Ride Height Adjustment Confusion

[piscator]

Ride Height Adjustment Confusion

I just installed 28mm rear torsion bars in my 1979 SC and the ride height adjustment has me confused.

I understand that the ride height is measured as the difference between the distance from the floor to the center of the torsion bar cover (measurement “B”) and the distance from the floor to the center of the wheel (measurement “A”).

According to the Bentley Manual, the difference between those two heights, (measurement “C”) should equal the specification for:

US/Canada spec …….. 37mm +/- 5mm (1.46 +/- 0.2”)
European spec ………. 16mm +/- 5mm (0.63 +/- 0.2”)

So which should I use? I realize the U.S specification was predicated on the 5mph bumpers, but the difference between the two is more than double. How do I choose a specification for my specific car?

Is the proper specification “anything between 16mm and 37mm based on personal preference?” That seems so very vague! I made an appointment to have an alignment done next week, so any advice on where to set my ride height would be much appreciated!

Question #2: Torsion Plate Angle.

What is the importance of the Torsion Plate angle? Mine measured 32 degrees with the original torsion bars. To compensate for the stiffer 28mm torsion bars, I need to change the ride height by rotating and re-indexing the splines on the torsion plate and/or bar.

But that’s going to change the spring plate angle to something less than 32 degrees. Is this important? This has me very confused! What am I missing?

Thanks for the help!

[Walt Fricke]

Let's say that with your loaded spring plate angle being X with your old T-bars, and the ride height what you want, you go to stiffer bars. What you want is your spring plate angle to be the same X. You need to move the unloaded spring plate angle up some to compensate for the stiffer spring.

With the adjustable spring plates, this is a bit tricky, since there are really two angles - the short one connected to the T bar, and the longer one aft of the adjustment pivot.

You are right about there not being a perfect spec for the height. Most use the Euro spec. When you lower a car too far, suspension dynamics can be adversely affected. And, of course, more problems getting in and out of steepish driveways. Suspension specs are almost always a compromise anyway - tire wear, cornering grip, a flat contact patch for best braking.

[Bill Verburg]

Originally Posted by piscator

Ride Height Adjustment Confusion

I just installed 28mm rear torsion bars in my 1979 SC and the ride height adjustment has me confused.

I understand that the ride height is measured as the difference between the distance from the floor to the center of the torsion bar cover (measurement “B”) and the distance from the floor to the center of the wheel (measurement “A”).

According to the Bentley Manual, the difference between those two heights, (measurement “C”) should equal the specification for:

US/Canada spec …….. 37mm +/- 5mm (1.46 +/- 0.2”)
European spec ………. 16mm +/- 5mm (0.63 +/- 0.2”)

So which should I use? I realize the U.S specification was predicated on the 5mph bumpers, but the difference between the two is more than double. How do I choose a specification for my specific car?

Is the proper specification “anything between 16mm and 37mm based on personal preference?” That seems so very vague! I made an appointment to have an alignment done next week, so any advice on where to set my ride height would be much appreciated!

Question #2: Torsion Plate Angle.

What is the importance of the Torsion Plate angle? Mine measured 32 degrees with the original torsion bars. To compensate for the stiffer 28mm torsion bars, I need to change the ride height by rotating and re-indexing the splines on the torsion plate and/or bar.

But that’s going to change the spring plate angle to something less than 32 degrees. Is this important? This has me very confused! What am I missing?

Thanks for the help!

There's no magic # that's right, it all depends on how you want the car to look and handle.

factory ride heights for the front of a 911 range from 103mm(high) to 160mm(low) and for the back from 37mm(high) to -38mm(low)

Choose the height that gives you a good driving car, lower is generally better but at some point other mods are necessary to restore good geometry and the stance that looks best, usually a slight nose down attitude.

If you changed the rear t/-bars to 28mm then hopefully you also changed the front to
21 or 22mm and want a superior driving car. Hopefully you have also upgraded wheels and tires to match the suspension. If all the above is so then the RoW stock 108+/-5 & 16+/-5 rear are at the high end of where you want to be, 143+/-5 & 14+/-5 is where the factory would put a sporting purpose car and 160 & -38 where they would put race car, then a corner balance and alignment would be done to finish the job, note that the corner balance will almost always modify at least 2 of the heights and you will rarely get the same measure on opposite ends of the same axle.

[piscator]

Good Evening Gentlemen,

Thanks so much for the clarifications. I suspected there was some latitude in the height specification. The ‘Euro-Spec’ is more in keeping with my modest suspension upgrades, so I think I’ll start there.

