Need some help with new spring plate angles

[tirwin]

I am going to 28mm rear torsion bars from stock on my '83. I'd like to know what my target number should be. I want the car to be at or maybe a little below Euro ride height.

I used Will Ferch's method to calculate the target spring plate angle.

Assumptions:
Weight of car:2740 lbs for '83 SC (is this high?)
60/40 rear/front weight distribution
28mm torsion bars

Results (assumes US ride height):
Static ride height droop: 3.80"
Weight-induced droop: 3.72"
Total droop: 7.52"
New spring plate angle: 23-24°

My understanding is that the ride height changes by .9" for every 3.2° of preload.

How much lower is Euro from US?

Anybody care to share the number they used with a similar setup? Am I in the right ballpark?

[tirwin]

Ok I found a suggestion for 14.85° but it doesn't say what ride height. The ride height difference between 23° and 14.85° works out to be ~2.25" (lower). Does that sound like the difference in US and Euro ride height?

[douglas bray]

Trial and error is your best calculation.

Did you note the spring plate angle before you took everything apart? Start one or two clicks higher and keep track. By some miricle I got mine right on the first attempt.

You can use blocks of wood under the bananas to approximate wheel height so you don't have to fight that. Calculators are nice, but dont account for stiction and component wear.

Slap 'em in and go. May take once, twice, maybe even three times.

[tirwin]

Thanks for the help.

Yes I have the before numbers.

I have the engine out for oil leak fixes and I'm redoing the rear suspension at the same time. I was just looking for something that got me in the right ballpark.

Last night I found a thread that says the Bruce Anderson recommended ride height of 25" from the ground to the wheel arch is lower than what Porsche considered Euro ride height. Interestingly enough the 14.85° number I found works out to be 2.25" lower than the 23° angle I got using the Wil Ferch method (which assumes US ride height). The measured difference between US ride height and Bruce Anderson's number? 2.25" That is either pure freaking luck or the right answer.

I'll post a link to the thread when I get back to my computer.

[BK911]

The Ferch method will get you close, but you may still need to make adjustments.
I use the BK method:
1. Measure the rear ride height.
2. Jack up back of car and put on jack stands.
3. Remove wheels and detach spring plate from banana.
4. Measure distance from end of spring plate to ground.
Every 1" the end of the spring plate moves, the ride height changes ~2".
5. Set spring plates to middle of its adjustment.
6. Pull the spring plate to reindex and reinstall.
Repeat 4 and 6 until you get the measurement you want.
7. Put it all back together.
8. Measure ride height.
9. Adjust spring plates as needed.
10. Vroom vroom

[javadog]

The US rear ride height varied somewhat. Some years, it was the same as ROW, some it was a little higher and for a brief period it was about an inch higher. For an inch higher, the spring plate angle went from 38 degrees to 42. The Porsche spec is 8-9mm per degree, although that obviously varies a little bit, depending on the starting point.

JR

[toddu]

Fwiw with 29 rear bars I was at 19.8 degrees. I'm a bit lower than euro, couple hundred lbs lighter than you.

Just for reference. I used the calculator and it was pretty accurate.
Todd

[tshebib]

I just checked and the calculator is back online

911 Spring Plate Angle Calculator

Too easy

[douglas bray]

Originally Posted by tshebib

I just checked and the calculator is back online

911 Spring Plate Angle Calculator

Too easy

Too funny. Thanks for the link to rebuilding and installing 930 calipers. Glad to see where all the fuss was coming from.

[tirwin]

Originally Posted by toddu

Fwiw with 29 rear bars I was at 19.8 degrees. I'm a bit lower than euro, couple hundred lbs lighter than you.

Just for reference. I used the calculator and it was pretty accurate.
Todd

Todd,

Good to know. Thanks!

Now that I look at the numbers again, 2700 lbs seems high. I have another reference that says a stock SC was ~2500 lbs range. I've done some things to lighten the car so maybe 2700 lbs is a little high.

The chart that I got the 14.85º number from for 28mm bars says 12.91º for 29mm bars. Hmmm.

I also found a note that says the chart is for "stock" ride height. Not very specific as to what "stock" means. But here are some examples for comparison.

911, 1968 & up
22mm = 38.97º
24mm = 27.52º
28mm = 14.85º
29mm = 12.91º
31mm = 9.88º

These numbers seem way off from the Will Ferch numbers.

I used the calculator link and it let's you select the ride height. Cool! I put in 2500 lbs in the calculator and the result is 22-23º. I played with the calculator and it looks like for every 250 lbs of weight increase the target spring plate angle increases by 1º. (I did this assuming 28mm torsion bars so maybe a different spring rate would generate different results.)

I think I'll give that a try as a starting point and see where I end up.

Thanks everyone!

[tirwin]

As another test, I used the calculator with my previous spring plate angle. I measured an angle of 22º with the car on a 10º incline on the jack stands. So that is 32º. Stock t-bars are 24.1mm and I was at, or maybe a little below, Euro ride height. Assuming 2500 lbs, the calculator says 32-33º.

Sure seems like the calculator is pretty close.

[RD911T]

I used the Will Ferch calculator on my 71 that I put 26mm rear bars on and it came out pretty close. I think I used 2200lbs. I bought it as a roller with no motor, so had no reliable starting point.

[brianlay]

Here's a method that worked for me years ago.
Set the spring plate adjustment to the middle of the range.
Make your best guess on a starting angle, then bolt the wheels on and let the car sit on its wheels. Don't really need torsion bar covers or shocks at this point.
Bounce the suspension a couple of times and measure ride height at the fender lips.
Calculate how much up or down it needs to go and then you can easily calculate how much the spring plate angle must be changed.
Jack it up and re-index the bars to get the target spring plate angle.
I can almost guarantee you'll only need to do this once to get it just where you want it.

[tirwin]

Originally Posted by brianlay

Here's a method that worked for me years ago.
Set the spring plate adjustment to the middle of the range.
Make your best guess on a starting angle, then bolt the wheels on and let the car sit on its wheels. Don't really need torsion bar covers or shocks at this point.
Bounce the suspension a couple of times and measure ride height at the fender lips.
Calculate how much up or down it needs to go and then you can easily calculate how much the spring plate angle must be changed.
Jack it up and re-index the bars to get the target spring plate angle.
I can almost guarantee you'll only need to do this once to get it just where you want it.

Does that assume the engine is in the car too? The back end is going to higher until the weight of the engine is in the car.

[brianlay]

Yes, assumes the engine is in the car.
No point in doing this until you get the car to final weight.

[tirwin]

Ok, thanks.

I'm waiting on some other stuff so I can button up the engine. I was trying to get the rear suspension done while I'm waiting.