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Replacing Formed Channel with Rectangular Tubing

02/26/2016 9:32 PM

Hello all.

I have a question in regards to replacing a formed channel frame with a piece of rectangular tubing. The channel is 3" x 8" .375" and 22 ft long. It is on a M-35 "Duece", army 6x6. I have attached a picture of the truck. I am going to add an extended cab and a pick up truck style box, rollcage and modern more powerful engine. The truck weighs in at 14,000 pounds and is rated for 5000 pounds off road and 10,000 pounds on road. I plan to replace the axles and brakes to modern, more safer designs based on a Ford F550. Disk brakes will be on all 6 wheels.

I was a welder and machinist in my past life and I have done work like this in the past on nodwells and other offroad equipment. I will be drilling out the sides of the tubing and welding in pieces of sched 80 pipe to prevent the sides collapsing where fasteners are needed. I want to cut down on weight and height (has to clear my garage). I would like to replace the channel with 3"x6"x.250 rectangular tubing for neatness, lighter weight and strength. From what I have been told and could figure out from the manuals the formed channel MAY have received quench hardening. The loading capabilities of the truck will be reduced to 3000 pounds on road use, NEVER off road.

I plan on bringing it to an engineer, but I was hoping I could at least be on the right track before I brought it in.

Thank You,

letssled

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#1

Re: Replacing formed channel with rectangular tubing

02/26/2016 10:02 PM
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#2
In reply to #1

Re: Replacing formed channel with rectangular tubing

02/26/2016 10:29 PM

Yes, that is it, I guess my pic didn't attach....Thanks!

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#4
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Re: Replacing formed channel with rectangular tubing

02/26/2016 10:37 PM

A lot of the stability and strength of the structure is dependent on the crossmember design...It seems like you are replacing everything, what are you keeping? Why wouldn't you just take a regular lighter duty truck frame, or perhaps two, and fabricate your frame from that?

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#3
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Re: Replacing formed channel with rectangular tubing

02/26/2016 10:30 PM
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#5
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Re: Replacing formed channel with rectangular tubing

02/26/2016 10:58 PM
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#6
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Re: Replacing formed channel with rectangular tubing

02/26/2016 11:12 PM

Mainly cost reasons. I looked at adapting a Ford F550 4x4 and/or Dodge SLT 5500 4x4 but the sheet metal work and paint was $30,000. I would like to put in Cummins 8.3l or a Cat 3306 engine to power it, that will be really hard to fit in a small truck. Plus I can do a lot more work on it myself. Just to clarify, this will be a crew cab extended cab to seat 8. I have 5 kids....so I need a fairly beefy frame and starting point. It is going to pull my holiday trailer too and carry my Polaris Rzr in the box.

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#7
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Re: Replacing formed channel with rectangular tubing

02/27/2016 3:16 AM
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#8
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Re: Replacing formed channel with rectangular tubing

02/27/2016 3:35 AM
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#9
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Re: Replacing formed channel with rectangular tubing

02/27/2016 3:43 AM

There is nothing like that up here in Canada. Plus I won't be able to fit in the engine that I want. The cat 3306/cummins 8.3l is rated 300 hp and 1100 ft/lbs and weighs an additional 900 pounds. I don't think that frame could take that long term.

But essentially that is what I am going to build, just on a larger truck. I want the safety of a larger frame, rollcage, and better brakes.

You mentioned about cross members, and those will be welded in with rectangular tubing where possible.

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#10
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Re: Replacing formed channel with rectangular tubing

02/27/2016 11:20 AM

Well I was thinking the way to go would be to get the body, or as close to it as I could find, then build the frame, or modify the existing frame, so that it would fit the body...If you could find an already stretched body vehicle you could separate the body from the frame, then modify the body or not, and build the frame separately, it would certainly need at least a 6" lift kit....That way you have a model of the existing frame to work with, and not so much body work....In any case it seems the body would need to be built before the frame...The mounting points of the body will have to match the frame...then you just need to make sure the engine fits....If you took the frame from the M35 and tried to fit another body on it, the mounting points are not likely to match....If you start out with a matching body and frame, even if you heavily modify them, it will be a lot easier to retain the fitment I think...

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#14
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Re: Replacing formed channel with rectangular tubing

02/27/2016 4:22 PM

The duece frame is roughly 22' and is not much longer then an extended cab truck. So it will be easy to modify for my use. Basically I just have a add onto the cab. I can keep the engine bay close to stock and just add in a few modern amentities like A/C, sound deadening material, air ride cab.

