Rear Frame Rail Fitment

[70 Challenger Lover]

Finally ready to get back to my 70 Challenger. Rebuilding the rusty rear half starting with new frame rails. I have the entire car sitting on a jig and all measurements are working off the factory diagrams but I noticed a discrepancy between all the Challenger frame measurements diagrams out there and the actual rear frame rails.

The diagrams show the rear frame rails with no upward curve but the actual frame rails do have a significant upward curve and that changes the measurement which is taken at the very rear of the rail. Wondering what others have done to get around this when rebuilding a rear body with new rails.

My original plan was to weld the rails in perfectly, followed by the floor, drop offs and wheel houses. Finally the quarters and rear tail panel.

If I cannot be certain of frame rail measurements because of a weird diagram discrepancy, then it looks like I'm forced to tack and screw everything together so I can fit the quarters and tail panel. Then if everything fits right, I could weld the frame rails in permanently.

Any ideas or other diagrams out there that show the actual upward curve?

[BFM_Cuda]

so it's not just me???

I thought it would be easier for me to assemble the rear rails with the front spring mounts, shock cross member and rear cross member, and then install the assembly in the car. So... I built a jig based off the factory service manual dimensions. If you lay the rear rails out according to the factory dimensions, the rear cross member doesn't line up with the bumper bracket holes.

I stopped working on it until I can bring the car home and compare the jig to the body. I will still pre-assemble the rails before I install them.

I wish I would have just bought the assembly Dynacorn makes.... oh well....

[70 Challenger Lover]

Unless someone has a different factory schematic with a different dimension at the very rear, I don't see any other way around it then to tack and screw stuff together temporarily just so I can be 100% positive the rails are sitting at the right height. The new rails fit good and follow the contours correctly but I'd be pissed to weld them in along with the floor only to have the height of the floor an inch too high once the quarters and rear tail panel are in.

[JS29]

Your better off pre fitting everything now. 

[jimynick]

I don't see the issue here? If you set up the datum line, as shown in the measurements and set the height of the rail to the shown measurement, where would you go wrong? Don't get me wrong here, I've been there and done that with the replacement panels and would likely hedge my bet by throwing the rails together with the rear crossmember. Toss the floor on too, while you're into it and see where it fits against the rails and wheelhouses. OR do it the way it was done at the factory and get high/pissed and just toss it all together! This works best on a Friday or Monday it's been discovered. Good luck 

[70 Challenger Lover]

The datum line of D shows it should be 18" but with the rail curving up instead of being straight as seen in the diagram, that measurement is closer to 19". The rest of the car is sitting on the resting points exactly as the diagram shows and it all measurements are just right. If I bring the rails down to the datum line measurement of 18" makes the rail fit horrible. I don't even need the quarters on the see how wrong it is. I'm certain it is all going together perfect but that datum line measurement at D can't be right.

Well, I'll get it together!

[Cuda416]

Quick question, is the car resting on the "seams" at the bottom of the rockers, or where the measurement holes are located? I only as because I've seen lots of folks who rest the car on a structure that amounts to beams going from side to side. The height of the seam would be around 3/4" and could account for the difference.

Just a thought.

[70 Challenger Lover]

I thought that at first but it's at the holes just like the diagram shows. I have a theory I'm going to check today. If the rail did not curve upward at the last 12-18" of the rail and remained perfectly straight as the diagram shows, then it would continue a slight downward trajectory. My theory is the measurement might be correct if the upward curve didn't exist. Easy to check. The rails are clamped in now so I'll put a straight edge along the underside and measure the datum line spot where the rail would be if there were no curve. If it checks out, and I think it might, then I'll know the height is correct.

[Cuda416]

What I was getting at, is the car supported in the yellow, or the orange? Orange, would have the car almost an inch above the yellow (where is supposed to be) and account for the difference. If you supported the car in the orange area, then you need to add that difference in height.

Moot now, but seems you didn't measure it all before taking it apart?

Anyway, that's all I got. Hope it works out.

-=C

[Cuda416]

Something else to consider is that diagram you have, and everyone has, isn't "drawn" well. The rails int he back aren't straight along the center axis. The dimensions, reflect it, but the drawing doesn't. Anyway, my point is, there is another diagram, that has different references and seems to indicate the measurements is taken 56.5 inches from the rear shackle on the same plane as the datum.

[70 Challenger Lover]

I'm not supporting the car at the rockers (the orange). Supporting at the alignment holes of the frame precisely where the diagram indicates. The other six locations have the car sitting perfect and all measurements check out so it's this last one at the back end that seems wrong. I've seen the diagram you posted but it doesn't help me since the car is just a shell. I would have taken measurements but the car was rusted apart and suffered a rear end impact throwing everything off. I'll get it. I just thought someone here might have noticed this rear frame rail discrepancy and had information to offer on it or another diagram with the correct measurement.

I know the one I posted simply cannot be right because it is showing a measurement at D given the frame rail as being perfectly straight when it is not. I don't think it's an artistic error, I think it's informational error. I used the same diagram when I rebuilt the front frame rails and afterward, I put the hood, doors and fenders on and found everything gapped out perfect.

[Cuda416]

Got it.

I have a vested interest in this as I'm working on learning the dimensions etc. before tearing into my vert.

What if you set it up so the measurement right behind the rear shock mount/cross-member is (18-9/16), where does the supposed 15-7/16 hit the rail?

I have the dynacorn rear assembly at home. I'll take some measurements and see what I come up with.

[Cuda416]

Measured my dynacorn assembly and have to agree, it looks way off. With the front of the assembly flat, the top of the frame arc is higher than the measurements show. if I tilt it back, the front section is at an obvious angle (not flat) and the rear is still about an inch high.

I'm not above learning I'm doing something obviously wrong, otherwise I'd think this would have come up before.

Anyone feel like measuring their car that's currently on a jig?

-=C

[70 Challenger Lover]

Well I measured the height off the datum line and realized I made the resting points about an inch high. Not sure how I did that but I did. I chopped them all down and repositioned the car like it should be and at first glance, the measurement at the rear rails is much closer. While the artistry of the diagram may not be accurate, it does appear the rear frame rail measurement might be correct after all.

[jimynick]

"And they all gave a sigh of relief!" I'd still hang the 1/4s and check my door gaps. Good luck