Swap steel trailing arms with alu trailing arms?

[Zithlord]

I am considering to change the steel trailing arms on my 71S with alu ones. What year alu trailing arms will fit without any modifacations?

Thanks
John

[Neunelfer]

That's one I wish I never did.

None that I can recommend. They all will make for a very tight shock fitment on your 71 car. You'll need new shocks as well or at least re-bush the lower attachment hole. Maybe even give up on the dust cover. I've heard Bilsteins work better...???

I was considering reverting back by getting a set of steel arms and putting the reinforcements on them and calling it a day.

Everything on my car is as original RS as I can get, this is one of the only things I question doing now.

YMMV

BTW - 74 through ?? works. Mine are 74's.

[Zithlord]

Thanks Neunelfer; I have Koni reds, dust shields already gone. Why rebush lower shock, different dimensions?

Any one done this?

BTW; what is the original dimensions of the rear and front torsion bars?

John

[John Z Goriup]

John,

the quick and dirty answer to your direct question is: Yes all except 930 Turbo arms will fit your car without modifications. There is no guarantee however that they will work without modification. That may sound like a confusing contradiction, but please let me explain. 70 & 71 MY cars only had a slight change incorporated into the steel tube welded into the upper body cross-member where the shock passes through for mounting the top of the shock. These tubes are of a smaller diameter and allow only a limited deflection of the shock as it swings through its arc due to vertical rear suspension movement. The curious phenomenon is that some 70 & 71 do not exhibit any shock interference at that spot when installing alum. arms, the majority however do.

Please don't listen to the negative voices of the easily defeated. There is a relatively simple, fairly inexpensive modification you can make which will allow everything to work perfectly with the alum. arms. The modification is solely to the alum. arm and therefore will not in the slightest affect authenticity and originality of the car / chassis if you chose to revert to the original steel arms. I would strongly urge you to definitely convert to alum. arms, because they are stronger. lighter, (which of course translates to lower unsprung weight) which in turn has a beneficial effect on ride quality. What it boils down to is doing the job, BUT, doing it right.

I'm momentarily busy as Hell on something else, but promise that this evening I will post photos, a schematic sketch of just what the problem is, how to solve it and many tips to ease the job.

Film @ 11, .....as they say

JZG

[Zithlord]

John, great! How much do I owe you ;-)

John

[Neunelfer]

Please don't listen to the negative voices of the easily defeated.

Thank you John. I appreciate and deserve that I'm sure...

Maybe I just haven't been "hipped" to the fabulous conversion my new best friend John is about to about to show me, or... maybe it's just that I'm negative and easily defeated. Joy. I'll go slit my wrists now.

[304065]

http://forums.pelicanparts.com/uploa...1158449132.jpg

[John Z Goriup]

John,

You owe me nothing, my pleasure entirely. In my opinion, this kind of question and exchange is precisely what this entire website and specifically the Technical Forum is all about.......and if you think it's bad advice, well, than by all means, don't do it to your car, or get additional opinions. I spent a lot of time and money and involved a lot of very good professionals to get this car right, and I see no reason on earth why I shouldn't share my experiences with others. There's no point in anyone else growing a thick callous in the middle of their forehead if I already have one on mine from 50 + years of banging it against walls trying to do things my way instead of the way that works.

When I tore down Ruprecht because it was because it was old and worn out. Consequently, I approached the building process with several "non-negotiable" modifications firmly in mind. One of these was the installation of a complete late-model, modified Carrera suspension set-up including all the aluminum bits and bigger, better brakes.

When it was finally completed, the car drove fabulously on its first drive to Treffen '07 to San Luis Obispo and back. When I got back however & we put the car on the lift to change the oil & filter and visually inspect the underside, I spotted large patches of bare metal on both the dust shields and larger, more deeply scratched areas on the lower rear shock housing itself.....on both sides. The rack the car was on allowed the wheels to hang free in the fully lowered position, which is important, because when its sitting on the ground you can't see the bare metal since the dust cover and dirt will pretty much obscure the area affected. My car is equipped with SSI heat exchangers and there were traces of Bilstain-green paint on the edges of the seams where top & bottom halves are joined. (I don't think that area is a problem spot with stock HEs, because their flange does not seem to stick out as far). During a 2-3 week time-out I did a lot of reading, studying, looking and talking to all the Porsche gurus in the area and was given the "straight skinny" by several of them.

