Bent frame--how bad is it?

[Bob C]

If you look at post #11 in the link Russell posted you can see where Brent
heated the frame.

[Russell Reay]

First of all I stretched a chalk line from a hole in the front of the frame horn back to a hole in the top chord just in front of the rear running board bracket. This showed me that I had about 1/8" sag between the middle crossmember and the rear hood holddown bracket. Per Brent's standard, the sag should be 0. With reference to Brent's pics in the cited post, I heated the bottom chord in that area using a #6 rosebud tip. No particular need to heat the web, and if so, only a short ways up from the bottom. DO NOT heat to red hot. Heat hot enough to make a wet towel sizzle and steam, but DO NOT quench it. Use the sizzle and steam description as a performance standard, not an actual practice. Shoot the heated segment w/ compressed air, or not. This is Brent's standard, not my own invention--credit where due. I let the heated frame cool overnight, and this morning I stretched the chalkline, and could just about see daylight between it and the frame. I have declared victory, and will continue with re-assembly. Note:--the first time I tried this method, the engine was still in, and I heated a length of ~8" in front of the middle X-member and the same under the motor mount. The next day the sag had increased from 1/8" to 1/4". Pulled the engine, removed the motor mounts, talked w/ Brent on the phone, and proceeded as described.

[Oldbluoval]

I found on another thread. #11 is my own thread…on this one!
Apparently this subject is addressed on various threads
Got it thanks

[nkaminar]

What Mike K wrote in May of 2010:

OK, I've heard this heat shrink/ cold bend debate about frame straightening too many times. Let's apply a little engineering/ math:

If a 4' long straight edge across the motor mount shows a 1/4" (0.250000") sag, the difference in the top and bottom chord will be 0.00260". I dont know if it will be all stretch in the bottom chord or all compression in the top. Likely some mix of the two, as the same sectional area of the same material makes up both chords.

Applying either straightening method, cold or heat, will cause one chord to compress (shorten) as the other stretches. Cold press will cause the chord acted upon (pushed with the jack) to re-compress while the opposite chord stretches. Heat shrink will cause the chord acted upon to shrink while the opposite chord will stretch. Whether the stretch/compress between the two chords will be 50/50 or 90/10 I don't know, but both chords will be affected by either method.

OK, let's assume the worst: 1) The sag caused only one chord to stretch or compress 0.0026". 2) The method or technique you choose and apply to straighten the frame acts only on the wrong chord. 3) Only one of your two frame rails was bent.
RESULT OF STRAIGHTENING: A frame that has one side 0.0052" longer.

If anybody thinks Henry's frames were accurate within 5 thousanths, think again! There's more slop than that in every bolt hole.
__________________

[nkaminar]

I checked the math. 1 minus the cosine of the arcsine of 0.25/24 times 5 inches is 0.00027 inch. .25 inch the amount of sag over 4 feet (48 inches) 24 inches is half of that. 5 inches is the frame depth So my math shows a factor of 10 less than what Mike K got. He used a different way of calculating the difference, using the chord. Please, if you can, check the math.

A stretch of .005 inch over 5 inches would result in the reduction of thickness of 0.1% or 0.00025 inch. If you take my figures then the reduction in thickness is 0.000025 inch.

In any case the difference in length (and thickness) of the frame is insignificant.

[BRENT in 10-uh-C]

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Originally Posted by nkaminar

I checked the math. 1 minus the cosine of the arcsine of 0.25/24 times 5 inches is 0.00027 inch. .25 inch the amount of sag over 4 feet (48 inches) 24 inches is half of that. 5 inches is the frame depth So my math shows a factor of 10 less than what Mike K got. He used a different way of calculating the difference, using the chord. Please, if you can, check the math.

A stretch of .005 inch over 5 inches would result in the reduction of thickness of 0.1% or 0.00025 inch. If you take my figures then the reduction in thickness is 0.000025 inch.

In any case the difference in length (and thickness) of the frame is insignificant.

Neil, a chain is only as strong as the weakest link. It doesn't matter if it is weaker by .0001%, ...however when there is enough stress, the likelihood of the next stretch or failure will be that one that is the weakest. Engineers tend to over-analyze and because I am not in that area of study, you can use figures that I cannot, -nor do I choose to understand. That is not to say I am wrong, -but it is to say that just because you have a theory using equations as your foundation does not make you correct. When there is an area that is compromised by whatever the amount of the stretch, -then it is weaker. I spoke with Mr. Reay by telephone for a bit, and he learned that after heat was applied, the sag in the metal became noticeably greater. In all likelihood, this was caused because the area that he heated was not stretched to begin with from any trauma, and therefore it expanded from the heat to exacerbate the sagging issue. My point is, by theory this should not happen, ...however those of us that have hands-on experience know differently. My feelings is if you do not agree with me in this area, then state your opinion and actual hands-on experiences, ...and then lets both move on.

[jerrytocci]

I would do it "cold". Put thick wood across the frame to raise the jack and protect the frame. The higher the jack the easier it can be done

[rotorwrench]

When a frame bends, the inside angle is compressed and the outside angle may be stretched. I would say that both application of pressure and heat to shrink can be used to affect the movement needed. If you only shrink, the metal that is compressed doesn't move very well. Shrinking and stretching in moderation can both be accomplished. 1/8" is a fair bend but I've seen worse. A person can discuss this with the frame man. One who knows how steel members react will understand.

Cold bending can be effective but it depends in large part on the profile, shape, & thickness of the member plus the extent of the bend. The experience level of the man performing the job is also important.

[nkaminar]

In response to Brent: There is nothing wrong with the method he is using. Heating and cooling with a wet rag is done all the time in body work to shrink metal and I have done that myself to straighten out door panels. I think that there is also nothing wrong with chaining down the frame and bending it using a bottle jack. Lots of frames have been straightened that way and that is what I plan to do to my frame when I get the time. One advantage that I can see with the cold bending is that it can be done on an assembled car. Brent may object to that but I have seen the frame on an assembled car straightened with a bottle jack.

[rotorwrench]

A nice big I-beam, chains, one or more hydraulic jacks, or other items such as wood blocks or other metal members can be used to straighten frames. It's been done this way since Christ was a corporal. Cracks and fatigue are the major concerns and should be addressed. Sometimes reinforcement plates or patches are needed to fight the sag of an old fatigued frame.

Straightening with the car assembled does not allow a person to check for other bends, twist, or squareness of the whole frame very well nor will it be easy to manipulate to correct all deficiencies. I would only try correcting a bare frame. Actions can have adverse reactions

[Tinbasher]

Use Brent's technique to straighten the frame. It works great and it lasts. I always use to straighten them cold on a frame rack. How I use the heat and shrink the side and lower edge and it works great. I've done it one driving cars by placing the axle stands about 8" behind the rear engine mount and heating the frame and let the weight of the front of the car bring the frame back into shape. Heating to a dull red and cooling works well. About 3 hrs. a side on a frame with 3/8" sag. JP