is frame section modulus the frame height?

[G-NUT]

I'm trying to find out the frame height under the doors of a 1984 chevy suburban. I think it's a C10 but it may be a C20 ... looong story. I won't have it in my presents for another month but I'm trying to do some preimptive stuff. I found a 1984 chevy suburban spec sheet from the gm heritage site. I has good info but there is a spec in there listed as frame section modulus with a numerical value beside it. I'm wondering if it is the frame height value I'm looking for.

[1967KaiserM715]

Modulus is just strength

C10 and c20 frame heights are the same, the thickness changes. Can't remember the value though.

It was my basic understanding all c20 had 8 lug wheels.

[slotard]

Section modulus is related to height and thickness but isn't either one directly.

For a rectangular section, bh^2/6 is the formula to calculate it. If there's another rectangle with the centroid at the same height that's missing (as there is on a sheet channel frame like we have, or a box section frame too for that matter) you can subtract one from the other: 8" tall 2.5" wide 1/8" thick channel is calculated as (2.5)(8^2)/6 - (2.375)(7.75)^2/6, for example, with the height of the section you subtract being height minus two thicknesses and width being width minus one thickness (width would be two if boxed).

And yes, if it's a C20 I believe it's 8 lug.

I suspect that on the Suburban they used a frame of equal height but a little thicker, but I could be wrong.

Looking at the gmheritagecenter specs there are some bizarre things there - I have no idea why the K20 is geared taller than the C20, for example.

[G-NUT]

Thanks for the responses guy.

I got the burb but will not have it in hand for another month. It's been a couple years since I physically laid eyes on it. A family member sent me pics and the title. The badge on the fender says C10 Silverado. It does have 5 lugs. But according to all the VIN # decoders I've used, according to the VIN it's a C20.

I currently have a 1985 GMC C2500 2wd that I was working on. I'll be selling it and focusing on the burb. BUT my C2500 had about a 3 inch taller frame under the cab than the C1500. I have the parts to bag it and lay pinch weld. So that's why I'm wanting to know about the frame heights.

[mongocanfly]

If it says c10 and has 5 lugs it's a sure bet that's what it is...not a c20..unless somebody went to alot of trouble...

[G-NUT]

Originally Posted by mongocanfly

If it says c10 and has 5 lugs it's a sure bet that's what it is...not a c20..unless somebody went to alot of trouble...

Ordinarily I'd think that, but we're getting this from an uncle that's passed and he owned his own body shop. I wouldn't have put it past him to swap that stuff out.

[mongocanfly]

I hear ya...a lot can change in 30+yrs

[SCOTI]

Originally Posted by 1967KaiserM715

Modulus is just strength

C10 and c20 frame heights are the same, the thickness changes. Can't remember the value though.

It was my basic understanding all c20 had 8 lug wheels.

C10's frame rails are not as tall as C20's or C30's of this generation. The extreme front/rear sections are similar but the rails under the body are taller on C20/C30 equipped trucks . Strength in the rail comes from it's height + thickness.

[RADustin]

Originally Posted by slotard

Section modulus is related to height and thickness but isn't either one directly.

For a rectangular section, bh^2/6 is the formula to calculate it. If there's another rectangle with the centroid at the same height that's missing (as there is on a sheet channel frame like we have, or a box section frame too for that matter) you can subtract one from the other: 8" tall 2.5" wide 1/8" thick channel is calculated as (2.5)(8^2)/6 - (2.375)(7.75)^2/6, for example, with the height of the section you subtract being height minus two thicknesses and width being width minus one thickness (width would be two if boxed).

And yes, if it's a C20 I believe it's 8 lug.

I suspect that on the Suburban they used a frame of equal height but a little thicker, but I could be wrong.

Looking at the gmheritagecenter specs there are some bizarre things there - I have no idea why the K20 is geared taller than the C20, for example.

don't forget about a little parallel axis theorem to subtract out the inside rectangle.

[1967KaiserM715]

Originally Posted by SCOTI

C10's frame rails are not as tall as C20's or C30's of this generation. The extreme front/rear sections are similar but the rails under the body are taller on C20/C30 equipped trucks . Strength in the rail comes from it's height + thickness.

Learn something new every day. I know the K20 and K10 frames are virtually identical except for material thickness, so I guess I assumed the c-series was the same way.