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#11

Re: Replacing formed channel with rectangular tubing

02/27/2016 1:48 PM
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#12
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Re: Replacing formed channel with rectangular tubing

02/27/2016 2:08 PM
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#13
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Re: Replacing formed channel with rectangular tubing

02/27/2016 4:14 PM

That is good information, thanks. However I am was looking more along the lines "yes the 3x6x.250 rect tubing is a good start" or "no 3x6x.250 is to light go to.....". I have to send the engineer a drawing with mounts, crossmembers, etc. So I would like to get it as close as possible to start.

Just to clarify, I want to replace the formed channel with rectangular tubing.

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#15
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Re: Replacing formed channel with rectangular tubing

02/27/2016 10:01 PM

You can't determine the yield strength needed without the total weight and weight distribution on the frame....The grade and type of steel used is every bit as important as the size and thickness of the steel tube....and the design and structure of the frame is every bit as important as either...

..."typical values are: 35,000 psi for mild steel, 110,000 psi for alloy steel, and over 110,000 psi for heat-treated steel. When mixing materials of different yield strengths, the lowest value must be used for calculations."...

I recommend someone who specializes in frame design....and has the software to do the job....otherwise you are just guessing, as I would be, to make selections....You need to nail down the components and weights involved, as well as distribution to calculate the bending moments involved....In closing your selection would probably work, the question is how long....

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#18
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Re: Replacing formed channel with rectangular tubing

02/28/2016 9:44 PM

Exactly, I want to make sure that this is a safe design and willing to pay $$$ to get it right. Admittedly, I was hoping to save a bit of $$$ getting it close at the get go.

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#21
In reply to #15

Re: Replacing formed channel with rectangular tubing

02/29/2016 9:40 AM

The most common steel grades for tubes in Canada are at 50,000 psi yield strength. Applies to those rolled in Canada and those in the US as well.

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#16
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Re: Replacing formed channel with rectangular tubing

02/28/2016 10:25 AM

Yes, the 3x6x.250 is a good start, and finish for what you want to do, maybe even overkill.

Remember, military vehicles were designed to be strong enough to withstand repeated abuse, overloading and any terrain you can imagine. That same frame was under all the WWII ducks used in the war as well.

Take a look at the relative strength and stiffness difference of a c channel vs a box rail and that should convince you.

I know what I put my bone stock Chevy 4WD vehicles through years ago and they had paper thin frames compared to this.

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#19
In reply to #16

Re: Replacing formed channel with rectangular tubing

02/28/2016 9:46 PM

Very true, I don't think my frame is ever going to see the abuse these trucks were designed for. I know they use the same frame on the 10 ton trucks. At best mine will carry 1.5 tons. Although it will have much more "twist" with the larger engine.

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#17

Re: Replacing Formed Channel with Rectangular Tubing

02/28/2016 10:54 AM

Using a Rectangular steel tube to replace the channel frame is a good start, but a 6-inch deep one may be susceptible to torsional loads.

Remember, truck frames are subjected to dynamic loads and hard to predict. I would suggest starting with a TS 3 x 8 x 1/4 tube or perhaps an even beefier TS 4 x 8 x 3/8 and let the engineer go from there.

I would not go with a steel Yield Stress any less than 50 Ksi. Might want to use a steel with Fy=85 Ksi and higher.

If anything, incorporate some X-Bracing (wherever you can fit it) between the parallel TS frame and the cross pieces to stiffen-up the entire chassis frame against torsion (twisting) if you're planning on upgrading the engine Brake Horsepower and corresponding higher torque.

Here is a case to find a Mechanical Engineer who knows dynamic loads and also knows how to run a very good Finite Elements software package.

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#20
In reply to #17

Re: Replacing Formed Channel with Rectangular Tubing

02/28/2016 10:11 PM

Exactly, I want to make sure this is safe for everyone on the road, not just me. I think I will see what is available in the higher yield limits. Personally, I like the idea of "softer" steels to resist cracking but a sound design should help alleviate that.

I am hoping I can incorporate part of the rollcage into the frame. That should add significant torsional strength. X bracing can take place under the cab once the driveling components start to slope away. Be nice to have an air ride cab, but I am willing to pass that up if it means a safer design.

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CaptMoosie (1); Kevin LaPaire (1); letssled (8); lyn (1); SolarEagle (10)

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