Preventing frame cracks

I recently came into enough money to do a major rebuild on my '75 CJ. It's getting 4.5" RE YJ lift springs, ARB's, 4.56's, 35's, big winch, V8 and 4-speed swap, etc. I know that the flex I'm anticipating will be hard on the frame. There are a lot of threads here about fixing frame cracks, but what about preventing them? Any suggestions for preventive mods to keep things together? I'm not planning to box the entire frame, I would think that strategic plating should do the job for moderately technical rock work.

 

build an internal cage that braces the frame. But that takes a lof ot work. Best and easiest method would be to plate over the known problem areas to reinforce them.

I have a very simular setup to what you are planning on doing. After I had the cage put in for the passenger area I could feel the front end twisting around while crawling over rocks and such. I was afraid I was going to tear the front end of the jeep off. To combat this I had a front hoop put on and then had stringers run back from the hoop through the firewall and welded to the A-pillar of the cage. Then to reinforce the whole thing the shop put braces down to the frame in the engine compartment and built a removable brace that crosses over the top of the engine between those. All told my cage now ties into the frame in 10 locations (5 on each side) and basically has created a tube chassis inside the sheetmetal to strengthen the frame.

In this pic you can see some of the additional cage work on the front.

 

Thanks for that info. I am planning to do a full cage with frame tie ins. It's good to hear that it is effective at reducing frame twist.

BTW, your build writeups have been good motivation and provided great information. Your spring write up in particular started me down this road

 

Glad I could help.

There is no perceptible frame twist since that work was finished. And oddly enough it actually improved my steering at speed because the steering box can no longer flex the frame where it is mounted.

 

This ties in with my questions about frame strength. How are you tieing the 10 point "cage" frame into the stock frame?? Are you butt welding with gussets or using the prothane bushing method (for flex and/or crack reduction) or just not worring about the stresses??? Inquiring minds want to know!!!

Dale

 

no butt welding. The tube is actually trimmed so that it slides down the side of the frame and is welded across the full width of the webbing on both sides of the tube and across the top of the frame rail.
1&2 - welded to the winch plate which is in turn welded to the frame.
3&4 - under the hood in the engine bay. Welded to frame reinforcing plating at the front main spring eye hanger.
5&6 under the seats just welded to the frame.
7&8 behind the seats in front of the fuel cell welded to the frame at the rear main spring hangers.
9&10 stingers for the rear tire carrier that run down theback of the jeep and are welded to 1" plate shapped into D-ring mounts that are slid into the frame in place of the rear cross member and then welded in place.

I will try to get pics.

 

Information update and pics referencing the post above. Pics are links because of the size.

1&2 - http://www.hudsonhawk.net/jeep/ft1.jpg
3&4 - This is a butt weld directly over the front main spring hanger. http://www.hudsonhawk.net/jeep/ft2.jpg
5&6 - http://www.hudsonhawk.net/jeep/ft3.jpg
7&8 - Above the rear axle. http://www.hudsonhawk.net/jeep/ft4.jpg
9&10 - http://www.hudsonhawk.net/jeep/ftrear1.jpg
http://www.hudsonhawk.net/jeep/ftrear.jpg

 

Looks good! Lots of weld length, how long has this been done and what size tubing are you using? Any problems (can't get the oil filter off or exaust header etc.??

Thanks, Dale

 

Nope everything is accesible although it is a tight fit to change the air cleaner element because of the brace across the top of the engine. The frame ties are all 1.5" DOM. Some of it (from the seats back) is a couple of years old. The front section is just over a year old. It was built in sections as I could afford the changes.

I will say that is not my work. I paid to have that done. My welding skills are not even close to being good enough for me to trust my life to them by working on a cage.

 

A lot depends on whether you want to continue to rely on the frame for part of the flex, or whether you want to change the design and make a rigid frame. This has been discussed in these forums in the past, so if you search you should find something about both approaches.

IMO you have to go one way or the other, because something in-between will lead to focusing the frame flex in one place (a "stress riser" of sorts). To take the factory approach, you need to keep the entire length of the frame equally flexy, so that any bend is distributed along its entire length. Again IMO, strapping is the best way to do this - that's the approach taken on the examples (various baja race Jeeps) that I've seen. In particular, a '72 Renegade that was prepped by the dealership where I worked (Brian Chuchua's) received a strap along the entire top and bottom length of the frame, among other changes. This was being prepared for magazine writers who were going to "run the baja" with the rest of the racers. Not a serious racecar, but they didn't want it to break either.

Tying the cage into the frame will eliminate the factory flex, and then you'll have to rely entirely on the springs for flex. From the factory, the springs are fairly stiff and the frame is flexy, minimizing body roll while allowing flex on the trail. The main problem, as I see it, with a stiff frame and flexy springs on one of these vehicles is the far inboard mounting of the springs, which leads to unacceptable (?) body roll on a streetable vehicle. After '76, the frames on CJs became stiffer and the springs softer, and other steps were taken to increase the roll stability (outboard rear springs, wider track, longer springs, lower 'stance').

Whatever you do, it will be a compromise. My conservative nature would favor keeping the vehicle as close to stock as possible, and trying to address some of the specific weaknesses without changing the overall design. This might include selectively strapping the frame, rewelding the frame joints, and very minimal boxing at the spring hangers.

Again, just my opinion and preference. It's hard to say what will and won't work, because noone has made a scientific study of this topic. We can only look at what works and extrapolate from there. However, the further you extrapolate, the the more likely you are to get a surprising result.

hth!

 

Also, IIRC Mike Boyink's CJ-6 is an example of a full cage that is intentionally not tied in to the frame. His design contains the passengers in a rollover and only ties in to the body, retaining the factory frame dynamics. Another approach...

http://www.boyink.com/splaat/ssdutch-comments/rollcage/

 

BTW a '75 is reputed to have a heavier frame than earlier intermediates, so you're already a step ahead if you want to keep the factory frame dynamics.

 

Good points Tim. In my case I wanted a rigid as frame as possible with a soft suspension. It still drives good on the street and has no problem with articulation.

 

Thanks for all the pics and opinions

I can certainly follow both lines of thought. It's going to be a tough decision. Both cage examples are obviously well thought out which makes it even harder :?

 

I agree, all sound logic, one of my thoughts was to do what some of the racers did to help stop frame cracking. It was to mount the cage to the frame with spring eye bushings. this is the method I am leaning towards, with a 10 point cage and maybe connect the front cage (or engine cage, the 1st two points) to the front passenger bar this way too!

Dale