Old Chestnut - Rear Height adjust

[POKDUB]

My rear height is way low. I have read through a number of threads and have a reasonable understanding of the spring plate angle adjustment, however for my particular job I need a little confirmation, verification, ....moral support.

I have entered info into the angle calculator on rennlist and the results are in the table below. I am not 100% sure about the info as it doesnt tally with my own figures and assessment below.



I need to raise the rear height by about 2 inches. Before plate removal, I measured the angle using an 'Angle Finder' device as recommended. I found the angle to be 23 degrees below the horizontal.

Based on info found in threads, 0.8 degree change = 1/4 inch change, see then my table calcs for a 2 inches change.

Based on my calcs I need to increase angle to 29.4 degrees to raise height by 2 inches.



BTW. If I was to set to 40 degrees, as per the angle calculator, for US ride height, then this would result in a 5.3 inch increase, this cant be correct.

Where have I made the wild mistake ?

[widebody911]

Does your car really weight 2950#?

[Elombard]

Did you have the shocks disconected? Was the spring plate resting on the bottom bolt of the SP/TBar cover?

[widebody911]

I think Peter's primary issue is with the 40 degree angle returned by the calculator. Unless he's running 30" spinners, plasma-screen DVD players for each seat, and a 12,000W stereo system fueled by a depleted uranium reactor, the weight of the car is a wee on the high side.

Find a commercial truck scale and get the car weighed with 1/2-3/4 tank of gas - and driver - and try again.

[Thrasher]

If you're not already doing so, I would recommend also using the "Tripod Method".

This would involve removing the front wheels to adjust the rear height, supporting the front end by the center of your crossmember.

This will remove the influence of the front end corners. Once you have the rear set properly, do the same for the front end by removing the rear wheels and supporting the rear end by the (eegads) normal engine jacking point. Then, set your front end heights w/o th rear end corners influencing them.

I got my corner balance to w/in 20 lbs on the diagonals by using this method.

[POKDUB]

Does your car really weight 2950#?

[pok] I believe this to be the suggested weight, based on threads read.

Did you have the shocks disconected? Was the spring plate resting on the bottom bolt of the SP/TBar cover?

[pok] Plate was disconnected and bottom bolt removed to allow it to swing freely.

Find a commercial truck scale and get the car weighed with 1/2-3/4 tank of gas - and driver - and try again.

[pok] So is the consensus that the angle calculator figures are misleading ? The weight is a guesstimate, the suggested entry in this field based on other thread advise. Is the 6.4 degree increase right in any case ?

[Thrasher]

Which of your P-cars is this? Even the 86 should be a few hundred pounds less.

My very stock 84 w/AC was under 2700 with me and 3/4 tank of fuel in it. Don't forget to include yourself in the driver's seat. I used bags of water softener salt to simulate my weight.

[Elombard]

Thom great answer LOL!!

[POKDUB]

The car has the effective weight of a 1976 SC chassis, it si not stock, engine is 2.0 Litre.

I picked up guidance on this thread.
Help! Rear Ride Height

Maybe a 930 is much heavier.

So even dropping the weight down to 2700 lbs the degrees are still on the high side.

[widebody911]

Originally posted by POKDUB
So even dropping the weight down to 2700 lbs the degrees are still on the high side.

I don't know off the top of my head, but we should also verify your t-bar size.

Quite a few people have used the calculator, and the feedback has been good. I count a smidge under 800 'usages' (unique IP's grouped together) in the server log.

I say double-check the weight and t-bar size, and give it a go.

[Wil Ferch]

Raising the rear "free droop " angle by 2" ..per your diagram...has no bearing on raising the car 2" once there is weight on the springs and the "at rest" angle is used...that's why the math behind the angle finder calculator was so difficult ( well... "noodly" actually, not so much "difficult") to do....

If you only go on " angle change".....it would be simple...but use a clock as an illustration to show that "angle change" alone is not sufficient....going "up" 1 hour ( say...from 3 o'clock to 2 o'clock) will net you a different height change compared to the same 1 hour change from 6 o'clock to 5 o'clock...

See ??.....

it depends on accurate rendering of weight, weight distribution, and torsion bar size....AND the start/end angle used. All are important....