Amazingly, there was almost total concurrence among all I spoke to, and they all agreed that:

1. '70s and '71s have smaller diameter "shock tubes" in the steel cross member than other model year early 911s. This of course makes it critical that the shock is mounted in a way that it does not contact any portion of the cars chassis while under way. Since the shock contacting the inside of the cross-memeber and possibly the HE it's not very a hard impact you can't always hear it. I suspect many owners of these two model-year cars have the problem of shock interefernce but aren't even aware of it because they can't hear anything and unless you are looking for it, tell-tale marks just aren't that prominent. The interferrence will however produce other noises, such as droning, sqeaking and Christ only knows what. All very annoying.

2. Aluminum arms will work just fine if you modify them by machining the lower shock -mounting location to duplicate the exact dimensions of the steel arms you removed.

3. Use Bilstein shocks. There was almost unanimous agreement that they are better preforming shocks, they are of superior quality, they last forever, and Bilstein Service is very responsive and will revalve and rebuild them for reasonable prices ( if they need it )and do good work.

4. You will most likely have to remove your dust shields to eliminate interferrence, but Bistein makes various blue silicone rubber bellows for their off-road shocks which fit perfectly and protect the hard-chomed shock rod from Schmutz und Dreck. You will require 2 pcs. Bilstein Part # E4-B46-666E, BOOT, BELLOWS, BLUE Lenght = 188 millimeters. Unit Price US $ 5.36. Please see photo #2.

Note: John, I don't know if your engine is out or not, or if you're planning to take it out for other work you may be planning, but I found something I unconditionally recommend to everyone who works on their 911. WEVO, Hayden Burvill's rescuorceful little outfit, makes a part which is the kind of thing , when you first see it, you slap your forehead and say to yourself...."Why didn't I think of this !" They're special bolts WEVO manufacturs & sell for mounting the control arms ( steel or alum ) to the brackets on the torque tube. They allow removal / installation of the trailing arms without having to remove motor / transmission ass'y. and will pay for themselves the first time don't have to drop the engine. It's the kind of thing you'll be glad you did, and this job is the perfect time to do it. They're called EZ-Pins and the photo #3 is copied & pasted from WEVO's site. windrushevolutions.com

5. Now, the most important step.....machining the arms. The shock attachment point is not in the same place on the alum. arms as it is on the steel stock parts. On the alum. arms that point sticks out further and material has to be removed to restore the relationship of shock, inside tube of cross-member and any other interferring components. I notched the seam on my SSI Heat Exchangers with a Dremel -type grinder. Before I took the arms to the machine shop, I installed them without torsion bars in place. The shocks were installed less their dust shields, as was everything else, so I could place a jack under the wheel and easily and safely move it it's full vertical travel to be able to check accurately just where and at what point in the travel there was contact where there shouldn't be any. This step will allow you to determine exactly how much material has to be removed from the shock-mount area of the alum. arm to give freedom of motion. Generally about 20 millimeters ought to be just about correct. Please see Photo #4 of my stock alum. arms and photo #5 after modification.

6. You'll note a small 5mm bolt installed in the arm near the lower shock mounting. The machining process will remove the hex-portion of the steel threaded insert which receives the lower shock mounting bolt. It is not hexagonal the entire lenght inside the arm. Think of the insert as a hollow hex-head bolt, threaded on the inside. The hex portion is intended to prevent rotation of the steel insert, but as mentioned above, is removed during machining. We therefore drilled a hole about 12-13mm deep ( you don't want to drill through to the inside threaded bore) making sure it penetrated into the steel insert. That will keep the insert from turning. Don't forget to shorten the shock bolt the same amount you removed.

7. If it hasn't been done in a long time, make sure you replace the inner trailing arm rubber bushings ( or get Elephant Racing Uni-Balls, they are by far the best)

8. Wash your hands, test drive, re-check everything, open a bottle of something good, sit back and enjoy.

JZG

[John Z Goriup]

Two more quicky shots: first one is of the shocks when I first removed them from the car, one with the boots a friend of mine recommended, to check for fit, and the second shot of the finished arm and the machined surface prior to re-attaching the shock absorber.

Thanks,

JZG

[Zithlord]

Great description John, this will probably also be helpful for others in the future!

The engine is out(se the picture above). I read how you recommended the inner bolts for removal of the banana arms with the engine in the car. Another solution is to dent the torsion bar tube and revese the bolt that attach the banana arm to the torsion bar tube. Grady recommended that mod in a tread he wrote on Pelican. Yes, I have ordered new bushings for the banana arms(inner). I will keep the original spring plates, the rubber seems to be in good condition. I will update the tread when I get the parts and start the assembly. In the meantime I will make the drain hole mod in the torsion bar tube, clean and paint the inside of the tube.

John