How to lower the rear end of your 911 without fail

This explains how to lower or raise the front end, but not how to lower your car predictably a certain amount in the rear. For example, how do you lower it an inch, or 1 Β½ inches? And the torsion bar spline comment was limited to the 84-86 Carrera. The 87-89 Carrera splines were actually 46/47. And you can’t really significantly lower your car using the height adjustment bolt, the closest to the torsion bar, since that is set at its lowest point from the factory. It was only designed to add height, as the suspension got more of a set with age, not to lower the car.
For what it's worth, I've found that the way to efficiently disassemble the rear suspension, dial in a lowering amount, and reassemble it is as follows. I didn't invent this I just read the Bentley and they don't give you lowering methodology, but they do give the facts about how height is adjusted and I took it from there. I'm not a mathematician, but I took a few minutes to figure it out, and I've done this more than once with repeatable success.
First, you should know the height you want your car to end up at. So, while sitting on a flat surface, determine the amount you want your fender lip to be in relation to the tire. Let's say it's at 25" from the ground, and you want it to be at 24, one inch lower, just for the sake of this example. After removing your body work, etc., detailed in many manuals so that the swing arm is totally relaxed, take a protractor and note the angle of the swing arm. Let's say it's 34 degrees.
Then place the protractor on the door sill, and note that angle. Let's say it's -2 degrees. Combine these and you get 32 degrees. With an 88 Carrera, and the 46/47 spline configuration, you have a minimum 0'10" adjustment capability which equals .055 or 1.4 mm. So, to lower your car 1", you'd need 1 inch/.055 = 18.9 x 10"= 189/60= 3.15 degrees. You then subtract that from the 32 degrees to get 28.85.
Now, remove the remaining bolts from the torsion bar cover, pull that guy out, and remove the swing arm. Rotate the torsion bar, and re-insert it, place the swing arm on it, and look for the 28.85 angle or close enough. It may take a few tries to fine tune it, but now, when you button everything up in reverse order and put the car on the ground, it's going to be perfect. Then repeat the process for the other side. If you did want to finally tweak it absolutely perfectly, you could adjust that height adjustment bolt on one side or the other to do that.

__________________
This post was auto-generated based upon a question asked on our tech article page here:

[Casey at Pelican Parts]

Wow!! I like the dissection of the process you've used, and honestly I am not too up to speed on suspension stuff. Would you mind clarifying a few questions I have about your process??

How did you arrive at the minimum 0’10” adjustment capability which equals .055’ or 1.4 mm? Is it the number of splines, diameter with distance between the splines divided by 360?

I am more of a visual learner so if you wouldn't mind posting a diagram that may help others understand as well, especially those with different spline setups, so they may calculate the minimum adjustments.


- Casey

[DRACO A5OG]

Not Bad, much easier than raising and lowering the car 50 times to get the perfect height

[Trackrash]

Two easier ways.

1. Measure the spring plate angle with car in normal height
Have someone sit on the rear of the car to lower it to the desired height.
Measure the angle again. Note the change in degrees from the first measurement.
Disassemble the suspension and adjust the torsion arm the amount noted above.

2. Use the spring plate calculator that is available on the web.
If you have adjustable spring plates you can fine tune if not perfect.
This is the way to go if you are changing torsion bars.

[Cenzzo]

Thanks for the feedback! I’m not sure how I’d diagram it, but in the meantime, here are some answers to the adjustment capability. For 78-83 SC’s, and for 84-86 Carreras, the splines were 40/44, and the minimum adjustment was 0’50”, or 6.5mm/.256 inches (About a quarter of an inch). I didn’t calculate that, or for the 87-89 Carrera adjustment, I just took it from what it states in the Bentley Service Manual. Though the later Carrera setup gives you more surgical precision, it’s really sort of easier to ratchet to your desired distance in quarter inches because it’ll take less try’s.

[Flieger]

Sounds similar to what I did, but it doesn't work if you change torsion bar size.

I made up a spreadsheet and used spring rates, corner weights, torsion bar to ground heights, swingarm length, etc. You can get the correct theoretical angle with a little math. The hard part is not going mad trying to use the vernier splines to get to the target angle.

[DRACO A5OG]

BTW, Cenzzo Welcome Welcome

Dang it Brother Flieger, I was afraid someone was going to write what you did. I upted my T bars . Oh well, Trial and error for me .

Question? Let say I have both sides angled equally but I adjust the plates to fine tune the heights, one being lower than the other, does this affect the tension of the two sides differently? Making one side have a different spring rate than the other?

[Flieger]

Using the plates is the same as the splines. You will be changing corner weights and ride heights.

[Walt Fricke]

More directly, adjusting corner balance or ride height does not alter spring rates. You do that by using different diameter T bars.

