frame sag

B

Bruskie

Member
good morning fellow barners. I have a sag in my frame im trying to straighten. it was not too bad to begin with, slightly less than 1/16th in. so I used a piece of c channel, chains & floor jack to straighten the rails. using a straight edge between the front & center crossmember, I now have 10ths gap on both sides. would this be sufficient or should I push it a little farther?
 
I'm not sure what 10ths gap means. 1/10 of an inch? That would be greater than the 1/16 you started with. I doubt if they were much straighter than 1/16 inch to begin with.
 
If you keep pushing, all you are doing is re-arranging the metal. The frame rail was very straight originally as that was the way it was designed. If you have a sag, it has been stretched, ...but not all of the metal in that area is compromised. Just the bottom chord of the rail and a portion of the lower section of the side chord. Use a torch to heat the bottom edge of the frame rail along with part of the side, and this will shrink the metal back when it cools to as it was originally manufactured.
 
BRENT in 10-uh-C said:
If you keep pushing, all you are doing is re-arranging the metal. The frame rail was very straight originally as that was the way it was designed. If you have a sag, it has been stretched, ...but not all of the metal in that area is compromised. Just the bottom chord of the rail and a portion of the lower section of the side chord. Use a torch to heat the bottom edge of the frame rail along with part of the side, and this will shrink the metal back when it cools to as it was originally manufactured.
Click to expand...

you have commented on this a number of times and undoubtedly done it more times than that. just curious as to why the bottom frame rail could be bent but not the top.....how can you distort one part of the frame but not another on top of it....
 
Think of bending the frame rails into a circle. You start with a straight
rail and the top and bottom are the same length, as you bend it you need
to stretch the outer circumference and shrink the inner.

Bob
 
Bob C said:
Think of bending the frame rails into a circle. You start with a straight
rail and the top and bottom are the same length, as you bend it you need
to stretch the outer circumference and shrink the inner.

Bob
Click to expand...

Yes, Bob has the right idea. Look closely at the drawing below and you will see the sag. Notice how the 'A' measurement remains the same as in the original frame rail yet all of the stretch is located at the bottom which is illustrated as 'B'.

Now here is what I keep trying to explain. Metal will stretch however it won't shrink without use of heat. Therefore when someone uses pressure to force the rail upward, the (stretched) bottom becomes the fulcrum and now the top chord is forced to stretch as the rail is pushed upward ( in an effort to make the rail straight). To prove this "re-stretching" method of using jacks/etc. to bend the rail does not work well, think about how many folks advise to "over bend" the rail because they find it settles. In reality, if the O/P would have used heat on the frame rail in the area of the triangle shape, as soon as it would have cooled, it would have shrunk back into the original shape. Then it will hold its' shape and no need to try to overcompensate the shape of the rail..
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Actually it a bit of both. The bottom of the rail with be stretched and the the top edge of the rail will be collapsed or made shorter. You can only do two things to metal to change it's shape. Stretch it or shrink it. And this happens in many ways. With that little bit of misalignment a little heat along the bottom should straighten it up.

John Poole The Old Tinbasher.
 
That's very interesting. Where would one apply the heat? Below the cowl area? And how much heat?

Not that I am planning on attempting this in the near future (altho I'm sure my car has some frame sag), just adding to my knowledge banks.
 
BRENT in 10-uh-C said:
If you keep pushing, all you are doing is re-arranging the metal. The frame rail was very straight originally as that was the way it was designed. If you have a sag, it has been stretched, ...but not all of the metal in that area is compromised. Just the bottom chord of the rail and a portion of the lower section of the side chord. Use a torch to heat the bottom edge of the frame rail along with part of the side, and this will shrink the metal back when it cools to as it was originally manufactured.
Click to expand...

Brent can you do this with the body on?

fred
 
I think these are pictures Brent posted.

Bob
 

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Brent:
Thanks for the good pictures. Does there need to be any force applied to the frame rails while they are being heated or is there enough force in the upper chord to self straighten the lower?

What is the position of the supports while the heating is being done? I am thinking that the ends of the frame need to be restrained from going up or have weight applied to the ends while the heat is being applied.

Thanks,
Vic
 
How much of an area needs to be heated? 1ft., 2 ft. etc.? It looks like in the pics a large area is heated. The do you just let it cool by itself or do you quench it?

Mike
 
BRENT in 10-uh-C said:
Vic, remind me and I'll touch on this when I come down to do the wheel seminar.
Click to expand...


I guess the rest of us who are unable to attend that seminar will not know the answers as to how much heat, what type of torch & flame and if pressure is applied anywhere if at all........
 
Bruskie said:
brent, how much heat do you suggest? & do I use a torch or a rosebud?
Click to expand...


To shrink metal, all you really need to do is heat the metal hot enough to make steam when water is applied. It does not need to be cherry red. Use a torch or rosebud, --or whatever you feel you can control the application of the heat with. It is a trial and error situation where you heat an area, ...and then let it cool to see how much draw was created. Look at those posted pictures to see how the heat was applied. There is other pictures I have posted here regarding this topic where you can see different scenarios.

Don't forget that when you are straightening the frame, in all likelihood, the rivets will need to be tightened too by heating and re-bucking them.

.
 
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