Pinch seem & Rock Rails?

[Jeep'nD]

I've been researching info on rock rails. It seems that nearly everyone builds these to utilize the pinch seem to do most of the dirty work. It also appears that the connections to the frame rails are simply intended to help avoid having the pinch seem get jerked around.

However, I have found one exception to this pinch seem rule. Kevin's OffRoad armour claims that his latest generation of sliderz do not need to use the pinch seem.

From his website: It's an old picture, so please disregard the pinch seam mount...it's no longer needed due to an update in manufacturing and an upgrade in strength.

http://www.kevinsjeepparts.com/xj/xj4.html

I'm interested in knowing where the brunt of the force is now being taken. Distributed along the rocker panel perhaps? I also assume that a cut in the pinch seem would be required as it would just get mashed in anyway.

I'm hoping to fab up my own set for my own rig this winter and hence I am collecting ideas. The Tech Article on Mesa4x4 was interesting in that the desgin was much simpler in that it used 2x6 box tube that required fewer welds and such.

Also, although I'm rambling at this point, I was also intersted in a comment that was made about using the internals of large rockrails (e.g. 2x6 box tube) as tanks for on board air. I'll be looking into that possibility as well since I'm hoping to have OBA of some sort down the road.

Any comments would be appreciated.

 

[93xjmark]

The 2x6 rock rails could hold a decent amount of air.
Set up an isolation valve for each side out of harm's way in case of a failure.

 

[geo78]

Isolation Valve

What is an isolation valve?

 

[93xjmark]

It's just a valve in the air line going to each rail that shuts the air off. If you tore a welded piece off, or punched a hole in the tube, you will have an OBA system that's un-usable. Shut the isolation valve off to the "leaky" rail and you're all set.

 

[Eagle]

Jeepn'D --

I think you are misunderstanding the way most rock rails are installed. The attachment to the pinch seam doesn't do much when the rock rail is scraping along a rock. The tube should be mounted tight to the bottom of the rocker panel, so under load the force is spread out along the length of the rail and then transmitted up into the rocker panel itself.

The pinch seam attachment is what carries the tube when it is NOT under load. Under load the pinch seam connection more or less drops out of the equation, and the loads are at the tube itself and at the inboard end where the braces attach to the "frame" rail.

 

[Kejtar]

'K Eagle... you lost me a bit here.... my rock rails (made for me by a local guy) are attached with 4 legs each to the rail( each leg attached with two bolts) and with 9 bolts to the seem... there is an actual rail welded across the legs that attaches to the seem to spread the load... in any case, I landed on my rails couple times... and I mean it was the teeth rattling, eyes popping hard landing to the point where I had to pause to catch my breath after doing so... and so far... no damage to anything, except I had to touch up the rails with some spray paint (scratched the paint of and the buggers started to rust! )

 

[JnJ]

I have not seen a rockrail for the XJ yet that didn't use the pinch seam (besides the ones that replace the rocker completely) and worked. From another post on rockrails;

"Any decent rockrail will tie into the pinch seam and the "Uniframe". The legs to the uniframe really only stablize the mounts at the pinch seam to prevent them from piveting at the pinch seam as viewed from the front/rear. The pinch seam takes the brunt of the forces. An example is a buddy of mine that went to tellico with me. He built is rails and ran out of time/material before he got the "legs" to the uniframe done, they only mounted to the pinch seam. Needless to say after each heavy contact, we were realigning his rails and checking out the contack they were making with his rockers. The only difference between his and mine was that mine attached to the unframe and pinch seam (9 bolts through the pinch seam and 8 self tappers per leg).
So, a well designed rail will be of material thick enough to withstand a good wack, and it will tie into the pinch seam and uniframe, after that, it's just about looks."

 

[Jeep'nD]

Well, in the time that I've been reading this board, it has occured to me that when Eagle talks, people listen. Just as an FYI, I did pass high school physics so I understand the basic concept of spreading a load over a wider area to reduce pressure.

