Question for the frame engineers
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I'll have the Roast Duck
Joined: Jul 2002
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From: Northshore, MN
Question for the frame engineers
It seems the standard size for the 3/4 ton and 1/2 tons is 6"x2.25" C-channel. I measured the same on my '48 F1, '47 KB2, and '79 Bronco, all of which are 125" wb or less. (I measured 6.5" or so on my '76 2wd frame, 133" wb)
Anyhow, if the C-channel is boxed, it would not only be considered more rigid, but stronger as well. So here's the question.... if the frame rail is boxed the whole length, what size would that be equivalent to in strength? IE: would 4x2 rect. tube be equivalent to 6x2.25 C-chan.?
wall thickness would remain constant @ 10 gauge
Also, for those who boxed their frames, have ya weighed them before and after? Big difference? or hardly noticeable?
Anyhow, if the C-channel is boxed, it would not only be considered more rigid, but stronger as well. So here's the question.... if the frame rail is boxed the whole length, what size would that be equivalent to in strength? IE: would 4x2 rect. tube be equivalent to 6x2.25 C-chan.?
wall thickness would remain constant @ 10 gauge
Also, for those who boxed their frames, have ya weighed them before and after? Big difference? or hardly noticeable?
Fleet Owner
Joined: Jul 2004
Posts: 27,299
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From: NM
Wow, two times in the last month I've had to pull out the AISC Steel Construction Manual! It's a great book, you can pick them up used for dirt cheap, too.
In terms of vertical load-bearing capacity, the relevant metric is Moment of Inertia.
The closest I can figure without resorting to some math I haven't done in years, 6" tall x 2" wide x 3/16" thick Channel would have a MoI of 8.55. Boxing it makes it 10.8.
4" tall x 2" wide x 3/16" thick rectangular tubing has a MoI of only 3.87. Not even close.
The height of the section is what gives it the resistance to bending, much more than the width.
In terms of vertical load-bearing capacity, the relevant metric is Moment of Inertia.
The closest I can figure without resorting to some math I haven't done in years, 6" tall x 2" wide x 3/16" thick Channel would have a MoI of 8.55. Boxing it makes it 10.8.
4" tall x 2" wide x 3/16" thick rectangular tubing has a MoI of only 3.87. Not even close.
The height of the section is what gives it the resistance to bending, much more than the width.
Thread Starter
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I'll have the Roast Duck
Joined: Jul 2002
Posts: 9,622
Likes: 110
From: Northshore, MN
I currently have T. Glover's pocket reference, and a bunch of formulas, including MOI from the ME at work, of which I also emailed the question to. But I can't find it, and the steel catalogs I have from work don't get into MOI, mostly just weight per foot.
I'll take a look for that book, as it would come in handy, no doubt.
My use for the answer is on my IH truck frame. I'm looking to use new rectangle tube for the really bent frame and IFS/IRS suspensions, but the rails are 2.25" wide, and the suspensions being used are 3" wide on the inside of the brackets. I'm considering using the 2.25" width and adding long oval 3/8" plates to the rail to make it 3" wide. I'm just wondering if 6x2.25 rect. tube would be overkill. The donor suspension's rails are not even 4" tall.
I'll take a look for that book, as it would come in handy, no doubt.
My use for the answer is on my IH truck frame. I'm looking to use new rectangle tube for the really bent frame and IFS/IRS suspensions, but the rails are 2.25" wide, and the suspensions being used are 3" wide on the inside of the brackets. I'm considering using the 2.25" width and adding long oval 3/8" plates to the rail to make it 3" wide. I'm just wondering if 6x2.25 rect. tube would be overkill. The donor suspension's rails are not even 4" tall.
Fleet Owner
Joined: Jul 2004
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From: NM
4x3x3/16" is a common structural tube shape. In front of the IFS, you only need strength to attach a crossmember to. 6" deep is used in the center (front-to-rear) of a truck frame, halfway between the support points (axles) is where you need the depth.
Lead Driver
Joined: Aug 2006
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From: Brentwood, TN
A fellow club member recently bought a Walton chassis for his 56 F100. The frame is constructed of box steel and from what I recall is the same outer dimension as the OEM frame. What is VERY different is the gauge of the metal...I am not sure what the actual wall thickness is but it feels like it is about half that of the OEM frame.
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I'll have the Roast Duck
Joined: Jul 2002
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From: Northshore, MN
Charlie, the donor Jeep's rails are pretty thin, too..... real thin.
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I'll have the Roast Duck
Joined: Jul 2002
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Likes: 110
From: Northshore, MN
Got my answer: Equivelant would be 5.5 x 2.25, same wall thickness., or 6 x 2.25 x .093 wall (13 GA.)
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Fleet Owner
Joined: Jul 2004
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From: NM
Great! Just have the steel mill roll you out about 10' of it! 
Did your source say that was commercially available? 0.093" seems awfully light, but it doesn't matter as long as you're comparing apples to apples.
Did your source say that was commercially available? 0.093" seems awfully light, but it doesn't matter as long as you're comparing apples to apples.
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I'll have the Roast Duck
Joined: Jul 2002
Posts: 9,622
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From: Northshore, MN
.140 vs. .093 doesn't seem worth the hassle. I'll just go with the 10 gauge. Heavier duty, with little weight gain. Plus the side impact from another car would be better handled with the thicker material. I'll be emailing the suppliers' salesmen from work tomorrow for availability. May end up with 3/16" or 1/8" wall.
Fleet Owner
Joined: Jul 2004
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From: NM
Keep in mind if you are going with IFS, the stiffer the frame the better. That way the suspension does the "flexing", not the frame. Complete opposite of the Ford concept with leaf springs. I had a good article off the HAMB written by one of the frame/IFS guru's but can't find it. Basically said that.
Thread Starter
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I'll have the Roast Duck
Joined: Jul 2002
Posts: 9,622
Likes: 110
From: Northshore, MN
.140 vs. .093 doesn't seem worth the hassle. I'll just go with the 10 gauge. Heavier duty, with little weight gain. Plus the side impact from another car would be better handled with the thicker material. I'll be emailing the suppliers' salesmen from work tomorrow for availability. May end up with 3/16" or 1/8" wall.
Fleet Mechanic
Joined: Aug 2010
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From: Morgan Hill, CA
Havi & Ross,
It's been a while since I've calculated a MoI, but I think the 2x4 3/16 wall tubing has a MoI that is around 11.96. That'd be somewhat higher than the stock 6" C channel.
I'll check my math.
Dan
It's been a while since I've calculated a MoI, but I think the 2x4 3/16 wall tubing has a MoI that is around 11.96. That'd be somewhat higher than the stock 6" C channel.
I'll check my math.
Dan
Fleet Owner
Joined: Jul 2004
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From: NM
My manual says 6x2x3/16 is 10.8. If you calculate it with sharp corners, it will vary. The book's tables use rounded corners. I used 6x2 in the tables for the value of a boxed stock frame, it's a lot easier than doing the math! For the stock un-boxed frame I took half the value of a 6x4 rectangular tube for the x-x axis MoI. That's right, isn't it? The only channel the book has is for forged/rolled structural channel which is not at all a good comparison (thick corners, etc).
Here is the page I got it from, it covers almost anything we'd be using. Ix is the MoI for vertical loading.
Here is the page I got it from, it covers almost anything we'd be using. Ix is the MoI for vertical loading.