Torsional Rigidity Measurement...warning pseudoscience involved
 

Wow, 11 replies based just on pictures before I even add the text, I'm surprised!

I am planning on a bunch of changes to this car's shell including shaving the gutters and seam welding along with some reinforcements and rust repair. I do not want to install a cage for a few reasons, not least being weight.

I thought it would be an interesting exercise to see if I can measure whether these changes along the way make the chassis measurably stiffer or more flexible. This is not an attempt to calculate with laboratory precision anything at all, just a measure of better/worse and hopefully with some measure of magnitude (ie. a little or a lot.) I am not going to calculate torsional loads or claim any level of precision or relate it to how other cars might perform.

I did a 'baseline' measurement yesterday afternoon. AS it would be the one to which I compare all my future measurements I was as careful as possible. I got repeatable results.

I'll load a diagram of the overall measurement when I get home but here are some narration of the pictures.

I started by stripping the car back to its bare shell. I then put it on the lift and strapped the rear torsion bars to the rear lift arms. I dont want the car to rock on the lift and I want the rear pivot to approximate the rear suspension loadpoints. The straps are very tight, there is no play or motion.

http://forums.pelicanparts.com/uploa...1405287571.jpg

Then I lowered the car so that the front of the tub came to a rest at its front/center on a jack which was in turn resting on a scale. I believe I want this front weight (54kgs) the same when I go and do future measures because a difference could indicate the car isnt resting on the pivot points the same as in the baseline case. I noted the front arms of the lift were 2 centemeters clear of the car so they wouldnt interfere with any twisting of the chassis.

http://forums.pelicanparts.com/uploa...1405287586.jpg

http://forums.pelicanparts.com/uploa...1405287602.jpg

Then I attached a 5mm (6 gauge?) angle iron piece across the front suspension at the crossmember points. Again, this approximates one of the suspension load points. Plus it was easily accessible.

I measured out exactly 2 meters from the centerline of the car and attached a ruler to the bottom of the angle iron.

Then I placed a laser level and recorded its unloaded measurement.

Then I put all the extra brake discs I could find on the end of the bar (41 kgs) and measures how much the bar had deflected. In this case it deflected 6.1cm.

http://forums.pelicanparts.com/uploa...1405287635.jpg

http://forums.pelicanparts.com/uploa...1405287648.jpg

http://forums.pelicanparts.com/uploa...1405287660.jpg

http://forums.pelicanparts.com/uploa...1405287683.jpg


Posted here for my recollection and your (constructive) comments!

I see what you're trying to do. However, suggest using rigid bar to anchor the chassis to the lift instead of nylon strapping.

Please continue.

Sherwood

Really interesting thread, very curious what you find out.

....but the really interesting thing is what to do to increase rigidity. There have been so many ideas, some of which make great sense (welding the drip rails), X brace in the front end, later G50 bracing of torsion tube, but anything new in that regard would be outstanding.

Dennis

That rig looks way too soft to me. Are you calculating the deflection of that beam, the rubber rear bushings, the nylon straps, and correcting your measurements? The rig looks to be less rigid than the chassis to me.

I'll be following this to see what mods will make a noticeable improvement.

I think the front crossmember mounting points are the right ones, although it would be interesting to see if there is any material difference between that location and the front A arm mounting point. It supposedly is part of the looseness of the early 911 chassis.

I couldn't tell what you were using at the rear. The logical point would be the torsion tube, but I think you would need to bolt some sort of plate to the chassis where the spring plate bushing is retained - in pictures of Celette mounting, they seem to use that location as one of the major points of construction.

Dennis

Dennis - I suppose he could bolt his beam to the front A arm mounts. But why:

My take on the load paths is that just about the loads go through those two bolts he is using, which is where the reinforcing cross member bolts into place. It holds the steering rack, the rear A arm mount, is where the torsion bars load the chassis, and is where most of the side loads and braking force are taken up.

About the only loads the front bushing of the A arm sees are the very attenuated (because of the long lever arm) side to side loads. Maybe the steering loads? Nothing vertical that I can see, except for what have also to be very small forces created by the front axle not always being directly in line with the reinforcing crossmember mounting points.

So I have not understood why the 934s and 935s or thereabouts had those elaborate X shaped tubes running forward over the fuel tank, and down. Perhaps they are just to protect the tank some, or make the front end distort somewhat less in the inevitable racing crash?

