1985 rear height adjustment math

[SpyderMike]

I am about to reset my height. I measured the angle unloaded with a digital protractor. I also measured the gap from bottom point to ground of the aft tip of the spring plate. I was hoping to come up with a way to know how many spines I need to move to get where I wanted to be. So, some math might help.

I have 40 splines in the inside end of the torsion bar. That is 360 degrees / 40 splines = 9 degrees per spline. I have 44 splines on the outboard end of the bar for 8.18 degrees per spline. So effectively, I am working this difference between the two torsion bar spline adjustments to come up with what I need.

I need to drop the rear about 0.50 inches in the back end.

The circumference of the effective swing plate circle is 2 x pi x radius. I measured the radius from the center of the torsion bar to the center of the wheel hub to be 19". Therefore, my swing plate circle is about 119.38" in circumference.

Each change of spline on the inboard side is 1/40th of this or 2.975" in circumference. Each change of the outboard spline is 2.713" in circumference. If I move the inner end one direction and the swing plate in the opposite direction the net effective change is 0.262" circumference in the direction of the inner spline change.

So, to move the car down 0.50", I need to move the inner torsion bar end two splines counter clockwise and then attach the spring plate two splines clockwise from the original settings on the Driver's side. On the Passenger's side I need to move two splines clockwise on the inner and two splines counter clockwise on the outer. That gets me about 0.524. Given that this measurement is on the circumference, that should result in about a 0.5 inch drop theoretically.

I measured about 35 degrees on the spring plate angle. Using this method, I need to subtract the 9 degree inner spline from the 8.18 degree outer spline change to net 0.82 degrees (34.18 degrees).

I measured 6.5" from the bottom of the lowest tip of the spring plate to the ground. I am hoping to get near 7" when done.

I will see how these three measurements work out when I try it. I realize that when I drop the front end it will affect the rear end. I am just trying to limit how many go arounds I have to do.

Is this correct? Are there errors in my reasoning?

Thanks,
Mike

[Neilnaz]

A different way of working this out:

1 spline change inner/outer as you say is 9-8.18 = 0.82degrees
drop for this =tan(0.82)*19" = 0.27"

Therefore 2 splines should drop you approximately 0.54"

Cheers, Neil

[SpyderMike]

Sounds good! Thanks.

Any additional minor change can be made up with the finer adjustment bolt on the spring arm.

[juanbenae]

do a search for "spring plate calculator". thom, widebody 911 hosts a calculator on his vintage bus site that you plug in weight, t-bar size & desired ride height and it will spit out a desired angle for the unloaded spring plate.

or you could blow my mind again with more of the goodwill hunting stuff...

[jjeffries]

Mike, cool project. Thanks for thinking this out loud; I will be doing the same thing soon.

Wouldn't you be looking for a 7" measurement from the ground (starting at 6.5")? Raising the spring plate to lower the car? Best of luck, John in CT.

[SpyderMike]

I tried to find that site juanbeneae, but no luck so far...I get an Internal Server Error.

jjeffries, you are correct, I was thinking the wrong direction! Thanks.

I guess I should mention that I am not changing torsion bars. I just had to disassemble my rear suspension to add new bushings and I also removed a couple of hundred lbs of excess weight on my backdate project.

[Mark Salvetti]

Originally Posted by SpyderMike

I tried to find that site juanbeneae, but no luck so far...I get an Internal Server Error.

I plan to do this soon too, and I bookmarked another recent thread. The second post has a link to Thom's page:

[suspension gurus] another spring plate angle question...

Mark

[SpyderMike]

Calculator is still not accessible....

[Lake Guy]

Subscribed as well - I need to do this too so keep us posted.

You’re math sounds reasonable to me.

[Jonny042]

If one spline inner and 1 spline outer gets you -.26, shouldn't it be 2 splines each to get .-52?

It might be worth mentioning if you are measuring starting angles, you should do so with the lower rearmost spring plate bolt and spacer removed.

Might also be worth mentioning you are looking at the left side, and that the right side has to be done in the opposite direction.

I know all that is obvious, but just saying

As always, thanks for sharing.... sub'd for the end result.

[chrismorse]

You mentioned loosing a couple hundred pounds, was all of that loss before taking measurements?
Are you using the same shocks? I have heard that new Bilsteines raise the suspension slightly.
Lastly, (and I don't think it makes much difference), did you glue in the rubber spring plate bushings at the original plate angle??
Do they want to get glued in at the full droop unloaded position, or at final ride height angle??
Chris

[Driven97]

The 40/44 spline count thing, plus pressing into that spring plate bushing, is such a total PITA to me. I use the guess & check with protractor on the spring plate method due to counting "one spline" being too difficult.

For a 1° spring plate change with your measurement:

19 * tan 1° = 0.3316"

~1/3" height change per degree of spring plate angle. This plus a free phone protractor app is my weapon of choice.

[SpyderMike]

Thank you all for reading my post - I am guilty of some errors in my typing. It is one thing I am good at.

To clarify some great points. Focusing on the rear of the driver's side, one spline counter clockwise on the inner and one spline clockwise on the outer should net .26 drop. So if I want .5 inch drop, it is two splines counter on the inboard and two spline clockwise on the outer. To further the example, If you want 1 inch drop, it would be four splines counter clockwise on the inner and four splines clockwise on the outer.

On the passenger's side, the directions change - for a drop, the inner splines are changed clockwise and the outer splines counter clockwise.

I will make corrections in my original post.

Thanks again! I should know in a day or two if this works out.

