cage mounting and physics?

i've been looking over various cage constructions and notice that additional diagonals are often tied to a main tube just above the frame mount instead of being directly connected to the frame & main tube in a compound weld. For example in the latest issue of Corvette Magazine there is a pic of the P & M GT2 chassis with this. The front frame mount is actually a short (~2") length of tube sticking up from the frame to which are welded the 4 diagonals that go the front cross bar. Why not have these diagonals meet on the frame welding pad? I guess that it makes the joint fitting and welding easier to go this way? I can also guess that some sanctioning bodies that limit roll cage contact points (NASA) may count it as '2' instead of '4' contact points. Is there a physics reason such as limiting stress at the frame or transferring the forces? I thought the ideal design was multiple triangles that meet at the vertices not near the vertices.

Steve

Do you have a pic of what you mean?

Twin_Turbo said:

Do you have a pic of what you mean?

Click to expand...

http://i254.photobucket.com/albums/hh104/beretta_07/utf-8BSU1HMDAzMDAuanBn.jpg

This is an exaggeration of what i'm talking about but you get the general idea. BTW, this cage had some other design 'issues' as well. The corvette gt2 case is much cleaner, of course. In that case the two angled members meet the 'upright stub' about 1" off the frame (small magazine picture made it hard to tell).

I believe the practicality of being able to get a good weld overrides the "ultimate" placement of the junction.

Ozzy said:

I believe the practicality of being able to get a good weld overrides the "ultimate" placement of the junction.

Click to expand...

that's what i thought. But, I have seen examples of airframe weldtest that require joining no less than 5 members at the same junction so it must be possible to do.

I'd think that the way they welded that in your picture would have way less strength that it could have. Here are a couple pictures of aircraft cluster welds.

http://www.cattrell.net/images/hatz_weld_cluster_10-2-08.jpg
and
http://www.weldingtipsandtricks.com/images/4130-cluster-weld.JPG

Steve J said:

Ozzy said:

I believe the practicality of being able to get a good weld overrides the "ultimate" placement of the junction.

Click to expand...

that's what i thought. But, I have seen examples of airframe weldtest that require joining no less than 5 members at the same junction so it must be possible to do.

Click to expand...

An airframe will not allow the trade off of speed and cost vs. ultimate strength. But race cars aren't airframes and trade offs occur.

Looking at the pic you posted, if the intersection was at a common location, then the weld on the diagonal bar on the bottom back side would be against the floor pan. The easy way out, raise the tubing. If this was an airframe that would not have been an option.

I agree, they took the easy way out. It would have been stiffer if they nested all the tubes on the base plate, There's no reason why you would not be able to get in there with the tig torch.

Especially the diagonal one, how much more work would it have been to nest it with the other 2.

is it just me or does that first picture look like a nasty overheated bit of welding? Looks like I did it!:crutches:

GT6Steve said:

is it just me or does that first picture look like a nasty overheated bit of welding? Looks like I did it!:crutches:

Click to expand...

Yeah, seriously. Not what i'd want to see on a roll cage.

Twin_Turbo said:

I agree, they took the easy way out. It would have been stiffer if they nested all the tubes on the base plate, There's no reason why you would not be able to get in there with the tig torch.

Especially the diagonal one, how much more work would it have been to nest it with the other 2.

Click to expand...

So is it agreed that there is NO GOOD REASON for these style of tube placements? I can accept the limitation of labor/cost for this but still wonder what is being given up in terms of cage strength and rigidity. Can one of the MEs out there explain what having the diagonal meet in a raised position would do to the strength? I would think it takes some of the tension out of the diagonal and turns it into a shear across the main tube betwee the diagonal intersection and the frame attachment.
damn, really wish i could find a pic of the gt2 chassis for everyone to see what i'm talking about.

steve

You are correct a well designed structure will react the forces thru a common point. The poorly executed designs in the pictures react the forces in mid tube which introduces bending loads it the tube and reduces the stiffness of the whole structure. Neither factor is usually desired in a cage.

Grampy

That's the reason why I did it like this, it was a PITA and I didn't feel like TIGing it all together, it's MIGed but it's fine and all the tubes are nested