Turbo arms in an SWB

[tadd]

Ok, I've searched and not found anything on the geometry, lots on the CVs and flanges though, so that should be sorted.

As per the title, I'd like to use turbo arms on a 67 to get both lighter weight and the double bearing in the drive hub/flange assembly. I've got some cool parts I'd like to use SWB style .

Bruce Anderson says the aluminum turbo arms are shorter than the LWB arms by 1.9". The SWB is shorter than the LWB bananas by 2.2", IIRC. This begs the question, would a 15" wheel offset by 7.5 mm be obviously, sore thumb not centered?

Now for the tricky part. BA also says the turbo arm is mounted 0.6" more center than the LWB. Are the LWB and SWB mounts located the same on the tube? All I found was this pic. It looks like the SWB are inboard the LWB points by a bit (maybe 0.6"?).

SWB vs LWB: Differences?

So don't know is would the mono ball just run 'crooked' since the spring plate would force it square? It does only rotate in one dimension thanks to the spring plate.

Thoughts on this would be greatly appreciated.

tadd

[Flieger]

You should contact chris_seven, he has made LWB into SWB and done RSR clones and knows about the geometry changes between SWB,LWB,RSR,930

The RSR and 930 arms have very similar trailing arm geometry, but have wider tracks than the SWB.

The RSR mounts are extended rearwards and outwards, with the 930s additionaly raised. That is an error in Brce's book. If you see how the extensions are done, just lengthening the stock flange, then the stock angle means that moving back means moving out as well.

[Steve@Rennsport]

Tadd,

930 trailing arms will not work on an SWB without the 930 torsion bar tube (or fabricating custom brackets). Further, you will need to widen the bodywork to fit as they are a lot wider than 911 ones.

[tadd]

Steve:
If I use the turbo torsion tube then that will push the mount points back towards the rear of the car, which I don't want. Wider I can work with as an increase in track would be a net improvement. IIRC it's an additional inch to the outside.

I'm assuming your saying the 'curvature' of the banana won't work with the SWB mount points (since the banana length is close), what about adjusting the 'attack' angle of the banana rather than the chassis? The arms are solid right? So one could think about cutting off the tubular end and drill/tap at the 'new' correct angle for a heim joint?

Flieger:
Thanks for the lead.

Keep em coming!

tadd

[gestalt1]

I wondered about his also. The swb arms do have 2 bearings and they are spaced wider from each other than on the lwb arms. The bearing life is better on the swb arms. I think the only advantage would be weight and brake mounting options with the aluminum turbo arms.

[tadd]

Gestalt:
Do you know the spacing? Could I get lucky and they be close??! I didn't realize the SWB arm bearing spacing was wider than the LWB.

Thanks for the tip!

tadd

[Steve@Rennsport]

Originally Posted by tadd

I'm assuming your saying the 'curvature' of the banana won't work with the SWB mount points (since the banana length is close), what about adjusting the 'attack' angle of the banana rather than the chassis? The arms are solid right? So one could think about cutting off the tubular end and drill/tap at the 'new' correct angle for a heim joint?

Tadd,

Yessir, thats correct. The arms are tubular castings but I would not be comfortable with welded ones.

[tadd]

Steve:
No welding. I'd be leery of that myself, although with the right preheat, welder, post heat I should think it could be done. Folks fix vintage aircraft parts that way and bad things happen if not fixed right . Not cheap for that kind of expertiese though.

I just am thinking about a simple drill and tap, just like is done for the 935 spring plates.

I'd really think the bananas should be solid aluminum for several inches from the torsion tube attachment point. Hack that off, maybe a hair more like 0.5" worth to get the length just right with a lock nut, then drill into the length, tap, and screw in a heim joint. With enough engagement thread length loading would be low on a 0.5" diameter. Could consider an insert if need be... Hmmmmm.

BTW, thanks for the clock cycles!

tadd

[tadd]

Steve:
I think ive just realized where my disconnect is... If stock, two part bushings are used, the banana curvature has to be exact to line up since the bushing is 'wide' and locks down the location. The spring plate is also a 'zero tolerance' locator. Between the two, the banana has to 'fit' perfect.

I put elephant mono ball cartridges in when I got the car so I've been thinking about the banana having some angular play in the plane of the floor of car... you wouldn't have that with the standard bushings. The desert had so deterated the bushings there was lots of play anyway!

I guess what I need to do is to get a turbo arm and remove one from the car and measure the included angle between the spring plate mounting point and where the bushing resides. If that angle is less than the angular misalignment tolerated by the mono ball, it should be a 'bolt on'. If not, then other, more drastic measures are required .

tadd

[tadd]

Ok, for those interested...

I now have one set of each type in my hot hands. One pair of turbos (930.331.511.05) and one pair of SWB (901.331.511.02).

Clamping each to the flat surface of a table by the spring plate mount I then used a bubble protractor on the torsen tube end to measure the angle of the arm by setting the bubble to 'level' (parallel to the table/spring plate). Early are 20.5*, turbo are 18*.

Two point five degrees is well within the angular mis-alignment of the elephant mono balls.... So that's one down. I'll get a pic up tomorrow since all I have at home is the iPad and I haven't figured how to post pics from that yet.

If I align the two sets of vertical spring plate bolts to match by 'stacking' the arms atop each other, the turbo arms are a bit longer. Eyeball says 0.5". I'll measure at work tomorrow. So best case looks like I'll need to either re-drill or slot the plate holes.

Finally the turbo arms run the outer bearing a bit wider. Again, the calibrated eyeball says 0.75".

Well see what I can do on the mill to get some measurements .

The only other thing I can say is that the turbo arms are massive !

tadd

[tadd]

Its been a bit for this, but somehow life gets in the way...

Regardless there is progress!

So instead of having to worry about getting good bearing alignment after welding on a new 'ear' to hold the larger outside 930 bearing, I decided to just lathe out a cassette to hold the bearings then go into each bananna and use a hole saw to slice off the old bearing bits. Picking the hole saw the same size as the cassette OD leaves a pocket for alignment. Doubly so given the brake caliper ear.

So here is the cassettes. Turned from solid 1018... big rocks to small rocks . I took the dimentions from the 930 arms.



Then I went at the bananas with a hole saw. 60 rpm, 1.5 hundreths per revolution on the bridgeport... worked pretty nice.


Added in just the bearing seals to 'hold' the hub and it will sit in the banana without touching... fit is pretty nice.

[tadd]

Here are a few with the hub 'installed'...




Now I just need to get them welded... and lots of questions for the pelican mind!

Anyone suggest a really good welder in the maryland/virginia area? Anyone want to take a stab at what the bananas are made of? How about a welding rod suggestion?

Im tempted to have them x-rayed after welding... is that just paranoid? They are thick pieces...


Thanks in advance,
tadd

[tadd]

PS... anybody know what the very top most hole in the spring plate mount is for? I think I may need to loose it... this guy here...



The flanges are two different sizes with the 930 a good bit larger. My options seem to be:
1. if the 930 CV is the same spline as the SWB drive shaft, just swap and trim the offending metal.
2. Make a 0.25" thick adapter disk to sandwich between the 930 and SWB CV. This would seem to be a good solution. I can push the bearing cassette out board a quarter inch to account unless there is more than enough play in the shaft for wheel movement since it has 'plunge' designed in.

Thoughts would be most appreciated for this adventure!

An SWB on centerlocks will be cool!

tadd