1972 car - Alumnium trailing arm upgrade - roll bar pin

[Magnum]

Morning all

I'm collating parts for the aluminum banana arm upgrade and have a set from a '82 arms which will fit onto the car fine. However I can't quite get my head round this small issue:

My confusion relates to the banana arm mounted pin onto which the early style of roll bar drop link attaches - whilst the roll bar style changed (in 1978 I think), the earlier alloy arms must have had a steel pin necessary for the "old style" roll bar drop links which they used, and these couldn't have been welded to the alloy arm.

The PET documents (earlier and later) completely omits these ball pins in their diagrams and listings, so it's not possible to tell BUT surely if the pins weren't welded into these early aluminum arms (ie pre-78), they must have been screwed into an insert (for strength)? Alternatively how was it done and what is the recommended approach when tackling this now - I now it's just too simple but has somebody made a threaded ball pin that simply screws into the later arm/roll bar mounting provision/insert (if indeed the position is correct)?

I need to resolve this as clearly I don't want a completely new roll bar set up and I can't complete the upgrade until I have a solution - your thoughts/experience welcomed!

[hot66]

I've got the aluminium rear trailing arms on my car & mike Bainbridge sorted me the custom anti-roll bar fittings to allow the early bar to be used

[Magnum]

Hi & thanks James

I was afraid of that...….so that's part of the answer....they're custom! Do you recall if they mounted to the existing insert onto which the more modern roll bar drop links bolt?

Does anybody know where I could purchase these pre-made or perhaps have detailed information (photo's, drawing etc).

Many thanks

[hot66]

as usual, I've forgotton the detail, but it was Mike who had got the fittings made

[fetuhoe]

I believe that the early arm has a Ball Pin that is cast into place.

The stud would be pre-fitted into the mould and would have features to both prevent rotation and to axially lock the pin into place.

There is no real method to fusion weld Aluminium to Steel as this would cause the formation of hard intermetallic compounds that would be very brittle and render the part useless.

It may be possible to use a solid-phase technique such as friction-stir welding but this technique would not have been available in 1978.

The steel insert in the later Aluminium Arms has an M12 x 1.5 female thread.

The standard ball pin fitted to the steel arm has a measured ball diameter of 18.5mm.

It is possible to buy a ball stud with a 19mm ball diameter but this has a thread of M14 x 1.5 and I am not sure they are quite long enough.

https://www.gebert-kg.de/produkte-kugel ... hp?lang=en

If they are long enough they could work as the ball is hardened - which is essential - but I would worry about them being a slightly tight fit in the drop link due to the extra 0.5mm diameter.

The studs only cost about £4.00-5.00 each so it may be worth checking the fit in the drop link.

It wouldn't be too difficult to increase the thread size of the insert.

If you change to the later ARB it is easy to make a suitable drop link using a pair of female rod ends.

[Magnum]

Hi Chris

Thank you for your thoughts - I'm 100% aligned and it should work unless it's more complicated than meets the eye. Essentially we have a solid base (ie the threaded insert into the later alloy arm) into which to attach something that uses the existing early roll bar mechanics/drop link - if a modified/bespoke ball pin can be mounted this way then everything will work as hoped for.....

But the risk is whether the orientation/direction/position of that threaded insert in the newer alloy arm is such that the ball pin, fixed such that it's extended the same distance from arm as the earlier 69-73 steel version, is positioned to allow the earlier drop link to be used (in unmodified form). I'll certainly give this a crack and either have an existing ball pin machined to spec or commission a new "job" but it would be really helpful if somebody has an early steel example to hand and can confirm:

1) The distance the early steel armed ball pin extents beyond the arm - ie total projection to end of ball?
2) Perhaps, if both a steel and alloy version of the respective arms are compared, whether the orientation/position/direction of the threaded insert in the alloy is comparable - ie a retrofitted ball pin could be made to end up in the same psition as the steel version?

Any help welcomed!

[fetuhoe]

The angle of the thread is the same as the angle of the ball pin and I will measure the postion of the ball in a steel arm in the morning.

If you custom manufacture a ball stud you will need to harden the ball end and I would think you will need around 56 HRC as a minimum hardness.

[Magnum]

Hi All

Brief update:

I think I have this ball pin thing licked with help from some very time generous specialists (really appreciated, and in fact the whole ethos that pervades DDK specifically is amazing) so I have ordered the standard Porsche ball pin (i.e. that is welded into steel arms) and will have the end trimmed and machined with the M12 x 1.5 male thread such that I can insert into my 1982 aluminum trailing arms. Will update when I get there!

Still struggling to come to terms with:

1) CV joints/rear wheel hub/stub axle(or just the axle flange?) relationship. Bloody confusing - it appears the drive shafts and CV joints are from the later 1984 915 gearbox I have in the 1972 car but they're presently mated with the stub axle/flange in a set of steel trailing arms. Am I going to have to replace anything with the aluminum arms.....

2) Handbrake mechanism - in the countless threads I've read to try and anticipate all problems in my "Frankenstein" upgrade, there are some that suggest the handbrake mechanism (at the rear wheels) will need to be updated. I can't understand why this is only mentioned as an occasional issue, but mostly not....

Any thoughts welcome!

Cheers

[hot66]

lower shock bolt is different too from memory

I can't remember changing anything handbrake or cv wise

[Magnum]

Thanks James

yes - I think i've got that one. I've read that the 1972 cars had 14mm which matches the later cars and I've ordered the newer shock bolt, so should be fine. Well maybe, maybe not....will double check .

Thank you

[911hillclimber]

FWIW:
I swopped my steel arms to early alloy arms about 20 years ago, so memories are faded, but I swapped the inner drive flanges over to allow the use of the larger CV joints on my 73T.
iirc I also swapped over the handbrake assembly but only because my arms were missing the parts.
I doubt the design changed from steel arms to alloy for the handbrake.
To check out date changes, I often refer to pars from Type911, and all their handbrake parts listed are the same from 69 to 89 inc the cables.



Thus I have the larger flange for the CV's and the 73 T handbrake parts.
I use 73 T calipers and discs on the rear.

I have the larger lower damper bolts too which will date the arms, but I think they are late 70's arms.
The bolts are on a different angle to the steel arms and make the fitting a bit more awkward. I have to jack the alloy arm up slightly to allow the bolt to be started by hand before using the socket set.
I imagine this ensures the rubber lower bush is under minimal deflection when at stock ride height.

I think there is a weight saving of 2.5Kg each and stiffer too than the steel parts, but must be cheaper to make!

HTH
Graham.