Help! Rear Ride Height

[wcc]

I'm in the process of lower my '87 930. I got it all apart and recorded the angle of the spring plate after it was removed from the trailing arm of 29 degrees. I went to Thoms angle calculator here:

http://rennlight.com/cgi-bin/spring.cgi

I got 26mm for my torsion bar from:

http://www.pelicanparts.com/techarticles/911_torsion_bars/911_torsion_bars.htm

Typed in my GVWR from the DS door jam of 3637lbs with a 26mm torsion bar at Euro ride height, normal dist. and it gave me a angle of 34-35 degrees. That isn't right! So my question is what am I doing wrong?

If I use just the rear weight of 2315 it gives me a angle of 24-25 degrees which seems closer but is it right?

[mark '87 930]

Bill,

I have had my '87 930 for 7 months now, and its been in storage for 6 of those (winter up here) months. In that time, I had LOTS of opportunity to do some stuff to the car, and one of my projects was to lower it. The rear is in fact a little more complicated, but I did it with a protractor I bought at the dollar store!

The adjustment bolt wasn't giving me enough, so I had to re-index the torsion bars. The process is the same as replacing the Torsion bars, except that I never pulled the torsion bar out to replace it. I simply drew on the car's wheel well where the spring plate rested (at angle with no tension on it). I pulled out the spring plate (from the Torsion bar) and re-inserted it on the very next notch on the Torsion bar. Once reassembled, I had to fiddle around a little with the adjustment on the spring plate (the first two LARGE bolts closet to the Torsion bar) to make sure both sides were even. I went for a spin (just recently) to make sure that he heights were as close as possible to each other (after the suspension has settled), and voila..

My wife thought I was nuts, as I was taking the entire rear suspension apart, minus the wheel hub.. and this is on our 'new' Porsche! I've been doing home mechanics for over 15 years (mainly on motorcycles), and like any other mechanical project, it's not rocket science. Take your time, and learn from you mistakes.

All that to say, I'm new, I try it out, even if it means having to redo your work. Take your time and measure both sides well. Use the adjusting bolts for final measurements.

YOu can do it. Good luck. It's a lot of fun and of course gratifying.

[wcc]

I'm not going to dig up a thread that I may or may not have read this in. But it seems like I read somewhere that 1 degree was equal to one inch. Well I just re-indexed the PS and got it lower by one degree and I put the height adjustment bolt in the center. So I think, the PS is done now. Time to re-assemble and measure. Thanks for the response though Mark....

BTW: Is wheel bearing grease sufficent for lubing the splines on the torsion bar?

I'm sure there'll be many more posts to follow...

[Chuck Moreland]

Your car doesn't weigh 3637. That is probably the max weight of the car, fully laiden with fuel, passengers and luggage.

The calculator is setup for the vehicle weight. You are probably closer to 2900-3000.

FYI your car had 26mm bars from the factory.

[ZOA NOM]

Don't use the GVWR from the door jamb, it's wayy heavy. If I recall, I used an angle of 28 degrees for my ride height, and I'm just a hair above Euro. I used trial and error (I know, long way), by dropping the car each time and settling the rear, then measuring the fender height. I put two thin squares of sheet metal under each tire patch, with grease between them so I could bounce the rear to settle it. Worked like a charm. I re-indexed three times, I think.

[javadog]

Bill,

When I corner balanced my '86, with a full tank of fuel, it weighed 3076. That's what you should use in the calculator. It has a provision for front to rear weight distribution; for a late turbo I'd use 39%/61%.

Having said that, I tried it using those numbers and it spit out a new angle of 30-31. Since you started at 29, the only conclusion I can draw is that you don't want to use the calculator. Either that, or your car is not truly horizontal when you measure it, or your protractor has some error.

The difference a one degree change in the torsion bar angle makes in the ride height is closer to 1/3 inch. Stock spring plate inclination on your car should be 33 degrees.