[DRACO A5OG]

Thank you Gentlemen,

The reason I ask is that when I indexed the rears. I verified several times that both sides were spot on even but when on the ground after corner balance with me in the car, my passenger side front sat lower causing the driver rear to go up. So Annoying after I killed myself trying to index it perfectly

I was hoping if I indexed the driver rear a tad higher in anticipate for the front corner balance, could I even her out to closer than it is now? Am I just being too OCD?

[garrett490]

Originally Posted by cenzzo (guest)

This explains how to lower or raise the front end, but not how to lower your car predictably a certain amount in the rear. For example, how do you lower it an inch, or 1 Β½ inches? And the torsion bar spline comment was limited to the 84-86 Carrera. The 87-89 Carrera splines were actually 46/47. And you canβ€t really significantly lower your car using the height adjustment bolt, the closest to the torsion bar, since that is set at its lowest point from the factory. It was only designed to add height, as the suspension got more of a set with age, not to lower the car.

For what it's worth, I've found that the way to efficiently disassemble the rear suspension, dial in a lowering amount, and reassemble it is as follows. I didn't invent this I just read the Bentley and they don't give you lowering methodology, but they do give the facts about how height is adjusted and I took it from there. I'm not a mathematician, but I took a few minutes to figure it out, and I've done this more than once with repeatable success.

First, you should know the height you want your car to end up at. So, while sitting on a flat surface, determine the amount you want your fender lip to be in relation to the tire. Let's say it's at 25" from the ground, and you want it to be at 24, one inch lower, just for the sake of this example. After removing your body work, etc., detailed in many manuals so that the swing arm is totally relaxed, take a protractor and note the angle of the swing arm. Let's say it's 34 degrees.

Then place the protractor on the door sill, and note that angle. Let's say it's -2 degrees. Combine these and you get 32 degrees. With an 88 Carrera, and the 46/47 spline configuration, you have a minimum 0'10" adjustment capability which equals .055 or 1.4 mm. So, to lower your car 1", you'd need 1 inch/.055 = 18.9 x 10"= 189/60= 3.15 degrees. You then subtract that from the 32 degrees to get 28.85.

Now, remove the remaining bolts from the torsion bar cover, pull that guy out, and remove the swing arm. Rotate the torsion bar, and re-insert it, place the swing arm on it, and look for the 28.85 angle or close enough. It may take a few tries to fine tune it, but now, when you button everything up in reverse order and put the car on the ground, it's going to be perfect. Then repeat the process for the other side. If you did want to finally tweak it absolutely perfectly, you could adjust that height adjustment bolt on one side or the other to do that.

__________________
This post was auto-generated based upon a question asked on our tech article page here:

Can you post a picture of how your using the protractor?


Sent from my iPad using Tapatalk

[Flieger]

Originally Posted by DRACO A5OG

Thank you Gentlemen,

The reason I ask is that when I indexed the rears. I verified several times that both sides were spot on even but when on the ground after corner balance with me in the car, my passenger side front sat lower causing the driver rear to go up. So Annoying after I killed myself trying to index it perfectly

I was hoping if I indexed the driver rear a tad higher in anticipate for the front corner balance, could I even her out to closer than it is now? Am I just being too OCD?

Not sure why that would be but the front does affect the rear as you know. I'm not sure what your before and after numbers are. Did you go for equal fronts or equal diagonals?

When I put my rear suspension back together I set the rear spring plates as equal as possible (the two sides had quite different eccentric positions but I didn't feel like messing with them so I just got them as close as possible with splines which I think was within .5 degrees. I set the balance with the fronts and the ride heights (torsion bar to ground) and rake ended up pretty close to where I wanted them and also pretty even. Probably partly by luck and partly by calculations.

I went for equal diagonals.

I think it helps that I have stiffer bars (for a given change in weight there is less height change) and Rebel Racing bushings (no spring rate from the rubber to account for).

[911pcars]

Google can be your friend.

tech.rennlist.com/911/pdf/settings.pdf
and for other methods:
https://www.google.com/?gws_rd=ssl#q=will+ferch+ride+height

Sherwood

[Trackrash]

I use one of these. Made by Starrett with a very accurate bubble level.

[Joe Bob]

That's why I added adjustable spring plates when I replaced my bushings back in the day......

[Elombard]

The problem with the whole counting splines thing is every time I go to pull the spring plate I can never get if off with out the T bar coming out of its inner mount so I lose my place....its a long day. But satisfying when you get it right.

[911pcars]

Originally Posted by Elombard

The problem with the whole counting splines thing is every time I go to pull the spring plate I can never get if off with out the T bar coming out of its inner mount so I lose my place....its a long day. But satisfying when you get it right.

Measure the angle of the free-hanging spring plate before removing the torsion bar. That's your starting/reference point.

Sherwood

[Elombard]

Ahhh wow the light bulb goes off. If it comes out just clean it up and put it back in and replicate the angle....I gave up to quickly.