Trailing Arm Bushings: Indexing Needed?

[Smoove1010]

I've read many posts on the topic of trailing arm bushings, most having to do with whether or not to ever change them. I decided that after 25 years, those bushings earned retirement, so I've got the old ones out (they weren't terrible, but they weren't good either...) and I'm mentally going through the process of installing the new ones in my '87 3.2 Cabriolet.

I'm replacing them with the neatrix rubber bushings from our host: PEL-915871; I'm looking for a stock ride. If I understand how these bushings are supposed to work they are held in place by the "ears" in the bottom of the tub, the nut and bolt are torqued with sufficient force to keep the outer flanges from moving, and as the arm pivots up and down, the rubber deforms as opposed to the bushing moving in the arm like a bearing.

It occurs to me that, after pressing the bushings in, if I bolt the arms in place with the car jacked up and the suspension in droop-mode, the bushings will deform to an extreme when I put the car's weight back on it's wheels. I did more searching and found this 11 year-old post to verify my concern: Rear trailing arm bolt torque?

So what's the best way to approximate the angle of the trailing arms when the car is back on its wheels at rest on level ground? If I had a pit or lift I'd leave the bolts loose until reassembly was otherwise done then get underneath to tighten the bolts, but that's not an option.

Any thoughts or suggestions would be most welcome.

Thanks in advance,
GK

[RD911T]

Interesting, I didn't set my car on the suspension before I torqued these bolts that go through the bushing. I thought they had a metal sleve through the bushing to allow it to do its job but still rotate. I'm look forward to what more experienced people have to say.

[Smoove1010]

Originally Posted by RD911T

Interesting, I didn't set my car on the suspension before I torqued these bolts that go through the bushing. I thought they had a metal sleve through the bushing to allow it to do its job but still rotate. I'm look forward to what more experienced people have to say.

I'm probably over-thinking it (but I'm probably not the first Pelicanite to indulge in a little over-thinking...)

Clearly the bushing isn't intended to rotate inside the trailing arm, and the inner and outer shells of the bushing aren't supposed to rotate against the rubber that's sandwiched between them. I did note that the metal outer shell of the bushing is smooth - there are no knurls or ridges to hold it stationary against the chassis mounting tabs. The only thing preventing rotation against those tabs is the degree to which the bushing is squeezed by the trailing arm bolt pulling those mounting tabs in. I also noticed that, although the bolts were fully torqued (and a chore to break loose) the trailing arm moved fairly freely up and down before I disconnected it from the car. I attributed that to the rubber shrinking, thereby decreasing the friction between the chassis mounting tabs and the bushing outer shells.

Bottom line, I'm thinking I want to get the initial arm mounting position somewhat close to where it will be when the car is at rest. When driving, if the bushing is pushed to deform past it's ability to do so, the smooth outer shell of the bushing will allow enough slippage/rotation to compensate, and will probably settle into a middle position.

Thoughts from some learned suspension gurus would be most welcome!

[Jdub]

Sure, this makes sense - I went through this when I R&R'd my suspension. The 911 does seem to have the "captive" bushing whose job it is to deform between the anchored internal/external surface faces, presumably glued or otherwise set to their respective surfaces.

Very interested in what other's have to say on this.

[E Sully]

No expert, but from my view once you unbolt the trailing arm from the spring plate you would need to have the allignment re-set. The ride height may change also, but since the spring plates are not being removed it should be able to be done with the adjustment on the arms, no need to re-index the torsion bars as would be necessary if those bushings had been changed.
You should be able to get the settings close, here are a few threads.
Alignment Pain and Suffering
DIY Alignment/Corner Balance ?
Question about DIY alignment & corner balancing

[Smoove1010]

Thanks Ed. I should've clarified that this is part of a rear suspension refresh project, I've got the whole rear suspension out and I'm redoing with new stock shocks, ER spring plate rubber bushings, neatrix trailing arm bushings, sway bar bushings and Lemforder sway bar drop links. The TB's are in excellent condition and will be re-used. I'm hopeful that I can put the TB's and spring plates back the way they were but I know the geometry will change just because of the new bushings. I expect to keep the ride height where it was. I'll do a string-job alignment to hold me over until I bring it to a pro.