Walt, I do have adjustable spring plates and you’re right – they are a little ‘tricky.’

Bill, this is a road car, not a track car. I liked the car ‘stock’ but wanted to try a modest upgrade. I paired the 28mm rear torsion bars with 22mm torsion bars in the front. Shocks are Bilstein HD’s all around.

My tires are Michelin Pilot Sports A/S – Rear: 225 50ZR 16 and Front: 205 55R 16. They are mounted on the original Fuchs wheels.

Regarding the ‘Spring Plate Angle’ – What does one do if they never recorded the spring plate angle? I’m thinking about guys who are building a car from parts. Is there a specification for spring plate angle based on model year? Or is the angle established by setting the ride height first; based on the goals for build? Road Car, Track Car, Rally Car – for example.

Thanks for all your help! I really appreciate it!

[Arne2]

There are baseline springplate angles published in the service manuals. I don't have mine handy now, but I recall that it was something like 36.5° for my '72. I suspect that will vary by year.

But take that with a grain of salt for your car, as the baselines are for stock bars. With stiffer bars the baseline settings are likely not correct.

[piscator]

Arne,

I measured 32 degrees on my spring plate. I'll look at my PDF copy of the service manual to see what can find for comparison. I'm always curious about this stuff.

Thanks very much for clarifying this for me!

[Driven97]

There's also the spring plate angle calculator.

https://www.elephantracing.com/tech-topics/spring-plate-angle-calculator/

Not an exact science, but it's been pretty close for me in the past.

[piscator]

Matt,

I bought all my parts from Chuck. Unfortunately, he wasn't available when I put a call into him. I should have thought to check the website.

Thanks much for pointing that out!

[FrankM_]

Originally Posted by Bill Verburg

There's no magic # that's right, it all depends on how you want the car to look and handle.

factory ride heights for the front of a 911 range from 103mm(high) to 160mm(low) and for the back from 37mm(high) to -38mm(low)

Choose the height that gives you a good driving car, lower is generally better but at some point other mods are necessary to restore good geometry and the stance that looks best, usually a slight nose down attitude.

If you changed the rear t/-bars to 28mm then hopefully you also changed the front to
21 or 22mm and want a superior driving car. Hopefully you have also upgraded wheels and tires to match the suspension. If all the above is so then the RoW stock 108+/-5 & 16+/-5 rear are at the high end of where you want to be, 143+/-5 & 14+/-5 is where the factory would put a sporting purpose car and 160 & -38 where they would put race car, then a corner balance and alignment would be done to finish the job, note that the corner balance will almost always modify at least 2 of the heights and you will rarely get the same measure on opposite ends of the same axle.

Bill,

Interesting info on setups.
For the rear on my car
a = 303 mm (wheel hub center)
X = 281 left / 283 right (torsion bar center)

The difference being 20 to 22 mm but the torsion bar is lower than the wheel center, does this mean mine is set-up to -20 mm (so reasonably to strongly sporty/race setup) or 20 mm ?
Front a-X = 142 mm , visually OK, not very low but a tad harsh I find on less than perfect roads.
This puts fender heights in front at 26 in, rear 25,4 inch.
Steering rack spacers installed.

Since I use the car mainly for cruising and Autobahn I find it a bit too hard on the springs and want to check if these heights are good for cruising setup.

[piscator]

Reporting back with curious results:

The Porsche Workshop manual shows the spring plate angle for a Bilstein equipped 1979 SC to be 39 degrees. The Bentley (I believe) showed a figure of 36 degrees. My actual measurement when I disassembled the suspension was 32 degrees, left and right.

Turning to Elephant Racing’s “Spring Plate Angle Calculator” I plugged in the following values – Vehicle weight: 2750, Ride Height: Euro, Torsion Bars: 28mm, Weight Distribution 40/60 front/rear (normal)

Chuck’s calculator determined my spring plate angle should be: 24.59 degrees

Curiously, this is very close to what I measured, yesterday, when I did a trial set-up and adjusted the ride height to “Euro” specification. So it seems that Spring Plate Angle is subject to a lot of variation..

But what still escapes my feeble cogitating machinery is the relationship between Spring Plate Angle and Ride Height. Is it a ‘chicken and egg’ sorta thing? Which comes first? Ride Height or Spring Plate Angle? Is one more important than the other? Do you set Spring Plate Angle at the calculated value and let Ride Height fall where it may? Or, vice versa? Set the Ride Height and let Spring Plate Angle fall where it may?

