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List of Chassis Stiffening Upgrades/Mods

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I'm not an expert, I'm just compiling a list of chassis stiffening upgrades I've found from reading online to make it easy on people who search for this topic in the future. (And so people can chime in, for things I missed)

In no particular order:
  • Aftermarket one-piece export brace (also, the reinforcement plate)
  • Aftermarket engine crossmember
    • Referring to the ones that tie into the lower control arm mounts
  • Monte Carlo bar
  • Torque boxes
  • Subframe connectors
  • X-brace (the ones that mount to subframe connectors)
  • Wishbone-shaped reinforcement plate for the shock towers
  • Convertible rockers
  • Honorary mentions (don't think these count as "chassis stiffening")
    • Metal plate between rear seats and gas tank
    • Gas tank reinforcement
    • Master cylinder reinforcement plate
Some of these apply only to certain years (like the torque boxes) and certain models. Hope this is useful, thanks.
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Dimples
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Looks like a decent list. Based on years of dealing with this stuff, here’s my prioritized list that highlights of what makes the most difference.

1: Export Brace
2: Torque Boxes (for those not already equipped)
3: Z-Ray crossmember

Honorable mention: panel separating the seat back and trunk on coupes. This, apparently can do a lot of work to square up the back of the car.
 

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I'm not an expert, I'm just compiling a list of chassis stiffening upgrades I've found from reading online to make it easy on people who search for this topic in the future. (And so people can chime in, for things I missed)

In no particular order:
  • Aftermarket one-piece export brace (also, the reinforcement plate)
  • Aftermarket engine crossmember
    • Referring to the ones that tie into the lower control arm mounts
  • Monte Carlo bar
  • Torque boxes
  • Subframe connectors
  • X-brace (the ones that mount to subframe connectors)
  • Wishbone-shaped reinforcement plate for the shock towers
  • Convertible rockers
  • Honorary mentions (don't think these count as "chassis stiffening")
    • Metal plate between rear seats and gas tank
    • Gas tank reinforcement
    • Master cylinder reinforcement plate
Some of these apply only to certain years (like the torque boxes) and certain models. Hope this is useful, thanks.
If you are interested in this type of thing, you should read this thread:

He doesn't cover everything, but he does cover most of the stuff on your list....with measurable results(though how applicable those results are to the real world is debatable, its still a measurement)
 

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the firewall to shock towers needs the most attention. Use Z-Rays lower crossmember or make your own weld on like i did. Use the heavy duty one piece export brace or make your own like i did. Use the one piece monte carlo bar like i did. Use the convertable torque boxes. Replace the the factory frame rail extensions with NPD 14 guage extensions. And if you want to make it stiff do like I did in the fotos below. That front frame aint never gonna flex.

Automotive tire Automotive exterior Bumper Motor vehicle Gas


Automotive tire Automotive exterior Motor vehicle Bumper Automotive wheel system


Hood Motor vehicle Automotive exterior Vehicle Bumper


Hood Bumper Automotive tire Motor vehicle Trunk


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67 Fastback T5, 331 stroker, TCI Frt End, Canted 4 link rear, 3.55 gears
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Seems like we give a lot of attention from the front end, back to the area of the rear torque boxes, but not much from the rear torque boxes to the rear leaf spring mounts. Occasionally I'll see where someone with a full cage will run the bars from the top of the cage hoop down to the rear frame rail, but without a cage to do that with I don't see much else done. Do you think this is an overlooked area for stiffening? When I look at photos of a Mustang with the gas tank out and the rear quarters cut off for replacement, it makes me realize how much is expected of those sheetmetal frame rails.

Random photo....
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Seems like we give a lot of attention from the front end, back to the area of the rear torque boxes, but not much from the rear torque boxes to the rear leaf spring mounts. Occasionally I'll see where someone with a full cage will run the bars from the top of the cage hoop down to the rear frame rail, but without a cage to do that with I don't see much else done. Do you think this is an overlooked area for stiffening? When I look at photos of a Mustang with the gas tank out and the rear quarters cut off for replacement, it makes me realize how much is expected of those sheetmetal frame rails.

