Question on reindexing rear torsion bars.
 

Before I tear the rear suspension apart again, I though I'd see if I could get some input from those who might know. Background: '69 911, w/ 2.7L, w/ Webbers, '74 915 trans, & 24.1 mm torsion bars. According to my weight guestimate (2,150 lb. - I've deleted things) and wanting U.S. ride height, Chuck's calculator indicated 31.5 degrees - which is where I set the spring plates. After measuring the rear wheel arch (after the car sitting & being rolled around a bit), the height seems to be about two inches too high (27.25 in.) Question is, anybody have an idea as to what angle the spring plates should be to lower another two inches (24.1 torsion bars)? I'm thinking of starting at 29.5 degrees. There is a possibility I might go back to stock torsion bars. I don't remember what size they were, so any guestimates on spring plate angle if I go back to stock. I check in here periodically. Thanks a bunch.

How old are the shocks? That can have a big impact on the spring plate height. If they are old, replace them and then set ride height from there. It's going to take some trial and error in my experience.

Also might be a good time to replace the sway bar bushings and the spring plate bushings if you haven't already.

Shocks aren't new but not old by any means - Bilstein HD. The sway bar bushings are new & the car has new Polybronze spring plate bushings. I've been through the reindexing procedure a few times, so I know what I'm (unfortunately) getting into.

What you might do is measure your spring plate angle now, then have someone push down on the rear. If you can get to the ride height you want, measure again and note the difference. That will be the difference you need. Actually, it will be slightly more due to the effect of the angle on the torsion arm but will get you close.

Or if you can't get it that low, use a jack to lift one of the wheels to the position you want.

You might want to consider replacing your torsion arms with adjustable ones, that way you will only have to do this one more time.

Gordon, great suggestion. I have the later adjustable spring plates & '74 aluminum trailing arms on the car now. I centered them the last time I put the suspension back together. I tried pushing down on the rear to try to settle the suspension more, but it doesn't move much. I will give the jack trick a try.

Who knows, the only way to get it right is to redo it 2-4x.

DId you all find it easy to pull the torsion bars out of the hole to re-index them?

No. I fabricated a tool to pull them that attaches to a slide hammer.

Yep. That's pretty much SOP.

You might want to measure your torsion bars to confirm they are actually 24mm. Could make a difference if they are something else.

Don't forget to grease the splines to prevent future rusting. Good luck.

here was my thought process:

http://forums.pelicanparts.com/porsche-911-technical-forum/981374-1985-rear-height-adjustment-math.html

given that you are just adjusting rear height, this might work for you.

Good discussion on that link. I'm wondering if I can measure or do a search on the length from the torsion bar center and the wheel hub center on my '69. I can just do the math to see how many splines I need to do the adjustment. I need two inches. The problem is trying to not pull the bars out too far to count your splines. Mine shouldn't be stuck since I've had them in & out a couple of times & greased them well when reinstalling them.

From my experience you are making this more complicated than it needs to be.
If you are just adjusting height without changing torsion bar size, this should be easy.

First measure ride height on each side and subtract the target height. I think you are looking at lowering approx 2 inches. ( I'm assuming you already have your adjustable arms set to mid-position)

You need to calculate the exact height difference on each side for your situation.

Now put the car on stands and disconnect everything from the trailing arms and let them droop all the way down so that they are fully unloaded.
Measure the angle of the trailing arm on each side with an accurate protractor...I would use a smartphone with the appropriate app.

Convert the height change to a trailing arm angle change. A change of 0 deg 50 seconds equals a change in ride height of 6.6 mm (0.256 in) so in your case this is about 6.5 degrees

So now you have the new target angle for each side. In your case since you are lowering the car the angle will be about 6.5 degrees less.

Now pull the trailing arm and torsion bar out and find the combination of inner and outer splines that gets you closest to your target angle. The smallest change you can make is 50' (moving one spline on the inner and back in the other direction one spline on the outer)

Do this for both sides, since your target for each side may be slightly different.

re-assemble everything and you're done. Now you can fine-tune the corner balance with the adjustable arms.

If you are changing torsion bar sizes it only takes one extra step.

install the new bars and bolt everything up just enough to lower the car onto the wheels. You don't even need shocks at this point since they are typically not carrying any load,except for gas shocks.

Bounce the car a couple of times and make a ride height measurement on each side. Calculate the height difference you need and convert this to a trailing arm angle difference and then proceed with the same procedure as previously described above.

Brian, your method is right, but I think you are forgetting the trailing arm bolts onto the end of the spring plate, and the spring plate is where the angle has to be measured. The trailing arm extends the radius from the spring plate to the hub, which figures into the angle you need.

Of course you are correct the spring plate angle is what you must measure. I was careless about the terms I used.

I was able to adjust mine without doing any math, other than measuring the angle and adding a few degrees.

Yeah, I might end up doing it that way. I have the idea in mind of changing from 31.5 to 28.5 degrees for my first shot. Seems like the more rigorous you are with your calculations, the more time it takes. I went with Chuck's angle calculator & ended up way off.

Trial and error with some educated guesses (even with the math). An adjustable spring plate makes it a little easier and allows for corner balancing.

Chris
73 E