To me, it doesn’t seem to matter, because – If I set the Ride Height to “Euro” (without any reference to Chuck’s calculator) I inevitably achieve a Spring Plate Angle of about 25 degrees. Conversely, If I pay no attention to Ride Height and use Chuck’s calculator to set the Spring Plate Angle at 25 degrees – I inevitably wind up with “Euro” Ride Height.

If this is correct, then it seems that the values for Spring Plate Angle and Ride Height are simply measurement tools that cross-check one another. And if that’s correct, is it even necessary to use both? This seems akin to having two specifications for Spark Plug Gap: 1) the gap itself And, 2) the overall length of the spark plug, top to toe.

As always, I’m probably missing something.

Thanks again to everyone!

[Driven97]

Originally Posted by piscator

If this is correct, then it seems that the values for Spring Plate Angle and Ride Height are simply measurement tools that cross-check one another. And if that’s correct, is it even necessary to use both? This seems akin to having two specifications for Spark Plug Gap: 1) the gap itself And, 2) the overall length of the spark plug, top to toe.

This is correct. When you've got the car apart, measuring the spring plate angle is the only thing you can do. Your end goal is a specific ride height. So I find the angle + calculator is the easiest way to get in the ballpark. Then fine tune with the cam on the spring plate (assuming your year has those.)

Without using the spring plate angle, you have to kinda guess, slap the whole thing together, and see how it ends up.

[piscator]

Hi Matt,

I just got to the shop and I'm starting the Ride Height Adjustment. I thought Chuck's calculator was pretty nifty and plan to go by that. I'm so glad you pointed it out! And I've got a new digital level to play with!

I really appreciate your confirmation! I need to write and read things in different ways before the subject starts to sink in and stick. Thanks so much for the clarification!

[piscator]

Thanks again gentlemen!

[Bill Verburg]

Originally Posted by FrankM_

Bill,

Interesting info on setups.
For the rear on my car
a = 303 mm (wheel hub center)
X = 281 left / 283 right (torsion bar center)

The difference being 20 to 22 mm but the torsion bar is lower than the wheel center, does this mean mine is set-up to -20 mm (so reasonably to strongly sporty/race setup) or 20 mm ?
Front a-X = 142 mm , visually OK, not very low but a tad harsh I find on less than perfect roads.
This puts fender heights in front at 26 in, rear 25,4 inch.
Steering rack spacers installed.

Since I use the car mainly for cruising and Autobahn I find it a bit too hard on the springs and want to check if these heights are good for cruising setup.

In the back when the t-bar is below the axle it's always minus, so -20 is correct

here are the factory diagrams



The height shouldn't contribute to harshness unless the geometry is too far out or the shocks run out of travel. At 142 in front you should at least have rack spacers

[fanaudical]

Originally Posted by piscator

But what still escapes my feeble cogitating machinery is the relationship between Spring Plate Angle and Ride Height. Is it a ‘chicken and egg’ sorta thing? Which comes first? Ride Height or Spring Plate Angle? Is one more important than the other? Do you set Spring Plate Angle at the calculated value and let Ride Height fall where it may? Or, vice versa? Set the Ride Height and let Spring Plate Angle fall where it may?

Here's one way to think of it:

Spring plate angle sets the position of the suspension when it is not burdened by the weight of the car. Once the car is on the ground, weight of the car loads the suspension and provides the resulting ride height.

Ride height too high? Reduce spring plate angle.

Ride height too low? Increase spring plate angle.

I hope that helps.

The calculator is essentially doing the "spring math" to determine the appropriate spring plate angle based on vehicle weight and stiffness of the torsion bars. It was dead accurate when I used it to set mine.

[FrankM_]

Originally Posted by Bill Verburg

In the back when the t-bar is below the axle it's always minus, so -20 is correct

The height shouldn't contribute to harshness unless the geometry is too far out or the shocks run out of travel. At 142 in front you should at least have rack spacers

Ok thanks,

After buying the car last year, I replaced the front Bilstein HD inserts and indeed installed the racks spacers. Especially the rack spacers made a big difference in enjoyment.
So I understand these figures (front 142 mm / rear -20 mm) are quite low in the range you've given. Car rides&handles very well on perfect asphalt (not a lot of that in Belgium) but any unevenness is felt relatively hard through the front.

