kN is Kilo newtons.

Rad = radian.

 

back in the early 90's ,we worked with john powell ( who built the escort challange vettes )to stiffen the frame .we reviewed the results with dave mcClellan.what we did was cut off the spotwelded flanges a section at a time and mig-weld the frame rails to make them more of a tube. it made a significant improvement in stiffness.my company produced all of the c4 frame except the front hinge pillars,a-pillars,cowl top and windshield header. those parts were made by MTD (MODERN TOOL AND DIE ) IN CLEVELAND (they're the same MTD that makes lawn tractors and yard equipment).

 

The "fundamental" structure has not changed, but when the C5 was developed, the frame was originally designed for a convertible. Apparently the idea was to do the engineering for a ragtop first even though the C5 'vert didn't appear until the second year of production. That way the chassis had the extra strength and rigidity built in for the coupe and the FRC.

The C4 was designed originally as a T-top and that changed to a targa design. But the initial design was that of a coupe and it's a major reason why the 'vert got the X-brace and is prone to flexing until the factory hardtop came along. A properly installed hardtop using the included steel brackets actually makes the 'vert stiffer than a coupe. The hardtop has a halo bar similar to the coupe but the frame around the top side where it attaches to the windshield frame is stronger than a coupe targa top assembly.

One thing about the C4 convertible X-brace compared to the R-D Racing version is that the factory brace has more attachment points. The back half of the factory brace is bolted to the floorpan in 6 places; in addition to the bolts at the outsice corners that attach to the frame. Each rear part of the brace attaches to 3 studs that are in the floorpan. The R-D brace does not attach to the floorpan, only to the 4 corners of the frame.

 

Thanks for posting those dimensions,

I'll use them to update my CAD model

 

Interesting thread. I'd really like to see some torsional rigidity numbers from the aftermarket solutions, though the same test would have to be done on a stock frame to ensure that the testing method is consistent.

There are 2*pi radians in a full circle, so you can convert using 2pi rad = 360*

EDIT: Brian, how accurate is your CAD frame now? I'm assuming that's in SolidWorks or something similar, which can run torsional rigidity tests. May be useful, if dimensions are taken of x-brace/camber brace/other solutions and added to the CAD frame, that would at least give an idea of the improvement. In theory at least...

 

Yep. There is ~6.3 rad in a full circle, the way that I under stand it. What I don't understand, is:
"Coupe (top on) 13 Hz and the torsional strength is 214 KN/rad"

If one Rad = 57*, are they saying that it takes 48,000 lbs-ft to twist the frame (from front to rear), 57*?? I don't think so. BY the time you hit I'd guess 45* the frame would have kinked already and be destroyed.

The number of degrees of twist is important (to me) to understand. How many degrees of twist are they talking, for that 214kn, or ~48,000 lb-ft?

 

^What he just asked^

 

It's a unit, like mph. You don't have to drive a whole mile to figure out how fast you're going. They only twist it a degree, or maybe even a fraction of a degree. pounds per degree is (I think) the Imperial unit for torsional stiffness, but the SI unit is kN/rad. Force per unit of rotation. Divide 48,000 lbs by 57* and there's the force that it takes to twist the frame one degree.

Also if the CAD frame is SolidWorks, or can be converted to a SW document, I'd be willing to help in some of the leg work to create the aftermarket solutions if given dimensions. I have a vert x-brace that I can take some measurements on, if you could send me the frame file to fit it to.

 

......One thing about the C4 convertible X-brace compared to the R-D Racing version is that the factory brace has more attachment points. The back half of the factory brace is bolted to the floorpan in 6 places; in addition to the bolts at the outsice corners that attach to the frame. Each rear part of the brace attaches to 3 studs that are in the floorpan. The R-D brace does not attach to the floorpan, only to the 4 corners of the frame.[/QUOTE]

I have the R-D X-brace & it was NOT worth the money.Someday I would like to design my own X-brace & attachment points & have someone weld it professionally with titanium or high strength aluminum to save weight.The R-D X-brace is heavy & not a good design in my opinion.I installed their Targa Truss & that made a night & day difference.It was the best mod I did to my car.Next on the list is the camber brace & the brace that mounts between the seats & mounts to the seat belts to box in that section.I would love to tear down the entire chassis & stitch weld it along with adding tubeing to some sections as well as welding in a roll cage that does not block or hamper getting in or not being able to store the targa roof in its place.With a solid chassis the C4 would be the best sports car or definately one of the best(my opinion)

 

Thank you for the info! I'm trying to picture what you did. Cut away the frame and add in tubing? I'd be willing to do this, just need more info.

 

Instead of the metal pices butting and being spot welded like this; ____||____
Their company cut off the || part and welded them like this: _____--\_____ <-over lapped seams, continuously welded rather than butted seams, spot welded.



Ahhhhhhh!! (the light bulb just went on). Thank you.

So the C4 is about 850 lb-ft per degree...end to end. This is good to know.

 

Brian you need to find some one local that has one of these. It will save hrs upon hrs of time and be spot on within .0002

 

It retains a small percentage of carbon so weldability remains like any carbon based steel.

 

That R&D Targa truss is beyond hideous. All their stuff looks like it was made by a fabricator in a barn with a MIG welder and a chop saw.

I've had some ideas for a bit but have been a tad money challenged lately to get down and take the TIG welder to it. Soon enough though. This thread justifies the need.

 

^I agree...and I'm in the same boat. Been "fantasizing" about how to address this on my own car for...too long. I'm finally prioritizing this. I want to move.


Copy that. Thank you.

 

Sort of. The force is applied at the suspension mounting points, where road force is applied to the frame, so using the wheelbase instead of car length would be more accurate for calculating ft-lbs. Unless you did that, in which case my bad (I didn't actually do the math).

 

Going to be hard to do in the limited amount of space. But I can give you a few Ideas. You need to think way ahead if you are going to do a project like this. You cant build something with heims joints in it and expect it to make the car stiffer.


"So the C4 is about 850 lb-ft per degree...end to end. This is good to know."

It will be less than that. Bout 400-500 or so.

 

My suspension model is pretty decent.

In order to do what you want with the frame model, I'd have to get in and hollow it out, it's a solid, to the wall thicknesses on the car. Which is not constant, not to mention the overlapping pieces.

I really just use it to see if I have the mount locations correct.

 

Guys, I don't know what the actual number is, but it's gotta be significantly better than 400-500 ft-lbs/degree. I got way better numbers than that on my C3 frame when I did a few tweaks to it a while back when I put the C4 suspension on it. (I anchored the batwing mounts to my hoist, and then twisted the frame at the front axle centerline.)

 

I do have plans...that they do include much more than heim joints and bolts. But what are your "ideas"?

Can you further explain your position on the torsional stiffness? I derived my number from the data that Aardwolf provided...