Handling performance: most important chassis bracing/reinforcement - per application?

[MRGTX]

This might be a good resource to collaborate on for this particular forum...but I'll need your help because I don't have enough first hand experience to say for sure what is needed. My guesses could end up wasting peoples' money.

This is what I'm thinking: For each application, which chassis reinforcements do you see as the most valuable? Which ones are absolutely critical? Perhaps list the type of bracing and what it accomplishes?

These are some suggested categories, feel free to create your own:

Thanks for any input at all.

Improved street driving:
Basic improvements to responsiveness and feel


Aggressive Street driving:
Improved handling limits/more predictable at limits


Autocross:
All of the above plus improved durability


Track Day/Road Course:
All of the above plus safety

-----------------
Straight line guys may have different priorities?


Street/strip-
Able to withstand repeated stresses from increased engine power/hard launches

Drag car-
Street/strip plus safety

 

[toolmanmike]

@Mattax

 

[Mattax]

@Mattax

You know I think too many people buy stuff that's not needed.

My opinon on the chassis is that an important factor is the base body, and secondly how well it was built. A convertible is obvious, but some of the others are not. @CPDave has found that his goldfish (66 Barracuda) is pretty darn good as is - even autocrossing on R-comps. The transmission tunnel to cross braces in my 67 coupe have had to been repaired several times. (and I added basic frame connectors before any autocrossing or drag racing).

Most know that Dodge and Plymouth added fender braces, and while they may or may not effect handling, they didn't do that just to spend money. Post cars ought to be stiffer than hard tops (generally) and some time around '70 cars got braces inside the doors.

TIRES
This is the biggest variable. On street tires none of this is going to make much difference.
Depending on the chassis may notice some difference with torque boxes and frame connectors. I won't guarentee it though.

First check for quality and condition of the factory welds. On my coupe, the torsion bar anchors and sub frames were all good, but some of the body and in particular the cross braces behind the seat were poorly fitted and therefore poorly welded.

Drag strip: If using drag slicks and the car is rising on launch (as it should) frame connectors would help. 4 point may help a bit more and obviously bracing to the front would be most effective.
I worked with a guy that would leave his '67 GTX on strategically placed jack stands ever week to get the car to settle back. This way he didn't permanently wrinkle the quarters. No connectors, no bars - Yes 10" slicks, pinion snubber and spring clamps setup for racing.

Autocross. As much as the rules allow and you are willing to tolerate. Street based classes are fairly restrictive on how much can be welded up. No seam welding (unless factory), only three attachment points allowed on frame connectors and only two may be welded (Street Prepared based classes). Serious C Prepared effort, running the max tire allowances, 6 point is going to be a smart starting point.

Racing. Again its the rules and the tires. Running vintage classes restricted to Street TDs, Blue Streaks etc is going to be different than a GT or open class.

Track days. Let others speak to that. Other than convertables, PDX and other non-timed events are generally pretty relaxed. Some of this will be driver tolerance. Dr. Bob Reed (former road racer not th edrag racer) found his a-bodies very soft compared to the mazdas he had been driving as a pro. But Bob can drive a car much closer to its limits on a track than most of us. The better you get, the more speed and grip and loads you will be able to sustain. Some bracing is going to help the car even if you can't feel it.

Flex or lack of feel has never been an issued raised by national level drivers when they've been instructing and driving my car. It's just not that critical to setting up a good car. I've had replacing the current frame connectors on my to do list for several years. I'm sure it will help, but hasn't been a top priority.

 

[MRGTX]

@Mattax
This is all excellent info.

I'm curious about this part though:

Flex or lack of feel has never been an issued raised by national level drivers when they've been instructing and driving my car. It's just not that critical to setting up a good car. I've had replacing the current frame connectors on my to do list for several years. I'm sure it will help, but hasn't been a top priority.

I don't get the impression that this is a common opinion. A stiffer platform theoretically makes steering inputs more responsive, preserves suspension geometry under load, etc...I believe you if you don't find it to be critical but adding bracing seems to improve how most old cars feel...especially unibody cars like these without much structure between the front and back of the chassis.

Another concern that I have is keeping the car from getting looser from the stresses or even twisting, breaking components or windows...this is a legitimate concern, especially if we start talking about sticky tires, no?

 

[Mattax]

I don't get the impression that this is a common opinion.

As you know, there is little correlation between how commonly something beleived and whether it is correct. This is so much more true when there are now many companies all pushing to sell product to the pro-touring crowd and anyone else they can rope in. Very very few opinions are backed by anything more than seat of the pants impressions and arm chair "engineering". We all know what happens when people buy something - they're invested in it. So that doesn't help evaluations. It doesn't help that many assesments of add ons are done at the same time as basic repairs - like replacing worn bushings.

My reference to national level drivers was autocrossing; which is one of the few places where one can push a street car to its limits against the clock. We can pretty much count on our fingers the number of people autocrossing rwd mopars. Over the years the guys I've had as instructors include Mike 'Junior' Johnson, Pat Salerno, Sam Strano, Tony Savini. (you can do websearch on them if you're curious). Chassis has never come up as an issue. In fact the only car related challenges are steering.

FWIW Mitch Lelito (ESP national level competitor) had no reinforcing on his Challenger. When @CPDave was running his '66 Barracuda it wasn't until his '08 season that its development and his driving had reached a point he felt subframe connectors would be beneficial.

