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Discussion Starter #1
Has anyone had issues because of narrowing the frame rails when doing a mini tub? See a lot of mini tubs installs done by narrowing the frame rail, but can't believe the narrowed rails are rigid enough to prevent any flexing.

I was only going to move the tubs to the outside of the frame rails, but am thinking I'm going to narrow the frame rails using 3/16" plate with some spacer plates in between new 3/61" plate and existing inner plate of rail. Ending up with a frame rail around 1/2" wide. Was thinking of adding reinforcing plate to the outside of the inner rails to add some bending strength. I am installing a TCI 4-link and plan to modify the ends to fit the narrowed frame rails. Move the panhard bar and shorten the panhard rod as required.

Thanks for any info or comments.
 

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I recently finished a DSE mini tub with TCI four link. I had no issues doing the frame pinch. I used 1/8" material as the box out plates. I believe if you use 3/16 " material you will run into an issue when welding the plates in , as the o.e. frame is roughly 16 ga. The frame should not flex, as your four bars tie into the front end of the frame which is still structurally as designed from GM. The links mount to the rear diff housing and this is tied to the rear shock mount , which is also mounted to the original width of the frame.
I did however, remove the stock shock absorber mounts from my car and I have fabricated a 2" .187 wall DOM tube which I went from inner frame rail to inner frame rail. I feel this will add rigidity to the area that I removed the factory shock mount assembly from.
You should have ordered your TCI 4 link for a pinched frame. TCI takes into account the dimensioning and sends the panhard bar shortened , and the rear shock mount narrowed. All you have to do is trim excess off of the shock mount and weld the ends on. Good luck with your project.
If you have any questions, contact TCI TECH SUPPORT (Tech Support 855-693-1259 ext. 221) . They are great to talk to.
In fact when I was doing my 4 link , I had a question , called TCI and spoke with Evan Dalley directly. Evan is the guy who drives the TCI Chevy II on the slalom events. I also questioned the frame strength, and he assured me his car was done per TCI instructions and was bolt in install. He tosses that car around real hard on the track. Call TCI & ask to speak with him. He is a stand up guy & willing to talk to customers.
Check him out on YOUTUBE. LINKS BELOW
https://youtu.be/VgXieeFAAo4
https://youtu.be/-x6XwtlplZc
https://youtu.be/lrFJNvwsutg
https://youtu.be/9foramfVyXU
 

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I guess you should determine what size tire you want to go with. I know a lot of guys are notching so it’s not something out of the ordinary that will compromise the strength of the frame.
 

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I recently finished a DSE mini tub with TCI four link. I had no issues doing the frame pinch. I used 1/8" material as the box out plates. I believe if you use 3/16 " material you will run into an issue when welding the plates in , as the o.e. frame is roughly 16 ga. The frame should not flex, as your four bars tie into the front end of the frame which is still structurally as designed from GM. The links mount to the rear diff housing and this is tied to the rear shock mount , which is also mounted to the original width of the frame.
I did however, remove the stock shock absorber mounts from my car and I have fabricated a 2" .187 wall DOM tube which I went from inner frame rail to inner frame rail. I feel this will add rigidity to the area that I removed the factory shock mount assembly from.
You should have ordered your TCI 4 link for a pinched frame. TCI takes into account the dimensioning and sends the panhard bar shortened , and the rear shock mount narrowed. All you have to do is trim excess off of the shock mount and weld the ends on. Good luck with your project.
If you have any questions, contact TCI TECH SUPPORT (Tech Support 855-693-1259 ext. 221) . They are great to talk to.
In fact when I was doing my 4 link , I had a question , called TCI and spoke with Evan Dalley directly. Evan is the guy who drives the TCI Chevy II on the slalom events. I also questioned the frame strength, and he assured me his car was done per TCI instructions and was bolt in install. He tosses that car around real hard on the track. Call TCI & ask to speak with him. He is a stand up guy & willing to talk to customers.
Check him out on YOUTUBE. LINKS BELOW
https://youtu.be/VgXieeFAAo4
https://youtu.be/-x6XwtlplZc
https://youtu.be/lrFJNvwsutg
https://youtu.be/9foramfVyXU
The rear shock mounting crossmember is not shorter. The end brackets just need to be modified to fit the narrower frame rails. The Panhard Bar is still the same. It is a simple modification to make so it will work with regular or DSE tubes.
Tim
 

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Discussion Starter #5
Thanks for the reply Hotrodguy. I bought my TCI kit a long time ago, but am just now getting around to doing the mini tub, so I'm sort of stuck with the panhard bar that I have.
Do you have any pictures of the piece you made replacing the stock shock mount piece?

ibuildm, I understand that the rear shock mounting bar is not shorter and that I just trim the ends for the narrower frame rails, but can you give me more information about the simple modification that would let me use the panhard bar I have?
 

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I used a TCI four link and notched my frame. It was my first time and I was also worried about frame flex. I went back with a little heavier metal to patch my frame. If you cut the pan hard bar make sure you cut the right end. The threads are different from end to end. I have a regular tap but don't have a counter clock wise one. I did have to narrow my pan hard bar.
 

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Mini-tub Frame Stiffening

I had concerns about frame strength also. I did some calculations comparing the stock frame rail strength to the narrowed frame rail and was not comfortable with the modification defined. After exploring some options, I decided to embed a 1/4 inch plate inside the frame the full length of the frame cutout. I then did the sheet metal closeout and there is no visible clue.

I feel much more comfortable with this frame mod.

I have attached a photo of the stiffener for the passenger side rail. It is two pieces with the pieces on the ends to transfer load to the stock rails. The two pieces are slid into the open ends of the stock rails and then final welded once positioned inside the the rail cutout. Fit between the top and bottom rail sections is very tight (press fit). It took quite some time to get the fit I wanted. The top and bottom rail surfaces were clamped tight to the the insert during welding the sheet metal close out to enhance the fit.
 

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Discussion Starter #8
MOD66, do these 1/4" plates go the entire length of the frame rail? From where the 4-link cross-member is to where the bumper mounts, or it only in the narrowed portion of the frame rails where the wheel tubs are? Are they welded to the existing frame rails or just welded together after inserting them in the existing frame rail?
This is similar to what I planned to do, only it would only be in the area narrowed for the wheel tubs. Plate is 3/16" with small vertical channel sections welded on inside that would be spacer against the inner frame rail wall. Plate would be welded to top and bottom of frame rail and spacer plugged welded to inner frame rail wall to help stiffen it. Does that make sense?
 

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I wish I could get it inside the whole rail! There has been some discussion on the site concerning quarter panel buckling possibly related to mini-tubbing. That may help with that or it may be hurtful.

As noted, I put "1/4 inch plate inside the frame the full length of the frame cutout". End pieces shown were tapped just inside of frame rail at each end of cutout. The two piece design allowed extra length to provide seating inside stock frame. So the embedded piece was just a bit longer than the cutout (approx. 1 inch). The two pieces are welded to each other after insertion in cutout rail. The end pieces were welded to the stock frame. Between there is no attachment to the frame rail. As noted, the frame rail is clamped around embedded plate and closeout patch welded assuring no relative movement inside rail.
 
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