I once straightened out a kink in a '60's car by chaining it to a convenient sewer grate and then jacking as needed, with the base of the jack on the grate. Jacking will do the job...BUT I think Brent's approach is superior and worth the trouble. Some parts of his way, like properly stringing the bend area to clearly define things, could not be done on an intact car...but if you have a pretty good idea of the problem area and find a way to measure results (perhaps just by putting the hood on) I think improvements could be made on a complete car. Intelligent measuring and estimation would probably get you to a better place, though of course without the fully documented precision of bare frame work.
For what it is worth, I have lifted an excellent post of his:
(Stolen from Brent here,
http://www.fordbarn.com/forum/showthread.php?t=26355&highlight=frame&showall=1 )
Re: Frame straightening
Torch/Heat shrinking is the same theory whether it is wheel spokes, frame rails, body sheetmetal, --or whatever. The amount of heat and the concentration are what changes. I honestly do not know if Mr. Andrews covers this in any of his books.
Since you referenced the pictures in the other thread, that frame was under a late '28 pick-up that as I recall was sagging in the area just behind the cab and under cab. This was likely due to overloading. We find that other bodystyles will bend in other areas depending on conditions in which they were operated in. Most would agree that the flywheel housing area is in essence another crossmember for the frame. Often times the motor mount block would crack/break causing this area to weaken significantly and thus you might find a sag there. Other times the sag might be caused from a car accident earlier in life. Therefore there is no certain place that you need to shrink the metal.
I also do not know the best way to determine the area of trauma simply because I don't know what you have to work with however I will tell you that we have a fixture to bolt the frame to, ...and we have done it on sawhorses and both methods seem to work well. The jig is probably used more for vehicles that have seen a rougher life. Some frames we see might only have a ¼" sag and thus it does not take much to correct it. Often times, many Restorers will choose to overlook that amount but we just make the repairs now while the timing is right.
For equipment, someone that has a roll of kite string, a tape measure, and a oxy/acetylene torch can likely do a quality job. The
laser levels, jigs, infrared heat gun, porta-powers, and steel rulers make the tougher jobs go faster, but I'm not sure all of them are necessary for a one-time deal.
While the frame rail has a bend in it, find an area where you can string a line tauntly and then stay away from the rivet heads. What you want to do is make the string go down through one of the holes in the frame rail in the front and back. If the string is stretched tightly, and it is setting flush with the rail on each end, any gap or space between the string &the frame rail will be an area where the frame is bent (--or sagging). Where there is the biggest gap between the string & frame is where the bottom chord is stretched the most. Understand that the upper chord became a fulcrum or pivot point for the lower chord of the rail to bend. This means if you were to imagine a pie shape on the side of the rail, the further downward towards the lower chord would show more stretch. Therefore more heat is applied on the bottom chord and some of the heat will rise upward and transfer into the side of the rail. As the bent areas are heated and then water and/or air is applied to those areas to cool, often times you will see the rail draw right before your eyes. One point to remember is that once an area or two has been shrunk, then the entire frame needs to be normalized to the temps of the room. For example, if you work an area around the motor mount blocks, then before an accurate measurement can take place, that area of the rail must be the same temperature as what the front & rear crossmember is. This is a situation where haste will make waste of your time, ...and Father Time seems really slow as you wait for everything to normalize.
After the frame rail has been straightened to level, then concentrate your efforts on making the frame measure the same diagonally. This is done with a Porta-power (--or a come-along) to get everything measuring within a 32nd. Next clamp a jig or something in a 'X' shape to the frame to keep it in alignment as you begin rebucking all the rivets tight again. For this task, you really need the proper tools. We use a 3X Rivet gun, which is not to be confused with the common Air Hammer. Also, this process is much more than heating the rivet head and then smashing it flat. We use a special torch that has a
very concentrated flame so that it heats the shank of the rivet all the way through. Then using special bucking bars that we have made that have lots of mass, we use the rivet gun to collapse the rivet even more which swells it in the hole. We have also found that if a crossmember or bracket has a rivet that has broken, ...or the crossmember shows evidence it has been moving within the frame, the only way to correct this is to remove the crossmember and weld up each hole and re-drill the rivet hole to the proper size and install a new rivet. Often times this is good because of rust that accumulates between the rail & the crossmember can be removed. Sometimes the center crossmember has enough rust between it and the rail where it will create a problem for the battery to make a good ground to the rest of the chassis.
My brain & fingers are getting tired but maybe this has been enough food for thought for you to understand the basic process where you can tackle such a task.
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BRENT in 10-uh-C
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www.model-a-ford.com
...(...Finally Updated!! 
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01-20-2011, 06:47 PM