The calculator uses a 3.84" ( from memory here) "droop" angle AT REST..which is EURO height...as a target. Thom may have added ( later) other numbers than 3.84" for other height "targets".

You say you want to raise static ride height by 2"...the key question is...how does this target compare to "Euro"?...that might also be part of the problem....where do you want to end up comapred to Euro ride height?

The calculator works well, as Thom noted...with accurate inputs....

- Wil ( co-author, "noodler") ...

[Early_S_Man]

Peter,

I think your weight number is 400 - 500 lbs too high!

[POKDUB]

Thanks for the good guidance as usual.

Wil, tks for the explanation on the droop angle, and your clock analogy is a good one. I know your calculator will give the correct answer with the right information input. Is more like 'rubbish in rubbish out' at the moment. I wont be able to get actual car weight and can only go by forum suggestions. Right now I will go with Warren's weight reduction program and lose a few pounds.

Here is a pic before I put her up on jackstands. You can see why I need to raise the rear. 1.5 inches would be perfect, 2 inches on the very marginal high side. (BTW I have already had to raise the front to avoid rim tear).



I take it that US ride height is the highest option on the calculator, so here is the latest inputs which are closing in on what looks like the actual setup.



I plan to increase the angle to 29 degrees and see how it looks, I am not looking for exact US/Euro spec heights, but a reasonable balance between front and rear.

[Wil Ferch]

Peter:

Let me help "hone" you in .....

try 2550 lbs....this is typical for mid to late 70's SC's and a slightly different engine will not change much....look at published engine wieghts..in general around 420 lbs ( on the SAME measurement basis..Porsche didn't always measure with the same ancillaries !). Magnesium to aluminum engine changed weight by 20 lbs or so.... a 2 liter was remarkably heavy compared to later larger engines ( relatively speaking...they were close in actual weight)

So...try 2550.

Also, make sure your car (itself) is horizontal...put the angle finder on the door sill and see...and adjust the "droop" angle accordingly. Verify the bar is 24 mm...as this has a major impact ( 4th power stiffness impact with diameter...huge !).


Also use a 40/60 weight distribution. Correct..."garbage-in / garbage out"...so if you can't weight the car or find weight distribution...this should bring you close.

From your photos...it really looks like you're aiming for a 1" height increase....2" seems huge compared to your pics...

Good luck !

Wil

[POKDUB]

Well maestro Wil, looks like we have a result.



You will see that apart from the weight issue the t-bar was way wrong. I have just taken a t-bar out and it has a 28 stamp on it. (call me a plonker)

So it looks like the calc and the actual setup are in sync.

I will try for 1.5 inch height addition, will tear into it tomorrow morning. Want to do some torsion tube cleaning / waxoyl'ing as well, (my last car's tube buckled).

Thanks for your spot on assistance, will post pics.

[Wil Ferch]

Best of Luck ..... let us all know how it goes ....

- Wil

[widebody911]

Originally posted by widebody911
I say double-check the weight and t-bar size, and give it a go.

What do I win?

[POKDUB]

I need to put a postscript on this thread with my experiences and outcome.

I reindexed the torsion bars, with mixed results. The left side worked out fine first time. I had to do the right twice and it still came out too low. Having scribed the side of the wheel well, turned inner/outer splines to get the angle change I required, the car seemed to want to go back to its near original height.

My final adjustment was on the spring plate eccentric bolt, and this finally gave me the balance I needed.

This is the before and after shot. The front is a bit high but between speedramps, rubbing, and bumpy corners in Ireland I thought it better go hi then low. I might drop a half inch at the weekend, easy to do.

Thanks for your help.

[Wil Ferch]

If I may ask....if all your work was on the rear...why does it appear that the front raised-up ? Did you also adjust that purposely? If it came along as a rear height change I'd be confused why that would happen.

Weird about the right-rear not wanting to behave... ( ???)

- Wil

[POKDUB]

Wil,

I raised the front at the same time, this was deliberate.

I had bad tyre rubbing especially on bumpy corners or hard cornering.

I think my first efforts on the rear height change were on the cautious side, ie half inch at a time. I thought the adjustments would be bigger.