Will Ferch wrote an article about all this here:

http://tech.rennlist.com/911/pdf/settings.pdf

Hope this helps,

JR

[wcc]

Well I'm done for tonight......

When the spring plate rested it was actually closer to 28.5 degrees according to the HF magnetic degree finder. On the PS I set it to 27.5 deg with the height adjustment bolt re-set to the center. I already knew that my torsion bars are 26mm otherwise the calculator wouldn't have been anywhere close if I guessed. I'll try the 3076 for the weight and see what it says.

OK, the car isn't horizontal AT ALL. The front would have to be up on jackstands to make it horizontal. I'll have to jack it up and put jackstands under it and see what the degree is then.

Where/how do you measure to see if you car is horizontal?

Or I'll have to figure out the angle of the car and take that into account but where do you measure that? The roof? The pan? Where?

[javadog]

The procedure assumes you have the car on a lift and the lift is level. Use the door sill to measure the angle of the car. It's close enough.

JR

[wcc]

I'll take into account the angle of the door sill and apply it to the angle I'm reading. That should make things a lot closer, I hope.

So, is Wheel Bearing Grease ok to use on the splines of the torsion bars? I would think so, but wanted to double check......

[javadog]

The main objective in greasing the splines is to keep them from rusting. I'd think wheel bearing grease would do just fine. There's a waterproof version of wheel bearing grease used on motorcycles and ATVs, if you want. Some people recommend a lithium grease.

JR

[wcc]

Alright javadog! I guess I'm not done for tonight... I'm addicted to these things so I'm going to go out and measure the angle and lube up the splines to the torsion bar with grease. What the heck it's only a little after 10 and I can sleep in to 5am. I won't feel this tomorrow, at least that's what I keep telling myself.

[javadog]

Attaboy!

I'm goin' to bed.

JR

[Wil Ferch]

1.) don't use Gross vehicle weight.. by *definition* ...that is the weight of the car with max number of passengers or load...use "real" empty-weight of car. An 87 930 should weigh about 2900-3000 lbs. If you need to....look up old Road and Track or Car and Driver test reports. They weigh the cars. My stock 85 Carrera was right on the money at 2760 lbs.

2.) Door sill is the accepted place to see if the car is relatively level. Adjust your angle numbers accordingly.

- Wil

[wcc]

I took the angle measurement on the PS door sill at 3 deg

Spring plate at rest was 28.5

that brings it to 31.5 deg.

Using 26mm, and 2950lbs with normal weight dist.
EURO ride height is 29-30deg
So I'm going to aim for 29.5deg

31.5-29.5 is 2deg. So that gives me a new angle of 26.5 deg for the PS Spring plate.

I just want to make sure I did this right before I put it all back together.

[Wil Ferch]

We ( all ) must make an effort to speak with *clarity* ...(ha !...I should talk !)...

Anyway...you say " I took the angle measurement on the PS door sill at 3 deg.." OK....which way? 3 degrees down-toward-front? 3 degrees down-toward-rear ??? See my point ?

I presume you mean 3 degrees downward-toward-front...based on your next few sentences. ( "That brings us to 31.5 degrees")... Are you saying that the measuement would have been 31.5 degrees *if* your car was level? That is what I translate this to mean, *assuming* a nose-down slope of 3 degrees.

Using 26 mm ( did you put a micrometer on the bar to *check* ???...maybe there is a non-standard part in there? Don't EVER assume anything on these cars)...and..."normal" weight distribution" ( again...what does this mean?...did you ever get your car on scales? The angle calculator works with "good" info...but can't perform magic. Normal Turbo's are closer to 38/62.....non turbos are generally 40/60)....you then say " Euro ride height is 29-30 deg.

OK...so you're taking a middle of the road apparoach and will try for 29.5. Fine. Are you then saying your actual "target" is 26.5 because your car is *still* canted 3 degrees forward/down, and you never levelled it since you found this amount off? That seems correct.

