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Old 04-25-2017, 06:05 PM
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Default Truck frame splice

So Trevor has gathered a bunch of parts for his truck project and before he starts mocking everything up, we think the first order of business is to get the wheel base set.

He is looking at taking somewhere between 12" and 18" out of the frame.

So here's my question: What kind of cut do we need to do on the frame rails? Do we need to a diagonal cut with radius's on each end to prevent stress risers? What about fish plates on the splices? Do they need to be diamond shaped to prevent vertical welds on the frame rails to prevent cracking?

I have no clue about this stuff as it is uncharted territory for me.

I ask that all the knowing shed some light.
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Old 04-25-2017, 06:14 PM
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What kind of truck?

My chebbys have a splice in them from the factory. Easy to undo it, chop out what you need, then re splice in the same spot. I think they do it this way to account for long bed vs short bed, crew cab, etc.
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Old 04-25-2017, 06:19 PM
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This '86 fire truck.
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Old 04-25-2017, 06:22 PM
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I used to shorten and lengthen truck frames many years back and always made a straight vertical cut. Always used a diamond splice plate same thickness and the frame web.
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Old 04-25-2017, 06:34 PM
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More details of the project might help... Why the shortening?
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Old 04-25-2017, 07:09 PM
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A lot of people do a step cut or a angle but I always cut a 90 degree angle. First I would put punch marks on each side where you want the cut, then if you were to take out 18 inches I would measure 10" in front and in back of those punch marks and put two new punch marks on each frame member. You would do this so that after you make your cuts you can keep 2" plus what you leave for a gap between your marks when you start tacking it up. (The cuts don't have to be directly across from each other as long as you take out 18" from each side). I then bevel the ends at a 45 degree angle leaving about 1/16" land. I bevel the legs from the top so there is no over head welding. I then leave from 3/16" to 1/4" gap and use copper backup and do a 100 percent weld doing all the flats first. Don't do all the welding on one side move from side to side. Then I make a fish plate for the inside leaving enough room to get a weld on the top and bottom and all the way around. Use lots of clamps and supports IMO
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Old 04-25-2017, 07:20 PM
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Similar to what digr said except I did all the beveling on the inside, then ground back into the inside weld and ran a fairly flat outside pass. Always did the top and bottom flanges first. Back then I was so much more limber than I am today so overhead then was easy.
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Old 04-25-2017, 07:48 PM
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The goal here is to shorten the wheel base for various reasons.

I guess I'm having a hard time describing what I'm thinking here so take a look at my sketch and direct me.
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Old 04-25-2017, 07:57 PM
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I would just cut the frame at a 90 and forget the fancy cutting. Its a waste of time IMHO There probably not enough room to make a cut like that anyway. Your a welder you know that frame isn't gonna bust after your done.
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Old 04-25-2017, 08:08 PM
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When I worked at the fire truck factory, we shortened and lengthened truck frames all the time. We always cut the frame at a 45 degrees, with the angle going from the top of the frame, angling down toward the front of the truck. That what Ford commercial tech showed us to do. No sure if it makes a difference.
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