Are the rail braces necessary?

[drcomanche]

As the title implies, I'm wondering if the rail caps on the floor of the truck are a necessary part of its stability. I'm working on replacing my floors right now, and putting the brace back on just leaves more places for water to get stuck in and cause rust again(which is why I'm here in the first place). If they are required, is there a way to make better ones? Or simplify it?

[Pete M]

nope.  buy that I mean that my 20 years of internet MJ fandom I've never heard of a single issue with removing it.  the truck was designed before modern computer simulation and apparently overbuilt in that area.  feel free to completely remove it in favor of a flat-ish floor.   

[Jeep Driver]

I suspect the bracing is there for a side impact. 

 

Your call. 

[drcomanche]

I now have built in 2x6 steel rock rails attached to the frame, so give and take as far as strength goes, I suppose.

[Jeep Driver]

I'm not arguing, just explaining my answer. 

 

My wife was hit broadside at about 35mph, totaled.

 

Floor creased and rose up, would have been much worse without any lateral bracing at all. In fact, you can see what looks like the end of what you are calling rail brace at the bottom of the door.......the impression of a brace. 

You can imagine what a 50+ mph impact might look like. Space gets real tight. 

 

 

 

 

[drcomanche]

That's a very informative picture. While my rock rails will definitely change where the brunt of the impact is, they do seem to be useful. I'll see if I can't come up with a better solution than the old rust channels. Maybe just weld solid steel plate on top. Anything's better than what's there.

[Eagle]

15 hours ago, Jeep Driver said:

I suspect the bracing is there for a side impact. 

 

Your call. 

 

I think you're talking about a different part. I'm pretty certain the question is about that "inverted hat" piece that's welded onto the top of the floor pan under the driver's and passenger's feet. If a side impact got far enough for that to make any difference, whoever was sitting in that seat was already dead.

[Jeep Driver]

Unibody- to the extent that it works, or doesn't work- designers and engineers.......that's another thread/discussion. Each piece adds strength, each piece deflects or transfers or absorbs energy, that's how it works. 

Regardless of what we think should or should not be. 

[Eagle]

2 hours ago, Jeep Driver said:

Unibody- to the extent that it works, or doesn't work- designers and engineers.......that's another thread/discussion. Each piece adds strength, each piece deflects or transfers or absorbs energy, that's how it works. 

Regardless of what we think should or should not be. 

 

Very true, but the Cherokee is also unibody and it doesn't have those above-the-floor hat braces, so to me that's proof that they aren't there for side impact protection. They are there because the Comanche is a pickup truck, a small, semi-unibody pickup that's either a 3/4-ton (almost -- 1,400 pounds capacity) in standard trim or a 1-ton (one metric ton -- 2,200 pounds) in optional trim. If I were using my MJ to carry loads at the full rated capacity, I would retain those braces. For a daily driver that's used more like an SUV with an open-top load bed, I agree with Pete. Not needed.

[AZJeff]

6 hours ago, Eagle said:

 

Very true, but the Cherokee is also unibody and it doesn't have those above-the-floor hat braces, so to me that's proof that they aren't there for side impact protection. They are there because the Comanche is a pickup truck, a small, semi-unibody pickup that's either a 3/4-ton (almost -- 1,400 pounds capacity) in standard trim or a 1-ton (one metric ton -- 2,200 pounds) in optional trim. If I were using my MJ to carry loads at the full rated capacity, I would retain those braces. For a daily driver that's used more like an SUV with an open-top load bed, I agree with Pete. Not needed.

As a retired mechanical engineer, I agree with Eagle.  

 

A "box section" (which is the technical term for the added piece we see in MJ floorpans) is not a part intended to improve crashworthiness.  It's intended to increase the stiffness of the "frame" to longitudinal bending loads.

 

Most of us probably rarely, if ever, tax our MJ's to the published limits from AMC/Jeep/Chrysler.   Removing this relatively small "hat section" from the top of the floorpan will have minimal impact to the overall structure, particularly in a vehicle that spends all it's time driving on streets empty or at best lightly loaded.

 

SIde note---as an automobile afficianado for 50 years, that "hat section" welded to the top of the floorpan is one of the STUPIDIST design features I have ever seen.  It TRAPS WATER (and other stuff) almost by design, and invites corrosion.  It is almost by design intended to destroy itself.

 

Knowning how engineering is often done, I suspect the addition of the hat section was an "eleventh hour" modification to the design to improve the basic Cherokee "frame" stiffness without forcing a major redesign of the lower "frame rails" that the MJ shares in common with the XJ.  

 

In other words, it was a "band-aid" to make the MJ more rugged without impacting the XJ's common parts.

 

I wouldn't worry one bit about taking them out, unless you are making a rock crawler, in which case you are doing other stuff to the chassis already.

[mvusse]

Actually, I'm pretty sure the lower "frame rails" on a Comanche are much taller than on a Cherokee in that area.

[drcomanche]

Yeah, the floor gets higher in the MJ cab to meet the boxed frame back end. In the end, with everything posited here, I'm going to go without them. I'll have far greater strength in the frame to rocker aspect with the welded tubes and everything. I'm also planning to plate the frame out too, so no lack of body roll strength.