Torsion Bars - Removing, Replacing, and Indexing

Introduction

Replacing the rear torsion bars is not particularly difficult but reindexing them is time-consuming. Common reasons to remove the torsion bars include ride height adjustment and upgrading to stiffer bars.

Before beginning, measure and record the existing ride height. The recommended measurement point is from the ground to the top-center of the fender well (centered over the wheel). A nominal target is approximately 25 inches (63.5 cm). Avoid going significantly lower than that. Front and rear ride heights should be kept equal for best results.

A small amount of ride height adjustment is available via the eccentric bolt on the trailing arm spring plate — approximately 1 inch (2.5 cm) total range. If the eccentric is centered, you can raise or lower the car roughly 1/2 inch without removing the torsion bars.

The steps below reference the 944 rear-suspension components by item number (an exploded-view illustration is a regeneration candidate). Each step also names the part it refers to, so the procedure is complete without the diagram.

Tools

• Jack stands and floor jack
• Metric socket and combination wrench set
• Pry bar

Other Procedures Needed

• SUSP-05, Rear Sway Bar Removal and Installation

Removal

Note: Record the pre-existing ride height before beginning. If you are simply reindexing existing torsion bars, skip to Step 6 — Steps 1–5 apply only when installing different torsion bars.

1. Raise the rear of the vehicle.

2. Remove the rear sway bar and drop links (see SUSP-05).

3. Lower the vehicle back to the ground and bounce the rear several times to allow the suspension to settle.

4. Fabricate a measuring device with two holes corresponding to the bottom two bolts on the torsion tube end cap, to provide a fixed reference point.

5. With the vehicle on the ground, measure the height of the spring plate relative to the torsion tube end cap and mark the loaded spring plate position on the measuring device. This measurement is awkward to take with the wheel in place.

6. Raise the rear of the vehicle and place on jack stands.

7. Remove the rear wheels.

8. Unload the torsion bar by removing the bottom rear bolt (item 5) on the torsion bar chassis mount (end cap).

9. If installing different torsion bars, mark the unloaded spring arm position on the measuring device.

10. Disconnect the rear brake lines at the calipers.

11. Unplug the brake pad wear sensors.

12. Unbolt and disconnect both rear axles from their trailing arms. Hang them out of the way with bungee cords or wire.

13. Remove the bolt attaching the trailing arm to the torsion tube (item 1 — one bolt per trailing arm).

14. Support the trailing arms from the transmission crossmember using a heavy-duty strap. The trailing arm and rotor assembly is heavy and will hang freely once the spring plate bolts are removed.

15. Remove the nuts and bolts holding the trailing arm to the spring plate (item 2 — three nuts and bolts per side). Note that the rear two bolts are eccentric bolts for height adjustment. Tap the end of the eccentric bolt to free it from the spring plate if necessary.

16. Remove the nut and bolt attaching the torsion tube cantilever arm to the chassis mount near the top of the wheel well (item 3 — one bolt per side).

17. Place the floor jack under the center of the torsion tube assembly and raise just until it contacts the tube.

18. Remove the nut and bolt attaching the torsion tube chassis mount to the body (item 4 — one per side).

19. Gently pry the torsion tube assembly alternating sides until it is free from the body. Lower the assembly enough to slide the torsion bar out once the chassis mounts and spring plates are removed.

20. The torsion tube chassis mount is attached to the torsion tube with four bolts. Remove the remaining three.

21. Remove the torsion tube chassis mount and spring plate.

22. Pull the torsion bar out of the torsion tube.

Replacing

1. Determine the old torsion bar loaded-to-unloaded range from the two marks on the measuring device.

Note: If replacing a solid bar with a hollow bar, use the solid-diameter equivalent supplied by the hollow bar manufacturer in the calculation below.

2. Calculate the new torsion bar's loaded-to-unloaded range. A larger-diameter (stiffer) bar deflects less under the same load, so it has a proportionally smaller loaded-to-unloaded range than the bar it replaces. Recompute the range for the new bar from its diameter relative to the old bar (the original's worked example — replacing 25.5 mm bars with 30 mm bars from a 50 mm existing range — and the accompanying formula diagram are a regeneration candidate), then re-mark your measuring device before indexing.

3. Measure down from the loaded mark (top mark) by the new bar's loaded-to-unloaded range and mark that point on the measuring device. This is the new bar's unloaded position.

4. Install the new torsion bar into the torsion tube.

5. Install the spring plate and torsion tube end cap. Use the measuring device to verify the spring plate aligns to the new bar's unloaded position.

6. Install at least two bolts into the end cap to hold position while ride height is checked.

7. When both torsion bars are replaced, raise the torsion tube assembly into position with the floor jack.

8. Bolt the trailing arms to the spring plates with the eccentric adjusting bolt centered.

9. Bolt the trailing arm to the torsion tube.

10. Attach the torsion tube cantilever arm to the chassis mount near the top of the wheel well.

11. Attach the torsion tube chassis mount (end cap) to the chassis.

12. Lower the vehicle, bounce the rear end several times, and check the ride height.

13. If the ride height needs adjusting within ±1/2 inch, adjust the eccentric bolt on the spring plate.

14. If the needed adjustment is greater than 1/2 inch, unbolt and lower the torsion bar assembly, reindex as described below, reinstall, and recheck. Repeat as necessary. It is common to require three or more iterations to achieve the correct ride height. Also allow the suspension to settle over a day or more before declaring final height.

15. Once correct ride height is obtained, install the remaining end cap bolts and reinstall the rear sway bar.

Indexing the Torsion Bars

The 944 torsion bar has 40 splines on the inner end and 44 splines on the outer end. This means:

• One spline movement at the inner end = 360° ÷ 40 = 9.00° of rotation
• One spline movement at the outer end = 360° ÷ 44 = 8.18° of rotation

The trailing arm on a 944 measures 16.5 inches from the center of the torsion bar to the center of the hub. With the arm length and the per-spline rotation known, right-triangle geometry gives the approximate ride height change for each spline movement.

For the inner spline movement (9° per tooth), the right-triangle calculation gives a first estimate. Because the hub end of the trailing arm swings in an arc rather than dropping straight down, that calculation slightly overstates the actual change, so a correction for the arc is applied, giving:

Actual ride height change per inner spline movement ≈ 6.6 cm (2.58 inches)

Applying the same method to the outer spline movement (8.18° per tooth) gives:

Actual ride height change per outer spline movement ≈ 6 cm (2.35 inches)

Practical Application

Standing on the driver's side looking at the torsion bar end and spring plate:

• Rotating the inner end one spline counter-clockwise (with the spring plate and chassis mount fixed to the outer end) raises the spring plate end, lowering the car by approximately 2.58 inches.
• Rotating the spring plate and chassis mount one spline clockwise (with the inner end fixed) lowers the spring plate end, raising the car by approximately 2.35 inches.
• Net result of these two combined moves: the car sits approximately 0.23 inches lower (about 6 mm).

Use these values to plan the number of spline positions needed to achieve your target ride height adjustment.