Well, the thread said measurement, not comparison.

I still think that he will have a hard time getting even comparative data with a digital level. He'd need to put one on the lift to measure the deflection of his fixture, so then he has twice as much uncertainty. I just don't think .1 degree resolution is enough. I think a plumb bob and pointer would be better. Fix a non-load bearing stiff beam to the chassis and a string with weight at the same spot, then you can either do some trigonometry to find the angle by measuring the much larger distance that the end of the beam is from the plumb bob. You could even use a laser for the beam and measure on the ground to the plumb bob.

I think the simplest method is a beam that is perpendicular to the loading beam that is a few feet long with a plumb bob at the end, as well as one where it meets the chassis. Measure the distance between the two attachment points of the strings as the beam length, then measure the distance between the parallel strings after the chassis is loaded. Then do sine^-1 of the distance/beam length. You will have to account for any offsets between the strings when the chassis is unloaded (they should be inline with each other in the unloaded condition).

Edit: nevermind. The separation of the strings for .1 degree with a 3 foot beam is only about 1/16th of an inch. Not much.

That was the title, I think. When you read his first post, I read the intent to be comparison. And the title warned us (pseudo science). Who knows, we will have to await results.

Burgomeister once posted the results of his testing of front shock tower movement (AKA, how much do I really need a brace). His results persuaded him, as a full on automotive engineer, that they really don't flex much even though he was quite aware of how his methodology fell short of what he'd have done at GM or wherever. And of people's anecdotes about paint worn off the sides of hoods and the like.

This is now getting down to some real science, though. Flieger and Paul Abbott (who also sounds a lot like an engineer) have made predictions based on engineering science. Will we discover if the sun distorted the starlight at total eclipse? Oh, wrong theories being tested.

I got back and resumed and revised the baseline measurement based on many but not all the suggestions made here. I know it could be done with more precision, but this is as much time as I'm willing to invest into it for now. On with the actual modifications soon.

I separated the loaded lever from the measurement process, and I am using degrees rather than distance the beam was deflecting.

I als strapped the chassis directly to the rear lift support arms via a spring arms mount bolt.

The front still balanced at 53 kgs. I zero'd the digital level across the steering mounts, then put 40kgs at the end of the 2m lever. THe car visibly twists, and I got a -0.3 degrees of twist across the steering arm mounts with the weights on.

I then took the weights off and checked that I got the original baseline setting the same and I did.

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more

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John - this is in the best tradition of shad tree race car engineering!

Did you put the level on the rear end too, to verify that it didn't move despite your ingenious attachment system?

The photos of the lift pads are to show the "visible" part?

40Kg @ 2M = 80 Kg/M = 579 lbs/F if I did the conversions correctly. So 173 lbs/ft per 0.1 degree, or 1730 lbs/ft per degree? Or is torque the wrong force? How does this translate into the units Porsche uses for chassis torsional rigidity?

Springs are normally specified in pounds per inch. Torsion bars, which also work as springs do, would usually be specified in degrees twist per unit of torque, and to translate to pounds/inch need to have A arm geometry factored in. You don't see those figures in torsion bar sales catalogs - just their diameter.

But if we take 200 pounds per inch of suspension movement as a guestimate (I think that's well above stock), what does the 0.3 degree value measured (if we ignore factors like measurement error, and how the level rounds up or down) mean as a spring rate equivalent? You'd have to know the distance from the torsion bar center to the ball joint center or some slightly more accurate measure, like the tire centerline on the axle). I don't know that, and it is Saturday morning.

Next in the analysis might be to learn what forces act more or less routinely on the chassis. For instance, if you can corner at 1.2G (not out of reach with good tires), how much torque does that put on the front? Since the rear also leans in a turn, would you also need to know what the differential is?

But that exceeds John's goal, I think, which was to learn if he has stiffened the chassis after doing what he plans to do?

I get 1925 ft-lbs/deg +14%/-20% using his .3 degree deflection. That means between 2300 and 1650 ft-lbs/deg. Seems a bit low compared to what we were expecting based on the 964.

