C-NOTCHING THE REAR NEED HELP

[57fordf100]

hey guys i found this on slick60s web site im getting ready to c-notch my frame but had a few different ideas i came across this picture of a c-notch dosent look reinforced just a piece of metal that follows the curve n thats it would that be safe or would i have problems down the road im not planning on carring heavy loads what do you guys think

[fordman]

any mods you make to your truck. i am not responsible for. that looks like too much to me. not without a lot more reinforment. i think he cut too high into the frame myself.

[td]

i agree with fordman on this one, you most definitely want more reinforcement than was used there. i'd spend the $ for a good aftermarket piece or have a good fab shop make me something before i did it that way!

[19FORD67]

Box the frame and add reinforcement to the bottom face and that might work.

[57fordf100]

ok so here is something of what i was thinking of doing sorry for my not so pro looking drawing i was thinking of cutting a square out reinforce from the inside box in that little piece would that still be safe for me to drive around

[Johnson]

What you posted will work fine. I did the same thing to my 70, it was notched a little bit wider than that. I never had any problems with it.

[57fordf100]

ok guys imma cut into it tommarow ill take pics n ill reinforce the best i can would my truck be to safe if i leave it open like that no reinforcement for about a week or would something happen

[ToughOldFord]

The bigger question is why in the blazes would you do something like that in the first place?

[57fordf100]

if i cut out 2 inches would i still need to reinforce the frame

[70_F100]

If you cut anything out of the frame at all, you're going to be cutting out the flange on the lower side of the frame.

As soon as you do that, you've basically destroyed all of the support of the frame in that area.

Remember, you've got a lot of force working here. The springs are supported on the ends. Since the rear has no support from the frame, the frame will naturally try to bend upwards in the rear past the notch. Just hitting a bump in the road increases that force exponentially.

Like ToughOldFord asked, "The bigger question is why in the blazes would you do something like that in the first place?"

This is a TRUCK we're talking about, not a Honda Civic!!!

[57fordf100]

C-NOTCH= LOWER STANCE thats y i wanna c-notch it not every one is into stock height or lifted trucks some of us like lower im a tall guy the lower i can go the better if im in a tall truck or stock height i feel ever taller i usualy leave the lifted stuff to the short guys with big egos thanx guys for the help

[71fordkid]

I have a lowered f100, and a lifted jeep. I haul virtually nothing in those trucks, i have a 93 f250 with one-ton running gear and springs in it.

Not all of us use our bumpsides for hauling, towing, or working. I like trucks, not cars, so i built a hotrod truck instead of a hotrod car.

[fitzwell]

Yikes!!! You need a piece of drill stem, about 5" in diameter. cut it in half. Weld that around where you c-notch the frane. 3/16 plate, cut to match the contour of the frame,and said drill stem, around the notch. One about halfway in the width of the frame, one on the other (inside) side of the frame. Do this with the truck on jackstands, and one end of the leaf spring mounts unbolted from the frame.


Overkill...maybe.
but it won't fail.

as always...

[MadMaxetc]

OK, here is my take on it.

You need to make as strong or stronger than the original frame. So how do you know? Well this is when it is good to have an Engineer Friend. (No, not the guy who drives a train)

If you look at it from a pure bending moment point of view we see that there is a direct correlation between Deflection in Bending (ie. strength) and Moment of Inertial.

Moment of Inertial is purely based on geometry. I can show the formulas if you want, but they are long and nerdy.

The major driver is Height between the Upper and Lower Flange. It is a Cubic Function {(y*x^3)/12}

Here is what you get...
(click to make larger)


If you want to keep it as strong, you should only do about 1/4" deep C-Knotch AND Box the frame.

I know this looks way off but it is true. The frame was probably designed with a Factor of safety of 2-3, so you could probably remove up to 25% of the frame height using a C-Notch and Box the Frame on a Daily Driver without ever seeing a problem. Also keep in mind that the frame was designed to haul a load, so if you are not doing that then that is even more you could remove without seeing a problem.

For me...I would not do more than a 25% notch before I started adding material to the top of the frame. Better Safe than Sorry.

Daniel K.
BS in Aerospace Engineering
Wichita State - 2004

[MadMaxetc]

If you have the dimensions of your frame I can calculate the Moment Of Inertia for you to compare to the Original.

Different Material Thicknesses make a difference.