While restoring an early Mustang can be, and usually is, a difficult task, it is basically clear-cut.  There is typically a great deal of research involved, especially when restoring something that is a bit rare or more exotic, but there is (for the most part) really only one correct answer to any specific question.

On the other hand, customizing your classic is a totally different project all together.  More and more people are doing it every day.  But, contrary to restoration, building a custom car requires a completely different set of skill sets altogether.

For example…

When you decide to customize your car, there are many different paths you can take and many, many decisions you need to make.  First and foremost on the list is the intended purpose of the car.  Street or race (Drag or road).  Pro-Touring or Pro-Street.  Radical, conventional, conservative, traditional, retro, futuristic, far-out, way-out, etc, etc, etc…

And, while all of these personal automotive expressions can (and do) vary greatly from one another, they do have one thing in common.  The chassis.

Your chassis is the very foundation upon which you will build your project.  And make no mistakes about it; the chassis has a direct impact on every aspect of your project.  And, while there are literally thousands of products available in the automotive aftermarket, chassis modifications are few and very far between.  This is the one area that has lagged behind every other aspect of the hobby.  In fact, some have become so dissatisfied with the early unibody that they have resorted to removing the entire substructure and dropping the body on a full frame.

But, contrary to what many may believe, installing a full frame under a unibody car is not the solution to the problem.  In fact it is taking a step backward.  If body on frame construction was the solution to the problem, you would see current manufacturers using this technique.  And I am hard pressed to name even one.

So come on a journey with us as we examine the basic chassis structure.  We will show you (in a three step process) how to modify the basic Mustang chassis.  These simple mods will have a significant impact on every other modification you chose to make.

NOTE: All of the modifications we describe are based on torsional stiffness studies performed on the baseline Hopkins NASCAR chassis designs. It is interesting to note that the major structural members of the Hopkins NASCAR chassis are very similar to those of the early Mustang chassis.  All these modifications can be made on any Mustang (stock or modified) without having to repaint the exterior of your car.
 

Phase one. 

The overall goal is to drive (or transmit) as many of the torsional forces seen by the chassis up into the roof structure.  In order to transmit these loads effectively, we must start with the base.  Adding rocker Reinforcements, installing torque boxes and replacing the individual driver and passenger seat pans with a seat pan that extends all the way from one rocker to the other (bridging the trans tunnel) are the bare minimum modification.  Fortunately for us Mustang owners, these parts were designed by Ford and are standard components in all early convertibles.  Installing these items requires a minimum of modification.  Anyone capable of replacing a quarter panel is capable of installing these items.

Cost of modification      2,800 

The above modifications will reinforce three key areas of your chassis.  First, reinforcing the rockers will net big results in regard to torsional stiffness of the chassis (Note: You will also see big gains in regard to “beam” strength.  Rocker reinforcement improves beam strength more so than the addition of sub-frame connectors.  You will note that we are not recommending sub-frame connectors as part of our chassis package.).  The rockers are rated as the sixth most sensitive structural member in regard to torsional deflection.  Second, adding torque boxes will tie the frame rails to the rockers, again, netting big results.  The torque boxes (or equivalent NASCAR structures) are rated as the fourth most sensitive structural member in regard to torsional deflection.  Lastly adding the convertible seat pan reinforces the rockers by tying them together and also reinforces the trans tunnel. These three simple modifications will drastically improve the base chassis. 

In fact, these are the very modifications the Ford engineers implemented in their black 65 fastback “Cammer” project car.

In our project, we had some minor floor damage.  We could have repaired the rotted out areas of the floor by installing patch panels, but we opted for a full floor replacement.  Ultimately this netted us a much cleaner and simpler installation.  If you have rusted floors, I would recommend this procedure in a heartbeat.

 

Above is a view of the chassis with the floor removed.  Note that we also had to remove a section of the firewall.  This area will be repaired after the installation of the torque boxes.

 

 

Above the photos show the inner rockers prepped and ready for the addition of the convertible rocker reinforcements.

 

 

 

Above, we are installing the convertible inner rockers.  There are very stout items and add a great amount of stability to the overall chassis.

