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ST4 Torsion Bar Suspension Conversion Legality


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Question about suspension legality for ST4 / TT4:

From the rules:

1d) Relocation of suspension mounting points is permitted in ST1, ST2, and ST3, provided that the modifications do not violate any of the other rules above. One possible method is via the use of modified mounting point brackets attached to OEM mounting locations. [Other than 1c. above, conversion from coil spring to coilover shock spring, and the additional of a panhard bar, torque arm and/or watts link, relocation of suspension mounting points, including “drop spindles” (spindles that have had the axle shaft relocated upward), is not permitted in ST4. Bump steer kits and replaced/modified control arm ball joints are permitted. Note: ball joint may be welded to the spindle for added strength].

My car:

1987 Porsche 944 Turbo (Same question would apply for all 944/968 and all pre-1990 911's).

944 rear suspension is a trailing arm with a traditional shock plus a torsion bar at the pivot point of the trailing arm.  Basically every modified 944 removes the torsion bar and replaces the shock with a coilover.  This coilover still uses the stock shock mounting locations.  This conversion makes it infinitely easier to lower the car, and enables corner balancing.

Would this coilover be ST4/TT4 legal?

It is still using the stock mounting points, but the location of the "spring" has been changed.
Regardless, the suspension of these old cars is worse than a more modern multilink rear suspension (bmw e36, etc).



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  • National Staff

Hi, I just posted this in response to a question about Corvette transverse leaf spring conversion to coilovers in the other thread, but it gives you the answer:


There were competitors out there ready to protest others for moving coil springs to the coil-over location.  There was a lively internal e-mail debate between NASA Officials as to whether such a protest would be confirmed or denied, because it was never intended for that modification to not be permitted in ST4-6.  No NASA Official argued that a change from leaf spring or torsion bar suspension to coilover suspension was not a relocation of suspension mounting points (which is not permitted in ST4-6).  So in the recent revisions, we clarified that it was permitted for coil springs.  There was never an intention for a vehicle in ST4-6 to have a complete rear suspension redesign, such as transverse leaf spring to coilover or torsion bar to coilover, which clearly require bolted in parts to be removed from OEM mounting locations and a completely new spring design.   It is not the same thing as moving a coil spring on each side of the vehicle (generally not actually attached, but just sitting on a perch) to the coil-over position (can't really mount a leaf spring or torsion bar around a shock...).   From a performance perspective, there are advantages of both the transverse leaf spring design (reason that Chevy kept if for so many years) and the coilover design.  However, we have not found that these vehicles (or the 944's with torsion bar design) need any modification to the rear suspension to be very competitive in the ST4-6 classes.   So, there are no Officials who supported this allowance.  Of course, it is permitted in ST3 and above.


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