Interesting torsional Rigidity Test on a 280Z

[largeorangefont]

Not an unreasonable theory.

What I am finding confusing is that if you're seam welding where there are existing spot welds, you shouldn't make a HUGE difference in rigidity either way, you should just add support to those not-that-awesome spot welds.

I always thought of seam welding as a durability modification more than a big rigidity improvement.

Agreed. There are not many cars that I can think of that are seam welded without a cage being installed which would make any weakness from seam welding moot.

Regardless if his data is right or wrong, considering the platform it is hardly applicable to a modern car.

I also agree with clocker, a 240 is as stiff as a wet noodle to begin with. I think all seam welding did was move where the structure flexes.

[DeathMetalKillsEmos]

It could be that the seam welding didn't make the chassis weaker, only less stiff. The strength/stiffness of a metal part is greatly influenced by how it is manufactured. Most of the time, a manufacturing technique is selected for a certain part in a specific application based on the loads that part will experience. If the part is required to be stiff, but does not require a high ultimate strength, the part will be manufactured or post-treated to achieve a high modulus of elasticity. If a part is designed to yield long before failure, it is possible to manufacture or post-treat the material in such a way to achieve this result. I am guessing that the stamped sheet steel they used on the chassis wasn't carefully selected to maximize the overall torsional stiffness of the chassis in the first place, considering it is a street car. Adding weld to the seams almost certainly added to the strength of the seam just by virtue of the added material in that location. However, introducing those kinds of temperatures definitely has a huge effect on the surrounding material (especially MIG welding where there is limited heat control), and it is more than believable that the material properties of the surrounding metal would have changed drastically after welding. Perhaps the result would have been different if he had used a TIG welder and controlled the heat better. Either way, I agree with the tester that seam welding would probably have a negligible (at best) effect on the overall chassis stiffness. Probably in large part a marketing gimmick. One would be much better served by properly designing a roll cage for maximum roll stiffness that is mounted (and properly triangulated) at the suspension pickup points.

[ZDan]

Modulus of elasticity isn't affected by heat-treating/annealing etc.  It's around 29-30Msi for steel, period.  Whether it's 50ksi ultimate mild steel or CrMo treated to 150ksi ultimate strength.

Heat treating drastically affects strength, doesn't affect stiffness.  I haven't checked, but I would say that 10% is probably due to measurement error.

[gc3]

i think you're right.
hardness/strength are affected by heat treat, but stiffness, which is what the torsional rigidity test is testing, should be the same.

id be interested in seeing a clockwise and a counterclockwise test ran and see if there is some strange preload built up.

[mattster03]

Modulus of elasticity isn't affected by heat-treating/annealing etc.  It's around 29-30Msi for steel, period.  Whether it's 50ksi ultimate mild steel or CrMo treated to 150ksi ultimate strength.

Heat treating drastically affects strength, doesn't affect stiffness.  I haven't checked, but I would say that 10% is probably due to measurement error.

I agree that 10% could easily be measurement error, but it still would be in interesting find (IMO) that on this chassis full seam welding made no appreciable difference.

[digitalsolo]

I still don't see why seam welding would make it stiffer anyway;   that would mean you had flex in your spot welds, which shouldn't normally happen.

I definitely seeing it preventing failures of spot welds through load distribution though.

That said, I have no plans to seam weld my cars.   I'll just subframe connector and cage them, personally.

[DeathMetalKillsEmos]

Modulus of elasticity isn't affected by heat-treating/annealing etc.  It's around 29-30Msi for steel, period.  Whether it's 50ksi ultimate mild steel or CrMo treated to 150ksi ultimate strength.

Heat treating drastically affects strength, doesn't affect stiffness.  I haven't checked, but I would say that 10% is probably due to measurement error.

My mistake, you are correct sir.  The material's elastic modulus should not change appreciably. I guess my overall point was just that welding the seams will most likely have minimal effect on stiffness of the chassis since it is just blindly adding weld to many locations that are probably not even be on the primary load path for a pure torque applied to the chassis. Since a unibody is not designed to be especially stiff in terms of load triangulation in the first place, seam welding just seems like an expensive and pointless way to add weight.

Another thing to consider for the test. I didn't think he made it extremely clear in his post, but is he applying an equal and opposite force at the same location on each side of the chassis? For example, is he applying an upward force on the same member on the other side of the chassis? If it was not a pure torque, some of the deflection he measured could have been due to the chassis bending. Maybe I just missed it in his posts? Either way, cool experiment.