frame sag

[Bruskie]

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?

[Tom Wesenberg]

I would get it straight or even a bit high because the weight of the engine and body will make it settle down.

[George Miller]

Originally Posted by Tom Wesenberg

I would get it straight or even a bit high because the weight of the engine and body will make it settle down.

Plus 1

[Jim in Wisconsin]

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.

[BRENT in 10-uh-C]

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.

[ericr]

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Originally Posted by BRENT in 10-uh-C

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.

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....

[Bob C]

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

[BRENT in 10-uh-C]

Originally Posted by Bob C

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

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..
.
.

[Tinbasher]

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.

[Y-Blockhead]

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.

[fredski]

Originally Posted by BRENT in 10-uh-C

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 can you do this with the body on?

fred

[Bob C]

I think these are pictures Brent posted.

Bob

[Vic in E-TN]

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

[BRENT in 10-uh-C]

Vic, remind me and I'll touch on this when I come down to do the wheel seminar.

[Bruskie]

brent, how much heat do you suggest? & do I use a torch or a rosebud?

[1930artdeco]

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

[Bruskie]

low spot is right at the motor mount cowl bolt area

[Mikeinnj]

Originally Posted by BRENT in 10-uh-C

Vic, remind me and I'll touch on this when I come down to do the wheel seminar.

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........

[BRENT in 10-uh-C]

Originally Posted by Mikeinnj

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........

Host a seminar and I'll try to come help!!

[BRENT in 10-uh-C]

Originally Posted by Bruskie

brent, how much heat do you suggest? & do I use a torch or a rosebud?

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.

.

[Bruskie]

thank you brent. I will give it a try tomorrow didn't want to get carried away & make it a humpback

[larrys40]

Excellent information Brent and good questions by all. I did this the first time I did it actually by deducting what to do by necessity to fix the problem correctly I hoped and a little common sense deduction in what to do with a car that actually had a more severe sag in the area and I gently heated with a 1/2 inch plate I had held up to the lower frame rail with very slight pressure . It did it perfectly and I was amazed how easy it moved. I'm not a metallurgist but you don't have to heat cherry red to shrink or move metal . I believe I started with a smaller area and kept working out . When all done I did replace motor mounts and rubbers and everything seemed to be just fine.
Yes I did it with the body on on one occasion . The other couple if times with just the bare frame. If course the bare frame is much easier to check your progress but you have to do what you have to do at times . Not sure if I was brave or stupid the first time with the body on but it got the job done
Larry

[hdmike]

I just purchased a 30 coupe that is shimmed up a lot at the cowl. So I removed the front fenders so I could put a straight edge across both rails. It looks good from the top but the bottom rail has a bend that you can feel on both sides at the rear motor mount on the bottom rails. Should I be concerned about the bottom rail or just the top rail. I'm new to all of this any help would greatly be appreciated.

[RonC]

Originally Posted by hdmike

I just purchased a 30 coupe that is shimmed up a lot at the cowl. So I removed the front fenders so I could put a straight edge across both rails. It looks good from the top but the bottom rail has a bend that you can feel on both sides at the rear motor mount on the bottom rails. Should I be concerned about the bottom rail or just the top rail. I'm new to all of this any help would greatly be appreciated.

That may depend. Are you having a problem with body parts alignment or drivability ?

[hdmike]

Yes, the cowl is shimmed up about 3/4" under the cowl. But everything aligns except the hood. Had a gap about 3/4" at bottom of hood. I just want to get rid of those shims. What I have read there isn't suppose to be little if any shims under there. I haven't driven it yet.

[Tom Wesenberg]

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Originally Posted by hdmike

Yes, the cowl is shimmed up about 3/4" under the cowl. But everything aligns except the hood. Had a gap about 3/4" at bottom of hood. I just want to get rid of those shims. What I have read there isn't suppose to be little if any shims under there. I haven't driven it yet.