Chuck Moreland must have thought about this when he designed his piece which connects to his strut tower brace stuff. Best I have heard is that they help in a crash. But that's not to say that they contribute to torsional rigidity.

Think about it - if the front mounts move some, they would move up and down torsionally. That would alter by a little bit the plane of the A arm. But not change camber, and I don't think even change toe, at least not much.

But this is a noble experiment. I don't think he needs to figure in the bending of the beam. If he has dial or other indicators under each mount, he can get the twist and easily convert that to degrees per unit of force, no?

Perhaps I can help with a vintage thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/259557-structural-reinforcement.html

Sherwood

Interesting...subscribed.

The 934 and 935 had coilovers that changed somewhat the points at which loads were fed into the tub.

Study Porsche race cars long enough and you'll conclude that about the last thing on the engineers' minds was crashworthyness.

I would agree with the comments about the test rig set-up.

JR

That is a good point- if you are only measuring the chassis itself and taking a difference in angle between the front and rear then the deflection of the rubber bushings at the rear anchor points or the beam in the front doesn't matter.

Agreed, I wonder though if the front A arm see's lateral movement, which being somewhat "hinged" with the inner fender and trunk floor could cause a bit of toe in/out on bump. The X frame would stiffen it laterally more than anything.....

Which in turn means that for this noble experiment it may be not all that useful.

Dennis

Yes, thanks Serwood. That one was one of the inspirations to try and get some quantifiable measurement.

Agree. Suspension deflection, while important, is an independent variable with chassis rigidity. Should measure the basic structure first and not muddy the calcs by including pieces that hang from the chassis.

John,
I overlooked you were the original poster. I hope all is well in the PNW.

Sherwood

The angle will fail by buckling the leg pointing toward the floor in compression with very little load application; it would be better if rotated 90 degrees and mounted using spacers to clear the leg from the chassis...better yet to use a rectangular tube. However, be aware that the mounting points were not designed to be used for torsional chassis stiffness measurements and as such may be limited to what load they can take. Your long lever arm will apply quite a force to the bolts in tension and compression and they were designed more for lateral and compressive loading; not for loading in tension.

I like the digital scale; simple to measure load applied to the lever arm.

Use two digital levels and take the difference in level readings between them to calculate chassis twist.

I expect the plot will be non-linear until you generate enough twist to get the entire structure working as a unit.

It would also be good to know what stiffness you might want to achieve. This can be deduced by estimating the ultimate grip at a front tire and then use this as a load condition for which you want to control chassis twist to some acceptable level such as 1 degree. These two numbers (tire grip and torsional wind-up) will provide a target for chassis stiffness. The distance from the road to the torsional centroid axis of the chassis could be assumed to be about 24" unless a better number is known. This is my guess for a coupe but the centroid height for a Targa would be more like 12". Certainly there are those with greater technical contribution for this but I'm just thinking the process through for your awareness.

I expect the 935 cross brace was to help stiffen top of shock tower by providing another load path to a support point. Also, by cross-bracing the open front compartment it becomes more rigid as a unit and helps reduce deflections at the suspension pick-ups.

It would be VERY interesting to have a coupe to compare your torsional stiffness numbers to. At least there would be a standard to aim for.

Still digesting your other comments but note thisis a coupe...a 1978 US market sunroof coupe.

For the same reason measuring the beam and bushing deflection isnt necessary I dont think doors are necessary...Im just looking to make internal comparisons to the same car over and over as I cut gutters, add reinforcements, add lightness, etc.

The doors have clearance at the latches so a torsional test would not have them contributing stiffness until the clearances at the latches were closed which would allow for the doors to share some load. Better to forget their contribution.

Glad to hear it is a coupe instead of a flexible flyer (Targa).

I have a 71 targa, next time its on the lift ill see how it compares.

The straps don't bother me as long as there is no gap to the lift. I would however agree with not measuring the same bar that's applying the load. Perhaps you could apply the load elsewhere? Or do two parallel bars with a gap in between, load one and measure the other?

Even if you're only trying to get relative measurements it'd be nice to see a more representative percentage difference. If bar flex is 90% of the deflection and you cut chassis deflection in half, your new measurement will still be 95% of the original. Repeatable perhaps, but better signal to noise would be nice...