Stiffening Up A Ladder Frame

This is all just conjecture at this point, but curious what others here would do.  I see a lot of information on my Online Googles about buying aftermarket frames, VetteCarts, and Ghettocets, but not much on strengthening regular old parallel ladder frames to improve handling. A full cage really isn't what I am looking for, for a few reasons.  And merely boxing the rails, while helpful, doesn't appear to add as much torsional stiffness as I would like. 

A space frame type of setup would be nice, but probably a bit beyond my capabilities at this point. (That doesn't mean I might not start a thread on that in the future)

So what would you do with an orphan ladder frame that you wanted to beef up?

Unfortunately, I think you just ruled out the simplest method and the most effective method (boxing and space frame/cage, respectively).

Think outside the box? Make the chassis a bigger box? Being unboxed blows. Adding ties more frequently might help? If there are sections where you can actually box all the way between the rails, that should help.

Making the rail more like a tube helps. Larger cross-section tubes get stiffer pretty fast.

When I think about this sort of thing (speaking of conjecture), I tend to think about how I can create partial monocoque sections.

Oh, and of course you've got a body to sit on this ladder, right? How much can you gain in terms of improving the body's stiffness, and improving its connection to the chassis?

Install an X member.

This thread is relevant to my interest. 

Im currently in the planning stages of a pro street style challenge car build.....

In reply to Ransom :

I haven't ruled out boxing the frame; in fact it is mandatory, IMO. But I'm not interested in stopping there. I am merely asking what can be done in addition to closing up the channel of the rails to stiffen it up corner to corner. I've looked at some of the offerings from various chassis companies for the C10 , and see how they add bracing to build a tunnel through the center of the frame, but I also see that they use what appears to be a taller rail in the first place.

  I'm kinda just taking baby steps at this point to understand the forces at work, and what to do.  I really wish I could find an online simulator to play around with, but so far have come up empty .

In reply to Ransom :

...continued-

And as far as a "space frame", per se, I might be on a different page, and not quite understanding the term.  I don't want to build a space frame from scratch at this point, (well, actually, I would LOVE to build something akin to the Factory Five setup, but I don't think I am anywhere near ready to understand that at this point; baby steps and all that) but incorporating that idea with factory rails is certainly on the table.  As long as I can get it inside of the profile of the body and not look like a tube buggy, I am fine with it.

The body is a roadster, and not too much help in reducing flex, unfortunately.I don't mind running a framework inside of it, but I'm trying to keep the entire package as light as possible.

The problem with ladder frames from an engineering perspective is that all the major structural members on the vehicle lay within a very short vertical plane. This means that all attempts to stiffen the chassis that focus on the frame are pretty futile in terms of strength to weight ratio. I would probably not spend that much time or effort trying to stiffen the actual frame.

Instead, investigate how the body attaches to the frame. For instance, the car probably uses rubber body mounts to isolate nvh. You could make find stifffer mounts or even rigidly attach the two. The body on a body-on-frame vehicle won't be as stiff as a normal unibody car but you can still use the stiffness it has to help with the overall stiffness.

I would also look into some sort of roll cage that could be bolted onto the frame. The closer you can mount it to the suspension the better, also keep it as tucked to the roof as possible. You want to have your major structural members on as great a vertical separation as possible and the closer you get to the suspension the better. Triangulation is your friend.

If you can't do any of that, then as a last resort I would build a big x-frame that goes from the center of the car to each corner of the car at the suspension pickup points. Look up the lotus elan frame as a good inspiration. Keep everything as vertically tall as possible! And it need not be made out of a bunch of tubes, a lightweight sheet metal structure will be more efficient!

If you can use solid body mounts then you can use bracing to triangulate to the body and even though you may not consider the body to be stiff you can use it to stiffen the chassis. You can triangulate to the firewall doing something similar to what's seen in the pic below. In the example below vertical deflection with and without without braces was measured and deflection was reduced approximately 1.5". I'm still trying to figure out a good method to test torsional rigidity improvements without the use of a body shop frame machine.

I make a line of products for 2nd gen GM F bodies to stiffen up the chassis and suspension mounts which are equally important.  https://lab-14.myshopify.com/  Depending on what year your truck is you may want to reinforce the steering box mount section orf the frame. 

[URL=http://s240.photobucket.com/user/NOTATA/media/Lab%2014%20products/20160622_191905_zps3siwdgab.jpg.html][/URL]

[URL=http://s240.photobucket.com/user/NOTATA/media/Lab%2014%20products/20160622_183527_zpsjgrgdr4x.jpg.html][/URL]

[URL=http://s240.photobucket.com/user/NOTATA/media/Lab%2014%20products/20160622_203836_zpsb6ywgxhi.jpg.html][/URL]

In reply to freetors :

Thanks.  That helps a lot.  