A wood block with a fabric reinforced pad tacked to the bottom side is a standard part, so are you saying in addition to this you have 3/4" more shims? Or, are you saying the very front body mount with no wood block has 3/4" shims?

[hdmike]

No wood block. I will post some pics this evening.

[Bruskie]

heated my frame rails this morning. pass side turned out nice & straight but drive side still has a slight sag will work on it tomorrow

[hdmike]

Here are the pics I said I would post. Sorry i didn't get them posted last night my internet was down till this evening. This is just rubber strips they put in as shims.

[hdmike]

This is the other side.

[Tom Wesenberg]

You weren't kidding about having a thick stack of shims. Have you run a string along the frame to check it for sag? They like to sag near the rear engine mounts.

[hdmike]

No, I haven't it still has the body on it. The only thing I have removed was the motor & transmission, and fenders so far.

[PatrickGrant]

Brent, I have a frame with 1/8th" sag. I'm going to try your heating technique, but I have a question first. I have a frame that the rear crossmember was bolted on and was narrower than stock by 4-5 inches. When I pulled the rear crossmember off the frame stayed in the same position. I spread the frame and bolted it to the crossmember in correct location (will be riveting). My question is should I heat the rails with the crossmember attached and if I do, won't that affect the inward tension of the rails? or shouldn't there be inward tension from the rails ? Frame is within 1/4" of square at a 8'10" measurement. Thanks, Pat

[BRENT in 10-uh-C]

Originally Posted by PatrickGrant

Brent, I have a frame with 1/8th" sag. I'm going to try your heating technique, but I have a question first. I have a frame that the rear crossmember was bolted on and was narrower than stock by 4-5 inches. When I pulled the rear crossmember off the frame stayed in the same position. I spread the frame and bolted it to the crossmember in correct location (will be riveting). My question is should I heat the rails with the crossmember attached and if I do, won't that affect the inward tension of the rails? or shouldn't there be inward tension from the rails ? Frame is within 1/4" of square at a 8'10" measurement. Thanks, Pat

Pat, I really am not sure what would happen in your instance however I do think I would install the rear crossmember with rivets where it is in its final location first. That way, you can tell what shrinking will need to be done, ...and where. Just remember this is a somewhat slow process because the metal is generally not finished "moving" until it normalizes with the surrounding metal. Cooling with air or water helps, but rarely does the heated area immediately normalize to the other metal.

[PatrickGrant]

Thanks for your response Brent. I'll get it sandblasted, primed, and riveted. Then I'll give it a try.....!

[ursus]

I have to wonder if the cars put in storage over the winter could benefit from being jacked up a bit and blocks placed to relieve the sag.

[rer_239]

how much is considered a normal sag? and how much can the heat method correct? also i don't think the passenger side sags, just the drivers side is that normal?

[Synchro909]

The chassis under my '29 Phaeton is badly sagged. One day, I'll get to it - maybe. The info in this thread has been great. It's all stored away in my neck top computer.

[Tom Wesenberg]

Originally Posted by rer_239

how much is considered a normal sag? and how much can the heat method correct? also i don't think the passenger side sags, just the drivers side is that normal?

I would want to straighten any sag over 1/8".

[rer_239]

ok, assuming i can get the driver's side level (pass. side ok) how do i keep it that way? do i have to reinforce it some way?

[Tom Wesenberg]

Originally Posted by rer_239

ok, assuming i can get the driver's side level (pass. side ok) how do i keep it that way? do i have to reinforce it some way?

No, we treat the cars better then the owners did in the old days. Hopefully it will outlast us.

[rer_239]

thanks for helping out a newby, in this case with a badly sagging frame.
rer_239

[Terry, NJ]

Rer_239,Our roads toady, are much smoother today, our cars are not driven/used every day. However, we're heavier today. But consider this, It took eighty years to move it this far (1/8-1/4") so maybe eighty years from now someone will be straightening again. I'm not worried about it!
Terry

Originally Posted by rer_239

ok, assuming i can get the driver's side level (pass. side ok) how do i keep it that way? do i have to reinforce it some way?