Pipe C Notch

[DWright]

stupid question, but does anyone have pics of a 60-66 with rear bags and just a pipe notch? (preferably aired out)

[shortbed70]

Heres my pipe notch welded in

[Chris65GMC]

I did a search, but does anyone have a link for a good 'how to" on the pipe notch?

[hothues70]

I've always just unbolted the rear hangers or coils and let the truck down on the axle to get my mark. Easiest way for me to figure out where to cut. Then just measure how much farther you have to go to reach the lowest point, laid out or wherever and cut ur piece of pipe to give you that much notch. Trace it on the frame, cut it out and weld it. It goes pretty quick that way. Has for me anyway.

[lakeroadster]

I thought about notching my frame with the simple cut out and welding in of a piece of pipe until I ran the numbers to see how it would effect the strength of the frame. It is really a dramatic change.

Notching a frame in the method shown above reduces the rear frame strength (behind the axle) by about 75%.

Now, that being said, depending on how you end up using your truck that may not be a big deal to you:

On a truck arm design frame, the coil springs are ahead of the rear axle, so the suspension load on the frame is negligibly affected by the c-notch.

You for sure don't want to use this method on

a leaf spring type set-up,
if you plan to haul anything in the truck,
if you plan pull anything in with the truck,
if you have moved your gas tank to behind the rear axle.

The above drastic strength reduction is the reason why purchased kits have heavy structural steel members that are added to the existing frame rail.

I am not preaching, do what you like, I just wanted to make sure you knew what these changes do to the strength of the frame, behind the axle.

[protrash64]

lakeroadster; with out going into a complete engineering seminar, how do you make that calculation? Just curious.

[SCOTI]

Originally Posted by protrash64

lakeroadster; with out going into a complete engineering seminar, how do you make that calculation? Just curious.

x2?

Also.... What if a pipe notch similar to what is pictured above is boxed on the inside? What effect would that have to restore that strength?

[lakeroadster]

Originally Posted by protrash64

lakeroadster; with out going into a complete engineering seminar, how do you make that calculation? Just curious.

Look at the equation below shown in the images. This is is the calculation used to determine the section modulus (S) of an open channel. Section modulus is directly related to the strength of the member.

The change in height radically effects strength because in the equation the height is exponentially to the 3rd power. Height is the major determining factor, which makes sense, but changing it effects strength to the 3rd power which derates its capacity really quick.

For a stock frame

b = 3
s = 0.16 in.
d = 4.75 in.
t = 0.16 in.
h = 4.43 in.

Crunching the numbers, S = 6.22 in^4

With a C-notch you have changed the height, d = 2.375, S = 1.30 in^4

So the notched frame has about 20% of the strength of the stock frame. That is the reason why purchased kits have heavy structural steel members that are added to the existing frame rail.

Originally Posted by SCOTI

x2? Also.... What if a pipe notch similar to what is pictured above is boxed on the inside? What effect would that have to restore that strength?

Yeah, I thought of that too. It doesn't help very much at all.

Modify a stock channel type frame, making it a tube frame:

S = 7.38 in^4

C-Notch modified tube type frame with d = 2.375

S = 1.41 in^4

Originally Posted by sailed2japan

Without detailed engineering experiments and only sense to work with. I would say that by notching half the height of the frame, using the same grade/thickness steel, and boxing the back side, to include going a distance equal to the height of the frame forward and aft of the notch, ther would be little to no degredation of the integrity of the frame. And lets face it, if you're notching your frame, you're not planning on heavy duty work.

Oh, and the gas tank behind the axle? Really? Even a topped off 40 gal tank only weights about 300lb!

I thought the same thing, that's why I ran the numbers and the numbers simply don't support this. Boxing a frame helps the structural resistance to twisting alot and that is why a lot of people box frames. It also helps the strength in bending, but adding the extra member doesn't double the strength, for a stock frame adding the extra member above the axle adds about 20% to the strength.

As far as the gas tank goes, it's not so much the weight as it is the moment arm. A 300 lbs gas tank mounted behing the rear axle on a long bed truck adds a lot of moment. Assume 3 ft to the center of gravity: 3 ft x 300 lbs = 900 ft-lbs.

Like I said previously I am not preaching, do what you like, I just wanted to make sure you knew what these changes do to the strength of the frame, behind the axle.

Just trying to help.... don't shoot the messenger.

[protrash64]

Ok, thanks John.........that's cool!! Now my question is what is the difference from boxing to '' heavy structural steel members that are added to the existing frame rail.''?

Isnt that just the same as boxing on the outside of the frame? And, do the bolt holes reduce the section modulus of the frame?

[SCOTI]

Originally Posted by daverod

Thanks for the time to figure this out John. I knew you just were'nt talking $hit. I even kinda understand it. Minus all the numbers and stuff.

Agree 100%. I would also like to add....

