For the Flatbed Gurus

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So a while back there was a very interesting discussion about flatbed design in AKweldshop's flatbed build thread that concerned building flatbeds with some flexibility in mind.  I learned a lot from that discussion, but I've also learned this myself, the hard way.  I build this bed back in 2006, it was my first big welding project, and about all I was thinking about is that I wanted it heavy duty.  I achieved that, but also caused myself some broken box mounts on my 2006 dodge 2500 after a couple years.The basic bed design is 3" x 1/4" square tubing crossmembers that are bolted to the truck frame (at the box mounts in the front), 4" channel long rails, and 3" channel deck crossmembers.The headache rack is 4" x 1/8" square tubing, and the rear bumper is framed up completely with 3" channel, with a 3" x 1/4" tubing step around the bottom.The bed is bolted in 4 places on each side, two towards the front, about 19 inches on center, and 2 in the back, probably about the same.  There's no attachment in the middle, but in line with the gooseneck hitch, there's 3" x 1/4" tubing stood on end to reach the frame, just to support the weight of whats on the hitch.The front box mounts on each side completely broke out, no other damage.So it's obvious that my bed is way too rigid, and the truck trying to flex and the bed refusing caused the mounts to break.  I've got some ideas, but was wondering what exactly in your view did I do wrong?  I'm about to build a bed for a new truck, and would like not to repeat this mistake.   Any help and suggestions would be greatly appreciated, thanks.
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Reply:That's common not matter how the bed's made. The metal is just to thin. Once broken to far I end up replacing with heavier metal. I think I've got some pics of that on this forum somewhere. I'll have to look.   My main cure for this from the start is to make washers that cover the whole inside of the mount from strap. I'll have to see if I can find pics of that as well.  Ok here's a pic of a fixed mount that I did.  Not a very good pic but you can see where I used 1/4" strap to make a new mount welded to the base of the old mount (not to the frame). It's got a piece of 1/2" on top for strength.  Here's a pic of the washers I use when mounting new beds to existing mounts. I've had very few problems with beds mounted with them. At least it takes a lot more time and abuse before they break out.  This shot shows both. For pics and some discussion on this subject the old thread is here. http://weldingweb.com/vbb/showthread.php...hlight=flatbedMillermatic 252XMT 304'sDynasty 280DXHypertherm PowerMax 1250Miller Trailblazer 302 EFIOptima PulserXR feeder and XR Edge gun and more athttp://members.dslextreme.com/users/waynecook/index.htm
Reply:Everyone knows everything on a Dodge is going to break... Airco Ac/Dc 300 HeliwelderMillerMatic 200 (stolen)Miller Maxstar 150STLMiller AEAD200LE (welding and generating power) Hobart MIG
Reply:OUCH but I agree!
Reply:Good info from Irish, he knows his stuff.Also, if that is a lock washer between the nut and mount, it is what caused the crack to start (or it at least helped).GravelThe difference between theory and practice is that in theory there is no difference.
Reply:I make mounts that trap the bed from moving side to side or front to back in a pocket and then use springs on the bolts to bolt it down but allow it to flex when it needs to. I don't think i have any pictures of them but will look.Mike
Reply:I'm not a guru.Sorry, can't help.
Reply:Originally Posted by leightrepairsI make mounts that trap the bed from moving side to side or front to back in a pocket and then use springs on the bolts to bolt it down but allow it to flex when it needs to. I don't think i have any pictures of them but will look.Mike
Reply:Never did anything that was made or used mainly for hauling hay so can't speak to that. Fertilizer will sure do some damage, glad i don't do much around it. To flexible is no good either. Also like boards and u-bolts if the truck has a straight frame.Mike
Reply:I think your problem came in by using cross pieces to the frame and then pieces going length wise. Every truck bed I've built, I use pieces running long wise on top of the frame and then the cross pieces welded on top of those. That way the bed can flex with the frame. Instead of channel running cross wise, I use flat strap usually 1/8" except under the stress points like the head ache rack, winch stand etc. I've never had a break like this on any yet.
