Rear T Bars - Spring Plates: Different degrees to make car level?
 

Hey all,

Has anyone ran into this situation before? To get the car "level" I compared the front-to-back and side-to-side... has anyone had uneven spring plate degrees?

For example, I just adjusted my rear 911's spring plates to 30 degrees on the rear passenger side.......... 24 degrees on the rear driver side. The car looks level!!!

Has anyone ran into a similar situation? The car is level... I took measurements between the concrete floor and top of the wheel well to see if they match... they do.

Just weird to me. I would've thought the spring plates would've been the same degrees to obtain the same rear ride height.

Any comments? Thanks!

You need to remove the pre-load, push the car forward and back and bounce on the bumper several times, walk away come back in couple of minutes and retake the readings.

My spring plates were not set equally when they came out. I just set them to the same degree measurement because the driver side was done first and it turned out to be at a lower ride height than the passenger side. Theoretically, you want the springs on each side of the car to either have the same preload or else have the driver side at a bit higher ride height setting on both front and back so that the car is level when the driver alone is in the car. The ride height looks level now, maybe a bit high on the driver side, but that will change when I get in and drive it.

The corner balance is what matters to me. If you have different degree settings in the rear but do not match it in the front you will end up causing the handling to be different when turning left vs. right because one rear wheel has more load on it and the diagonally-opposed front wheel has less load. I did not have corner balance scales with me when I did the recent mods, though, so I don't know how the corner balance was before and after, but it is easier to make the front match the back than the other way around!

Yes, the spring plates should be pretty much the same angle. Measure the angle of the bottom of the door opening and add it to the spring plate angle (assuming the rear of the car is up on jackstands) to get the true spring plate angle. If the shock is connected, it may limit how far the arm droops.


Like Flieger mentioned, what really matters is the corner weights.

Normally they're pretty close. You could have a failing torsion bar, or dis-similar torsion bars. Also, to accurately check angle, the lower shock bolt and lowest rearward spring plate bolt must be out
-C

I'm working on the car also...a few more factoids

'80 S/C in great condition, purchased the car about 15 years ago. Now converting to wide body with turbo and wanted to start with a level car. Started to lower the back and found 27 mm TB's in new condition. Found wore out trailing arm bushings and replaced with ER sport bushings from our host this weekend.
With all hardware disconnected, spring plate at full droop - the car is level with 30 degrees on one side and 24 on the other ?
The spring plate is not sitting on the resting spacer on the TBcover.
Again, all the hardware is off with a free floating spring plate and the larger TB's appear brand new. Shocks disconnected.

Unless the t-bar has a sizable crack in it the rate of the bar (how much it flexes at a given load) will not change. The modulus of elasticity of steel does not change with age. It is a commonly repeated wives tale about springs. The steel can yield, it can wear down at the attachment points, the heat treatment can be taken out of it (so it yields easily) but the Young's modulus for steel will always be 29,000,000 psi. Springs don't sag.
The bushings in the suspension and where the springs attach can wear and/or cold flow which will make a car sag but that is less of a problem the way the 911's rear is setup.

---------------
Back to why the angles might be so far off for a level car...
Assuming everything was disconnected so it hung free: If your front suspension isn't adjusted evenly you would need to adjust the rears out of wack to get the car level. This is exactly why corner balancing is so important on a car with a 100% adjustable suspension.

Thanks for all input.
Forgot to mention the car is up on a dead level 2 post lift. The door sill bubbles to level eliminating a least one variable.
The 16M bolts on the TB cover were not loosened and appear to be in the same position on both sides.
Time to learn more about corner balancing....

There isn't an elephant in the room I hope. Has the car been in an accident severe enough to rack the tub?
-C

Probably not, I've owned the car for 15+ years with no accidents and with the car up on a lift we've taken a close close look underneath and everything appears pristine.

can you get a set of car scales?

http://forums.pelicanparts.com/uploa...1370891564.jpg

Live in SoCal. Does anyone rent the scales ?
I'm thinking not, but figured I'd ask. TRE in Van Nuys is my next phone call.

Measure your ride height at someplace on the frame, like at the suspension attachment points. The fenders can be uneven, so measuring to the top of the wheel well might give you an incorrect reading.

I like the tri pod method. I cut a little block of wood, say 1/2 by 1/2 inch, center it on my front cross member, jack front up high as can go, measure from center of rear torsion bar to ground. You can do the front same way, gets it pretty close.

