Is frame twist an issue with an FSJ?

[KYJ10]

I read alot about the SWB jeeps and others doing thier cages to the frame to help make it more rigid. Im wondering if the FSJ frames are already more ridgid? And I also thought the J trucks were a little stronger anyway.

The reason I ask is that I have solid tranny and motor mounts, so a twisting frame would not be real good on my tranny/tcase. Im thinking I have made my frame alot stronger already by doing the following.

I have shortened the wheelbase considerabley to 108". My front and rear bumpers are welded to the frame in about a million spots. The engine mounts are solid. The crossmember is solid to the frame and tcase(no rubber). My flatbed is actually a 4x5 sheet of diamond plate that I welded to the frame in a million spots. The supports going across the frame under the bed are also welded to the flatbed.

So it's all tied in pretty well, plus I have eliminated a ton of weight. What do ya think?

[jode]

I think that, yes, frame twist is a problem, and also that it sounds like you may not have as much of a problem as others of us.

Other 4x4s behind me on the trail frequently comment about the huge amount of "travel" or "articulation" my rear bumper has on my 76 wag. I haven't ever measured it, but I bet I get a good 4" travel at each end of the bumper

However, the J truck frames are WAY beefier (at least my 73 was

[Bob Barry]

As stiff as you might have made your frame, the drivetrain is still going to want to twist and vibrate, both in small and large magnitudes. Why did you mount the drivetrain up solid? Something is going to fracture, most likely the metal where the mount is bolted to the transfer-case adapter or the engine-block.

You can actually think of frame-twist as a suspension-component; that's why riveted crossmembers can last longer than welded ones without cracking.

[KYJ10]

I know on tube frames, alot run solid mounts. And I know some run solid when they tie a cage into the frame. I would think that with enough weld and support, it can be done successfully?

[Bob Barry]

I suppose that you COULD, with enough gusseting and triangulation.

I'm just not sure why you would WANT to in your application? It's a HECK of a lot easier to convert the stock mounts into inseparable rubber mounts, if you're worried about them falling apart.

[fsj1978]

I would bet dollars to donuts that you're going to have problems with those solid mounts, especially with a trail rig. A regular frame like ours is almost a "flat" plane, you can twist them pretty easy. The body of the vehicle helps take some of the twist out of the frame. If you incorperate a roll cage into it then that makes the frame MUCH more of a 3 dimensional object so theirs FAR less flex, if any. If it were me, and it's not, I'd at least change the tranny/T-case mount back to rubber.

[scotty]

maybe just run a rubber or polly tranny mount. i have been running solid engine mounts for abut 4 years now,and ive not really had any trouble with them. ive also got chains that help take some of the motors desire to twist away from just the mounts.

on the rest,id try and mount the cage to the frame,to help with the twist,and for safety reasons during a rollover. my cage ties into the frame at 10 spots [img]smile.gif[/img]

[robselina]

if I'm not mistaken, Dennis does get some flex from his crossmember. The tranny/t-case is attached dirrectly to the crossmember, but the crossmember has bushings where it meets the frame right? I'd say that leaves you in decent shape. When you make your cage though, tie it to the frame anyways, can't hurt to add some ridgidity to what you've already got

[scotty]

Originally posted by robselina:
if I'm not mistaken, Dennis does get some flex from his crossmember. The tranny/t-case is attached dirrectly to the crossmember, but the crossmember has bushings where it meets the frame right?

ohhhh,in that case i agree with you completley,mr selina. [img]smile.gif[/img]

matter of fact,im planning on making a crossmember just like that-bolted directly to the tranny,and with bushings where it attaches to the frame. the rubber mount on the stock xmember is still letting it move more than id like.

[KYJ10]

That thing has some bushings, but they don't hvae much give, if any. My thinking would be if your running solid engine mounts, you would also want solid tranny mounts? Seems like one not moving and the other moving would tend to want to brake something. The reasob I did solid tranny mount is because if I didn't, my fabbed up crossmember would hang much to low. With my setup, it actually is tucked up further than my frame. I know Andrew ran a solid mount on his waggy. I guess my next step is to weld my new cage to the frame.

[Stuka]

The frame on my cherokee has twisted enough to cause cracks in the body. For a cage, I would use may a 1/4" rubber piece, but thast it. With the way your rig is, I would bolt it right tot he frame.

And the cross member you have actually encourages chassis flex, because it has no way too stop it, it just rootates along with it. Although if you put 2 mounts on each side, one above the other, it would stiffen it quite a bit.

[testuser]

my frame has twisted enough that it popped a factory weld on the right rear quarter panel. someone has to push the panel in so I can shut my tailgate

[scotty]

Originally posted by KYJ10:
My thinking would be if your running solid engine mounts, you would also want solid tranny mounts? Seems like one not moving and the other moving would tend to want to brake something.

my thots also.thats why im gonna make a crossmember like youre runining. [img]smile.gif[/img]

[rockjeep44]

After hearing what you described, I think your frame is gonna be SUPER beefy and trussed. My frame really didn't flex at all and that was mainly due to the front and rear bumper being welded to it as well as the rock sliders. I did bend it in quite a few places but that was just from sheer gettin it. You've done a lot more stiffening up than me plus the fact that J-truck frames are a hell of a lot beefier. Also, you'd be real surprised how much flex those MORE bushings in the crossmember can provide. I used them on the axle end of my traction bar mount so they HAD to flex and did a darn good job of it. They are plenty stiff but can flex if need be. I think your setup will be just fine.
-Andrew

[ February 03, 2004, 03:46 PM: Message edited by: rockjeep44 ]