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Box or span the fame, neither?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by BowtieBlazer, Apr 5, 2005.

  1. BowtieBlazer

    BowtieBlazer 1/2 ton status

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    Here's the deal, I have really beefy bumpers front and rear and I'm worried about a solid front or rear impact doing damage beyond the plate or channel mounts of the bumper. Possibly tweaking the frame out or collapsing it in. I have my rig down to the frame, weight is almost no issue, everything is accessable, however boxing the frame doesn't look like fun, and the frame sure isn't straight, and I'm worried about the weak points. Its near impossible to box around all the mounts and get everything just right, seams like the 2 points that are weakest when boxed that thick would be the stress points and eventually give. I've read many post on this board about frame integrity and how not to tie a cage into the frame but impact damage is my question.

    Do any of you rock crawling guys box the front or the rear at all?

    I'm thinking of just upgrading my crossmembers and maybe using a V design to connect the rear and maybe front most point to the frame rails at a 45. The front already seems to have lots of structural integrity.

    Opinions, Pictures?

    I bought a blazer with a wrecked front, and the frame tweaked around the front pass shock mount, and i wrecked another blazer with a trailer attached and it pummelled my class V hitch and the frame was fine, minus the bottom where the 4 grade 8 bolts mounted.
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    From what I've seen of GM reinforcing the weak A/G body frames, a V from the engine crossmember to the front framerails should be effective. So should another crossmember as far forward as possible, but on a truck frame, a beefy bumper that is mounted solidly should serve that purpose.

    My opinion of what GM tried/attemped is that early on they tried to make everything rigid. 205 bolted to the frame, crossmembers that wrapped around the frame rail, and crossmembers that were bolted to the top and bottom frame rails.

    Then, they went to the later style that was only bolted to the lower edge of the frame rail. The construction of those crossmembers and how they are attached leads me to believe GM KNEW that portion of the frame was going to move, so the crossmembers are designed to handle the twisting stress without doing much to combat it.

    I would think that an "X" in the center, if it would fit with the t-case and drivelines, would be the best option. A clocked t-case would make that option more possible I think. This is something GM did when building convertible cars, so you know it's effective.

    Boxing the frame without rigidly tying the two framerails together will do little to combat the twist that seems to be the most common problem as I look at it. But boxing the frame around the suspension mounting points would probably be a good idea if you succeeded in stiffening the rest of the chassis up, since the suspension would actually be required to do all the work, and absorb all the loads you place on it.
     
  3. CyberSniper

    CyberSniper 1/2 ton status Premium Member

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    As far as I'm concerned, trying to eliminate frame flex on our trucks just doesn't make sense. You'll end up causing the frame to fatigue in critical spots. I think the best you can do is try to keep the rigid crossmembers at the suspension points and let the rest flex.

    Boxing the frame is something hotrodders do because they have nothing better to do with their time. The frame on our trucks is simple enough where with a little ingenuity and some time you could make a box steel frame from scratch. 3/16" thick 3x5 box steel is going to be 10x stronger than the best hillbilly job at boxing is going to do. By the time you weld on that heat treated frame and mix mild steel with the frame you haven't gained much in strength. The cost difference isn't going to be much either because what you pay for the extra steel will be offset by 300 rolls of wire and 400 cutting discs.

    I know if I had the money I'd have made a custom frame for my project rig. By the time I got done cutting out rivets, prepping and painting, and moving/redrilling/fastening I would've been done with a custom frame.

    My 73 had this. But it also didn't have a cab crossmember. It used two "legs" to mount the cab like the front cab mounts. So, on the 73, it used a multiple piece transmission crossmember to tie the frame rails together and used nothing back by the back of the cab.

    By 77 it seems, they'd gone to a one piece crossmember for the engine and a cab crossmember.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    I don't know much about heat treating, but are the frames actually heat treated? Does that tend to make things more brittle?

    I'd have to agree on creating "fatigue points" by stiffening the chassis up, BUT that is one thing that manufacturers are doing big time now that they are able to. They can call it all the fancy names they want (hydro forming, etc) but the end results are chassis' that are VERY rigid, and force the suspension to do the work it's supposed to. For a purely offroad truck probably a rigid chassis isn't so important, but in my case, where the truck sees very little time offroad, when it DOES start to flex even a little bit, the doors don't line up, the body panels pop, etc.

    Thats one of the reasons I'm not a HUGE proponent of polyurethane suspension bushings...if the bushings don't give, that force is transmitted to the next weakest point.
     
  5. BowtieBlazer

    BowtieBlazer 1/2 ton status

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    I agree about trying to eliminate frame flex being stupid, however I'm talking about impact. In a sense I almost see it designed best as it can be in stock application, all the load is sent in a fairly straight line down each frame rail, but to keep those two rails square is what I am worried about and have a feeling the crossmembers should do.......as with anything seems the furthest point from the bracing is at risk to bend, hence the want to add a V suppor I would think people have boxed the front or rear alone....however thinking to how unibody 4x4 parts are made they just use longer brackets to achieve strength.
     
  6. BowtieBlazer

    BowtieBlazer 1/2 ton status

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    I'm pretty sure its mostly how they cool it....heating the metal rids it of impurities and almost fuses the metal together better, especially when compressed(forged), its been awhile since i tool a materials class
     
  7. blazinzuk

    blazinzuk Buzzbox voodoo Premium Member GMOTM Winner

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    I don't know about the frames being heat treated but I have seen a truck before and after the boxing of the frame and the difference it made in ride quaility and the overall feel of the truck was huge. The only thing that was changed was the frame although if I was going to take on such a project I would get a torch or plasma cutter the plasma would make it easy. Also I can't see why frame flex is good or even alright. The frame is there to support mounting points for the suspension drivetrain body etc thats all not to act as a secondary suspension. Also when boxing a frame wouldn't it be kind of stupid to use steel that is thicker than the frame I would think this would cause more points of weakness. My thought is the more you can make each part of the truck do its job the better I would imagine it would be easier to make a suspension do what you want it to do if you don't have to worry about how the frame flex is going to affect the suspension travel. It might be just me but I think boxing and putting stronger xmembers in the frame is the way to go.
     

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