Reply:Looks like you did ok on your build. I would just weld in a thicker plate on the bottom side of those mounts so they don't break again. Looks a bit high tho, if it was mine I would want thedeck a bit lower.Yeah, I know, but it'll be ok!Lincoln Square wave 255Miller Vintage mig30a spoolgunThermal Dynamics Pacmaster 100xl plasmaSmith mc torchEllis 1600 band saw
Reply:Most flatbed I have seen, including the one I have now use a piece of wood the frame rails and the bed rails.    Not sure if that adds any flexibility or not.   If they are U bolted together it probably does.Tiger Sales:  AHP Distributor    www.tigersalesco.comAHP200x; AHP 160ST; MM350P,  Spoolmatic 30A; Everlast PowerTig 185; Thermal Dynamics 60i plasma.  For Sale:  Cobra Mig 250 w/ Push-pull gun.  Lincoln Wirematic 250
Reply:The factory Dodge front box mount is notoriously weak. Ive had 2, A 96 and a 98 with service bodies. The front mounts didn't last a year before they had to be rebuilt.
Reply:Originally Posted by BobI think your problem came in by using cross pieces to the frame and then pieces going length wise. Every truck bed I've built, I use pieces running long wise on top of the frame and then the cross pieces welded on top of those. That way the bed can flex with the frame. Instead of channel running cross wise, I use flat strap usually 1/8" except under the stress points like the head ache rack, winch stand etc. I've never had a break like this on any yet.
Reply:Originally Posted by soutthpawMost flatbed I have seen, including the one I have now use a piece of wood the frame rails and the bed rails.    Not sure if that adds any flexibility or not.   If they are U bolted together it probably does.
Reply:Lots of good info Irish, thanks very much.
Reply:Originally Posted by irish fixitThat's hard to do on some of the modern frames. To much curve and not enough room for much else. Short of cutting the original mounts off and bolting to the side of the frame not much else to do but use the factory mounts. If done properly I've seen it work for a long time even under adverse conditions. Hauling cake feeders, and two bales of hay on rough ground is pretty adverse. Combine that with some people are just simply hard on equipment (some farmers are high on this list) makes for some rough times on truck frames. I've fixed a lot of different broken frames, and remounted a lot of beds. I rarely have much trouble once I mount them with my method. Which is not to different from how he mounted his above. I usually use 2" x 3/16" wall or worse case 2" x 1/4" wall tubing run across both front mounts like above. Using the washers most have never cracked out but some people are hard enough on them to break the whole mount off the frame so those get redone with heavy duty mounts.  I also run a strap down from the beds channel to the side of the truck frame above the axle. I piece is placed between the two frames to make up space and a single 5/8" bolt is usually used to connect the strap to the frame of the truck. I then have spacers on the back end of the frame made from 2" tubing. Depending on whether I need to miss the hay lifting mechanism or not those will either go across the frame like the front or along the frame. Two 1/2" x 4" straps are then used to bolt the back end to the frame. Either two 5/8" hard bolts or three to four 1/2" bolts are used back there depending on the frame.   The truck in the picture is actually my dads feed truck. Somebody else mounted that bed to the truck and used the channel across it. It works ok like that but not really my favorite. The bed got taken off and a welding bed off another truck was put on for my nephew who never used it (at least not for making any money). In order to fit the welding bed on I had to cut the front mounts off the frame to clear the tool boxes. When I put the flat bed back on I had to make new mounts to replace the original mounts. But I've done the same on trucks with broken mounts. Actually when I took the flat bed off I found that the mounts had cracked out before I cut them off.
Reply:Originally Posted by welderjLooks like you did ok on your build. I would just weld in a thicker plate on the bottom side of those mounts so they don't break again. Looks a bit high tho, if it was mine I would want thedeck a bit lower.
Reply:Mine is 39" high deck.Very low.
