flexability question

alrighty, got the new front frame horns reinforced and the new bumper on (i know, i know.......gotta post some new pix) and after getting that done i figured kool, strengthend the frame really good.

But that got me to thinking about flexability......

I'm guessing the frame is made to flex a tad......which is probably a good thing, but will adding poly bushings through out (springs, body mounts, etc) and adding strength to the frame actually hurt the frame more as it will hinder flex?

By adding "strength" to the frame, i mean the new frame horn bracing & bumper, and i have to replace the rear crossmember where the body mounts. I was planning on using "C" channel for the rear member and then building off of that for my rear bumper & tire carrier.

Any insight/input?

 

hmmmm, must have put this in the wrong section.....thanx for moving it!

common fellas, somebody hasta have input on this :?

 

While Jeep frames are known for their flex I personally think that your flex should come from your suspension, not your frame. When you bend (flex) metal, eventually it breaks. As many here on this forum can attest to. My frame has had numerous cracks repaired. I now have it almost entirely boxed and haven't had a problem in the last two summers with cracks (knock on wood). It also depends on the kind of trails you will be running. I run trails that are rocky, gnarly frame twisters. Also, when the frame flexes, so does the body. This contributes to cracks in the body and floorpans. I'm sure there are a lot of opinions out there on this. Early Broncos had frames that were fully boxed and they are great offroad vehicles.

 

I looked at your repairs in the other thread. The frame is meant to flex, and it will flex "a lot." when I had my '73, I developed a frame crack at the junction of the front crossmember and the frame rail, right behind the steering box. Part of the reason for this crack may have been a collision, but I think that location is a common place for cracks on these frames. The frames in '75 were made stronger, I assume by increasing the thickness of the rails. I can see no obvious differences in the '75 and '72-4 frames otherwise.

Regarding your repair, you have reinforced the corner where the most stress takes place. If you think about the frame material, the corner forms a stress point between the flexible frame rail and the rigid front crossmember. There is added stress on the driver's side corner due to the forces from the steering, and the rigid steering bracket. Your C channel will obbiously reinforce this area, but I would worry that you have moved the stress point back to where the C channel meets the original rail. Also, you've made the front crossmember much more rigid with the addition of the welded shorty bumper. The original bumper is bolted to the frame, and the rigidity mostly comes from the crossmember.

Sorry if this sounds like criticism, but I thought it better that I tell you my impressions rather than not comment. JMO - I kinda sorta think you have to go one way or the other - either keep the overall flexibility of the original frame design or go with a very stiff frame, like the aftermarket tube frames, and depend entirely on the suspension for flexibility. To keep the original flexibility, you probably shouldn't add much material to the frame - just a carefully chosen small, few pieces of steel.

 

mornin guys ~

Tim.....you may be right about moving the stress point on the frame. I did notice that once everything was in place and i pulled on the new bumper there seemed to be some flexing in the frame rails down under the firewall just behind where the boxed in section ends

I'm starting to think that by adding strength to the front, i may have unintentionally added more stress in other parts of the frame......do you think boxing in the complete frame would help?

 

Yes, fully boxing the frame will help a lot. It will make it much more rigid as well so your suspension will do most of the work.

I am approaching this problem from a different perspective. Eventually I will have a cage that extends from the front to the rear of the jeep. Most of the front will be hidden inside the sheet metal. This will create a rigid box made of tube and the stock frame that will keep it from flexing. I have the suspension set up so that it will handle any articulation I may need.

 

hmmm, i was planning on tying my full factory cage into the frame as well......that in itself may help the flex in the frame rails!

 

I 'spose my main insight here is that the cracks occur where you have a stiff section connected to a flexible section. If you look at the spring hangers on a pre-72 CJ, you see cracks at the same sort of location - where the stiff spring hanger connects to the relatively flexible frame rail. You can get rid of the flex everywhere and solve the problem that way (boxing with lots more metal), or you can try and solve some of the specific problems by adding a little more metal. In the case of your C channel, you might try to distribute the stress between rigid and flexible with a strap along the top and bottom frame rails at the junction of the C with the rail, or else maybe add a small boxed section.

Not to diss the CJ, but the whole idea of flexy frame, stiff springs and light body goes back to the WWII prototypes. That concept was pretty much carried through unchanged until AMC took over. Then, they had a transition period where they made do with the design they had and added some more modern features. From 1976 to 2000, you can see the changes in the vehicle, trying to update the overall design while retaining the basic character of the vehicle (wider stance, springs softer and more outboard, stiffer frame).

JMO - the design is a compromise of traits: streetability, comfort, durability, off-road capability. Any change you make in these vehicles will push the compromise in some other direction. In this example, you want to stiffen the frame. This means that the suspension must be more flexible in order to get the same articulation you had with the flexy frame. The springs are mounted far inward, so making them more flexy increases body roll. Increased body roll makes for less stability in daily driving. Recall that roll-over is a big concern for the manufacturers of these vehicles.

So, again JMO, the stock configuration is a pretty good compromise for most people. You can make some significant changes IF you can be more specialized in your use of the Jeep, on trails only for example. If it were mine, I'd make only minor changes at first, and then be sure that the outcome is what I expected it to be.

 

If you do this without extending the cage to the front you will create a rigid back section with a flexible front. This means there will be a lot of stress on the frame from the front cage tie-ins to the front of the jeep. This is currently how mine is set up. On the rocks I can literally feel the floor flexing under my feet as the suspension cylces right now. This is not good for the frame or the sheet metal. This is why I am extending my cage.

Point of note: My CJ is a trail rig that can be driven on the street/highway. I have sacrificed some streetability to make it work well off-road. So if you are looking for a daily driver this is probalby not the route you want to take.

 

you guys really have me thinkin now

Ok, lets say she's going to be more of a trail rig than anything. Basically meaning it would see the occasional trip to the corner store for milk or something. Would boxing the frame be ok then?

OR-maybe run some angle iron on top & bottom of the frame rail......would that help?

As for the cage, i see what you mean H-Hawk......didn't even think about the front end

All this thinkin is hurtin my head LOL

 

The body roll affects high speed evasive maneuvers most. Tall, wide tires with lots of lift affects rollover stability too. Even from the factory, these vehicles were not immune to rollover.

Shouldn't be a problem as long as you realize your vehicle's limitations and drive accordingly.