Originally Posted by lakeroadster

Just trying to help.... don't shoot the messenger

There's a difference when someone criticizes a modification and/or method w/o providing the benefit of their logic on how things are affected. 'Just because' as reasoning is when people grow irritated. We can all learn from each others backrounds, educations, & experiences throughout our years.

I did consider the strength impact when reducing the height of my frame channel. I figured the use of the heavier walled material (.250" steel plate c-notch) w/boxing it in (using .1875" steel plate) would restore the majority of the strength. Seeing those numbers crunched, I'm going to re-evaluate/scrutinize my set-up & consider the possibility of plating the outer wall of my frame for additional assurance.

[lakeroadster]

Originally Posted by protrash64

Ok, thanks John.........that's cool!! Now my question is what is the difference from boxing to '' heavy structural steel members that are added to the existing frame rail.''?

Isnt that just the same as boxing on the outside of the frame? And, do the bolt holes reduce the section modulus of the frame?

Yep, from a "calculation standpoint" adding the structural steel member is about the same as welding in a boxing plate. Most of the bolt on kits use a steel plate that is considerably thicker than the stock truck frame thickness. That helps but I still would like to run the numbers on one of the kits to see if it is as strong as the original uncut frame when all is said and done.

As for the bolt holes, they don't effect the strength very much as long as they are spaced away from each other.

I took a little time this morning and made a solid model using Inventor Cad software. (See attachments below)

First I made a model of a stock frame rail.

Second a model of a c-notched rail using 4 inch pipe (0.237 wall) as the notch. This configuration is about 40% weeker than the stock frame.

Third was the c-notch with a 1/4" thick boxing plate. This configuration is about 25% weeker than the stock frame.

Using FEA is more realistic than the simple channel and tube calculations that were discussed in the previous posts because it looks at how all the parts work together as a system. And the cool thing about this type of analysis is it gives you a warm fuzzy feeling about how different modifications effect the strength of a part.

Kind of neat, don't ya think?

[lakeroadster]

Originally Posted by protrash64

I guess to make a more extreme example, using the 4.75'' frame rail from page 1 (d).....what if, centered on the frame rail, one was to holesaw a 3'' hole thru the frame. What does this do?

My father was an engineer at Caterpillar for 40+ years so I'm used to physics, equations, and math blowing holes in my 'common sense'.

Looks like with a 3" hole in the center of the channel it is about a 40% reduction in strength over the stock configuration. But if you were to add a reinforcing pad to the hole, lets say 4 inch o.d. x 1/2" thick this configuration it just about as strong as the uncut frame.

But regular bolt holes, lets say 9/16" dia. don't effect the structure much. I plugged in (4) of them and it reduced the stock frame strength by about 10%.

Normally though if there are bolt holes, there is some structure being bolted to the frame, and that structure will likely stiffen things back up... depending on how strong the structure is.

So using the re-pad concept I added a half re-pad to the c-notched frame that had the boxing plate and it is as good as the stock frame. Use a 3/8" thick boxing plate, a 3/8" thick 7" dia. half re-pad and 4" 0.237 wall pipe.

[DWright]

is this workable? sorry its so crude. and yes, the extra 1/4'' wall pipe is used above the frame and plated on both sides with 3/16" steel.

[lakeroadster]

Originally Posted by DWright

the pipe was 6" od. i wasnt too concerned with the bed floor as mine is currently a sheet of plywood. i was thinking of raising it about 2" to clear the diff. when aired out anyway. what kind of steel do you recomend be welded to the top of the frame? i was only asking about the remainder of the pipe because i already had it and hate wasted steel.

Dwright,

With the 6 inch pipe notch the frame has about 17% of the original frame strength.

Modified in the configuration you sketched out, with the remaining 6 inch pipe on top and boxing plates, does little to strengthen the frame. This configuration has about 22% of the original frame strength. The frame still needs more section strength.

I took the liberty of proposing an alternate design. This has pieces made from 3/16" plate and this version is stronger than the the original frame. Basically it is a box structure that extends 2" above the frame, 12" long and ends are angled at 45 degrees and they are also boxed in.

[lakeroadster]

Originally Posted by DWright

thanks for the work Lakeroadster. i notched the frame lastnight and installed the 6" pipe notch. i'm definitely gonna add a piece of heavy wall 2x4 steel to the top of my frame.

No problem, glad I could help. Make sure to box the inside of the frame with the 3/16" also.

[lakeroadster]

Scoti sent me some data on the c-notch he did on his frame. This one is a bit tricky. The notch isn't an arch, it's kind of squared off at the top. The tight radius at the bend makes for high stresses at the bend line.

The frame itself is strong since Scoti added a boxing plate and the materials used for the arch is 1/4" thick. Take a look at the attachment below of the stock frame and of Scoti's frame. Take note of the colors and the corresponding scale at the left hand side. Notice that most of the areas have a safety factor of between 1 and 5, which is good when comparing to the stock frame.

