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Re: Removing rear engine crossmember
Rocket man #1349284 Sun Mar 08 2020 11:00 PM
Joined: May 2005
Posts: 7,165
General Purpose
To your original question:
If I read this right. Truck has no motor or trans. You want to mock up a new motor and trans. If you intend to cherry pick sling/jack stand and block the new stuff in there to design the motor and trans mounts, you can remove the crossmember without any worries. You can design, build and tack weld (and maybe finish weld) the various mounts while motor and trans are mocked in place, if you wish as long as new motor/trans are not "loaded" onto the frame per se. You will want to measure the exact dimension between frame rails before removing the crossmember to observe any slight change afterwards. (probably none)

To the "new" crossmember phase:
I hope you are looking at those universal tubular motor and trans mount crossmembers. I like to use those.

Your idea of modifying the existing crossmember may need some engineering thought/advise. Not sure if you mean for the tranny or the motor. Modifying/dropping existing crossmember may not be advisable strength wise. Probably best to derivet the crossmember and use above linked or equivalent. I have used them several times on 55-59. 350's, 454's and TH350. If there is some special reason they won't work on AD, learn me. I have also used an existing crossmember with mods. I am a Mechanical Design Engineer FWIW, like Kevin. I would also probably go with Mike's deal. It has a lot of bolts and hard to design. If you want to hear why my linked crossmembers are cheaper, better, easier, faster, better looking and more accurate. Let me know. If you wish to do a modification to the existing, maybe I and others can help but would have to see pics/drawing of the idea. All of us are just giving you input. Only opinions.

I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.

Re: Removing rear engine crossmember
Rocket man #1349386 Mon Mar 09 2020 07:39 PM
Joined: May 2005
Posts: 7,165
General Purpose
I see all these efforts for crossmember modifications. I read that the poster wants to make a temporary one to support the frame from "shifting". Not having a good look at the AD frame in person or what different motor/trans combos are contemplated, I am giving a general engineering opinion mixed with my crossmember experience.

It seems the issues folks are solving are:

1. Removal of original motor crossmember to allow a new motor/trans clearance. The removed member is near the rear hanger of the front leaf spring. Which, when removed, some believe would causes the frame to deform during operation and cause something to happen.
2. Adding/designing/fabing a (dropped for clearance) replacement for this crossmember to clear a new V8 and auto trans on their way back to a new support member.
3. Adding/designing/fabing a crossmember and support for a new transmision which is located at X distance rearward of the original.
4. Adding/designing/fabing a new motor mount for the V8 Mouse or Rat.

So the challenge is presented. Many ideas. Many designs. Many theories. Here's another batch of those from little ol' timid me.

1. The tubular crossmembers that are available are cheap, already fabricated to the approximate desired shape. Dropped. Universal. Few pieces. Boltable. Strong. Elegant. Adjustable. Not much fab work. No welding unless desired. They are crossmembers! Tubes are used all over hot rod suspension, roll bars, supports. Provides a strong one piece custom shape.

2. Some folks abandon the original crossmember and do not replace it. A "C" shaped channel frame is pretty strong. The side loads from a leaf spring hanger are not huge. If the new crossmember is strong and not too far away from the old crossmember, my engineering gut tells me everything will be fine. Some of the designs I have seen are not doing much. My first reaction was the same as Hot Rod Jerry's. No gussets. True, they have been used for years and no issues. That could be because they are just extra weight metal and have no function. I won't judge because I don't know for sure. The one's that are not carrying a motor or trans load, are not hurting anything.

3. If you want to replace the original because you feel it is necessary. That's fine. Derivet the entire old member and install a universal tubular crossmember. Either one for a transmission or one for a motor. Whatever has the best shape. Bolt it on. If the adjustable flange ends of those tubular crossmembers can be welded to the tube and the whole thing can still be removed, that's the best method. Maybe it would never have to be removed depending on the other clearances. I know it can be extreme pain to derivet the old crossmember. But schlepping onto the cut off part is not the right thing to do. Takes a lot of time and effort. Can't stick it up there, mark and mount it like you can a Universal Tubular. You have to measure, design, cardboard it, and keep testing, cut some pieces, tack it, clamp it up there, then final drilling and welding.

4. Use two more of those universal tubular crossmembers for the motor and trans.

5. Some builders are also changing to IFS. The leaf spring side load support issue goes away. If there is a IFS frame boxing requirement that interferes with a universal motor crossmember, work it out/mock it up.... or use a different motor-mount-to-frame part/design.

Contrary to popular belief, I am not a Cross Member.

I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.

Re: Removing rear engine crossmember
Flatblu4748 #1350074 Sun Mar 15 2020 01:14 PM
Joined: Jan 2010
Posts: 109
Shop Shark
Originally Posted by Flatblu4748
52Carl: I was able to find your picture that you referenced in your above explanation. I wanted the other Bolters to appreciate your effort....”a picture is worth a thousand words”. Carl, like you, “I’m not an engineer”, either, and everybody’s got an opinion....I’ll just leave it at that...

That idea looks doable but I would've made my cuts at 90 degrees to the ground. Looks like these ar 90 degrees to the area cut.

If you cut 90 degrees to the ground and welded flat plate to each side of the cut, you would basically be able to move it up and down easily.

This is basically how the bigger trucks are set up.

1955 GMC 150, 1980 Chevy C10, 2011 Sierra 3500 HD SLT Duramax Dually Crewcab, 1960 Chevy BelAir 2dr sedan, 1978 Pontiac Trans-Am
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