Without frame dimensions, you can only check with a tape measure. You need to check for square and parrallesim (sp). It is better if the frame is still attached to the suspension, but if not put the frame level on jackstands. You need to find locations on each side of the frame that match side to side, often there are holes in the fame that were used in manufacturing to position the frame in a jig. You can use crossmember bolt holes or whatever, but there has to be a corresponding hole on each side. With your tape measure from one location on one side to a different location on the other ( the further apart the better) and write down the measurment to within 1/16". Do this several times to ensure you are accurate. Now using the same locations, reverse the measurements side to side, like an × , the two measurements should be the same. If they are not, then the frame is not straight, and it would dogtrack if you used it. Do this check with as many locations as you can.
You can now check the centreline of the frame. Measure the crossmember centres and run a taut string down the centre of the frame, and measure to the frame rails from this centre line, the distance should be the same at each side, for each measurement. If they are not the frame is bent to one side. You may need to drop a plumb bob to the ground from the frame rails and crossmembers if there is obstacles preventing you from getting a line down the centre on top.
To check if the frame is bent up or down, you can use a long carpenters level accross the frame. This should be considered as field measurements only, but will help in identifying a bad frame. If you find what seems to be a good frame, I would still take it to a shop to verify.
How can you trust an atom, they make up everything
Good question. Which unfortunately I cannot answer definitively, as I don't know what angle the frame would sit at normally. And that's not to mention how much the stance of a TJ can change with relatively little change in weight. I think the FSM states that the beam alignment should be done with a full tank of gas and a specific amount of weight should be added above the tank for each gallon the tank is short, which gives some idea of how much the angle can change. You've got me thinking now. I'm wondering, just out of interest... If I can find where I put my digital angle finder I can compare how much the angle changes when i fuel up on the way home tomorrow. I'm also wondering how much of an angle my headlamp beam adjuster (it's an export thing) drops the beam aim.
The only two measurements that are the same there are the center body mount and the one behind it. So for the purpose of the measurements, the center box section of frame must be sat level, which anybody checking a bare frame could do, with stands on a level floor.
If I had a bent frame... I would just buy 8 adjustable CAs and go do a 4 wheel alignment. As long as all the doors shut, I wouldn't bother with it anymore than that.
Life with Christ can be explained like a Jeep...
Being in the Word... grows your tire size.
Prayer... gains more ground clearance.
Fellowship... Is like having a good spotter.
The more of these you have... the easier it is to get over obstacles in your path.
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