over 10 deg and the pinion and TC angles relative to the shaft are not equal.

I have tried shimming the rear 2 degrees, which improved the vibration but did not eliminate it. It is drivable in it's current condition, but probably not healthy for the TC or diff long term, not to mention it makes my e-brake handle vibrate noisily, which is very annoying.

Just dealing with the rear for now: (interestingly, the front doesn't vibrate in 4H or 4L even at 30-40 MPH... it is shimmed too, to get the caster back near zero)

- The TC flange is angled down at -0.8 degrees (relative to the horizontal).
- The rear pinion angle (shims in place, fat end aft) is angled up at 4.8 degrees (relative to the horizontal).
- The drive shaft angle is 12.5 degrees.

I'm completely new to solid axle stuff, but after reading and watching videos on the topic, and assuming my diagnosis of the vibration is correct, it appears the best option to eliminate the vibrations would be to:

- Install a double cardan (DC) drive shaft and
- reverse the shims to bring the pinion angle up to approximately 8.8 degrees. I haven't done the math on this, but this would also significantly reduce the shaft angle (relative to both the horizontal as well as the pinion) because it would lift up the aft end of the shaft.

My understanding with the DC is that, to account for wheel wrap, you start with a 2 degree angle (relative to the DS) at the pinion so that, under torque, the angle moves through about a 4 degree range, i.e., 2 degrees to -2 degrees). With some added weight (to compress the springs further) and the shims flipped, I'm thinking I might be pretty close to that.

Other contributing factors and information:

- The ride height difference after the EMU was 4" up front and 6.5" out back.
- I still have some weight to go onto the truck, including a heavier bumper and 8274 on the front. Fuel can, tools, spares, camping junk, etc... on the rear. Orion case, ARB lockers with compressor, etc...
- The new spare is 21.5 lbs heavier than the old spare (as are all four corners, but those are un-sprung, so not part of the sus geo calcs).

Another option would be to eliminate the shims by cutting and rotating the axle housing. I'm guessing this would be more expensive than a DC shaft, but there is another train of thought at play here, which is to have someone like Torfab or Cruiser Outfitters do this mod, and at the same time, install the lockers, thus making my life less about working on the truck and more about driving the truck ;0).

What say you mud bugs?? Am I missing any options? Given how steep the shaft is, I don't see how the original drive shaft will every really work in the rear.

Thanks in advance for your input!

 

[cruisermatt]

First, put new OEM or Matsuba (OEM-quality made in Japan) U-joints in it

 

[mrldcty]

First, put new OEM or Matsuba (OEM-quality made in Japan) U-joints in it

Hey, thanks for that... so, I've had the shafts out and on the bench... there are no signs of ware/looseness in the U Joints or splines, so I'm thinking these are OEM in good shape.

 

[cruisermatt]

Post up some pics of your driveline, axles. etc.
Most times I've had vibrations immediately after putting a lift on a Cruiser it's a unhappy U-joint.

 

[mrldcty]

Post up some pics of your driveline, axles. etc.
Most times I've had vibrations immediately after putting a lift on a Cruiser it's a unhappy U-joint.

Gotcha... here you go:

 

[Skreddy]

I’ll take a stab at this, but could be wrong. With stock driveline and u joint setup (single u joint at each end) I *think* you want no more than 2 degrees difference between the t case plane and driveline plane. Sounds like you are at 4 degrees (4.8 up and subtract .8 down), which would likely cause vibration issues, maybe popping, etc at certain speeds. One way to tell for sure is just remove the rear driveline and drive it, but I’m pretty sure that’s unnecessary. I would think you need to shim to get a better parallel between the drive flanges, which, with a short fj40 driveline, means a very steep angle and a very short u joint life. So in short, at the end of a long post, I’d be looking at the double cardan driveline.
Also, that much higher on the front, I’d imagine you are at or just past the limit for geometry where you’d need to be looking at cut and turned knuckles, as you were thinking.

 

[mrldcty]

I’ll take a stab at this, but could be wrong. With stock driveline and u joint setup (single u joint at each end) I *think* you want no more than 2 degrees difference between the t case plane and driveline plane. Sounds like you are at 4 degrees (4.8 up and subtract .8 down), which would likely cause vibration issues, maybe popping, etc at certain speeds. One way to tell for sure is just remove the rear driveline and drive it, but I’m pretty sure that’s unnecessary. I would think you need to shim to get a better parallel between the drive flanges, which, with a short fj40 driveline, means a very steep angle and a very short u joint life. So in short, at the end of a long post, I’d be looking at the double cardan driveline.
Also, that much higher on the front, I’d imagine you are at or just past the limit for geometry where you’d need to be looking at cut and turned knuckles, as you were thinking.

