Frame sag - common ?

Special Coupe Frank

Special Coupe Frank

Well-known member
This subject came-up on the "'28-'29 Brake set-up" thread, when I saw Tom W.'s very nice picture of the ****** area of his '28.

The "finger" on the upper u-joint cover is resting hard on the cross-member lip of my '28 Coupe, and it looks like it's been that way for a long time.

That issue, plus a significant gap between the rear, lower corners of the hood and the raised bead at the bottom of the cowl, all suggest that the frame of my car is sagged in the area of the rear motor mounts / cowl pillar.

How common is this ?

What causes it ? ( '28 Special Coupe )

What other related damage / danger signs should I look for ?

:confused:
 
Frame of the Model A will usually sag over time in the area of the flywheel housing engine mounts. Quite often when this condition of sag occurs, the doors no longer stay in alignment, and depend a large amount on the dovetails to ensure closing.
 
I think the frame sag is the root cause of my 7 tooth column being bent and the steering shaft binding.
 
I acquired my 29rpu as a basket case. My frame had a severe sag at the point mentioned. It has been very interesting how, as I have gone about restoring it piece by piece, I can attribute damage to various body pieces to the sag that occurred when the car was together. It actually validates the various sheet metal pieces came off that frame.
 
Of the 25 model A's that I have had, all but one had frame sag to some degree. Every frame needs to be checked before you restore your car.
 
One of the worst cases of frame sag I've seen was at a body shop. They had restored and painted the body on the frame with no engine. This was a 1928 Special Coupe with multiplate ******. After the paint job was done they called me to install the engine. After I installed the engine the hood went from a perfect fit, to a large gap at the bottom rear. The shop owner asked me what was going on, and I explained how the weight of the engine made the frame sag at the rear mounts. I told him he'd have to tie down the frame ends and jack up by the rear motor mounts. I didn't see him fix it, but the hood fit right the next day.

Another bad frame sag on a 1931 Widebed PU (with no body or box on yet) made the driveshaft hit the brake cross shaft, so the brakes couldn't operate.
 
To further answer your original question. You will notice that the rear engine mount (flywheel housing) is mounted approximately mid-point between the front cross member and the center crossmember. The greatest weight of the engine, clutch assembly, drive shaft, radius rod torque, are applied to the area of the prevalent frame bend. Add to this the weight of front cowl section, with a fuel tank loaded with 10 gallons of gasoline, doors etc., that is a lot of weight for a Model A light channel frame to withstand humping over rough roads prevalent of the time.
 
Tom's statement PLUS another thread a few months ago about having a " flat on
the top , level frame ", got me thinking. Should the frame have a slight crown
in it, bare, before starting a restoration ?

Dudley
 
We show some of the sag and point out the reasons it happens at that point in the Summer 2011 issue of the Model A Times. We are doing a two part article on preparing a frame to use under our new roadster pick up.
 
d.j. moordigian said:
Tom's statement PLUS another thread a few months ago about having a " flat on
the top , level frame ", got me thinking. Should the frame have a slight crown
in it, bare, before starting a restoration ?

Dudley
Click to expand...

I've wondered the same thing?:confused:

I can't explain why that 28 Coupe had such a sag from just installing the engine, because I've never seen that problem on other cars.

I wonder what Brent's experience has been on frames?
 
Tom Wesenberg said:
I've wondered the same thing?:confused:

I can't explain why that 28 Coupe had such a sag from just installing the engine, because I've never seen that problem on other cars.

I wonder what Brent's experience has been on frames?
Click to expand...

Sorry guys that I initially missed this question. Based on what I have experienced, it is likely caused by a frame that was "re-stretched" back into position instead of the "stretched chord" metal shrunk (with heat). When an area is stretched, the metal becomes weaker. Then when force is applied to 'correct' the sag, the stretched metal becomes the fulcrum which stretches the metal in another area, further compromising the strength of both upper & lower chords. Sometimes it is best to go with what works for yourself but I truthfully am of the opinion that if you use heat to shrink a frame rail back into specifications, then there really shouldn't be a need to "over-stretch" or to create a little crown in the rail.

.
 
BRENT in 10-uh-C said:
Sorry guys that I initially missed this question. Based on what I have experienced, it is likely caused by a frame that was "re-stretched" back into position instead of the "stretched chord" metal shrunk (with heat). When an area is stretched, the metal becomes weaker. Then when force is applied to 'correct' the sag, the stretched metal becomes the fulcrum which stretches the metal in another area, further compromising the strength of both upper & lower chords. Sometimes it is best to go with what works for yourself but I truthfully am of the opinion that if you use heat to shrink a frame rail back into specifications, then there really shouldn't be a need to "over-stretch" or to create a little crown in the rail.

.
Click to expand...

That may hold true in severe cases and maybe it requires 10k miles or more to return. With that said, I don't know personally of any reoccurrences. Doug's Roadster isn't exactly high mileage but there has been no shift in 27 years. How many miles do you think it would take to show up?
 
Marco Tahtaras said:
That may hold true in severe cases and maybe it requires 10k miles or more to return. With that said, I don't know personally of any reoccurrences. Doug's Roadster isn't exactly high mileage but there has been no shift in 27 years. How many miles do you think it would take to show up?
Click to expand...

Hey buddy, not sure what you are specifically disagreeing with me on (--maybe all of it!! :D) but what is your opinion on it then based on your personal experiences?? I'm open to other theories.

.
 
Fist of all I'm a rookie at restoring As (old rookie) but reading this series of posts about frame sag and previous posts about frame sag motivated me to check my 28 tudor out. Mine has absolutely no signs of frame sag. To my way of thinking, it's a lot easier to reinforce the frame now than try to fix it later after the fact. Especially now that I have a float-a motor kit on the back of the motor and top of the ******. I reinforced that area of the frame with 3/16 angle iron and 3/16 plates welded to the frame on both frame rails. Don't know if that's over kill, but as I mentioned, to my way of thinking it's a lot easier than trying to straighten a frame. Espcially with the body on it.
 
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