Porsche 944 turbo torsion bar replacement
Doug Donsbach:Here is what I have done. Start with the car in a good state of alignment.
Remove the anti-roll bar and shocks from the rear. Bounce the rear of the
car or drive back and forth a couple of times to settle the rear. Leave the
car down on the ground. Make a little gauge out of very stiff cardboard or
thin sheet metal that will allow you to measure the position of the center
of the spring plate as close as you can to where it bolts to the trailing
arm. This measurement should be in relation to the torsion bar bearing cap,
the alloy part with four bolts holding it to the torsion tube. I hate this
but here is a picture:
-----
-------------| o o | <-- Torsion Tube Bearing Cap, 4 bolts
) o o
| o | with torsion
bar end in center
-------------| o o |
-----
The gauge needs to align with something on the end cap. I cut
holes in the cardboard for two of the bolt heads so it looks
something like this:
---
| |
| -----------
|
o o |
|
|
---------------
Error is minimized if the gauge is long enough to get as far out as possible on
the spring plate.
Now, when the gauge is placed on the bolt heads the left hand side of the gauge
can be used to mark the position of the spring plate on the gauge. It is good to
measure and mark with pencil the center of the spring plate out near the
trailing arm to aid in this. Make the same measurement on each side of the car
with the car in the desired load configuration (driver, 1/2tank gas, etc.)
This measurement will be the loaded ride height reference. Jack up both sides
of the rear, use brush or spray paint to mark the position of the trailing arms
on the spring plates (don't scribe!), remove the trailing arms and make the same
measurement as above on each side of the car. This will be the unloaded ride
height.
The difference in the two marks on the gauge will be the suspension travel
between completely unloaded and loaded. Your goal is to install the new torsion
bars so that the correct unloaded measurement is obtained for the new torsion
bar stiffness.
I assume you are putting in larger torsion bars. So the desired new unloaded
height will be less than the old unloaded height because the new bars will have
to twist less than the old ones to support the same amount of weight. Simple,
yes?
For example, say the distance between the two marks on your gauge is 2 inch. Multiply
this distance by the ratio of the old bar stiffness to the new bar stiffness.
Stiffness for solid bars is proportional to the diameter to the 4th power.
Hollow bars will have a "solid bar equivalent" stiffness so use that.
Say the old bar was 23.5mm and the new one is 28mm (both solid). Then calculate
the new unloaded position (on the gauge, relative to loaded):
2 inch * (23.5 ^ 4) / (28 ^ 4) = .99 inch
So you measure down 1 inch from the "loaded" mark on the gauge and
index the bars to align the spring plates with that mark. This is a little
tedious because my gauge requires you trial fit the bearing cap and at least a
couple of the bolts (not completely tightened but not loose either) to get a
reference.
Maybe you can think of something better to use as a reference but the important
thing is that it has to be referenced to the torsion tube axis when the tube has
been dropped from the car. Don't make the mistake of taking measurements only to
find your reference
point gone when the bar is down!
A couple of other points. After you make the measurements and before you drop
the tube, reset the ride height eccentric to the center of the range. This will
provide you with a little adjustment range for corner weighting, ride height
trimming, etc. At first you might think that you'll blow your measurements but
remember we are making everything relative to the spring plate out near the
trailing arm, removing the eccentric from the measurement.
Replace any fasteners that show any signs of corrosion. White oxidation on
whatever they plate them with is OK but orange or red rust means the thing
should be trashed. Definitely replace the lock nuts on the height eccentric and
the fixed bolt there. Also the bolts and lock nuts on the trailing arm to spring
plate, regardless of their state of corrosion, because they usually get munged
up during alignments.
You will have to remove the alloy chassis brace parallel to and forward of the
torsion tube. It also helps to rotate the torsion tube a little CW (when viewed
from right side of car) as it comes down to clear the dogs on the transaxle.
Inspect the new torsion bars for nicks or scratches in the paint and re-paint
any problem areas. Grease the bars with bearing grease and coat the splines with
moly-based CV joint grease or similar before installation. Take care not to
scrape the bars on the torsion tube
during insertion. Any scratch through the paint will allow for the formation of
rust creating a stress riser and increasing the odds for sudden failure.
