Angle of the trailing arm dangle

[caproader]

Guys, I am installing alloy swing arms from a 75 911 on my 1973 S (with sporto and a/c). I am also replacing the rear bushings with Neatrix.

The 75 section of the manual (source of the alloy 75 swing arms) says that the angle of the trailing arm at reassembly is 42 degrees. The 1973 section of the manual says that the trailing arm angle is 37 degrees. So which figure do I use when I put the rear suspension back together? I suspect that the 73 body/torsion bar combo overrules the 75 swing arms and that I would use the 37 degree angle. However, prior to proceeding I though that I might ask for help.

New question: the 75 section of the manual says to add half a degree for sporto and half a degree for a/c. Assuming that I use the 73 firgure of 37 degrees, do I add a degree for the sporto and a/c?

As always, thanks for the help. I have been out in the garage working on the car and so have not spent much time listening to all the good advice. But I do appreciate the help. Thanks

Caproader (The Capitalist Roader)

[REGFOUNDERS]

You don't say why you are using the 75 arms, but assuming it is performance-oriented, you are probably lowering the car as well. The free hanging trailing arm angle basically determines the ride height - more angle=higher ride ht. Since the 75 cars probably were spec'd with a higher ride ht due to bumper laws, my guess is that's why the 42 degree spec is used.

The best thing to do is measure the angle you have now before you pull the spring plates off the torsion bars. Start there to reassemble. If you want to lower the car say half an inch take about 1 degree of angle off (as I recall). Be careful with two points: since rotating the torsion bar one way and moving the spring plate the other way usually is referred to as one degree change, the change may be a bit more or less than that. You don't want to end up with less, because as the car is driven and settles, it will go lower anyway and you could end up frying the paint on the fender with the outer edge of the tire.

Of course there are a lot of variables here: Neatrix bushings are a bit harder than stock rubber so may let the torsion bar and spring plate "rotate" easier as it all settles, giving a lower ride ht. Don'y know that for sure, but I do know that replacing hard plastic bushings with stock rubber and the same angle gives a higher ride ht, so...

Also depends on how carefully you measure the angles, where you set camber and toe before bolting it up solid, tire size, and on and on. This is not rocket science but sometimes it is easy to overlook all the relationships of movement. If someone experienced was looking over your shoulder the first time, it would be much easier for you the second time. And there will be a second and probably a third time. There is the matter of the front bars being equally set so that the rear settles evenly side to side. And after you get the rear aligned properly the ride ht may be different than you want. And if all that is OK you still want to balance the corner wts with the driver in the car and that could change the the ride ht and alignment again, so you get to start over. You'll get it eventually, just don't think it's something you'll do on a Saturday afternoon and go win at an autocross the next day. Possible I guess, but unlikely.

BTW one of the earliest issues of The Esses had an article describing the whole process. Good luck.

Vern

[REGFOUNDERS]

Just read my previous post and need to clear up a few things. One degree change in angle is approx 1/4 inch, not half an inch as I said. Actually the smallest change you can make is 50 minutes and that works out to about 0.3 inches. So two notches of bar and plate rotation is not 1 degree and half an inch in ride height, but 1 degree 40 minutes and approx 0.6 inches.

So be careful if you were trying to go from say 37 to 35 degrees to lower the car half an inch. One notch will still read more than 36 degrees (actually 36 d, 20 m), add another notch and it will read 35 d 30 m. Thinking you haven't gone far enough you add another notch til it reads under 35 (34 d 20 m). Now you drive the car and find that it's 0.9 inches lower and if you've been very lucky there is no burn mark on your fenders! Been there, done that!

What I meant by the change possibly being more or less than you calculated, is that you don't know how much the car will settle. You need to drive it more than around the block, say like 20 miles over some uneven roads. Hope that is more clear.

Vern