My question was specifically about the trailing arm bushings, and whether they would be harmed if I bolted the trailing arms in and fully torqued the bolts with the arms in the "droop position." Assuming that it's best to torque these bolts with the arms in the "settled" position, I just need to figure out what the approximate angle of the arms/spring plates is when the car is on its feet.

GK

[Jeff NJ]

So I am actually doing pretty much what you are with the rear suspension as we speak. I have a 930, so the parts are a little different, but the theory is the same. I pulled my motor this winter, so I figured now was the time to do the suspension refresh. I left the nut loose for the trailing arm bushings and will jack up the rear of the trailing arm to the height that matches what it will be at when the car is on the ground. Easy to do with the wheel on and the shock disconnected. I am sure that it just needs to be close. An inch either way in height at the wheel is a very small degree of twist in the bushing itself. I would make sure the shock is back on before letting it fall too far once tight, otherwise, you are just doing the same thing to the bushings, but in the reverse direction. I am not sure if this really is an issue, but since it is fairly easy to avoid, no sense in not doing it, IMO.

BTW - My Porsche factory manual states that they increased the torque specs for my car on this nut. I believe it was 43 ft/lbs and it was increased to I think either 80 or 90 ft/lbs. I cant remember now and the manual is at home. This is specifically for the turbo, so please check your manual for your specific car as they may well be different since the trailing arms are different.

[Smoove1010]

Thanks Jeff. As I was over-thinking this today I decided how I'd go about this:

I took the "crude" ride-height measurements (bottom of the fender lips) before starting this project. I'll subtract the radius of the rear wheel from that, subtract another inch for tire squash, and that will give me the rough distance from the wheel center to the fender lip. I'll jack up the trailing arm with the tire mounted as you suggest until the wheel center is at that distance from the lip, then torque the bolt down.

As far as the torque goes, Bentley says it's 72 ft lbs for the NA Carrera:



It would be a whole lot easier to apply that torque with the pesky engine out of the way - I envy you!

Thanks for the input,
GK

[moneymanager]

IIRC correctly (always suspect), the rear wheel travel is something under 6" total. That would convert to a negligible amount of twisting at the trailing arm bearing... probably not worth worrying about.

[gtc]

You guys are over thinking this.

[moneymanager]

It seems two of you are doing this as we speak. It sounds to me from your descriptions that it would actually be possible to pull the trailing arm without removing the spring plate from the car and disturbing the ride height/corner balance settings. What do you think?

[Smoove1010]

Originally Posted by moneymanager

It seems two of you are doing this as we speak. It sounds to me from your descriptions that it would actually be possible to pull the trailing arm without removing the spring plate from the car and disturbing the ride height/corner balance settings. What do you think?

It would indeed be possible to remove the arm and leave the spring plate in place, but the eccentric bolt that holds the plate to the arm has be be removed and disturbed. I think you could preserve ball-park alignment settings though. I'm refreshing all the bushings anyway. Everything is already apart and waiting to be cleaned up, painted, and re-bushed.

...and gtc is probably right. Once I'm done over-thinking I'll get started on second-guessing.

GK

[Jeff NJ]

Originally Posted by moneymanager

It seems two of you are doing this as we speak. It sounds to me from your descriptions that it would actually be possible to pull the trailing arm without removing the spring plate from the car and disturbing the ride height/corner balance settings. What do you think?

I was able to pull the trailing arm without removing the spring plate from the torsion bars. The alignment is toast once you do this, and so are the height and CB settings. I marked the position of the 4 bolts that come out of the trailing arm and connect to the spring plate, so I was able to get it back close to what it was and driveable to get to the alignment shop when I'm all done. At least you probably wouldn't have to reindex the TB's unless the bushing replacement changes the ride height so much that you cant adjust it back on the spring plate height adjusters.
I cant imagine doing what I did with the motor in, but who knows...