The parking brake cables can be left attached to the trailing arms if you can
stand to leave the arms under the car while you work. This will save you a
little time. Almost forgot, when you put the tube back up, one side of the
chassis has a hole for the bearing (the torsion tube to chassis bearing, down
low with axis perpendicular to the tube axis) and the other side has an
elongated hole. I think the right side has the hole (gotta start writing some of
this down). Be sure to align the tube/bearing to the side with the hole or you
will get the tube up there and find that you can't get the bolt in!
Great. You might want to think about other maintenance while you are doing
this. I usually leave the trailing arms attached at the axles, just wiring them
up in the wheel well using the upper shock mounting hole to secure them. But if
you need CV joint maintenance, this is a good time to do it.
Torsion Bar Replacement for the 944 created by Marc Belanger.
The most challenging (read: difficult) task in upgrading my car's suspension was
the
procedure for replacing the rear torsion bars.
step by step procedure I followed during my recent successful experience at
torsion bar replacement in my 1986normally-aspirated 944 (from 23.5 to 26mm).
Be warned that torsion bar replacement for a 944 is not a job for the faint of
heart. In fact
even experienced specialists will reluctantly admit that they have never
replaced the torsion bars on a 944. So youve been warned, proceed at your own
risk.
Please note that this particular procedure applies to my 1986 944; your
procedures may vary slightly (i.e. different calipers, ABS sensors, etc.).
Please also note that this procedure will require that you disconnect brake
lines, so all
usual brake system safety procedures will apply, i.e. bleeding, etc.
Procedure
1.Lift car onto axle stands on all four corners (approx. 16-18 inches above
floor will be
required).
2.Remove rear wheels
3.Remove rear anti-roll bar and associated drop links.
4.Disconnect rear brake lines (from caliper) at hoses.
5.Disconnect brake pad wear sensors, if your car has them.
6.Unbolt rear axles at CV joints on transmission housing. Place plastic bag
around each CV joint and seal the bags with tape around axles in order to avoid
contamination of CV joints by dirt and debris.
7.Temporarily suspend the CV joints and axle shafts (use any kind of strap) from
the transaxle mount cross member (it is located directly above the transmission
housing and runs right to left)
8.Remove the trailing arm to torsion tube mounting bolts that run through the
trailing arm pivot bushings (see diagram item 3).
9.As done with the CV joints, suspend the trailing arm and brake rotor
assemblies from the transaxle mount cross member. I find that a good quality
cargo hold-down strap works best. Wrap the strap around the rotor assembly then
up to the cross member.
10.Loosen the three trailing arm-to-spring plate bolts (see diagram items 7, 8,
9, etc.). Please note that some are actually alignment adjustment eccentrics and
must be gently knocked out from the outside to the inside. Attention: as you
remove these bolts the trailing arm and brake rotor assembly will now be free to
drop. So make certain youve tightened up the hanging strap that you prepared
in step 9.
11.With the trailing arm free, by slowly loosening the hanging straps now
attached to the trailing arm and the CV joint, you can gently lower them to the
floor (approx. 16"). The parking brake cable should be long enough to let
them down to the floor.
12.Remove the nut and bolt which holds the torsion tube cantilever arms to their
mounts (see diagram item 5), center-top of the wheel well.
13.Place a spare tire (or suitable substitute) under the torsion tube assembly.
The best thing to use is a mechanics hydraulic car jack with a suitable jig - my
jig is a 12" x 12" piece of plywood with a small sand bag on top of it
- just raise it up to the torsion tube housing and proceed to the next step.
14.Remove nut and bolt at torsion tube chassis mounts. The bolts run front to
back through the torsion tube chassis mount bushings (see diagram item 14)
located in the lower front of the rear wheel wells.
15.Once the mounting bolts are removed gently pry torsion tube down onto spare
tire (or
hydraulic jack jig).
16.Make sure torsion tube assembly is dropped low enough below the chassis so
that end
caps can be easily removed.
17.If you want to remove the entire torsion tube assembly from under the car
(optional) you must remove the three E clips that hold the parking brake cable
onto the torsion tube. Note: you do not need to remove the torsion tube from
under the car to
replace the torsion bars.
