Ok guys this is a very frustrating problem that's getting worse and I've been fighting it for years. My memory serves this started happening 15 years ago when I first put 35s on it. When I went to 35s I did a 2" body lift with all new poly mounts and hardware.

What happens is the cage (8pt family style) and body just start independently shaking. And it gets very bad. This is not death wobble. The wheel is solid and still completely under control but the column is shaking with the dash. I've replaced literally every suspension component along with getting it aligned and two sets of wheels and tires. Still does it.

Cage is not tied to the frame but that's my next step.

Thanks in advance

is it at a specific speed? acceleration/ decel? on-road or offroad or does it matter? can you replicate it or does it "just happen" whenever? I would say things that come to my demented mind would be: tire balance, driveline U-joints

Could you post up some pictures of your truck and cage? I think what you are experiencing is similar to a problem I had when I first purchased the Bronco.

The PO had trimmed the rockers and it only had a roll bar with a soft top. When I put 35's on it (which had a dynamic balance issue, but more on that later) I could look out towards the passenger side while driving at a certain speed (60mph) and see the gaps in the door growing and shrinking at a certain frequency. I could also see the headrest of the passenger seat bobbing fore-to-aft.

What became apparent was the front half of the body, forward of the door opening, was moving somewhat independently of the rear part aft of the door opening. What another member pointed out to me was that the rocker panels add a lot of structure to the body and the PO removing most of them left the body "flimsy" in the area of the door opening because the only support left was the sheet metal floor.

From a dorky engineering standpoint, most cages don't solve this problem. If you look at the image below, you can see a typical cage with 6 attachments to the floor of the body. The blue border is the rear of the tub and the green the front half. The cage ties nicely to the rear half, but leaves the front un-supported and it can still bend about the floor plates below the A-pillars. On my rig, you could see the movement at the top of door relative to the softtop behind the door.
Body01.jpg


You can probably help your situation by adding cage tie-ins near the "belt-line" like I did. This will use the cage to tie the front and rear sections of the body together. I noticed a 10-fold improvement in body stiffness. No more door or windshield shaking at any speed. I highlighted all my attachment points in red. There are 14 in total.
Body02.jpg


Additionally, even with all those points of attachment, you can only stiffen the body to itself. You can still experience flexing between where the cage attaches to the body and the body attaches to the frame. (shown below)
Body03.jpg
The only fix here is directly attaching the cage to the frame or adding crossmember to the body to bridge the gaps between body mounts and cage mounts.



All that aside can you post a picture of your alignment sheet and did you have your tires dynamically balanced? OR even better road-force balance? Sadly many aftermarket tire companies make large diameter tires that have dynamic issues that affect ride quality and alignment. You won't think a set of tires could make a vehicle pull, but they do. I was ready to swear off BFGs after the issues I had with their KM2s.

Given the magnitude of shaking you're describing, I would look closely at the tires / axles. It sounds like something is slowly failing. What condition are the tires in?

Have you had your Rear Driveshaft balanced?

I believe Digger has the winning answer. I now remember that I cut the rockers off the same week I installed the 35's all those years ago. I will take some pics of my setup but it is basically exactly like the drawings but with a diagonal bar from the B pillar down to the top of wheel tub. I have not had the hard top on it in 15 years and last summer I took the hard doors off and put inserts and soft doors in.

It does it from 50-65 mph on any highway and is very repeatable. Doesn't do it on dirt roads probably because I'm not doing 50+ doesn't do it on the trail either.

Last year I rebuilt both axles %100. brakes, bearings, gears, ball joints, ARB's, new 1ton chevy steering, new lars bars and bushings, new adjustable trac bar with stock mounts, new Deaver springs front and back, new bilstein remote reservoir shocks, new heavy duty rear shackles, wrap trap, plated frame and installed 4x4x2 steering box and borgeson shaft. Current tires are 35" MTR's on champion bead locks and have balance beads. I've had other tires on it. 37" & 35" pro comp mt's. 3 different sets of wheels different shops do the balancing. shake was still there. Current tires were on my old XJ, which i pulled them off and put them on the bronco. i drove both vehicles that day. XJ had no shakes..... bronco did.

2 months ago I pulled out the np435/D20 and installed NV3550/Atlas. I had both drive-shafts rebuilt/balanced then. This fixed a drive-line buzz I had but shake persists. However the 5 speed now allows me to do 75-80 and I've found it doesn't shake near as bad at this speed. drop to 55-60 and its terrible. I really do think its a harmonic thing, it just gets into it and starts bouncing and just gets worse if I stay at those speeds.

I thought about tying the firewall to the cage but haven't had time to really look at it. I did buy the the wild horses things to attach the windshield to the cage but it shakes so bad I'm kinda scared to install those. well guess I'm tying the cage to the frame and firewall to cage.

What if you tried something like this shoring up all 4 corners tying the frame into the roll cage?

Digger?

I will end up doing something like the bottom picture. My main problem is, every time I do something like this I cant ever just leave it simple. I'm not happy with the sliders I built 15 years ago when I cut the rockers so I would like to build a new set and use them as part of the cage tie in, but of course that means more time and planning. My current rock sliders are hidden under the rockers and as such i got into a rock two weeks ago in Tennessee and crunched my door insert pretty good so its time for some rub rails that aid in body protection. I think i'm going to try and tie the firewall to cage and the top of the tub to the cage as digger suggested and see if that helps get me by till winter when maybe the shop slows a bit and i can have the bronco here for a couple days.

I had cage shake when I installed the cage in the 1/2 cab. It only has 4 mounting points. I added a dash bar and the shake went away.

I was running 30 but ive also noticed that when im done wheeling and I only air back up to 20 with the powertank the shake seems better but then again it can still get in rythym and get get bad even at 20psi. Do you have any pics how of how you tied your a-pillar bars to the firewall? I was also going to add a dash bar and probably some floor bars too. I'd like to mount the seats to cage eventually.

Those are perfect thank you! Will you be at the swap meet sat?

My cage has lateral bars in the back across the cargo area floor. Another about two feet above that. One across the floor right behind the front seats. Another across the top of the dash. I also have bars that follow the contour of the floor on either side of the doors from the wheel wells to the front leg tie ins. In addition my rockers are cut off and I have 2x4 sliders welded in. I also have my windshield anchored to my cage at the top front which eliminated a slight windshield shimmy.

I have virtually no shake of any kind and as digger has noted I attribute that to the lateral supports and large sliders welded in giving the body support.

I know those lateral and door opening bars cut down on already cramped quarters but it's been a stable running rig with them in there.

I have thought about tying in the seats to the cage to make a safety cell of sorts and then tie the cage into the frame but haven't gotten there quite yet.

Anyway just sharing, sounds like you already have a plan of action

I'm always open to ideas. So do you have your rockers welded to the body or to supports and then your frame? I've been doing non stop reading about hard mounting the cage to the frame or use bushings. I'm leaning towards bushings since i still drive it on the street.

My bronco hasn't had rockers in years and never had a shake that wasnt the tires. So i'll be following along. I plan to build a set of sliders this year sometime. I cant decide either run them off the frame or weld them to the body like landsharks. I also plan to tie the cage into the body some more at the upper sections of the quarter panels.

Gotcha yea I've been down that road before with my Cherokee and it doesn't work for the type of wheeling I do. I buckled the entire floor of the Cherokee. I'd have to tie them to the frame and idk how I'd do sliders welded to the body but tied to the frame with bushings....mmm guess I'll sleep on it.

Well as of right now I'll be there with Bronco.