Torsional Rigidity Measurement...warning pseudoscience involved

[ganun]

Did anyone read the Pano article on the development of the Targa about a year ago?
Too much to cover here on the forum, a must read for its how P did what your trying to do with about as much resources. Very interesting and informative.

[Walt Fricke]

JohnJL - do I have this right: You put weights 6' out toward the end of your long bar, and measured how much closer to the floor the bar was with and without the weights?

If you did that, I would suggest that what would be a better measuring point would be from the bolts holding the bar to the floor. That is chassis twist, and you don't have to worry about how much your long bar bends. And you can measure how much down one bolt moves, and up (or whatever it does ) the other bolt moves.

I know you aren't worrying too much about quantifying and precision, but couldn't you use two dial indicators on some kind of stand to do this measuring? They are not expensive, maybe you have a couple, and surely you have friends who do and might bring some over, and maybe those magnetic mounts with rods and thumbscrew clamps which often come with them? This should be easier to read than a tape measure or the like.

As to the nylon straps, they are going to stretch. I'd be inclined to use chain somehow, and a chain tensioner like truckers use, if it could be used in the space you have. Maybe wrap it around the torsion tube?

But there is rubber bushing stretch. Perhaps you can compensate for all the stretch back there with measurement? Treat the unstressed tub as a plane. If one rear side moves up, deduct that from how much that side moves up in front. Ditto with the side moving down?

I think this is a really great thing.

[1990C4S]

I am in the GTA, if you need material let me know. I have a bit of everything...

[JohnJL]

Originally Posted by Walt Fricke

JohnJL - do I have this right: You put weights 6' out toward the end of your long bar, and measured how much closer to the floor the bar was with and without the weights?

If you did that, I would suggest that what would be a better measuring point would be from the bolts holding the bar to the floor. That is chassis twist, and you don't have to worry about how much your long bar bends. And you can measure how much down one bolt moves, and up (or whatever it does ) the other bolt moves.

I know you aren't worrying too much about quantifying and precision, but couldn't you use two dial indicators on some kind of stand to do this measuring? They are not expensive, maybe you have a couple, and surely you have friends who do and might bring some over, and maybe those magnetic mounts with rods and thumbscrew clamps which often come with them? This should be easier to read than a tape measure or the like.

I cant quite visualize what you mean. Both my dial indicators are gravity-based. I guess you mean mount them upside-down and push up the plungers to measure the change in distance with/without the load? I would have to build a solid rig to hold them too, something that can be moved away and stored, then brought out again in a few months as things progressed.

[1QuickS]

Use a digital level on repeatable mounts at each end of chassis, better two levels that are not moved during your measurement process. Zero them and record readings during loading intervals. Remove load and see if they return to zero. Total chassis twist is difference between level readings, no need to deal with motion at your support points. Total spring rate is difference between level readings (twist) divided by load times the distance from center-line of chassis to point of load application on the beam.

By the way, the digital scales will indicate less than the actual load applied since load is applied between the supports of the rear torsion tube and the single point at the front where the scale is located. Actual weight applied can be calculated from the digital scale reading and the geometry (location of supports and load application beam) but easier to measure individual weights.

If you could attach loading beam to the tops of the shock towers then I think there would be less risk for damaging your chassis (pulling out a threaded mount). Also, the loading at these points of application will be lower in magnitude than at the threaded mounts for the steering rack for a given torque about the chassis. The upper shock mounts are designed for vertical loads so they are a better structural interface.

[JohnJL]

I've got a longacre digital camber/caster level. I'll give that a shot too and see what angle changes I see when placed across the croaamember lcoation where the beam is bolted.

[Walt Fricke]

I've never thought of dial indicators (or their digital successors) as being gravity based. They have springs pushing parts around, and are independent of gravity. Yes, I envisioned some kind of moveable support - a big tool chest, just about anything. Plus you don't need the chassis high off the floor to do your measurement.

Paul's idea of dispensing with distance measurement and using angles directly makes more sense. Digital levels are fairly inexpensive, can be zeroed at any actual angle with a tangent to the earth, and would, as pointed out, go directly to what you want - degrees of twist per units of weight.

I think the crossmember mounts are strong enough for this. They are welded into the lower pan sheet metal, and again in a higher level you can't see because the fuel tank is in the way. For lateral loads, the crossmember has a recess around the bolt holes, and the chassis mounts have a collar sticking down. The two mate, and thus take up side loads. But you aren't using much weight. I suppose you could build up a structure over the shock mounts to clear the inner fender metal - but you have one side pulling up, for which the shock towers are not really designed.

So the only thing I think is wrong in your protocol is to be measuring out at the end of the flexible beam.

[djb25]

I agree with Walt - you're really measuring the flex in your piece of angle iron and the twist in the chassis.

Actually - you may not have reached the point where the chassis is twisting. There is really no way to tell based on the way you have things set up.

If you follow Walt's suggestion and measure movement at the suspension mounts, you'll only be measuring the chassis movement.

Great experiment! Can't wait to see more results!