18.Take a look at the torsion tube end caps and note that their are four bolts
that fasten it
to the tube. First remove the bolt which holds the spring plate end stop located
on the lower rear location; you will notice that the spring plate rests on this
bolt. This will unload the torsion bar. Before proceeding to the next step you
should make a note of the position of the spring plate in the
"un-loaded" condition relative to the torsion tube assembly as this
will be valuable information if you decide to put the stock torsion bars back
into the car at a later date (doubt that will happen cause after youve
done this job once youll never want to do it again!).
19.Now proceed to remove the three other bolts holding the end caps on.
20.Pull off the end caps.
21.Pull out the spring plates and the torsion bars. The next step is to insert
and properly index the new torsion bars. In order to properly accomplish this
task you will require a sound understanding of the index adjustment procedures
for splined-end torsion bars. 911 (pre-964) guys and gals have understood this
for years - God bless them. At this stage, after having removed the torsion bars
from your 944 as described above, hold the torsion bar in your hand and take a
good look at it. You will immediately notice the splinned ends. Take a closer
look and you will notice that one end has more splines than the other. In fact
the inner end has 40 splines and the outer has 44. Now here comes the
fundamental concept of the torsion bar adjustment (i.e. indexing). Knowing that
a complete circle has 360 degrees, imagine that each end of the torsion bar is
actually 360 degrees divided by the respective number of splines. The inner end
has 40 splines therefore each spline is separated by 9 degrees (360 divided by
40) and the bars outer 44 splines are each separated by 8 and 1/6th degrees
(360/44) or 8 degrees and 10 minutes. (Remember your high school geometry? There
are 60 minutes in a degree.) Here comes the real leap of faith. Lets image
you are at the drivers side rear facing the spring plate. If we fix the outer
spring plate on the outer end of the torsion bar and then move the torsion bar
inner by one spline (in the counter-clockwise direction - moves the spring plate
up and lowers the car), we will have changed the position of the spring plate
(relative to the ground) by 9 degrees. Now if we fix the inner splines and now
relocate the spring plate (in the opposite direction, clockwise - moves the
spring plate down raising the car) we will be essentially removing 8 degrees 10
minutes from the 9 degrees we advanced on the inner splines, thus resulting in a
net 50 minute change in the spring plate position. Think about it, and reread
this paragraph until you get it. Due to the 4-spline difference between the
inner and outer splines of the torsion bar, adding then subtracting these
relative angles allow us to adjust the ride height to within fractions of an
inch.
If you understand the meaning of the preceding paragraph you are ready to index
your own torsion bars. If youre lost or confused at this point, get help.
Your next question will probably be "But how much do I raise or lower the
spring plate in order to get the ride height Im looking for?" If you are
not changing the torsion bars and you are starting from
the factory torsion bar position (you did take notes when you took it apart didnt
you?), you can estimate the ride height change (approx 6.5mm for a 50
minute change) by using basic principles of geometry a the rules of the great
queen SOH-CAH-TOA. If you are replacing the torsion bars with ones of different
rates or did not note the location of the spring plate (dumb-dumb) before
removing the torsion bar, the answer to this question, Im sad to say is:
"Trial and error my friend, trial and error." When I bought my new
torsion bars, and dared ask this stupid question, the customer service rep at
Automotion gave me that answer. I thought he was nuts! He also told me it would
probably take three tries to get right. I whimpered then cried. My better half
Kristine smacked on the head and snapped me back to reality. "Thanks Kris,
I needed that".
Unfortunately, he and she are right. It will take about three tries and a good
smack on the
head (maybe more) to get the ride height right. This effort is aggravated by the
fact that you must basically re-assemble the entire rear torsion bar assembly,
remount the wheels, then roll the car along enough to settle the suspension
before you can take the a valid ride height measurement. Once you take your
measurement you then have to disassemble the rear suspension once again to make
any necessary corrections to the ride height. You repeat this process until you
get it right. (Lord take me now!) The 944 is a wonderful car, but the design of
its rear torsion bar housing does not allow for easy adjustment. While I have no
first hand experience with a 944 rear end coil-over setup, it would certainly be
a great if you plan to change your rear ride height on regular basis.