But even if we add material to the top to make the overall frame section stronger that darn tight radius is the "hot spot". This is called a stress-riser in engineering lingo. And the kicker is, depending on how much the frame is loaded, this hot spot may never rare it's ugly head. But this spot is likely where a problem would occur in an overload condition.

I will "noodle" over this a bit more... maybe add a post later with some updates pending any good ideas ya'll might have.

[lakeroadster]

Scoti,

How is you overhead welding abilities? If you were to add a piece of 1/2" solid round bar to the notch as shown in the attachment and then grind the welds such that they feather into the notch it helps a lot. Plus I would suggest that you grind a radius on the notched edges as shown in the attachment, that also helps. As a rule, radius edges when you can, that always helps reduce stresses.

The upper truss is 10-1/2" overall length, is boxed in, made from 3/16" plate, and ends are angled 45 degrees.

I hope this helps.

[SCOTI]

Originally Posted by lakeroadster

Scoti,

How is you overhead welding abilities? If you were to add a piece of 1/2" solid round bar to the notch as shown in the attachment and then grind the welds such that they feather into the notch it helps a lot. Plus I would suggest that you grind a radius on the notched edges as shown in the attachment, that also helps. As a rule, radius edges when you can, that always helps reduce stresses.

The upper truss is 10-1/2" overall length, is boxed in, made from 3/16" plate, and ends are angled 45 degrees.

I hope this helps.

It's a frame on a quasi chassis table/jig. I suck @ overhead stuff but flipping the frame is very do-able . To make sure we're on the same page, what height did you establish on the truss?

[lakeroadster]

1-1/2" high on the upper truss... see attached.

Be sure and let me know how this works out for ya.

[SCOTI]

Originally Posted by lakeroadster

I got ya, the tops of these frames are not straight, good thinking on using the other frame for the additional truss.

However I don't know that I would use an additional 3" to the frame height. If you were to maybe cut the frame section such that you end up with the 1-1/2" to 2" of additional height that should be adequate and it would still follow the contour of the frame. And for the ends I would angle them at 45 degrees and not use structural angle. Remember, abrupt changes mean stress risers. Try to make it flow without any hickups, if that makes sense. Smoother be betta.

I might have not been clear enough in my description. The angle pieces would be underneath the outer structure simply to assist tying the 2 sections together; the 'ends' of the truss would be 45° angled from new to old frame sections.

[lakeroadster]

There was previous discussion here about commercially available c-notches.

From what I have seen Beltech has a very good notch kit, but they don't offer them for our 60-66 trucks.

Take a look at the pictures below and the followig link: http://www.belltech.com/trucks.php


Basically they have a heavy formed c-channel with a welded notch. Then this bolts through the frame and there is a heavy backing plate.

This essentially triples the wall thickness of the frame. Interesting. If any body has one of these I would be interested in knowing how thick these components are.

[lakeroadster]

A couple things to keep in mind:
The base line I used was my stock c10 frame geometry above the rear axle. I believe this geometry is also the same for a C20. So I created a model in cad and loaded it using FEA to get a safety factor of a stock non-modified frame. Then I modified the model based on scot's c-notch dimensions to determine the safety factor of his frame as modified.

So all we are doing is comparing a stock frame to a modified frame.

The stock frame is very very strong.

Also be aware that modifications to a long bed truck puts more stress on the c-notch area.. Longer lever arm bending this area.

But everything relates back to how you use your truck. For me I want any modification I make to be at least as strong as it was when the truck left the factory. Might be overkill, but that is my baseline.

As for the "kit" notch, see posts #89 and #90 above, I re-ran the model assuming formed kit channels made out of 1/4" plates and the 1/4" backing plate.

The FEA says this "kit" it is still a 40% reduction of frame strength.

From an "Engineering" standpoint of beam analysis the height of the beam is the driving factor of the strength. Thickness matters, but not nearly as much as vertical web height.

If you want the frame to be as strong as stock and you want a notch deeper than the 2" one shown in thread #35 there is simply no substitute for adding some sort of truss above the frame, or replacing the frame section with another structural member that has the same section height as the original frame.

Whenever someone starts talking about a deep c-notch and state that just by adding thicker plates to the sides of the frame rails this makes the frame stronger than it was in its original condition, yet they are and not adding vertical height, they do not understand beam analysis.

[N2TRUX]

Edited version for FAQ. Original version located here- LINK

[SBTork]

I know this is kind of off topic, but how does a step-notch factor into this? Would they not suffer from decreased strength because they're still the same section (frame) height through the notch?

[Notch1988]

Originally Posted by SBTork

I know this is kind of off topic, but how does a step-notch factor into this? Would they not suffer from decreased strength because they're still the same section (frame) height through the notch?

That's what you want... the same section height to maintain the strength. Plus the step-notch is fully boxed.