frame flex

[Danielg]

Been working on and off on the cobra, body work is very close, but door gaps are now the issue.
The cobra is on a two post lift, engine and suspension, wheels tires, installed, body is bolted to the frame with the hard rubber 1/8 body gasket, roll bar installed.
The doors close perfect,
now lower the car onto all for corners and the door gaps at the latches closes up enough to have to make it harder to close the door, maybe a 1/16 at the most.
Do your cars flex that much? So do they flex this much while driving, let alone spirited driving?
My intent will be to add X brace accordingly, etc. but is this normal flex and how do you address it.
Thanks
Daniel

[ERA Chas]

Patrick will now post a picture of his ERA jacked in one corner of the frame and the doors and all are unmoved. For the 1000th time.
But to answer your question, really, ERA's don't flex. Bob has the stiffness figures-like zero deflection per 1 jillion pounds or something crazy like that.
I'd guess JBL's don't flex at all either.
Having said that, if you're not competitively tracking, why not live with it rather than the construction of chassis X's and stiffeners? That's lots of work

[patrickt]

Originally Posted by ERA Chas

Patrick will now post a picture of his ERA jacked in one corner of the frame and the doors and all are unmoved. For the 1000th time.

The artistry of that pic was the mellow golden hue from the trouble light reflecting off the garage floor under the raised wheels. I also have a pic of three wheels off the ground, with one jack point, and no flex at all. If I could figure out how to do four, with one jack point, I'd really impress myself.

[Danielg]

Okay, ????? I can do the same thing with my trans am , Ferrari , and Detomaso, however......it's a Contemporary Classic Cobra not an ERA or any of the above.
I am concerned it may flex enough to chip paint or .......?. if I adjust the doors to a weighted position on all fours, what happens in a corner when you unweight so to speak one side, will it pull hard enough mess up or pull off a striker? or the other side close up enough to to some kind of cause and effect damage?

[patrickt]

Originally Posted by Danielg

... if I adjust the doors to a weighted position on all fours, what happens in a corner when you unweight so to speak one side, will it pull hard enough mess up or pull off a striker? or the other side close up enough to to some kind of cause and effect damage?

Honestly, I have never heard of that happening with a CC -- they are well built, frame-wise.

[patrickt]

Here's a little blurb on the CC's frame and body from the original literature. I almost bought one in the early 80's, but didn't. I saved the literature on it, though... of course.

[Wbulk]

DanielG, I think you have a valid point. I had a similar situation, at one point I started thinking about frame flex as I have an expensive aluminum radiator that was designed to be hard mounted to some welded on ears on the frame. I was concerned with eventually cracking the radiator so I did some testing. When I jacked up one front A arm the frame flexed more that .025 until the other front wheel came off the ground. My car has a K member and I found that the maufacturer just went from 4x3 to 4X2 steel for the K member. At the suggestion of an engineer friend that builds cars I decided to add an X brace to the K member. I made it out of 1 1/4" .120 wall DOM tubing and it took a couple of days.
After it was all in I did the same test again and the frame did not move at all. In fact when I jacked up the one side all the way the other side stayed on the ground and the whole side with rear end lifted off the ground.

I have seen a lot of frames that if I was building the car I would have X braced the frame between the seat area and X braced the K member.

This was how I did mine. One of the best decisions I made with building my car.

[scottj]

Originally Posted by Danielg

Do your cars flex that much? So do they flex this much while driving, let alone spirited driving?
My intent will be to add X brace accordingly, etc. but is this normal flex and how do you address it.
Thanks
Daniel

I think these cars flex more than people think... or want to believe. Maybe not to the extent to be problematic on street tires, but certainly noticeable with slicks on the track. With slicks I've pinched the passenger door in the opening, tight enough to split the inner and outer panels above the striker. How do I address it? I accepted the fact that a ladder-style chassis is a terrible design when it comes to torsional rigidity... then I increased the spring rates to the point that the chassis performs as a spring as much as the actual springs do... and the car got faster.

[legenmetals]

Interesting Post. Yesterday I noticed a 3 inch indentation on the fender of my Contemporary. This indentation is barely noticeable, I only noticed it because of the lighting. This car has been on a jack stand (one stand only same corner as fender) for three months. The other corners are on the tires.
So today I went to the body shop that last did the work for me and the owner explained that on a fiberglass Cobra where any two panels are bonded (as the fender and foot boxes are on this car) you can over time see the movement in the fiberglass. The tension between the two fiberglass panels is causing the line that I am seeing.
He then showed me a steel fender with an inner support that was installed so that it backed up the outer skin of the fender but was not attached directly to the skin itself of the fender. He explained to me that the heat of the sun on the outer skin would cause the skin to expand quicker than the support and create a flaw in the finish of the fender.
He told me fiberglass will always move.
I would have to assume that a stiff chassis with the latest suspension and the body firmly mounted to the chassis is the trick way to go.
Just a note....... Henry Ford designed his early cars so that the chassis would flex with their buggy spring suspensions and rivited frames.

[Wbulk]

I read through this thread again and realized there are two issues to address.
First to be safe when setting door gaps during construction the frame should be dead level front to back and side to side. If it is not the frame flex could be on one end of the flex spectrum or the other.
Second after reading Patrick's advertisement on your car it appears to be a 4" round tube frame. I have never driven an original cobra but I have talked to a friend who has driven a couple, and was told the originals were not good at all. Some that use that design just up the thickness of the tube, others also add a lot more bracing to significantly improve the strength.

Take a look at this Kirkham frame and note the extra cross bracing to stiffen the frame. Hope this helps.

[legenmetals]

Are the Kirkham aluminum body cars built with a birdcage type structure within the body that is tied into the chassis?

[Danielg]

Good input, thank you.
Yes the frame is flush on a welding table, body bolted down, body work and gaps set accordingly.
Set it on the ground with suspension, wheels etc. it more or less sags in the middle so to speak.
I think the weak link is where the rear frame is connected to the main rails, effectively the leverage placed there weighted is flexing the frame.
Image grabbing a long paint stick and holding the end with vise grips and push down on a table, the grip point does not bend but flexes the length of the stick.
I will add a X member like I did on my old race frame, with and X to the rear frame upper cross bar.
I never imagined this car would flex this much. For kicks and giggles I see if I can set up a before and after. Engineering 101, I know it will reduce torsional twist which reduces flex but how much in the real world?

[patrickt]

Do you have the sway bars and torsion bars?

[patrickt]

Here's a pic of the CC frame:

[Danielg]

Mine is a late CCX, it has coil over front and rear, floor pedals and front and rear sway bars.

[legenmetals]

would the prelaod of the torsion bars cars add rigidity to the chassis?