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Old 02-21-2007, 04:26 PM
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"C" notch vs. Squared notch vs. no notch

I'm building another chassis using an F-150 twin I-Beam front suspension and I am attempting to use an under slung frame. This creates a challenge to stay within the scrub line and at the same time keep the axles from contacting the frame on the down stroke. (If not familiar with the F-150 set up you can click on my journal and go to the front suspension section).

I could create enough travel space if I notched the frame on each side. If I used a "C" notch, I'd need to use 1 1/2" pipe to gain enough space. However, my frame rails are 2x3 box tubing so I'd be notching half way through the frame.

If I used a "square" notch, I would only need to cut down 1"...thus leaving 2" of the original box tube intact.

So my first question is, is there a major strength difference between using a "C" vs. a "square" design (my gut tells me the curved notch would be stronger even though it is a deeper cut. Also, the "C" would sure look a heck of a lot better.)

My second question is, a lot of you have recommended in other threads against notching at all because it weakened the frame substantially. On my particular set up the strength issue involves the bracket which holds the pivot end of the I-Beam.

On one side of the car it is of no consequence because the bracket would be attached to the frame rail before the notch. (The only other thing the frame has to hold after that is the radiator.) But on the other side there would be a notch and then immediately in front of the notch a bracket to hold the pivot end of the I Beam (I hope that makes sense to you guys who know twin I-Beams). I'm not sure how much abuse the pivot end takes but I'd imagine plenty. Any thoughts on just how much strength I might lose with either type of notch and if it would hold up to support the axle.

My thinking at the moment is to go with a "squared" type notch but then while I had the notch cut open, weld a separate, smaller, box tube inside the frame or reinforce the notch on the inside with 1/4" plate welded on the inside of the frame tubing (These could actually run a foot or so both front and rear and be "hidden welded" by drilling a series of 1/2" holes in the frame to make the welds.)

Any other thoughts on this potential solution or the problem in general?

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Old 02-21-2007, 10:19 PM
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It sounds like you could use the square notch like you described without any problems, Or you could use the round notch and reinforce the inside with a couple of plates if you really feel that it needs the strength. I am a big fan of over engineering.

I won't C notch anything more than 1/2 the thickness of the frame, and that depends on where the load is. If a frame needs a notch and the crossmember that supports the weight is located a relatively long way from the notch, I use less of a notch.

I have cut plates and welded them inside of the frame if I wanted to reinforce it. If I can't reach inside of the rails I drill holes and plug weld the reinforcing plate to the main rail. (same as what you said you'd do with the 1" box tube )

With what you are doing I don't think that any kind of notch, round or square will hurt. The load is almost right above where your notch will be, (it isn't like the spring mount is 2 feet away and the load is on the opposite side of the frame from the notch.

I don't think the pivot end of the I beam takes alot of abuse, think of it as the end of the crowbar that is in your hand. It takes a relativly small amount of strength to hold it where it needs to be.

A notched beam is ultimately as strong as it's thinnest section. The rounded shape spreads the stresses out along the whole notch, and it has no stress risers that you would get from the corners on a square notch. So in effect it really isn't that much stronger, just tougher, and able to "spring back" better.. But, if the notch is close to the load, it won't matter.

BTW, don't use "pipe" to reinforce anything. Pipe is usually low carbon, crappy steel. Use "tube". I like using hot rolled flatbar, bent over a pipe to shape it. That way I know what kind of steel I'm getting.

I hope all that makes sense to you.

Notch it!


Later,
mikey
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Old 02-22-2007, 07:25 AM
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Rather than notching the frame why not 'Z' the frame just behind where the twin 'I' beam axles mount? This will enable you to lower the main section of the frame as much as you want while not reducing frame strength at all.
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Old 02-22-2007, 08:27 AM
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Quote:
Originally Posted by powerrodsmike
BTW, don't use "pipe" to reinforce anything. Pipe is usually low carbon, crappy steel. Use "tube". I like using hot rolled flatbar, bent over a pipe to shape it. That way I know what kind of steel I'm getting.
Thanks for this tip and also the rest of your analysis. Exactly the info I was after.
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Old 02-22-2007, 08:40 AM
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Quote:
Originally Posted by Frisco
Rather than notching the frame why not 'Z' the frame just behind where the twin 'I' beam axles mount? This will enable you to lower the main section of the frame as much as you want while not reducing frame strength at all.
If I understand what you are saying correctly(Z the frame up OVER the I-Beams rather than under-slinging the frame) that's how I did it on the roadster chassis. But I was never totally happy with that "look". A little cumbersome and cluttered to me. That's why I wanted to attempt an under-slung frame on this project...to see if I could make it safe, simple, and a little better looking than the roadster chassis. (And add a bit more to Twin I-Beam knowledge base.)

I know you were one of the folks warning against the perils of notching frames in some of the other threads, so I'm glad you jumped in on this one. My inclination at the moment is to continue ahead with the under-slung concept and reinforce the inside of the frame rails with 1/4" plates as outlined by Mikey. Do you see a major catastrophe ahead with that plan?
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Old 02-22-2007, 08:56 AM
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Quote:
Originally Posted by cboy
My inclination at the moment is to continue ahead with the under-slung concept and reinforce the inside of the frame rails with 1/4" plates as outlined by Mikey. Do you see a major catastrophe ahead with that plan?
Mikey gave excellent advise for "C" notching the frame and using the correct material.

