3rd gen chassis stiffening

[Wicked72]

I building a beast out of my 72 and it will do more then just a straight line as well. so far I am going with s&w race cars 6 point. sub frame connectors, xv inner fender braces, engine compartment brace, and lower rad support. maybe boxes Ive never seen torque boxes before. I was wondering if there would be any major diff if I full or partial welded all the seams? yeah thats a ton of work but the car is completely apart so if im gonna do it then now is the time. would love some insite thanks guys 

[HPP]

Yes, welding up seams do make a measureable difference in overall rigidity. There are some sanctioning bodies that totally prohibit seam welding in stock classes. I'd concentrate the effort from front to back with emphasis on the cowl/a piller area.

[Mike DC]

            
A rollcage can work wonders to stiffen the car, but not any rollcage will do so.  

I personally can't imagine wanting to use a rollcage out of a catalog for any car of mine.  Custom built or forget it IMHO.  Mail order cages can definitely save your life, but they won't fit the car as comfortably and they usually aren't designed with chassis stiffening in mind.  
 

[FLG]

           
A rollcage can work wonders to stiffen the car, but not any rollcage will do so.  

I personally can't imagine wanting to use a rollcage out of a catalog for any car of mine.  Custom built or forget it IMHO.  Mail order cages can definitely save your life, but they won't fit the car as comfortably and they usually aren't designed with chassis stiffening in mind.  
 

Mike,

I agree...BUT...I can say Ron (firefighter) has one of their cages in his 68 and from my understanding and first hand experience in his car...it fit great.

[firefighter3931]

S&W make the best fitting rollbars/cages.    My chassis guy has been building rollbars, cages and rail dragsters for over 30 years and he was totally impressed with the S&W kit that i had purchased for the 68. His words ; "by far the best kit i've ever seen"

Mine has the "Pro-street" rear bars which follow along the roof line/C-pillar so you can't see them from outside of the car. The door bars were the only custom bend item to retain the arm rests. He did recommend installing the down bars through the floorpan and welded them into the subframe connectors which really helped triangulate the whole unibody into one rigid SOB ! 



Ron

[Wicked72]

I am actually going with the same cage ron has   Whats the deal with torque boxes btw?

[firefighter3931]

The torque boxes are redundant if you're installing a roll bar....you won't need them.


ron

[Mike DC]

   
My comments on mail order cages apply less to 6pt jobs than to the 8pt+ cages that involve A-pillars & halo bars.  It's usually the stuff in front of the main hoop that really gives you fitment trouble with pre-bent cages. 

I also tend to favor running the A-pillar bars through the dashboard, especially if it's a street car.  Dashboard frames aren't all that hard to obtain and it gives you a lot more room than the alternative.  Pre-bent kits generally never do this.     

[Wicked72]

Hey ron can you post up your link of your cage?

[RallyeMike]

Out of curiosity, what seams are you talking about welding?

If you are going for more than straight line performance, be sure to brace your steering box mount. It makes a noticeable difference.

[Wicked72]

I am getting my front suspension from Reillymotorsports so I think I should be ok at least with that. as far the seams I would love some insite with that. Im not sure how much weld I should go with or even where I should weld. I was thinking just about everything up front but if anyone has pics or some good insite on it that would be great! I am planning on running at VIR and Summit Point once shes done. Its not going to be a race car but I do plan on playing so i figure since I have it completely tore down I can start getting into things I can get to now.

[firefighter3931]

Hey ron can you post up your link of your cage?

Here ya go Wicked, pics in this thread :

http://www.dodgecharger.com/forum/index.php/topic,1940.0.html



Ron

[HPP]

as far the seams I would love some insite with that. Im not sure how much weld I should go with or even where I should weld.

Well, anywhere there are two pieces of metal overlapping each other, weld the seam shut. This is a tedious and time consuming mod to perform, but it firms things up considerably. I read an analysis of this exercise on a Mustang. Using a method to measure degrees of deflection front to back, they were able to reduce deflection 65% over stock before adding any subframe structure or roll bars/cage items.

Think of it this way, how well would your clothes hold up if they were all buttoned together instead of sewn? Think of the factory spot welds as buttons holding the car together. Your stiching all the seams shut.

[Mike DC]

           
A 65% increase?  Wow.  That's most of the way to doubling it.  I wonder if the Mustang in question was a freak case or not. 


I've often wondered how it would turn out if you compared a factory unibody versus a hand-built tube framework done exactly the same way.  Subframes made of rectangular tubing rails in place of the factory U-shaped sheetmetal rails, rocker boxes that have butt-welded edges (where the inner & outer stampings join) rather than spotwelded flanges, etc.

