It would seem the resident GM engineer doesn't think the backbone works and has issued a challenge of sorts over on the BB. Here is what he posted. By the way 3.5 millimeters = 0.137795276 inches or 1/8 of an inch, and it was steel they used.
Is the DDM Backbone made from steel or Al?
Some things to consider:
-When does a vehicle encounter significant torsion loading?
-How does the tunnel section generate "stiffness"?
-Can you pick the change out of several blind swaps?
I am getting a lot of questions in Private Message system about the benefits of the DDM Backbone.
We did work with increasing the tunnel belly pan thickness during the Z0K development, and tried double thick steel plate, approx 3.5mm. The predicted change in torsional stiffness was predicted to be minimal, and was not detectable blind.
Tell someone what part is in the car, though, and they definitely "FEEL" it. It would be interesting to spend a day in a garage, have several people evaluate a controlled blind study (the study would be randomized, the conductor of the study would not be an evaluator, and would consist of 6 evaluations where the belly pan was removed and reinstalled). No cross discussion allowed, as consensus can truly ruin a blind ride. It would take the better part of a day, but would be very worthwhile.
During the development of a vehicle, we do not always have the luxury to do blind evaluations, but I love to do them when possible. Nothing resets your ability to evaluate a vehicle than a blind evaluation. Nothing can be so humbling, either.
We also did a series of blind evaluations when increasing the rear crossmember stiffness, during the development of the GXP Z0K. The results of stiffening the rear crossmember were so dramatic that we went through the trouble of producing a specific beam for use on the Z0K club racing models. Expert evaluators could consistently, 100% of the time, determine whether a stock or a "stiffened" rear crossmember was in the vehicle. Normal drivers, on one evaluation run, had 11 of 12 able to pick the differences between identical vehicles, WITHOUT cross discussion.
My conclusions from this development work are that it is critical to increase stiffness of the rear crossmember.
DO THIS FIRST before moving on to other structural improvements. There are beams, braces, etc., but the critical part is that the rear crossmember must increase the stiffness of the beam and must attach the structure to the CENTER OF THE BEAM itself. If you do not do so, as in attaching a gusset at the corner of the beam and the drop-down bushing bracket, you will have wasted your time.
I know this from experience, my first attempt at strengthening the rear suspension structure was exactly this scenario. It did lead on to understanding how the crossmember functions in the system.
From people using the example of a thin Coke can, remember that Al has a fatigue limit no matter the strain. Steel has a strain limit, under which the fatigue limit is basically infinity. Consider too that this small can, the thickness of two sheets of standard paper, holds at times in excess of 100 PSI. It is all about the proper constraint and optimization of how the material is stressed.
The tunnel belly pan, and the trans belly pan both operate primarily in SHEAR, not bending. It closes out the tunnel, and provides the fourth side of a mostly square "torque tube" that is connected to the rails. That is why the number of fasteners (a dozen by memory) are so important. Constraining the edges is critical. The technicians used to mess with me (in jest, of course), removing a bolt or two in the tunnel belly pan during evaluations and changes. I would come back after and evaluations, puzzled by the change in steering or ride shake, and they would admit they were messing with me and put the bolt or two back in. The center bolts seem to be the most critical. So is cutting into the trans belly pan, so do not mess with this pan as it seems to be critical in providing the proper structural tie in to the front parts of the side rails.
The car is very stiff in bending, there is a large diameter torque tube, two large rails, and good structural tie-in to the loading points. Also, the tower offset at front is so close to the very large rail that we determined (again, by analysis and later evaluation rides) that a front tower brace is basically a waste of mass.
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Second Post
In fact, regarding a blind ride, I will volunteer my personal time some weekend in August to run a blind ride for the DDM backbone, if someone wants to sponsor it.
We could run a couple of them, depending on the results of the first one, I would suggest a comparison of the thickest one to the stock panel.
If that is successful, we could run a second one on the thinnest backbone vs. the thickest one, or even vs. the middle one.
If it passes both rides, I would consider it very conclusive that there is a detectable effect. It would be fun for me, I always love interesting puzzles. LOL
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