Thanks for that... I'm asking vendors for quotes on cutting and rotating axles. Since I wanted lockers too... maybe I get a sort of two-fer out of that route. Plus, then I can eliminate the shims all together. Might need to get some weight on the truck to get accurate measures.

 

[Skreddy]

One other thing I’ve heard but never experienced myself, if the driveline slipping apart and getting reassembled out of phase, which could cause vibrations.
I believe you are in good hands with torfab.

 

[mrldcty]

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.

 

[mrldcty]

One other thing I’ve heard but never experienced myself, if the driveline slipping apart and getting reassembled out of phase, which could cause vibrations.
I believe you are in good hands with torfab.

Yeah... I actually did that! But caught it, took it off and re-phased it. It did eliminate some of the initial vibration, but not by much.

 

[B y r o n]

"- The TC flange is angled down at -0.8 degrees (relative to the horizontal).
- The rear pinion angle (shims in place, fat end aft) is angled up at 4.8 degrees (relative to the horizontal)."

You want the flanges parallel, or at least as close as possible, which means that if you reversed the rear shim and put the fat end forward it would be 2.8 degrees which is closer to the -0.8 degrees for the transfer case. If you had a 4 degree shim you would be even closer.

 

[mrldcty]

"- The TC flange is angled down at -0.8 degrees (relative to the horizontal).
- The rear pinion angle (shims in place, fat end aft) is angled up at 4.8 degrees (relative to the horizontal)."

You want the flanges parallel, or at least as close as possible, which means that if you reversed the rear shim and put the fat end forward it would be 2.8 degrees which is closer to the -0.8 degrees for the transfer case. If you had a 4 degree shim you would be even closer.

Well, the pinion is pointed up 6 degrees without the shim, so putting the shim in fat end forward would point it up even more, which would be going the wrong direction. The thing is, even if I could get the pinion down to 0.8 degrees (parallel with the TC flange), the shaft would be so steep that I think it would be near the limits of the U joints. My reading is that 15 degrees is really the limit without a DC setup, right?

 

[mrldcty]

Here's my little cheat sheet:

 

[cruisermatt]

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.

 

[B y r o n]

Here's my little cheat sheet: View attachment 2796774

Seeing your drawing I stand corrected with regards to the fat end being forward. I still think a 4 degree shim should replace the 2 degree shim you have in place now. Yes your driveshaft angle would be right at 14.5 degrees but under acceleration it would become less. Shims are cheaper than a new driveshaft.

 

[mrldcty]

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.

OK, makes sense... I'll get some ordered. Thanks for the input!

 

[mrldcty]

Seeing your drawing I stand corrected with regards to the fat end being forward. I still think a 4 degree shim should replace the 2 degree shim you have in place now. Yes your driveshaft angle would be right at 14.5 degrees but under acceleration it would become less. Shims are cheaper than a new driveshaft.

Good point... It would definitely be less $$ for sure, and it's relatively painless to swap them. The current center pin is barely long enough to engage the spring perch with the 2* shims, so I'm guessing I would need a longer center pin?

EDIT: Ah, I see how it works... you bolt through the shims with the center pin. Since the 2* shims are thin enough for the pin to go all the way through, bolting through them isn't needed. Looks like I might want the 6* shims though huh?

 

[-MaTTi-]

Other contributing factors and information:

- The ride height difference after the EMU was 4" up front and 6.5" out back.

You got 6.5” lift with 2.5” lift kit? Ok, sagged old springs, but still so much.

I would take a leaf or two out off the rear packs (drive is harsh now, right?), would help the driveline angle too.

 

[mrldcty]

You got 6.5” lift with 2.5” lift kit? Ok, sagged old springs, but still so much.

I would take a leaf or two out off the rear packs (drive is harsh now, right?), would help the driveline angle too.

Yeah, I was surprised at that difference in ride height too, and the ride is pretty stiff for sure. I have a good track near my house to test it on. I think you are right... either take some leaves out, or perhaps set some weight on it to simulate "final weight" and see what that does to the ride height and pinion angle. Good call.

 

[ryanwk628]

- The ride height difference after the EMU was 4" up front and 6.5" out back.

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.