Got in touch with a sprint car friend racer and mechanic.
He fancies the tri-pod method too - to isolate the problem front or back. He said to call him when I get to that point.
We're learning as we go and it's got me thinking....
The driver side measured at 25 degrees which I think is the bad side. We found an axially out kilter trailing arm bushing and replaced them with stiffer ER units. The bushing and tire all look perfect again. Would the problem be in the passenger front putting undue forces on the driver rear ?
We'll be tripoding tomorrow night.

I use the same method, removes any influence from the more easily adjusted front torsion bars. Once you have the rear heights sorted the front is easy.

I have to disagree with your comment that springs don't sag.

The 'sag resistance' of spring steels has been the subject of huge amounts of both commercially sponsored and academic research for the last 50 years and improvements in performance are still being sought.

It is true to say that at a microscopic level the Young's Modulus is steel is reasonably fixed although I think 195 GPa is a bit low compared to the 207GPa which is the Young's Modulus of pure Iron. I would imagine 195 would be a reasonable value for an Austenitic Stainless Steel with about a 30% alloy content.

I would think a typical Spring steel with high silicon would be closer to 210MPa but it depends how this value is measured and Ultrasonic techniques.

The precise chemistry of the alloy being considered and the manner in which it has been heat treated will have an influence on how fatigue loading can influence microstructure and hence mechanical properties.

The most common impact on a Spring's ability to resist sag is due to Fatigue Softening.

It is generally well accepted that annealed low strength steels can exhibit increases in yield strength due to fatigue loading. High strength steels, however, typically undergo a softening process which causes them to lose strength and

The sagging of a spring tends to be a gradual process with small amounts of ride height being lost incrementally over relatively long periods of time.

This phenomenon is commonly observed in both leaf and coils springs and there is no reason to assume that Torsion bars are immune.

Materials with a ratio between the Ultimate Tensile Strength and Yield Strength of less than 1.2 will soften.

Where this ratio is greater than 1.4 the material will harden.

Between these two limits it is difficult to predict behaviour without testing.

There is also another, less common issue, which concerns the reduction in the 'Bulk Modulus' of a material.

Material which is subjected to stresses that are approaching the region where Low Cycle Fatigue occurs suffer from reasonably significant changes in their Bulk Modulus.

In simple terms voids in the material are created due to dislocation interaction and these voids effectively reduce the amount of material present in a given volume and hence affect modulus.

Reductions in the nominal Young's Modulus of typical Heat treated 0.45%Carbon steels subjected to LCF type testing of between 10 and 20% have been measured depending on the number of cycles and the plastic strain imposed.

When you consider these effects along with corrosion on the surface and the potentially small surface defects that can occur due to corrosion or wear if the bushes have been bad it is not surprising that a few old torsion bars will sag.

Performed a tripod test to begin the isolation of the problem

- both rear radius arms set at 30 degrees
- jackstands under the front with the tires a few inches off the ground
- jacked the rear at the centerpoint with a large socket
- rear tires lifted off exactly at the same time
-sitting on the ground with the fronts off the ground - dead level

http://forums.pelicanparts.com/uploa...1371097040.jpg

Removed the jackstands and put the front tires on the ground.
The driver rear raised up by a few inches. I'm guessing this makes the problem in the front ?

here's the pic with the front tires back on the ground
http://forums.pelicanparts.com/uploa...1371097107.jpg

Here's a pic of the front torsion bars.
With the front tires up off the ground at full rest - the passenger TB looks like it rotated to the bottom of it's travel
while the driver's side only rotated about 1/3 ?

http://forums.pelicanparts.com/uploa...1371097335.jpg

Setting the front tires on the ground - both TB's traveling to their respective stops

The position of the adjusters doesn't mean anything. Pulling that cap off and putting it on in a different place/rotation would change the setting of that adjuster screw. EDIT: On second thought, from the picture, if you assume that the control arms both droop until the shock stops them, then they are set at different ridge heights judging from the adjuster.

I would make the rear plates dead nuts equal angle and use the easy to adjust fronts to tune your heights and corner weights.

If you put your location in your profile, one of your Pelican neighbors may offer to let you borrow his scales. I loan tools out to friends/mechanics in the area (I'm in NH) but I don't necessarily want to broadcast that to the world.