[540964]

You need one of these
SPMM4000 - Moving Body Suspension Parameter Measurement Machines: Kinematics & Compliance Test Machines

[911pcars]

Originally Posted by 540964

You need one of these
SPMM4000 - Moving Body Suspension Parameter Measurement Machines: Kinematics & Compliance Test Machines

Oh yeah. Let's order up a SPMM4000. Can you quote us a price.... out the door?

Djb25 I agree with Walt - you're really measuring the flex in your piece of angle iron and the twist in the chassis.

Actually - you may not have reached the point where the chassis is twisting. There is really no way to tell based on the way you have things set up.

If you follow Walt's suggestion and measure movement at the suspension mounts, you'll only be measuring the chassis movement.

Agree. If X ft.lbs of torque is applied at the end of the bar (angle iron), then that is an element that must be considered in the calcs, especially if it flexes.

Here's a link to improving the chasis rigidity of an Ultima GTR, a very quick kit car. Of interest are some chassis rigidity numbers of various vehicles at top left.
Untitled Document

Sherwood

[TCracingCA]

How the engineering study is translated, is when it gets really interesting. Maybe we can get some Las Vegas types odds and pay-outs going on strategically where we all would guess they are going to go!

[1QuickS]

Everything flexes unless at absolute zero when molecular movement stops.

Use two digital levels and any flexure in bars or movements at attachment points is irrelevant.

[Flieger]

Dial indicators will give better resolution.

[1QuickS]

Dial indicators give better resolution but you need four and then the trig to resolve to angular rotations. Absolute accuracy does not seem to be the end requirement but a method to determine progress in chassis stiffness modification. Also, indicators must be mounted to a ground reference that is not affected by the torque applied. In this case that ground reference would be the garage floor which is somewhat below the points of interest so reasonably substantial mounts would need to be constructed just to locate and support the indicators.

Then again: The stiffness of a 993 is around 24,000 ft-lb/degree and a cabriolet/Targa is about 1/3 of that. A 911SC is probably somewhat below that figure but would still be pretty stiff. So, to load your chassis to generate meaningful deflections you would probably need to use something more than a few hundred pounds of weights which implies that a battery of indicators might be required for a home garage stiffness test.

[JohnJL]

So if the expected ballpark of stiffness of a 78sc was say 15000 ftlbs per degree, someone willing to estimate the weight i would have to put at the end of a 2 meter bar attached to the crossmember points? My digital level is pretty good, has 10th of degree resolution.

[JohnJL]

So if the expected ballpark of stiffness of a 78sc was say 15000 ftlbs per degree, someone willing to estimate the weight i would have to put at the end of a 2 meter bar attached to the crossmember points? My digital level is pretty good, has 10th of degree resolution.

[jpnovak]

Originally Posted by JohnJL

So if the expected ballpark of stiffness of a 78sc was say 15000 ftlbs per degree, someone willing to estimate the weight i would have to put at the end of a 2 meter bar attached to the crossmember points? My digital level is pretty good, has 10th of degree resolution.

So 15000 ftlbs/deg x 0.1 deg is 1500 ft pounds. If your lever is 10 ft long you need 150 lbs to get that 0.1 deg deflection.

[Flieger]

Originally Posted by jpnovak

So 15000 ftlbs/deg x 0.1 deg is 1500 ft pounds. If your lever is 10 ft long you need 150 lbs to get that 0.1 deg deflection.

And you want well over .1 degree of twist to get any sort of accuracy (otherwise you will have a result that is 7500 ftlbs/deg plus or minus 7500 ftlbs/deg since the level will round up or down). Which is why I favor the dial indicators.

[JohnJL]

Thanks guys.

Wouldn't it be 75 lbs though, since the deflection is across the plane? Half the twist is "up" on the far side of the beam and half is on the same side as the beam?

....Banker doing physics.....

[1QuickS]

No, the torque will cause one attachment point to rotate "up" and the other "down"; their total rotation will equal the torsional stiffness of the chassis (in degrees per foot-pound of applied torque) times the torque applied.

One data point is not a curve so like Flieger said you will want a few data points to plot a stiffness response curve. Your data will be scattered and most likely non-linear, especially for the first few points. If you have only the one level then you will need to have very good mounts for it. The mounts must have very good locators so when the level is shuffled between them they will provide repeatable readings. Thermal distortions will affect your readings so stay out of the sun.

The 150 pounds applied at 10 feet from the centerline of the car will generate approximately 825 pounds of vertically applied load to each of your steering rack mounting bolts; one in tension and one in compression.

It would be easier to load the chassis if you used concrete anchors to provide a point in your garage floor to attach a turnbuckle to your lever arm. Your digital scale will resolve the applied load and you don't need to shuffle weights around or have an excessively long lever arm.

By the way, since you are using the digital scale to provide loading data you might want to correct its output to show actual weight/load applied. Easiest way is to add 200 pounds to the steering rack mounting points and observe the scale's output. This will be a constant factor for whatever load is applied, the digital scale will read perhaps 85% (guess) of actual weight/load applied.