It has always seemed to me that the rocker panels were always intended to play a part in the sliderz design, however, there is so much conflicting information out there that it makes it difficult to discern what all the designs are aiming to achieve.

In many cases it appears that the rails are not flush with the panels, but that doesn't make sense, because it would either end up putting a load on the pinch seam, or the rails would be accelerating right before they impacted the rockers. Clearly you don't want movement of the rails. From having looked at the pinch seam of the Jeep, I can't imagine that any engineer would design rails to utilize this point to be the primary shock absorbtion point for the type of stress that dropping a two ton vehicle on it would produce.

I guess that in the end what I can surmise is that a proper design attempts to spread the load over the entire rocker panel section, that the pinch seam is used primarily to support downward loads, i.e. people standing on it to reach the roof, and finally that the unibody frame connecting points provide support for lateral loads, i.e. trees and other pivot points such as rocks and also protect the seam from getting twisted up.

I suppose not everyone will agree, but this seems to make the most sense to me.

Now I need to figure out how to get the inside of the rails not to rust so that I can make them into tanks for an OBA system.

Eagle, do you have any nuggets of wisdom WRT creating tanks out of these assemblies? Presumably I'd paint the inside of the rails while the ends are still open, but after I weld the end caps on, surely the paint on the inside will be damaged. How then can I finish the inner-surface preparation for these tanks?

 

[mattk]

Generally, Eagle is spot on with his info but I think something is missing here. I can see that if you are referring to lateral forces acting on the rails, Eagle is correct.
I think the question applies to vertical force. In that case, most of the impact is transmitted to the pinch seam and the "frame" mount becomes nothing more than a pivot point. Most rock rails do not have a tall enough frame end mount to provide any real vertical support. That is, in part, why I junked my Olympic rails after I came down on a rock and the rail tramsmitted the force upward, bent the pinch seam vertically and crushed my rocker panel. This happened without the rail touching the rocker panel. Pinch seam and its mount only.
The problem is that the existing body is ill-suited for rocksliders. They will take some impact but not a whole lot. Eliminating the pinch seam and the rocker panel entirely and building replacements is the only real way to improve the design short of building a sub-frame. There are several posts on how to do this if you do a search.
The pinch seam is stronger than you might think and will take a fair ammount of abuse. Rock rails are great for most moderate wheeling.
Good luck
mattk

 

[dmillion]

rails

A lot of the rock rails out there do not transmit the force of any impact to the rocker panels. They are mounted out away from the rocker panels (the type that function as "step rails," for instance). In these cases the majority of the forces are absorbed by the mounting to the pinch seam. The legs connecting to the frame rails are there primarily to prevent the mount to the pinch seam to act simply as a pivot point around which everything bends.

 

[footdale]

So what is better? Have the slider butt up against the rocker, or should there be a small gap? Based on web research I've seen it done both ways. I'm planning on building some this winter and I'm new so I have no personal experience to go on. It seems like it will be better for the two pieces to mate, but like a lot of things you won't know until you test it.

 

[Rufies97XJ]

Ask OneTonXJ about his rocker rails he makes.
I believe they will take any abuse you can throw at them and still protect the XJ from most if not all structural damage (that a normal rock rail would face)... and you don't lose any clearance from stock.
The stock rocker panels are cut out... a 3x5 rectangular tube is cut and placed in and then welded to the cut, which is hidden by the door. and also plug welded to the pinch seam and sealed to prevent rust in this area.
Myself and few other have his rails.... I'll try to get up a few pics for ya.

 

[csudman]

Im as close as I can get them, Basicly touching my rails to the rockers. Beat on them pretty good so far and I see no way that I m gonna cave in my rockers w/ these rails. But do what you want.