[HPP]

 I don't know if that result is typical of unibodies, typical of mustangs, or an aberation. It will provide a sizeable increase for sure, the actual percentage will probably vary from car to car. FWIW, SCCA  does not allow any seam welding in their stock level classes. They feel it provides too big of an advantage to allow it.

That would be an interesting exercise in the tube design vs factory stampings. I know when I was building race cars, we stitched every  seam and capped every tube.

[Mike DC]

 
For that kind of increase (and without a weight penalty!) I'm surprised the OEMs aren't employing seam-welding.  It would cost more but I could see them doing it on short-run jobs like special hi-po editions of the ponycars.   

Or you could also think like an OEM with efficiency in mind - if you can increase the rigidity like that for no weight penalty, then you could also reduce the amount of stiffening efforts that go into the original design and then use seam welding to make up the difference.  Same stiffness, less weight.       

[HPP]

Well, new OEM stuff is a bit of a different animal than our older stuff. With 3D modeling of CAD/CAM and finite element analysis, I'd imagine newer model cars have redesigned unibodies that have the additional structural stiffness with out the associated labor of stitching the entire car together.  The few I have poked around in have pretty different layouts in structural components and quite a few have longer weld runs in some critical areas without having to do the long welds over the entire structure.

[Mike DC]

             
So are there any particular spots that are known to be beneficial?  Of course it depends on the specific car's design, but surely there must be some spots that can be counted on to help more than others.  Just saying "it helps to weld all the seams on the car" seems unnecessarily vague.   


I remember once carrying around the rear subframe of an old Mopar (without the trunk/floor skin on it) and thinking it seemed less torsionally rigid than a rectagular tubing version of it probably would be.  I wonder how much difference the floor skin would have made by forming the top side of the U-shaped rails. 

[HPP]

Most of the reading I've done on the subject points to the engine bay/cowl/A piller area as needing the most work. You've got several major structures that all come together there- roof, floor, firewall, front stub, door supports, and inner fenders and they are all subject to very high loading from the weight and torque of the engines as well as front suspension loads. XV's analysis on the four post rig confirmed the front third of the car needing as needing additional support, which is why they developed their lower radiator and inner fender braces. They believe the back third of the car is reasonably well supported for performance driving.

The shortcoming in their research is that it was all handling oriented. Anyone who has spent enough time at the drag races can tell stories of rippled quarter panels or broken joining seams resulting from high torque, high rpm launches. Any drag cars I've ever been involved in usually had fabricated rear clips and/or roll bars/cages. The additional support of these items seem sufficient to eliminate any unwanted flex in the rear. Would seam welding back here help out as much as additional uprights, I don't think it would, but if you have an engine with enough torque to lift the front end and rotate the body enough to flex the quarters on take off, you should probably consider some additional bracing in the rear. In this case, goods sub-frame connectors are a requirement. Torque boxes might even be a good idea too because they will tie the leaf spring mounts, floor, and rocker sill all together. For additional stealth approach, maybe some diagonals tied in from the rear package shelf structure, contacting the wheel housing and terminating at the leaf spring shackle area. This would provide bracing similar to a roll bar with out protrudig in to the passenger compartment.

Was the missing floor board the reason for the rear clip flex you had experience with, I think so. Even with the floor, I doubt it would have been as strong as a tube frame. Think of it like a shoe box - without a lid, you can twist a shoebox quite a bit. Put the lid on it and the deflection is reduced considerably. Tape the lid down and it moves even less. Put a tubular structure inside the box, it becomes highly rigid.

[Mike DC]

                       
I'm sure the front end needs more help than the rear end.  At least for handling. 



For drag racing I imagine the stresses are much more on the rear half of the car compared to handling purposes. 

A wheelstanding launch is pretty intense if you think about it.  Imagine fastening both of the car's rear leaf spring shackles to the pavement with heavy chains.  Then put a pair of floor jacks underneath the front eye bushings of the leaf springs on both sides.  Then start pumping both the floor jacks upward until the car's front wheels are hanging in the air. 

And that's just the front-to-back loads of a wheelstanding launch, let alone the torsional twisting involved. 

[dangina]

most drift cars benefit from seam welding the seams - i do it  on all my race cars - its a pretty big difference in the way the car feels - especially if your putting her sideways..

[Mike DC]

most drift cars benefit from seam welding the seams - i do it  on all my race cars - its a pretty big difference in the way the car feels - especially if your putting her sideways..

I assume you are talking about cars that are fully rollcaged (including bars linking to the suspension areas) as well? 

Interesting. 



The main design principle for stiffening a car is that it's all about WHERE you put the steel (rather than how thick it is, what alloy, etc).  It surprises me that the standard factory joining methods really still leave that much stiffness on the table to be gained.