2300 ft-lbs/deg = 3126 Nm/deg
1925 ft-lbs/deg = 2616 Nm/deg
1650 ft-lbs/deg = 2243 Nm/deg

You need to read chassis at back and at front and subtract the two readings, strapping to your lift is not a rigid mounting and deflects during your loading process. Also, better to place your level on the structure and not on your beam, your beam assumes a curved shape during loading and if your level isn't placed in the same location then it will measure slope of the deflected beam in addition to the chassis twist.

The figure reported earlier was for the 993 at 24,000 lbs/ft/degree. An order of magnitude stiffer?

Keep in mind this car has some nice rust damage along the rocker panels and I suspect kidney bowls.

This would be fun to test on a Targa.
My friends say that they can see my car twist in the roof line when I auto-x.....

Doesn't sound right, as the 996 was much stiffer, and quoted as just over 20,000 nm/deg. What was the source?

Here's what I'd previously backed out:

'88 911 7000 nm/deg (measurement by burgermeister)
964 11500 nm/deg (993 was 20% stiffer)
993 13900 nm/deg (996 was 45% stiffer)
996 20120 nm/deg (Excellence was Expected p.1381)
997 33000 nm/deg
991 40000 nm/deg (porsche used 20% torsionally stiffer as a design target, has claimed "up to 25% stiffer")

Early car chassis had less steel to begin with, and if there was any rust... Short way of saying I don't think this is an order of magnitude out, and in fact it would be close to what I'd guess. Unfortunately the measurement resolution doesn't look like it will be enough to divine subtle differences...

I know you're done, but... laser pointer strapped to the chassis pointing at the wall. Mark the spot with an without load, measure the distance between the spots... If you want to get really fancy, strap a second pointer at the back of the car to take out the movement of lift.

Laser pointer accuracy and measuring distance? My el cheapo construction laser pointer is accurate to maybe 1/16" at around 20 feet. Could laser resolution slant end-calcs significantly?

Sherwood

You measure from the center of the spot, but 1/16" at 20 feet is .015 degrees. Order of magnitude better resolution...

Two thumbs up for two laser pointers. Accuracy isn't a problem since it is motion differential between beginning dot location and ending dot location and not pointing accuracy.

Donohue in The Unfair Advantage reported the 73RS coupes had stiffness of "over 2000 ft-lb/degree" without a cage.

In post #34 I stated 993 torsional stiffness as 24,000 but it was just a grab from a quick internet search trying to put some kind of number to the question at hand.

Some progress....

In another thread I documented how I cut out my old sunroof and made a patch from the old sliding portion. Here's what it looks like posing with the 2 products I used...

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I also mocked up a set of brackets I am thinking about welding in around what I am guessing are weaker areas prone to flex. Interested in comments about these.

I also got myself a set of dimple dies and a press and played around a bit in anticipation of making the brackets.

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also a knee bar with gussets and a bar across the sills behind the seats, with gussets to the sills and the center tunnel.

also moving the shifter up 3" and back 3"

Mine is a coupe.

I don't think the top B pillar braces will do much. Seems to me that the arch of the roof is one of the strongest/stiffest parts of the shell and it forms a sort of triangle with the A pillar so there isn't much load going through the B pillar, at least in shear. So I think the windscreen pillar brace will do much more than the B pillar corner gussets.

The bottom gussets for the lock post should be worth their weight. They would be approximating a diagonal door bar from a roll cage.

Hmm, I think these gussets would be horrible, put in a cage instead.

If you don't like that idea, I have another one that I have been playing with in my mind:
Make an extra layer of sheet metal to cover the sills, a-pillar down to the sills, b-pillar down to the sills, roof line all along the c-pillar to the shelf, over the windscreen. Make lots of dimple holes and bevel the edges so you get a nice distance to the existing metal and weld it in.
Sort of an integrated roll cage.

I have been borrowing ideas from the other caged cars I have here, incuding an identical 83 with a Bond full weldin cage.

I don't want a cage as I wont be wearing a helmet often in this car.

I am going down the road of what you suggested, though I don't plan on completely "reskinning" the posts and sills. Just reinforcing and gusseting.