Measurement of frame sag

[flybob08]

How should I measure to see if I have frame sag as opposed to the body blocks being incorrect and causing door alignment issues? Can I use a laser line and measure at various points?

Thanks

[nkaminar]

Good question. Most of the time the frame sag is measured with a straight edge while the body is off. But with the body on I am not sure how to do it. Can you put a straight edge on the bottom side of the frame? The sag is usually located at the rear engine mounts.

I have the same issue. What I am going to do is replace the rubber pads first to see if that corrects the alignment and then, if that does not do the trick, work on the frame sag. My plan is to embed a high strength stud in my concrete garage floor and strap the front part of the frame to it and then use a bottle jack at the rear motor mount. I probably will not get to this until mid summer. It is not a high priority issue in my case. The bottle jack probably needs to be 20 ton and the rear of the frame probably needs to be strapped down too.

[flybob08]

The problem using a straight edge on the bottom is all of the bolt protrusions and the cross shafts. Will try the laser and see if that works. It may not be a precise enough dot, however

[Synchro909]

I'll be looking into the sag on my Tourer very soon. I plan on using two blocks of wood of equal thickness. One at the front and the other towards the rear of the chassis and using a pair of "G" clamps, hold a stretched string between them. I expect to find that the distance from string to chassis rail will be less at the sag.
Keep it simple.

[Oldbluoval]

So body still on the frame? If so that’s tough. Maybe there’s a way with new tech, lazers, etc, I just don’t know it.
Easy with bare frame. Just do it like a carpenter lays out a house…stretched string from ends of rails and measure gap. Gap usually at the motor mounts.
At same time measure “X” with the frame to see if square..sometimes not.

[rotorwrench]

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The top of the side rail remains relatively flat till it passes over the front cross member. The side dimensions of the rails tapers in two areas so it doesn't work well to use the bottom surface as the one to check. The only real way to get a good check is with the body off so the top surface can be checked.

[old ugly]

i think i would try a laser , although i have not used one on a model A frame.

with the car in the air supported by the suspension or the point at which the suspension hold the car up. the natural sitting stance of the car.
mount the laser at the front of the car aimed down one of the frame rails.
mount it lower than the rail with no obstructions
take your tape measure and adjust the laser so the front distance the light hits the tape measure is the same distance as the point the light hits the tape at the rear of the frame, keep in mind that the measurements have to be on the part of the frame that is flat, no taper. you may have to measure from the top edge of the rails.
once you have the front and back measurements correct the laser is set.
now just measure along the rail and see if there is a variation in height and at what point it varies.

the string will do the same thing. if you have a way to mount it the laser will probably work good.

if you had a perfectly flat floor you could level the car frame front to rear and just measure off of the floor.
would not work on my floor, its wavy

datum line.

[old ugly]

always work on the car with it sitting natural. in the air or on the ground.
-stands under the axles
-stands under the frame rails where the axles mount if the axles are out of the car.

don't make adjustments to the drive line, frame or body with the stands halfway down the frame rails.

[BillLee/Chandler, TX]

Ken Parker and I measured my A this way:


Clamped a block of wood under the frame at one end, a second (same dimensions!) under the frame at the other end.


Stretched a string line under the blocks from end-to-end.

Used a third block as a measuring tool, placed it in various spots on the frame next to the string.


Any frame sag is quite obvious.

[katy]

How much sag is acceptable?

[Conaway2]

I straightened a sagging frame a couple of months ago. With the body off, it was fairly easy to do using the string method. I;ve never tried to do this with a body on the frame, but would imagine it can be done with a laser and some ingenuity.

As mentioned in post #2, you will need a 20 ton bottle jack at a minimum. I estimate it took 8-10 tons of pressure to straighten the frame on my car, based on the effort put into the jack. I also repeated the process 3-4 times per side to be sure the sag was gone.

Good luck - Jim

[Oldbluoval]

Simply jacking it up straight doesn’t necessarily solve the problem long term. Once sagging, the metal is stretched. I didn’t search it but on this site, I think, Brent did an explanation of what happened and what needs to happen.