[SpyderMike]

Originally Posted by chrismorse

You mentioned loosing a couple hundred pounds, was all of that loss before taking measurements?
Are you using the same shocks? I have heard that new Bilsteines raise the suspension slightly.
Lastly, (and I don't think it makes much difference), did you glue in the rubber spring plate bushings at the original plate angle??
Do they want to get glued in at the full droop unloaded position, or at final ride height angle??
Chris

Good questions Chris. Same shocks, weight lost before taking spring plate measurements, bushings glued while spring plates were off the car - so fully unloaded. Here is my project:

so this is how a backdate project starts...

The bushings have to be glued that way. I understand that I will apply some dish soap to the outside of the bushing to insert the torsion bar into the spring plate. In my head, that soap will allow the bushing to find its home and rest in a position with the car on the ground ant ride height. From there the soap will dry and the bushing will finally seat. I could be wrong though as I am doing something I have never done before.

[SpyderMike]

Originally Posted by Driven97

The 40/44 spline count thing, plus pressing into that spring plate bushing, is such a total PITA to me. I use the guess & check with protractor on the spring plate method due to counting "one spline" being too difficult.

For a 1° spring plate change with your measurement:

19 * tan 1° = 0.3316"

~1/3" height change per degree of spring plate angle. This plus a free phone protractor app is my weapon of choice.

Except with this spline arrangement you can never get a nice even degree of change in splines alone. You have to make up for it in adjustment elsewhere. I hear you that it might be difficult to move just one or two splines. Having never done this before, I don't know. I plan to mark the initial torsion bar inner spline orientation and give it a go.

The important thing for me to understand in this exercise is that I need to move both ends and I need to move them the opposite directions and the same amount (of splines).

[juanbenae]

Originally Posted by SpyderMike

I tried to find that site juanbeneae, but no luck so far...I get an Internal Server Error.

try PM'ing thom, widebody911 and see if he can address the server issue, or run your data and report back. he is a regular contributor to the random pic thread in the OT area.

[Driven97]

Originally Posted by SpyderMike

Except with this spline arrangement you can never get a nice even degree of change in splines alone. You have to make up for it in adjustment elsewhere. I hear you that it might be difficult to move just one or two splines. Having never done this before, I don't know. I plan to mark the initial torsion bar inner spline orientation and give it a go.

The important thing for me to understand in this exercise is that I need to move both ends and I need to move them the opposite directions and the same amount (of splines).

Yes, exactly. When making a change, I measure before pulling apart and note the values on each side. I then pull it apart and reset the fine adjusters to the middle position. I get "as close as possible" to my new angle with the splines, then dial in the final angle with the adjusters. There's a fair amount of adjustment on the spring plates themselves that you don't have to nail it.

I've never done the math before to know the relationship between angle and final height, but it's very useful for my technique.

[3Ddesigns911]

We have been working on getting the rear ride height right as we speak. We had the torsion bar bolted in and pulled out a few too many times and now we can get it in and out in about 30 minutes per side. Here are some observations in no specific order.

Use plenty of liquid soap on the rubber bushings when installing it back. That will make it much easier.

When you insert the assembly (with the torsion bar) back in, your original bolts might not reach, so get a couple of longer bolts (if you replace your rubber bushings from Elephant racing) they will provide those. Use these long bolts until your cover plate get close enough so your regular bolts reach.

Before you remove the torsion bar, mark the spring plate angle twice when its unsupported. First when everything is still bolted together, then one more time after you unbolt the spring plate from the control arm. The spring plate will sit lower after you unbolt everything.

Don't forget to make note of the eccentric wheel alignment bolt positions. Even if you are having your wheel alignment done after you're done you might have issues with your tire rubbing, not fitting (depending on your ride height and tire size). Ask me how I know.

When you are removing the torsion bar, you most likely will have to pry it and pull it out with some force, and even if the torsion bar slides out easily on both ends you most likely will not know what spline position it was in so, you might want to try to re-insert it without pushing the bushing in just to get the position where you had it, so you have a starting point.

The spline counting is not that big of a deal, but it sounds confusing. My car is different because it's an 87 and have the G50 with the 47/46 splines, but I ended up making a graphic tool, that I can rotate and it shows me exactly the height I get and how many clicks it takes to get that.

Lastly, use the proper measuring methods to get your ride height. Measure the distance from the center of the torsion bar to the ground, then measure the distance of the center of your wheel to the ground. The difference is that gives you the ride height measurement. Then do your math of the adjustments you want to achieve to this number.

I'll be back at this later today. Good luck with yours.

[3Ddesigns911]

One more thing about the spring plate adjustment. You have about 3/4" wheel movement you can achieve from the lowest to the highest point of the adjustable spring plate positions. That means if you have it in the center you can balance your left and right sides it if there is a slight difference in height, so if you set it in the middle you can go up and down about 3/8 on either side.

[suckho]

I did rear height adjustment for my SC last spring. I read somewhere that the smallest possible adjustment is 0.8 degrees (1 inner spline, 1 outer spline to another direction) and it affects car height approx 6 mm. My target was to adjust car about one inch (2.5cm) lower since the rear was raised too high with new Bilstein shocks. So.. 2.5 cm is around 4 time 0.8 degrees.

When the car was on lift with everything but the torsion bar removed I took exact measurement of the angle of free hanging control arm. For that I used my Android phone with this Bubble Level application. Then I took torsion bars out and started experimenting with different position to achieve that 3.2 degree difference to the original orientation. The end result was perfect on the first try, no need for fine tuning!