 

[xjaddiction]

i ran some olympics that attatched to pinch and to the uni via self tappers x 2 per leg, 2 legs only. simply put, they were fine for gentle scraping but under hard impact, slammin' down on rocks, they sucked. i've got bent rockers from the sliders themselves. the self tapping bolts also would shear.

i went to tennessee off road, and they built some rocker guards out of 2x4 stock and some 2 inch tube. i'll get some pics up soon. the 2x4 protects the bottom of rocker and the tube helps push the xj away from rocks protecting the sides. the attatchement is what is really cool. there are 4 legs that attatch to the uni via flat 1/4 inch plate with another plate 1/4 inch on inside of uni rail, and grade 8 bolts going through uni to inside plate. this essentally sandwiching the uni rail between to 1/4 inch plate in 4 places per rocker guard. there is no pinch seem attatchement at all. this is a temporary set-up, as i am installing tor cage to the rocker guards through the floor of xj. but... in the mean time i've found the rocker guards to be alot stronger than the olympics could ever dream of being. i've landed hard on som of the same rocks that bent the olympics with no damage. i've lifted the side of the jeep using a high lift on the rocker guards, no problem. they would work fine with no other modifications. i'm much harder on these guards than i ever was with the olympics and they won't budge. they also are stiffenning the uni up some too.

there is a rocker alternative with no pinch seem involvement and no crappy self tapping uni rail bolts.

Greddy

p.s. sorry about all lowers, i have a future 4 wheelin' babe on my shoulder... and i mean babe, she's just 6 weeks old.

 

[Jeep'nD]

Pictures of this rail system would be great. With these rails, where is the vertical load being absorbed? Do the rails come up against the rockers at all? I can't imagine that rails that have a 6+ inch pivot arm wouldn't move when a 2 ton vehicle drops on them. As the saying goes, a picture's worth a thousand words.

Thnx much

 

[JKTXJ]

I have built numerous XJ rock rails at this point in my life (always striving for the perfect design). The pinch seam is critical, it is a very strong area of the unibody. You also need to use legs to the frame though. When you build outward from the pinch seam, the seam becomes a fulcrum and a hard hit merely pushes the rail up and pulls your pinch seam out. Use some legs the frame with self tappers though, this counters that and you have a strong design. Self tappers work fine for this because the force is in the downward direction, it is not trying to pull the bolts out. The pinch seam is not invincible however, so spreading the load is important. Look at ORGS rails.. they use 2x2, as do most that I build. This spreads that load over the entire pinch seam area and is very strong. If you have small areas contacting the pinch seam such as other rails out there, you will push the seam in when you hit it hard enough. Pretty soon your pinch seam is a mess. I do not make my rails touch the rocker though.. you don't need to. The main thing is, spread the load over the pinch seam. Go full length and using something beefy, not 3/4" angle iron Then tie into the uniframe with legs and self tappers. Last set of rails I made for a guy were 2x2" @ 3/16" for the pinch seam, with a 1.25" schd 40 pipe (.144" wall) for the outer. A hard hit on Proving Grounds bent that .144" pipe slightly, but the rocker and pinch seam stayed perfectly intact. Success in my opinion!!

 

[JKTXJ]

Ah Ja!!! I still love these rails. The design is labor intensive, but I do love them. My friend painted them to match his rig (they are still in primer here) and they looked sweet. I need to get some pics from him.

 

[RyanSullivan]

'K Eagle... you lost me a bit here.... my rock rails (made for me by a local guy) are attached with 4 legs each to the rail( each leg attached with two bolts) and with 9 bolts to the seem... there is an actual rail welded across the legs that attaches to the seem to spread the load... in any case, I landed on my rails couple times... and I mean it was the teeth rattling, eyes popping hard landing to the point where I had to pause to catch my breath after doing so... and so far... no damage to anything, except I had to touch up the rails with some spray paint (scratched the paint of and the buggers started to rust! )

Gotta love Ryan B. I have the same set of rails that Kejtar uses.

(shhhhh i'm missing a bolt )

 

[JKTXJ]

Crude diagram... it's getting late... Anyhow, more surface area on the seam area the better. My first design had little contact with the upper portion of the seam/rocker area and therefore pushed the whole area up slightly where it makes that 90 degree turn. If you are gonna use angle, I'd turn it the other direction and rest that flat area on the underside of the rocker/seam. Just my opinion of course..