Double Bars Blues

[DMC-81]

Sorry to hear about this problem. It seems you got good advice so far on the adjustment. The DCF Club did mine a couple years ago and used a jig. Here is what it looks like in position:



When you get the new bars in and adjusted and if you choose not to apply heat shrink tubing as mentioned, you can opt to protect them with vinyl tubing after they are in place. I used a 3/8" i.d., 1/2" o.d. clear vinyl tube slit horizontally. I found this thickness allowed enough clearance between the bar and the rear hinge. The overall length was 26 1/4". I oriented the slit at the bottom, and fastened them with 2 small zip ties at the ends. This way, the tubing can be easily removed for bar cleaning, or whatever:



Good luck with the job.

[HollywoodF1]

I store my car with the doors open all winter to
release pressure on the bars,,,

It's not the static stress on the bar that ages it, it's the working. Every time you cycle the bar, you age it by one tick. You get to enough ticks and it fails. Assuming it were protected from corrosion, you could leave a torsion bar stressed for tens of thousands of years and nothing would happen. But if you cycled it day and night, it would fail in short order. Metal fatigue is strictly a function of the number of stress cycles (load-unload) and the amplitude of the stress cycles relative to the material's yield point. Trust me-- I'm a structural engineer. You can feel free to keep the doors closed this winter.

[AugustneverEnds]

The more I think and investigate the more I believe the roof box separating contributed to my bars failures. My T-panel was popping up for the past few years and the passenger side would make a creaking noise when it opened and closed. From where the bars broke it's possible they were coming into contact with the rear hinges especially when the doors were closed. I remember reading somewhere Grumman promised the bars would be good for 50,000 opening/closing cycles. Of course I have no idea how many times the doors were open and closed. Presumably the drivers side should have more use and should be the first to go. Maybe we should all install counters for the doors like an odometer or lambda counter.

[Rich]

Grumman's lifetime estimate may not have included any provision for DMC installing the bars in such a way that they rubbed against the rear hinge. Some of them interfere with the hinge, some don't and none of them should touch the hinge.

In case your roof box deterioration helped either of the bars hit its respective rear hinge then, yes, the roof box deterioration could be linked to one or both failures.

Had your bars both been protected/cushioned before they hit the hinge too many times they may both may still have been in one piece. Nothing you can do about that.

Recommendation: After your rebuild see where each of the replacement bars is. And cushion whichever one is against the rear hinge.

[David T]

It's not the static stress on the bar that ages it, it's the working. Every time you cycle the bar, you age it by one tick. You get to enough ticks and it fails. Assuming it were protected from corrosion, you could leave a torsion bar stressed for tens of thousands of years and nothing would happen. But if you cycled it day and night, it would fail in short order. Metal fatigue is strictly a function of the number of stress cycles (load-unload) and the amplitude of the stress cycles relative to the material's yield point. Trust me-- I'm a structural engineer. You can feel free to keep the doors closed this winter.

The "heat treat" (cryogenic) and twisting that Grumman did was supposed to induce more stress into the bars then they would see during their design life. The fact that all of the bars I have seen broken all happened at the same spot would seem to indicate the formation of a stress riser in that area and the most likely possibility is that rear hinge. Any surface defect (scratch) will cause the stress to concentrate in that area overstressing the metal, leading to failure. I don't believe they fail because of too many cycles (openings and closings). If that was so you would not expect all of the failures to occur in the same general area of the bars.

[Nicholas R]

The "heat treat" (cryogenic) and twisting that Grumman did was supposed to induce more stress into the bars then they would see during their design life. The fact that all of the bars I have seen broken all happened at the same spot would seem to indicate the formation of a stress riser in that area and the most likely possibility is that rear hinge. Any surface defect (scratch) will cause the stress to concentrate in that area overstressing the metal, leading to failure. I don't believe they fail because of too many cycles (openings and closings). If that was so you would not expect all of the failures to occur in the same general area of the bars.

It seems like what you're saying is contradictory. The generation of a surface defect comes from repeated cycling as HollywoodF1 said. Rubbing the hinge once wont do anything; but rubbing it 2000 times may. As he said, statically loading the doors isn't what ages them, it's the cycling (the point being that having your doors closed wont pre-maturely age your bars).

[David T]

I don't think what I said was contradictory. if you continually damage the surface then every cycle will do more damage and concentrate the stress. My theory is if the torsion bar rubs against the rear hinge it damages the surface creating the problem. If the bar never rubs or gets scratched by the rear hinge it should not fail prematurely. Another point. The number of door closings and openings does seem to have an effect, we see more driver's side bars broken then passenger side. That would make sense because you would figure the driver's side door would be opened and closed more often than the passenger side. My strong suggestion to every owner is to protect their bars by getting a piece of rubber in between the bar and the rear hinge. It can't hurt and may save them a bar! Rubbing (scratching) the hinge once may be all it takes but if you continue then every time you open and close you are making the scratch worse and reducing the strength of the bar.

[Farrar]

When you're done with the roof cage, you may need to adjust the door striker pins. Take your time and be patient. You've got this.

[AugustneverEnds]

When you're done with the roof cage, you may need to adjust the door striker pins. Take your time and be patient. You've got this.

Thanks a million! I know your car is giving you grief but you have already come so far with it so now every day is one more closer to it being all squared away. We shall overcome

[Farrar]

I know your car is giving you grief but you have already come so far with it so now every day is one more closer to it being all squared away.

Thanks, I appreciate that. It's hard not to get frustrated and doubtful, but it's good to try to treat every setback as something to learn from rather than something to be angry about. And if not, well, it can wait. The best lesson I've learned is to never work on the car when I'm in a bad mood. Take that for what it's worth. LOL