I have always noticed that the 80s and 90s era trucks had a lot of frame flex, or frame torsion I should say...I was thinking of a way to fix the problem but I dont know if it would make it better or cause more problems in the future...My thought was that when they do twist its not just one rail sagging more than the other, the one frame rail moves toward the front of the truck while the other moves towards the back...its pretty noticeable if you look at them in a ditch, one corner of the bed will be closer to the cab than the other, like the bed is cocked sideways a bit...Id imagine this is due to the way the cross members are designed. They all just go straight across with no braces on the side to keep the frame rails from sliding opposite each other...I was thinking a simple steel X in the frame right over the axles would help, Im almost positive it would...but I think it would cause more cracking issues in the future...would it be best to let the frames do what they do or make them stiffer?

 

I plan on stiffening mine idk I guess I see how it goes

 

I guess so...I dont have an 80 or 90s truck, but I plan to get a 92 -97 for a work truck some day. I never liked how much they twisted, maybe its not that bad...Another thought I had was to bolt a 1/8 or 3/16 plate along the inside of the frame rail.

 

I'd suspect the Ford engineers designed it to flex. Remove that flex and something else may break/crack. Yeah, it looks bad but your truck has held together all these years.

 

My thoughts are these:

1) Suspension travel is not great on these trucks, less so with 3/4 and one-tons. I think a VERY stiff frame would be more appropriate for a very flexible suspension.

2) Recalling the Three Stooges plumbing episode, every time they diverted the water with a pipe, it just shot out somewhere else (gas stove etc.).

And similar to a powertrains weakest link, once you install badass u-joints, the axles are next in line, then maybe a ring gear. Point being is that you may just want your u-joints to remain the weakest link because the other alternatives upstream are much more expensive and a bigger PITA.

When flex is elimated successfully, that stress is going to find the next weakest point where ever that may be. It may indeed be the metal itself and cracks will develop at invisible weakpoints along the frame or perhaps a spring mount will tear off.

I'd let the frame flex as it was engineered to do and not worry much about it.

 

Frames were originally designed to flex. Some crazy world we live in where frame flex is bad these days? Well according to GM. Now there 3/4ton trucks are boxed. Horrible idea for us New England truck owners. Plus flex isn't bad at all. We all know how the YJ jeep wrangler boxed frames hold up here. They don't they rust from the inside out.

 

I don't think the rails slide in relation to each other. If they did that, your bed and cab would buckle, plus your bumpers would swing in and out on the ends. What you're seeing is the frame twisting, which is good because it distributes the stress among several components, rather than concentrating it at various connection points.

Jason

 

Thanks for all this info, It always looked bad to me, mainly because the 70s and older trucks didn't twist as bad as the 80s is why I was concerned with it.

I always thought it was funny how frame twist wasn't a concern until gm boxed their frame...I just always thought the 80s and 90s trucks flexed a bit too much, but the 70s and 80s chevys were just as bad, and I guess I have never actually hear an issue with it.

I just had to let someone else answer my question, more cross members and triangulation would stiffen it up, but would cause much stress elsewhere... at least with the flexibility it can rest into a non stressful position

 

Agreed.

In my pulling truck, I wanted to loose the flefor obvious reasons). I trussed the frame with 1.5" square tubing from the trans cross member to the rear axle