I will try to replace the bushings (no indication when they were replaced in the past) and getting a full alignment & corner balancing done by a shop then. I want to drive it more often but the harshness up front is keeping me from doing that.

[piscator]

Originally Posted by fanaudical

Here's one way to think of it:

Spring plate angle sets the position of the suspension when it is not burdened by the weight of the car. Once the car is on the ground, weight of the car loads the suspension and provides the resulting ride height.

Ride height too high? Reduce spring plate angle.

Ride height too low? Increase spring plate angle.

I hope that helps.

The calculator is essentially doing the "spring math" to determine the appropriate spring plate angle based on vehicle weight and stiffness of the torsion bars. It was dead accurate when I used it to set mine.

Fanaudical,

Yes, that helps a lot! And it’s well stated. Thanks!

It makes clear how I got a Ride Height value for “C” of 20mm on the left side and 10mm on the right, when I remounted the wheels and dropped the car to the floor – even though the spring plate angle on both sides was an identical 24.59 degrees. There, I guess, is a concrete example of a difference in corner weight.

If I now understand this correctly, I need to reduce the spring plate angle on the left side (by raising the spring plate) in order to reduce the ride height value for “C” from 20mm to 16mm.

And on the right side, I need to increase the spring plate angle (by lowering the spring plate) in order to increase the ride height value of “C” from 10mm to 16mm.

Fwheeeeewwwwww! That’s a lot to keep straight!

And if it’s wrong, I hope someone will point that out!

In addition to just ‘gettin’ the darn car back together’ – I really enjoy learning about the engineering and appreciate everyone’s superb tutoring!

[fanaudical]

10mm difference side-to-side with the same spring plate angle seems like a lot to me for some reason. The rear of the car is relatively well-balanced side-to-side. Are you taking care to set the rear camber and toe-in as close as you can before dropping the car back on the ground? Also, be sure to roll the car forward/backward a few times to let things "settle" before taking measurements.

And an interesting thing about corner-balance: In most cases, you can adjust the suspension for near-equal weight on the corners (side-to-side) OR you can make the static ride height look even. If you're very lucky then both dial in close.

[piscator]

Hi Fanaudical,

Thanks for following up! No, I’m not trying to adjust the camber and toe-in. It didn’t occur to me that that was necessary. I’m not familiar with how to do that, but I can give it a try tomorrow.

While you're here, let me ask what I suspect are a couple of dumb questions.

Today I re-did a bunch of stuff to better understand things and to try and insure I got it right. I had some concern that the unbolted trailing arm pressing on the Spring Plate, might have effected my previous measurements. I only worked on the left side.

I made sure the trailing arm was free and clear of the Spring Plate, removed the Torsion Bar Cover and reset the angle by rotating the Torsion Bar. Testing different inner and outer teeth, the closest I could get to the Elephant Calculator’s suggested angle of 24.59 degrees was 24.24.

Without further adjustment, (I fully torqued ALL the bolts) and dropped the car on the deck. My Ride Height value for “C” calculated to 12mm. Which is 4mm short of the 16mm ‘Euro Height’ I was shooting for.

So now the dumb questions:

To make further ‘fine-tuning’ adjustments (by rotating the eccentric adjustment bolt) do I disconnect the Trailing Arm – again? In other words, are all the adjustments made with the Spring Plate ‘un-loaded?’

If so, that could mean an awful lot of un-bolting, bolting, and re-torquing a lot of fasteners – to get the ride height just right! The Bentley manual is not quite clear on this point.

Lastly, is it necessary to fully torque ALL the bolts to specification before dropping the car? I’m pretty sure the answer is ‘YES’ having learned that the ‘hard way’ today. But as I’ve always told my daughters, I got most of my eduction in the ‘school of errors.’ :-)

Thanks so much for your help!

[fanaudical]

I'm not sure that I'm going to explain this correctly/clearly, but here goes:

- The attachment of the rear trailing arm to the spring plate affects both the camber and toe-in of each rear wheel.

- Camber adjustments might affect how you measure the ride height (depending on where you take your center-of-wheel measurement). Just as adjusting the angle of the spring plate adjusts right height, adjusting the camber makes small adjustments in ride height.

Admittedly, when I did my suspension rebuild, I set the adjustable spring plates in the center of their adjustment. I put them on the torsion bars per the calculator. I set the camber/toe as close as I could get it using levels and the string method. I then let the shop with the real alignment rack make final adjustments and adjust the spring plates to get reasonable corner balance.