A stiffer platform theoretically makes steering inputs more responsive, preserves suspension geometry under load,

Yes.

...I believe you if you don't find it to be critical but adding bracing seems to improve how most old cars feel...especially unibody cars like these without much structure between the front and back of the chassis.

It depends on the make and model. Compared to other unibodies of the time, our cars are good. At some point go up the ladder of competition the tire traction will be high enough that the chassis flex will be noticible under loads. A car that will be built to compete nationally (like the Sandberg's CP 'Cuda) will need serious chassis stiffening. But please look at the tires we're talking about in that class. The front rear connections are useful but the real need to triangulate as high as possible - especially to the front corners. Herb Adams Chassis Engineering has a great series of photos using balsa wood models to illustrate this.

Another concern that I have is keeping the car from getting looser from the stresses or even twisting, breaking components or windows...this is a legitimate concern, especially if we start talking about sticky tires, no?

Like I wrote before, if ther is bad weld or bad joint, then its a weak spot. Other than that it all depends on what one thinks is 'sticky.'
At the moment, my coupe is the only car I can think of that showed some flex damage. That was at the tunnel as mentioned before. I know drag racers who had damage in that area from the pinion snubber and wrinkling quarters. Again, I'm describing chassis that are actually being lifted by rear suspension at the launch.

Mitch competed for years on Hoosiers. Pic from the 1990 Mopar Performance catalog

Dave's Barracuda - mostly on Toyo RA-1s

No broken windows in either car.


I'm not suggesting that reinforcement, seam welding and caging won't help. I'm just saying its not always a great bang for the buck.
And if someone is serious about competition, then the first thing to do is check the rules. It really sucks to be bumped into a much more highly prepared class because of a modification that is difficult to undo.

 

[Mattax]

You may recall XV Motorsports did some real chassis modeling under loads. Some of the videos may still be floating around. IIRC the front end was the area that twisted the most when a front wheel was loaded. I think Pete Bergman worked there for a while. Peter also has track time so its not all theoretical. Maybe he'll pipe in with his thoughts when he sees this.

Closer to the other end of the spectrum from Mitch's ESP Challenger, check out cage work on Jess Neal's 'Cuda. Even includes bracing in the engine compartment.
‘Cuda to the Clouds

 

[mbaird]

If you are serious about making your chassis stiffer you might start by completing or adding to the existing body and Kmember welds. If you look at the stock body it is hard to believe they don't fall apart with so few spot welds.

 

[BergmanAutoCraft]

If you are serious about making your chassis stiffer you might start by completing or adding to the existing body and Kmember welds. If you look at the stock body it is hard to believe they don't fall apart with so few spot welds.

Running through some older articles and this is a good one. For a unibody Mopars are pretty good. I've worked on all types of muscle cars. You want fimsy, check out early Mustangs.

 

[BergmanAutoCraft]

We can keep this in one thread. I've had my Dart 35 years now. It was purchased from the original owner when I was in high school in S. Florida. My car had never seen winter, so the chassis was very healthy despite body rust from failing window seals, being near the water and the south florida swamp like atmosphere.

Fast forward as I've competed in track days at NJMP (both courses), Lime Rock, Pocono and Monticello as well as Run To The Shore (NJMP) and two times in Optima Batteries Ultimate Street Car.
Two years ago I did a complete teardown from the firewall forward. This started as I restored my instruments and was replacing a cut dash frame and reinstalling the factory AM/FM (updated). My situation was some rust in the drivers kick panel. However, the amount of cracks all over the front sheetmetal was concening. These track events are typically four to five sessions of 20 min each. This might not seem like a long time, but at speed its pretty extreme. I founds cracks at the inner fenders, firewall and bottom of the front frame rails. Lots of cutting and welding to repair. Since seam welding is very difficult due to all the crud that builds up in seams through the years, I find it easier to add spot welds, as many as possible helps quite a bit. At XV the torsional testing proved adding reinforcements ahead of the firewall are the most effective.

 

[Mopar Tim]

We can keep this in one thread. I've had my Dart 35 years now. It was purchased from the original owner when I was in high school in S. Florida. My car had never seen winter, so the chassis was very healthy despite body rust from failing window seals, being near the water and the south florida swamp like atmosphere.

Fast forward as I've competed in track days at NJMP (both courses), Lime Rock, Pocono and Monticello as well as Run To The Shore (NJMP) and two times in Optima Batteries Ultimate Street Car.
Two years ago I did a complete teardown from the firewall forward. This started as I restored my instruments and was replacing a cut dash frame and reinstalling the factory AM/FM (updated). My situation was some rust in the drivers kick panel. However, the amount of cracks all over the front sheetmetal was concening. These track events are typically four to five sessions of 20 min each. This might not seem like a long time, but at speed its pretty extreme. I founds cracks at the inner fenders, firewall and bottom of the front frame rails. Lots of cutting and welding to repair. Since seam welding is very difficult due to all the crud that builds up in seams through the years, I find it easier to add spot welds, as many as possible helps quite a bit. At XV the torsional testing proved adding reinforcements ahead of the firewall are the most effective.

View attachment 1715517107

View attachment 1715517108

View attachment 1715517109

View attachment 1715517110

View attachment 1715517111

That is unexpected

 

[mbaird]

That is unexpected

More common than you might think .
I have some similar cracks .