I feel like I should have seen an Elan chassis at some point, but darned if I can remember it. 

Just in case I forget again-

That really brings home the idea of chassis height, as well as sheetmetal for me.  I do have quite a bit of room in the body vertically for a taller frame, and can raise the floor several inches.  Hmmm...

In reply to NOT A TA :

This is all very good stuff.  The cowl area is pretty strong on the tub I would be using.  As far as the steering goes, I would likely use a C4 front suspension setup, rack and all.  I want to get away from the original suspension and steering entirely, and so it seemed like a waste to put the 'vette stuff on a floppy chassis. 

If it doesn't become a weight issue, what about making some metal skid plates of sorts that serve to smooth out the underbody for better aero?  Depending on their attachment points and thickness those might offer some stiffening effect. 

I like that idea a lot. I would think there wouldn't be a whole lot of weight added compared to the extra chassis rigidity, plus, who doesn't love belly pans?? 

I will need to figure out how much of my drivetrain would protrude, and if I could clear the driveshaft. This might be where an IRS would be helpful...

Oooooo

Like a torsion box table.  That actually makes a lot of sense. 

Cooter said:

In reply to freetors :

Thanks.  That helps a lot.  

I feel like I should have seen an Elan chassis at some point, but darned if I can remember it. 

Just in case I forget again-

That really brings home the idea of chassis height, as well as sheetmetal for me.  I do have quite a bit of room in the body vertically for a taller frame, and can raise the floor several inches.  Hmmm...

Here's a later Lotus Elite type 75 chassis if it helps to see how it evolved.

In reply to Stampie :

It does. Quite a bit, actually. 

So would it be safe to say an "X" between the suspension bits would be miles ahead of an "H", or ladder.  Assuming that the cross section of said "X" is tall enough, of course.  How would engine torque factor into the equation?

Keep in mind that Lotus used the fiberglass body and sheet metal chassis as a conjoined structure. Definitely use it for inspiration, but understand that in a Lotus the tunnel was in contact with the body on 3-sides(and in early cars was bonded to it). If you can implement something like that too, even better. 

Can you incorporate the transmission tunnel into the chassis and make it a closed tube? If you can incorporate the firewall and a rear bulkhead into the design even better.

In reply to NOHOME :

Quite possibly.  I was wondering about the bulkheads as looking at the Cobra replica chassis GTXVette was selling, and it took me down one of many rabbit holes I have been down trying to figure this stuff out the past couple months.  I've seen a lot of approaches, but I have been having difficulty with figuring out the "why" of each one. 

In reply to Pete Gossett :

Excellent point.  Thank you.  

I've learned more here in just a few hours than I have in nearly 3 months of Googles. 

Based on the limitations you provide, I think you are looking at something like this sketch with a tunnel build in. That would actually be pretty good.

What I would want to do is replicate the tub in this picture and integrate it into the existing chassis. While I bet this is carbon, a sheet-metal version could also be effective at a bit of a weight increase.

Here is an example of a frame with no stiffness in the floorpan but built around the tunnel and a front and rear bulkhead

This place never ceases to amaze me. 

So, all I would need to do (after the frame mods) is make sure the tub was more rigid so as to distribute the chassis load from the front tubes to the rear bulkhead with minimal deflection, it appears. 

Easier said than done, of course, but this is light years ahead of where I was when I started this thread. 

NOHOME said:

 

 

A Google reverse image search helped me to understand the idea behind the bulkheads, and taught me a new term that helps me to understand the importance of my firewall in this application- "Shear Plate"

Just be careful, as that image is also tied to a multitude of porn sites, for some reason. 

Just remember that the central x-frame built around the trans tunnel is still less effecient than a conventional monocoque/unibody. Moving that sheet metal as far away from the car's centerline, side to side and vertically, dramatically increases stiffness. This is just the same way that a large diameter thin wall tube can have the same stiffness and less weight than a small diameter thick walled tube. Also the lotus frame really isn't all that stiff, but for such a lightweight car and the cornering forces of the day it was stiff enough. The x frame was a compromise that fit within the car's design well. And everything in car design and engineering is compromise. It's up to you to understand the compromise that you make, their benefits and their drawbacks.

In reply to Cooter :

It should be shear plane.

It’s a complete pita, but this is why I greatly dislike chassis builders completely cutting out the floor and firewall then pop riveting aluminum/steel back in. All that potential stiffness is lost. Welding is ok but the underlying metal is changed. Oh well.