Frame Sag, How much acceptable?

[BRENT in 10-uh-C]

Originally Posted by BillCNC

Ever since the thread was posted, I have been searching for my paperwork from when I did my frame but for some reason, I can only find page 1 of 2.

To get my frame square I used page one. Looking at the sheet for a few minutes it should be self explanatory. The ledger on the right side is the guide on measuring left to right and right to left on the frame at the rivets. The chart on the top left is the rivet location on each rail. The colored boxes are color codes of the dimension ranges. As you can see, my frame was a project to start with.

Page-2 is pretty much laid out as page-1 for squareness. I'll keep looking for the other page. To get it level basically I leveled the frame on jack stands and laid 1/8" stranded cable over the top of the rails rivets and hung 50lbs weights at each end tensioning the cable. I then measured between the cable and the frame.

Is my method overkill, You Bet! But I possibly have the straightest model a frame around. It's within 1/32" to 1/16" from perfect anywhere on the frame. My thinking is that if your going to go through all the trouble to straighten a frame, you might as well be #all's-deep into it, if not, what's the point.

Regards
Bill

EDIT: I'll get the page scanned and repost it so it's more legible if someone needs it.

Geez, you are right about overkill! You definitely have the Grand Prize award for achievement locked-up for that worksheet!! Even we do not use such a detailed method!!

To begin with, straighten the rails to be flat. Once that has been done, measure from the center of the rear crossmember tie-bolt hole (-the square hole) up to the center of the #1 body mount hole in the frame on each side, and you should find the measurement to be matching at 73 57/64th inches ...which is a tad over 73 7/8". Then measure the opposite side and see that those two numbers match. To verify, then place a string from the center of the square hole in the rear crossmember (-or a laser) to the center of the square hole in the front crossmember. Next measure at various points from the string (-or laser) line between the rear and front crossmembers to each area along the frame rail to verify the measurement on each side match. When both measurements match the entire length, the frame is square. At that time you will heat and re-buck all of the frame rivets. After you have re-bucked about ½ of hem, recheck all of your measurements and correct as necessary. Once the frame is finished, you should easily be able to match less than 1/32" sag.

Originally Posted by Rob Doe

Brent, in your pictures from the MAFCA threads, you had four places to repair/straighten.

1 I assume you used a rosebud acetylene torch?

2 Did you heat on the inside, flame focused on the bottom rail?

3 Did you measure the general temps of the area with an IR temp gun? Were the temps 230 ish? Or?

4 Did this require multiple iterations of heat to get the frame straight? If so, how long did you let things cool before putting the string back on and testing with the blocks?

I've already learned a ton from this. 3x rivet gun, heat the rivet bright red/orange from top until red on the bottom, then buck.

Thx.

1 Not always. We use a pencil-tipped torch (Henrob brand) for the rivets as we can concentrate the heat into the rivet and not the surrounding areas. That torch is plenty adequate to heat the rails.

2 Probably heated in multiple areas just trying to spread the heat out over a larger area. Experience from straightening other frames generally dictates how area we heat and quench.

3 Nahh, using a heat gun is just a waste of time for us. Somewhere between straw and blue colors will cause the quenching water to turn to steam when applied to the hot metal. If it makes steam, then that is hot enough to do what we need to shrink the metal.

4 Let the metal temps normalize back to the entire frame area being room temperature. Sometimes a couple of subsequent heat cycles in surrounding areas are needed for minute corrections.

As far as heating the rivets, the key is transferring the heat down into the rivet shank so that it will swell during the bucking process to fill the entire hole of the frame rail and crossmember. If you just heat the head until red but not the shank, all it does is deforms the rivet head but does not tighten the rivet down in the drilled hole. If you use too large of a flame, it heats the surrounding metal of the crossmember and frame where the hole is enlarged by the heat just like the rivet. When it is re-bucked during that scenario, you don't allow the rivet to swell enough to be effective.

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