Reply:Originally Posted by leightrepairsI make mounts that trap the bed from moving side to side or front to back in a pocket and then use springs on the bolts to bolt it down but allow it to flex when it needs to. I don't think i have any pictures of them but will look.Mike
Reply:Originally Posted by 12V71The factory Dodge front box mount is notoriously weak. Ive had 2, A 96 and a 98 with service bodies. The front mounts didn't last a year before they had to be rebuilt.
Reply:They used to use wood between frame and beds back in the olden days , but I have heard it was outlawed buy some states . You just don't see it anymore.
Reply:I did a flatbed mount on a Dodge the same way, only difference is, instead of bolting through the entire tube I only put the bolt through the bottom side of the tube, it may give you some more flex.
Reply:I always bolt through only one side of the tube as well. It makes sure that the connection is tight and stays tight.  I do it by doing my cross pieces first. Drilling and bolting them to the frame. I then weld the bolt heads inside the tube (so no back up is needed when taking the bed off).  I then put the flat bed on the truck and line it up where it's even with the body and frame. Tack weld all the cross pieces in place, take the bed back off, finish weld everything, and then finally bolt the bed on the truck.Millermatic 252XMT 304'sDynasty 280DXHypertherm PowerMax 1250Miller Trailblazer 302 EFIOptima PulserXR feeder and XR Edge gun and more athttp://members.dslextreme.com/users/waynecook/index.htmI use grade 8 bolts for the welded bolts so there less chance that they will twist off after they've had time to rust.Yeah, I know, but it'll be ok!Lincoln Square wave 255Miller Vintage mig30a spoolgunThermal Dynamics Pacmaster 100xl plasmaSmith mc torchEllis 1600 band saw
Reply:Originally Posted by welderjI use grade 8 bolts for the welded bolts so there less chance that they will twist off after they've had time to rust.
Reply:Another tidbit learned the hard way,  if your going to weld in a bolt the parts should be in place and tight before welding. If not you can end up with a situation where the weld is holding the bolt rather than the bolt head.Yeah, I know, but it'll be ok!Lincoln Square wave 255Miller Vintage mig30a spoolgunThermal Dynamics Pacmaster 100xl plasmaSmith mc torchEllis 1600 band saw
Reply:I've not built any on trucks newer than about 02 or so. Are the frames curved or just angled inward?
Reply:Usually curved down and sometime angled inward at the same time. It depends on the truck but many just aren't made for a flat bed making it difficult. Cab and chassis are nice straight flat frames making a good foundation but I rarely see any of them myself.Millermatic 252XMT 304'sDynasty 280DXHypertherm PowerMax 1250Miller Trailblazer 302 EFIOptima PulserXR feeder and XR Edge gun and more athttp://members.dslextreme.com/users/waynecook/index.htm
Reply:Originally Posted by irish fixitUsually curved down and sometime angled inward at the same time. It depends on the truck but many just aren't made for a flat bed making it difficult. Cab and chassis are nice straight flat frames making a good foundation but I rarely see any of them myself.
Reply:Craziest thing I've ever seen, it's like they've taken a step backwards.  This style of frame was pretty much the norm back in the 30's, and 40's.  Deep curves over the axle.  Crap way of building a truck body.  My Fords have a small bend, but they're mostly flat.I'd sh9tcan the factory mounts, and just use traditional straps on that thing.  Straps straight off the flatbed mainframe rail, bolted to the side of the channel.  Get rid of that crap that's on there now.  Just cut it off.  Straps allow for some flexibility, but still hold the bed rigid enough sideways if you use 3/8" material.  Add a strongback to the outside of the straps where they have a large clearspan between truck frame, and bed frame.Yet another reason I like my 90's Fords, the frames are made right, and they're probably 30-50% thicker than what's out there today.  All that BS with torque tubes running across the frame nowdays tells you that the frames are substandard, and flex like rubber bands"Any day above ground is a good day"http://www.farmersamm.com/