I would double check bar size and re-run the numbers for a 62% rear weight bias...to get a "fine-tune" on this. I see you put the spring plate in the "middle" of its adjutment range/slot before going further. This is a good idea as it gives you a final +/- 1/2" range (or so) when you're done. I hope all the other "stuff" is not attached to the spring plate assembly, like sway bars and stuff...right..and is truly hanging "free"?

Just coverin' all the bases.....

- Wil

[wcc]

Originally posted by Wil Ferch
We ( all ) must make an effort to speak with *clarity* ...(ha !...I should talk !)...

Anyway...you say " I took the angle measurement on the PS door sill at 3 deg.." OK....which way? 3 degrees down-toward-front? 3 degrees down-toward-rear ??? See my point ?

I presume you mean 3 degrees downward-toward-front...based on your next few sentences. ( "That brings us to 31.5 degrees")... Are you saying that the measuement would have been 31.5 degrees *if* your car was level? That is what I translate this to mean, *assuming* a nose-down slope of 3 degrees.

Yes, the back of the car is jacked up and the front tires are on the ground. The 31.5 deg is what the spring plate would have measured if the car was level.

Using 26 mm ( did you put a micrometer on the bar to *check* ???...maybe there is a non-standard part in there? Don't EVER assume anything on these cars)...and..."normal" weight distribution" ( again...what does this mean?...did you ever get your car on scales? The angle calculator works with "good" info...but can't perform magic. Normal Turbo's are closer to 38/62.....non turbos are generally 40/60)....you then say " Euro ride height is 29-30 deg.

Everything on this car is still stock with low miles and only a couple owners since new. I'm sure the bars haven't been changed.

I have never had the car on scales. I used the "Normal" weight dist on the calc cause it was that or 45/55. This 38/62 wasn't an option on that calculator.

OK...so you're taking a middle of the road apparoach and will try for 29.5. Fine. Are you then saying your actual "target" is 26.5 because your car is *still* canted 3 degrees forward/down, and you never levelled it since you found this amount off? That seems correct.

I would double check bar size and re-run the numbers for a 62% rear weight bias...to get a "fine-tune" on this. I see you put the spring plate in the "middle" of its adjutment range/slot before going further. This is a good idea as it gives you a final +/- 1/2" range (or so) when you're done. I hope all the other "stuff" is not attached to the spring plate assembly, like sway bars and stuff...right..and is truly hanging "free"?

The bar size is correct based on the history of the car.

Is there a spreadsheet or the calculators formulas that I can modify for the 62% rear weight bias?

Yes, I removed all four bolts so the spring plate is only connected to the torsion bar and not the trailing arm. It's hanging free.


Just coverin' all the bases.....

- Wil

[Wil Ferch]

I think Thom ( Fitzpatrick...aka "widebody911" )set the calculator up to allow a specific rear weight ( in "lbs")...to get any % on the rear that you want....

...maybe he can chime-in here as he took my "math-process" and created the spread sheet. Absent his furhter input, I would say calculate 62% of your weight ( say 2950 lbs) = 1829...and use 1/2 that "per wheel"..or 915 / wheel. That will mimic a 62% rear weight bias.

So...you *didn't* mike the TB size, eh? ( just doin' the prosecuting attorney thing.... ) . Don't forget, the bar spring rate increases to the FOURTH power on diameter...meaning?...there is a big spring force change with a *small* diameter change.

- Wil

[widebody911]

The weight balance is a drop-down list. I just added a 38/62 entry for ya.

[wcc]

Cool! Thanks Wil and Thom for changing that calculator for me. I re-ran the numbers and it came out to 30-31 deg so 31.5 minus 30.5 is 1 deg diff. So I'm going to set it to 27.5 deg.

[Oh Haha]

Hey Bill,
You should post pics when you are done. Before and after would be cool if you have them.