Random photo.... View attachment 869051
That's why the rear seat divider(14g not 22g) is such an effective mod....since it adds so much in that area. Then of course those of us with some sort of coilover rear end also have a truss or reinforcement of some type running from one frame rail to the other to mount the watts-link or panhard bar(well, anyone running a panhard bar would have this, but I am not sure I have ever seen a panhard bar outside of a 3 or 4 link suspension)
 

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One that wasn't mentioned, the convertible seat pan adds strength to the floor area.

On the rear divider, I'm thinking about building an X brace from tubing instead of a piece of sheet metal.
 

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1968 Mustang Coupe
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Someone in the past told me that when Ford started putting high HP/TQ engines in the early Mustangs (mid to late 60's) that torsional problems started showing up in the bodies, in particularly the coupes, in the form of stress cracks around the rear windows (quarter and rear). In some cases the rear window in the coupe would dislodge.

FACT or FICTION? Sounds reasonable but is it true?
 

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Someone in the past told me that when Ford started putting high HP/TQ engines in the early Mustangs (mid to late 60's) that torsional problems started showing up in the bodies, in particularly the coupes, in the form of stress cracks around the rear windows (quarter and rear). In some cases the rear window in the coupe would dislodge.

FACT or FICTION? Sounds reasonable but is it true?
Unlikely. Of all the body styles, the coupes would be the most rigid by design(not to say they are super rigid, but certainly more so than either the convertible or the fastback)
 

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Someone in the past told me that when Ford started putting high HP/TQ engines in the early Mustangs (mid to late 60's) that torsional problems started showing up in the bodies, in particularly the coupes, in the form of stress cracks around the rear windows (quarter and rear). In some cases the rear window in the coupe would dislodge.

FACT or FICTION? Sounds reasonable but is it true?
The quarter panel crack at the quarter window is super common & independent of the power plant. It is due to body flex, but it's also where leaded joints meet.

My '65 6-cylinder even had them. The cracks came back after paint & new quarters...But probably a lot faster due to a healthy 351W. Front torque boxes & rear seat divider (like convertibles have) would go a long way to limit this flexing.
 

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Discussion Starter · #16 ·
Thanks everyone for chiming in, looks like I have a lot more material I can learn from now.

@supershifter2 mentioned replacing the frame rail extensions with NPD's heavier gauge ones, my current one's are fine so I don't want to do the work to remove and weld in new ones. BUT, I am replacing parts of my floorpans and have them exposed at the moment. I'm thinking about bending some sheet metal such that it can fit right into the frame rail extension and welding it in. Not sure if it would make any real different, but any thoughts on this?

I'd basically be making a square tube but with just 3 sides (missing the top), kind of like a U shape if you took a cross section, and fitting that in.
 

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Seems like we give a lot of attention from the front end, back to the area of the rear torque boxes, but not much from the rear torque boxes to the rear leaf spring mounts. Occasionally I'll see where someone with a full cage will run the bars from the top of the cage hoop down to the rear frame rail, but without a cage to do that with I don't see much else done. Do you think this is an overlooked area for stiffening? When I look at photos of a Mustang with the gas tank out and the rear quarters cut off for replacement, it makes me realize how much is expected of those sheetmetal frame rails.

Random photo.... View attachment 869051
Too much is not enough... or, the while you are at it gremlin took hold.

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I've got jacking rails and stringers joining my subframe connectors to the the rocker. The squared tubing is welded to the rocker and the round tubes are welded from the subframe to that squared tubing.

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Seems like we give a lot of attention from the front end, back to the area of the rear torque boxes, but not much from the rear torque boxes to the rear leaf spring mounts. Occasionally I'll see where someone with a full cage will run the bars from the top of the cage hoop down to the rear frame rail, but without a cage to do that with I don't see much else done. Do you think this is an overlooked area for stiffening? When I look at photos of a Mustang with the gas tank out and the rear quarters cut off for replacement, it makes me realize how much is expected of those sheetmetal frame rails.

Random photo.... View attachment 869051
The Tank Armor guys told me how the racers like the Tank Armor because it stiffened the back of the chassis. I took it a step further and, instead of using sheet metal screws and mounting it to thin sheet metal, I stitch welded in 1" x 1/8" steels strips around the tank opening, re-drilled the holes and mounted the Tank Armor and gas tank using grade 8 1/4" bolts.
 
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