Hot Rods 53 Truck Frame Notch Help!

I am looking for a little advice on a truck that was just purchased by my friends dad. It is a 1951 Chevy 3100 truck that has had a lot of work done to it by its previous owner; 350/th350/10 bolt. I went over there tonight to check it out and he got excited and wanted to dive in so we decided to take out the rotted wood floor in the bed. Once all the wood was removed we saw that the old owner built some sort of frame in the bed that is a little shady. Then I noticed the frame notch that was done. Looks pretty decent, but kind of thin. Looks to be about half of what the frame used to be, but still small looking. I crawled under the truck and look it all over and it looks solid. No signs of any issues. The old owner claimed he towed a trailer and hauled a bunch of stuff in it all the time, but I don't know for sure.
My question is, how sturdy is this frame really? Or I guess I should ask how does it look in the notch section? Is this normal? Is it strong? Is this frame notch going to hold? If my friends dad tows his car trailer with it, is the frame going to break? I am used to the typical frame notches and was hoping I could get some information to pass along the my friends dad. We are all learning and need help from the professionals!
Is there anything he can do to strengthen it the way it is? Or is he going to have to put a better notch in?
Any help or advise is greatly appreciated!
-Jeff

 

It should have had an internal gusset to strengthen it before it was boxed. Given that the shocks are mounted on a tubular cross member and it still has leaf springs that's a relatively small area to rely on for strength, especially when carrying a load or towing. I'm surprised it hasn't broken its back already as those 'C' notch vertical sides are the weakest part of the frame. Another reinforcement plate on the inside or outside will strengthen things up.





It's wider on the horizontal than verticals and that's the weak link.

 

I am having a hard time understanding the pictures. It almost looks like I can see thru the frame like it has no vertical section. But I can give you some facts about how a beam works. Assuming a beam loaded vertically

1. The metal furthest from the neutral axis does the most work i.e. Provides the most strength. in the case of an I-beam, the neutral axis is parallel to the top and bottom flanges and in the middle of the web of the beam.

2. So what this means is the top and bottom flanges provide the majority of the strength. Having a taller beam moves the top and bottom flanges further apart (farther from the Neutral axis) and makes the beam stronger exponentially. (Doubling the height makes the beam 8 times stronger)
3. Doubling the amount of metal in the top and bottom flanges would only double the strength of the beam.
4. The web (vertical section) of the beam does not really provide much strength. It's basic job is to keep the top and bottom flanges apart since when you load the beam it would want to "kink" the top and bottom flanges together. Think of a piece of thin tubing when you bend it, it is strong until the inner radius of the bend kinks and collapses. What basically has happened is the top and bottom flanges have gotten close together and the strength of the tube / beam has reduced to zero.
5. Because of item 4. You can cut holes /openings in the web of a beam and not really reduce the strength. (Especially if the holes are near the middle of the web/neutral axis). Think of a bridge truss where the web of the beam is really only a series of vertical members often arranged to form triangles. Basically 99 percent of the web is gone with only enough to keep the top and bottom flanges apart and from slipping in the horizontal direction. (horizontal shear)
6. Considering item 5, the holes can't be so large or close together that the top and bottom flanges are no longer held apart.
7. Adding more metal in the web of the beam helps a little but not much. The portion of the web that is far from the neutral axis (closest to the top and bottom flanges) is the only part that actually does any thing.

8. Summary: you need enough material in the web to keep the top and bottom flanges apart and from slipping in relation to each other.
The fastest way to increase beam strength is to increase height. (The opposite of what has been done by notching the frame). If you can't increase height, increase metal thickness or width of the top and bottom flanges.

9. Considering that the notch is over the rear axle and the leaf springs attach to the frame in front and behind the notch, half the wheel load is transmitted to the frame in front of the notch and half behind. What this means is the amount of load in the frame (or how strong the frame needs to be) is significantly reduced after the front spring shackle. You can see this if you look at a new pickup truck. The frame is pretty deep (and they use high strength steel) under the middle of the cab and to the front spring shackle, after the front spring shackle, the depth of the frame is only about 1/3 as deep and they switch to thin mild steel for the rear portion of the frame under the bed.



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Thank you for the very informative response guys! I thought it might be an issue and need fixing. I will pass the information along and see if we can’t work on getting it fixed.


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...that truck must sit really low in the rear as these orig. AD frames have a good arch over the rear axle to begin with, usually no need for a notch,...personally i don't like the way those look but I'd just add a metal plate to the framerail, inside or outside to stiffen it up. good luck.

 

Do you think adding a plate to the outside and inside will help make it safe? He will be towing his car trailer a lot and has the occasional scrap metal, heavy car parts haul. He bought this truck to use as a truck. He doesn't want to baby it. He wants to make it work hard, like it should!
I am new to this frame notch stuff and just trying to help him out. Most likely I will be the one to tackle this project. What would you guys do if you were in this situation? I guess my question is what is the most logical next step to take?
Thank you for everyone's help so far. It is greatly appreciated!
-Jeff

 

I forgot to mention that the pictures I took are a little deceiving. It was taken from a top angle. The notch is not angled like it looks in the pictures. It is squared. The back of the frame is about 4" and the center of the notch is 2".

 

Towing a car trailer with a 1/2 ton?
Gutsy.
I don't tow anything bigger than a single axle utility trailer.

 

put a plate on both sides of the frame and if you can make them taller than the top of the frame now, then box the two plates in over the top, a bit of overkill but it'd be plenty strong enuf to do what you want with it. got any pics of the whole truck?
...as far as towing with a halfton p.u,, I do that all the time, you just have to know your limits...

 

This basically says that the frame is 1/2 it's original height so it is therefore only 1/8 as strong as it was. (.5 X.5X.5=0.125).

Not knowing how strong it needs to be, I would target increasing the strength to as close as possible to where it was originally. Therefore I think that you need to do more than adding doublers on the vertical sides. You need to add doublers to the top and bottom flanges as well. On the top flange since adding metal on top may interfere with the way the bed fits, you can add the doubler on the under side of the top flange.


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