[old ugly]

if you are ambitious

tie the front and back and jack at the point of the sag.

you need to protect the frame rails at the tie down points and at the jacking point.

if you simple wrap a chain around the frame ends you are going to create more problems.

and if you use the round head of the bottle jack on the frame flange you are going to dent it in.

a pc of thick flat plate between the jack and the frame will work at that point. the anchor points will take some inventiveness.

you will have to jack it, release it, measure it over and over until you know how far past straight it needs to be forced before it comes back to the point where it is straight.

if it is sagged, it is most likely both rails are sagged and i would say to tie the whole frame down solid and jack both rails at the same time , if possible.

i have straightened frames using a steel beam under a frame rail. it can be done. it is not for the faint of heart tho.

i would say if you are not used to doing this sort of thing,
-do your measurement to see if it is bent and by how much.
-1/4" or more you may want to address the problem.
-stopping by a body shop that works on frames will let you know how much to fix it.

[rotorwrench]

A bend in a frame rail causes a compression on the inside of the bend and a stretch on the outside of the bend. A person can relieve that area during straightening by heating the stretched area and quenching it. A person shouldn't get to carried away with the heat though and a cold water quench may be too fast. A compressed air quench may be enough. Folks that do this get a feel for how well the shrink will work and someone that isn't familiar with heat shrinking steel should learn from a pro before trying this.

Cold bending is generally enough but few people try to stress relieve the repair area, Shot peening is another way to relieve the stress after straightening. A lot depends on how badly bent the member was before the repair.

[old ugly]

heat is not used that much for a slight bend/sag in a conventional ladder frame. if the damage is substantial to the point the web or flange is distorted, kinked, mashed or dented then the use of heat is probably needed.
i think this is not the case here. the OP is probably new to frame repair so i don't think, like you suggested, that he should start heating the frame rails.
on an old style conventional frame we would never quench the heated area with a liquid. let it cool naturally then re introduce the work hardening with hammer and/or hammer and dolly. i would say if it is bent to that extent leave that to the repair shop.

relieving the stress in the damaged area by the used of a hammer while making the pull is a good idea and is a common practice. the frame will retain its wanted shape better after the pull.

[nkaminar]

There is an article about removing frame sag with the body on in the March-April 2017 edition of the Model A News, Volume 64, Number 2. The article, entitled "Model A Hood Alignment" by Don Bowne of Cambridge City, Indiana is on page 24. There are 13 color photos and a complete description of the process. He used a 10 ton jack placed at the motor mounts, straps tying down the front shocks, and a strap with a vacuum plate at the rear. The straps apparently were used instead of a chain so as to not damage the shocks. He over sprung the frame a bit so that when he took the force off it was straight. He had a tie down anchored in his garage floor to hold down the frame at the shocks. The head of the jack was extended lengthwise along the frame to keep from denting it. Don removed the choke rod and loosened the steering column clamp before straightening the frame. He also made sure that the radiator shell was in line with the cowl before starting.

Don Bowne passed away on October 24, 2020. He was 83 and a body and frame man most of his career.

In one old Barn thread I read, a member who straightened his frame, no body attached, welded a 16-inch long strap to the bottom to reinforce the frame at the engine mounts. He blended it to the frame and tapered the ends to make it less obvious.

[old ugly]

one more issue before i stop blabbing.

if some one is going to straighten their frame with the car assembled.
loosen or remove of any items that may bind and break during the jacking process.
things like the bell housing bolts, you wouldn't want to crack that bell housing when you are jacking the frame. watch for areas that might cause damage to other components like the fan hitting the rad.
all i am saying is be careful.

[rotorwrench]

The body will likely need to be reshimmed anyway if the frame has more than just a tiny bit of straitening done. Removal of the body, even if it is just suspended over the frame for a time, would allow for a reshim of everything to get best fit of doors and hood.

In a U shaped structual member where the open section of the U is vertical such as a model A frame rail, the majority of the stress is actually on the closed vertical section of that member. The upper and lower flanges do provide strength but they either pucker or stretch when heavy force is applied. The vertical section does both.

To reinforce a rail without boxing it would require reinforcement of the vertical part of the member with added lower flange reinforcement such as an L-shaped patch in order to keep from adding a thickness to the area where the body rests. A simple strap on the lower flange would make it harder to stretch but that is only a small part of the structure. To spread the load more evenly is a better way to go. It would likely bend somwhere else if it were ever to bend again.

The area where the cowl sets has always been the most vulnerable part of the frame for frame sag due to fatigue. Crash damage is a whole different ball game.

[nkaminar]

rotorwrench, For a C section, most of the bending stress is taken up in the flanges. To strengthen the frame a piece must be welded at the top and bottom. As you stated, the problem with welding a piece to the top is the interference with what goes there.