If I understand your original post; the only thing mounted forward of where you want the notch to be is the radiator and a bracket to mount the pivot end of the 'I' beam. If that is so, I am concerned about the forces on the bracket for the 'I' beam working against the "C" notched section of the frame. I wouldn't use a square notch. It wouldn't be as strong and looks bad.
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Old 02-22-2007, 09:09 AM
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Quote:
Originally Posted by powerrodsmike

it isn't like the spring mount is 2 feet away and the load is on the opposite side of the frame from the notch.

I don't think the pivot end of the I beam takes alot of abuse, think of it as the end of the crowbar that is in your hand. It takes a relativly small amount of strength to hold it where it needs to be.
Unless I am thinking wrong; this is exactly what CBOY will have with the twin 'I' beam setup. See picture below of his original twin 'I' beam setup with the previous frame.

The pivot end will also be subjected to side loads as the suspension comes in contact with irregularities (potholes) in the road surface.

Adding plates on the inside of the frame to provide additional support would help in an up and down direction but add little support to any side to side forces.

Just some things to consider. Not saying what is being considered is incorrect.
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Old 02-22-2007, 09:52 AM
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I agree, Frisco. I could be misunderstanding just what Dewey is doing. (say that out loud, haha)

I wish there was a picture or drawing. I would like to see just where the notch needs to be, what is in front of it and what is behind it. . My powers of comprehension get taxed pretty easily on this kind of discussion.

I'll bet this discussion could use a structural engineer, ( I'm strictly shadetree), maybe a PM to willys36 is in order.


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Old 02-22-2007, 04:03 PM
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Quote:
Originally Posted by powerrodsmike
I wish there was a picture or drawing. I would like to see just where the notch needs to be, what is in front of it and what is behind it. .
I've got to head out to a meeting tonight but I'll try early tomorrow to draw and post something to describe what I'm attempting to do. My hand sketches might be more confusing than helpful at this point since I'm about the only person that can understand them.

One possibility to counter Frisco's concerns about the side loading is to place a cross member (probably a 1x2 box tube) at the point on the frame where the pivot bracket is welded. I'd have to look the design over in more detail but I don't think it would interfere with anything except possibly the lower radiator hose. I could then even weld a small triangular brace from the cross member up the pivot mount bracket.
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Old 02-23-2007, 07:57 AM
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You guys have pretty well analyzed this question already but here are a couple quick drawings to make sure we are on the same page regarding what I want to do. The first sketch is the total frame side view which shows the position of the axle pivot mounts. The second sketch is a close-up (not to scale BTW) showing the driver's side frame rail, pivot bracket, and proposed notch in the frame. I did not include a passenger side frame rail sketch because that pivot bracket mounts to the frame BEHIND the notch (when viewing from the front of the chassis to the back)...thus the notch should not carry any particularly heavy loads.

Does this help visualize things?



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Old 02-23-2007, 08:06 AM
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And just for the fun of it...here's a sneak preview of the project:

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Old 02-23-2007, 08:17 AM
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Quote:
Originally Posted by cboy

Does this help visualize things?
That's what I assumed you had in mind when you mentioned an "underslung" frame.

The notch would apparently be for clearance of the axle when the axle is deflected down (primarily when under acceleration due to weight transfer).

I don't think that much of the suspension travel will be in a downward direction, but rather will be inclined to get compressed (upward direction).

That being the case, do you really need as deep a notch as you suggest?

Have you enough ground clearance up front to install the brackets that the twin 'I' beams mount to up higher? This would give you more clearance between the bottom of the 'I' beams and the frame. Increase the distance of where the 'pivot center' is located.

The problem I see with the notch is that the portion of the frame in front of the notch will be supporting much of the weight of the entire front end. ( engine, radiator, frame, steering, all frontal suspended weight) That's a lot of force and the notch would be a very weak point.

Last edited by Frisco; 02-23-2007 at 08:20 AM. Reason: added pivot center info
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Old 02-23-2007, 08:24 AM
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Where does the upper spring mount go, and how is it tied into the frame rail?
Can you make an upside down hoop that connects to either side of the notch, and straddles the I beam with the shock mount directly above the I beam, that would relieve any stress on the c notch.

Now that I see the pic, I don't think that the weight itself is an issue to the notch, but how you mount the spring may put some twisting forces on it.

I could see if the upper mount is far enough away that it would act as a big lever,with the c notch as it's fulcrum.

More info please sir..

Mikey
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Old 02-23-2007, 08:36 AM
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Here is what I am talking about



What do you think?..Dewey? Frisco?

Mikey
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Old 02-23-2007, 08:57 AM
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Quote:
Originally Posted by powerrodsmike

What do you think?..Dewey? Frisco?

Mikey
Mikey, that would give the additional strength, but would probably not give the clean appearance that Dewey is looking for.

I just did a search on "underslung frame" hoping to find a picture or two of a few I've seen on-line using semi-elliptical springs. I didn't find them but found some interesting reading. Links below. The article on the Healey is very interesting pertaining to "body roll" and frame interference. The other articles are written about the "Underslung" car from the early 1900's. Seems that it could actually tilt up to 55 degrees to the side and would not roll over. Had monster tires though for ground clearance. What's old is new again.

Underslung #1

Underslung #2

Underslung #3

Underslung #4
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