 

 

Above we see the installed inner rocker reinforcements.

 

  

Above you see the installed firewall patch panel.  You will note that the patch panel is installed via a spot welded lap seam.  This technique is stronger than a butt seam.  The finished seam is typically covered with seam sealer, but the seam can also be welded and ground to achieve a seamless installation.

 

  

Above we see the installation of the convertible seat pan

 

 

Above is a bottom view of the drivers side torque box

 

  

You will note that we moved the seat pans back 3” to give those taller drivers a bit more leg room.  Also we seam welded all of the torque boxes and suspension mounting points.

Phase two. 

Now that we have made the “base” stable, it is time to address the forces applied to the chassis by the front suspension.  In the case of the early Mustang, all front suspension loads are carried by the shock towers (and to a lesser extent the front frame rails).  These loads are carried to the roof structure through the sheet metal of the engine compartment aprons and the cowl.  To reinforce these areas we must box the cowl area with structural tube and then triangulate that structure to the “A” pillars and the top of the shock tower.

Cost of modification      1,900 

Phase two modifications will reinforce two key areas of your chassis.  First, tying the shock tower to the cowl will greatly stabilize the front suspension.  This area is rated as number two on the list of most sensitive structural member in regard to torsional deflection.  Second, reinforcing the cowl will carry the forces applied by the front suspension directly to the roof structure (as well as the rockers), again, netting big results.  The cowl structure is rated as the ninth most sensitive structural member in regard to torsional deflection.  Now, when you add that expensive aftermarket front suspension, it will perform exactly the way it should greatly improving road feel, grip and responsiveness.

Completing phase one and two (in addition to the installation of an export brace and Monte Carlo bar) will reduce over 87% of the deflections seen in an un-modified chassis.

  

Above is a good view of the cowl reinforcements.  This entire structure is designed to transmit all the front end loads through the cowl to the rockers and the “A” pillars.

 

  

After the installation of the structural tubes, trim panels are installed.  These panels not only keep debris from accumulating in all the nooks and crannies, but also sandwich the structural tubes between two sheet metal panels helping to stabilize the entire area.

 

  

Here are some views of the final trim panel installation.  These trim panels are installed to keep these areas clean and help prevent the accumulation of road debris in the structure.

 

 

Here is the finished installation

 

Here is an example of just one of the welds holding the front structure together.

Phase three. 

Completion of Phase one and two is all that is typically required for a high performance street driven vehicle.  But for those of you who are looking at installing engines rated at over 600 RWHP, we recommend installing a full cage.  The cage will not only increase overall chassis stability, but also help protect the passengers in the event of a crash or rollover. 

But installing a cage that will strengthen the chassis and reduce torsional deflection is a bit different than installing your typical run of the mill roll cage.  Most roll cages are designed with one purpose in mind; satisfy the safety regulations of a race sanctioning body.  They are designed to protect the occupants in the event of a crash or rollover. 

A properly designed cage will reinforce as well as help transmit loads to and from key structural elements.  In our case, the roof structure just above the drivers and passengers side windows, the “A” pillars and the roof structure at the top of the windshield.

The major differences between our design and the typical roll cage are as follows.  The cross bracing of the main hoop.  The triangulated 3 bar rear frame rail braces.  And the additional bar braces at the four halo connection points.  In addition to these differences, the “A” pillars, and the roof “halo” are gusseted to the existing Mustang structure.  Tying the cage to the existing unibody structure is extremely important if reducing torsional deflection is your goal.  If the two structures are allowed to float independently, minimal gains, if any, will be seen in regard to overall chassis stability.

Cost of installation      4,800 

 

 

 

Phase three modifications will reinforce three key areas of your chassis.  First, tying the cage to the roof structure over the drivers and passengers side windows will generate the single greatest return in overall chassis stability.  This area is rated as number one (by a large margin) on the list of most sensitive structural member in regard to torsional deflection.   

Second, reinforcing the cowl will carry the forces applied by the front suspension directly to the roof structure (as well as the rockers), again, netting big results.  The cowl structure is rated as the ninth most sensitive structural member in regard to torsional deflection.