Homebrew Subframe Connectors

by Paul Campbell

Big thanks to John Pyle and Chris Neighbors for all their contributions!

Note: I suggest you read ALL of this article (parts I and II) before you begin. This will help you prepare better and get you thinking of how the final product will look. If you have any questions feel free to email me.

Introduction

In the grand tradition of hot rodding, Mustangers often become mesmerized by the many flashy go-fast goodies the strong aftermarket tempts us with. Intakes, exhaust, cams, gears, and/or heads are usually the first things we seek in our quest to outgun the local heroes. After all, it is power that makes the performance world go round, right? Well that is partly true. The fact is, without the ability to transfer that power to the pavement in an efficient and orderly manner, all you have is a very cool sounding chick chaser that is likely to get embarrassed by some of the local rice burners at a stop light. Even if it is not that bad, you can still be cheating much of the car's true ability through inadequate chassis setup.

The Fox chassis is at best designed to only take the abuse of a stock motored Mustang. It is a flimsy unibody design with poor weight distribution among other flaws. Even a stock powered Mustang (or any unibody car for that matter) will benefit from attempts to counter the twisting force placed on the chassis as it accelerates. The power used to wrap up the car can be regained by focusing on minimizing as much flex as possible. Top of the list is the installation of subframe connectors to tie in the front and rear subframes, creating somewhat of a full frame car. Best of all, if you have some decent fabrication skills, access to a welder, and a free day, you can build your own for a very meager price. I have about $15 and 8 hours in mine. There is still some welding to be done, but they are very functional.

Also, I should note that the second thing every Mustang owner should consider is the strengthening of the torque box area. This is where the lower control arm attaches to the body and is very weak. The torque boxes often will rip off the car when slicks are run at the track. At minimum, the sheetmetal will distort and fatigue from abuse, ripping as illustrated here. I look at these two areas (subframes and torque boxes) as a system where each are tied into one another. Thus, some forethought must be taken as to how the rear of the connectors attach to the torque box...more on that later.

Options, options....

If you have browsed through a few different aftermarket companies' catalogs, you may have noticed there are several different design approaches to subframe connectors. There are round ones, square ones, some bolt on, some weld on, some require cutting the floorpan, and there are various lengths. So which is best? Good question that is hard to answer. The ideal setup is an in-floor connector that completely replaces the stock subframes. This requires a complete gutting of the car and a lot of cutting but since you weld the entire length of the connector to the body, it is the strongest without question. Next is something like Wolfe Racecraft offers where the floorpan is cut but the connector runs only from the stock connector to the torque box. They also offer additional bracing to allow the installation of a cage that fully ties in to the subframes. (Go here for a good pic). Anothor option that I highly recommend you consider is the installation of seat mount brackets such as the ones found on the Kenny Brown Super Subs. There are at least two versions I have come up with. If Kenny Brown's are copied, they must be done before the connectors are welded on. I will return to this subject later when we get to that point. Then we come to the shape debate. In my opinion, the verdict is still out on whether square or round is stronger in this application. Theoretically speaking, round may be better at transfering the twisting force, but in reality, they both work. I did mine with square tubing, but you are obviously more than welcome to do yours as you please. One benefit of square tubing is that it can be attached much easier. You be the judge. One thing is for sure:
 
WELD THEM ON!! DO NOT BOLT!!!
 

Enough of the talk! Let's get down to business...

Some things you will need, and some things that would be nice to have:

Need:

  • Tubing - I used 1.5" x 2" x .120" wall mild steel tubing. It is possible to use 1" x 2" but unless you have severe ground clearance problems I would go with the larger. It will take about four feet per side (add about 16" per side for full length connectors).
  • OPTIONAL: about 1.5' of 2" x 2" tubing for seatmount brackets, depending on version. (highly recommended!)
  • 4 - M10-1.50 nuts and flat washers for bolting on seatmount brackets.
  • 3/16" or 1/8" plate - put on each side of connector as doublers to allow for greater stability and to cap off the ends.
  • MIG Welder
  • Paint
  • Hand held electric grinder
  • Jackstands or ramps - If you are doing this alone (not recommended) you need a couple of extra jackstands or jacks (bottle jacks are best) to hold them up against the car while you fit and weld.
  • Floor jack
  • Bandsaw, or cutoff saw, or hand held cutoff tool (die grinder with discs), or even hacksaw if you are desperate - You need something to cut the tubing.
  • Soapstone or marker - to mark the pieces for cutting.
  • Tape measure
  • And of course, eye protection!
    •

    Nice to have:

  • A helper - This is really a need but you could make do.
  • 3M weld-through coating. Makes a zinc coating where you weld. (Really a need, but I guess you can do without.)
  • Air compressor
  • 90 degree die grinder with sanding discs for cleaning
  • Camera - to show off your creation (I might even put it here if you do something creative)
    •

    The first thing is to get the car up off the ground. If you are using jackstands, it is imperative you place them under the rear axle and front A-arms and the car is as level as possible. This puts the car under a similar load as when sitting on the ground as all the weight is transferred through the suspension. Just make sure they are solid at ever place. Next, get under the car and survey the situation. You will see that the transmission crossmember is connected to the front subframes. You want to start the connector 1/4" or so from behind that point unless you are doing the full length design. Make note of the locations of brake lines and/or fuel line now. You obviously want to avoid these items while welding. In the rear, examine your torque boxes. Make note of all the seams around this area and check them for tears, bending, or any obvious distortion. If they have been badly abused, you might consider going ahead and reinforcing them while you have the car in the air. At least weld up all the seams if you can't do some reinforcements at this time. Chris Neighbors and myself are planning a good writeup on reinforcing this area for the near future. The rear of the connector will attach to the bottom of the large flat area in front of the notch for the lower control arm. I left mine about 3/4" forward to allow the lower plate that will be fabricated for the reinforcement of the torque box to be attached a little more solidly.

    WARNINGS

    Before you do any welding consider how close you are to the interior of the car. If you care about it at all and want to forego the winter wonderland effect left by the fire extinguisher, remove the seats and peel back the carpet where you weld. Have someone keep an eye out for too much smoke and/or flames. This is especially true if you weld the seams on the torque boxes. The metal at this point is thin to say the least. I left my carpet in for the entire subframe connector portion, but melted some of the padding when the front plates were welded at the top. Be safe...peel yours back.

    Also, before doing any welding on a vehicle, you should disconnect the grounds from all sensitive electronic devices: battery (to isolate alternator), EEC computer (if applicable), stereo amplifiers, timing/nitrous control/BTM's, etc. Welding on a vehicle with the grounds attached may create a ground fault loop that could send a spike to theses devices, causing damage and erratic function. 

    Take the time to clean all the areas where welding will be done down to the bare metal. You can use a variety of tools, but I found a 90° die grinder with sanding discs work very nicely. I would caution against a stone type grinder as the metal isn't overly thick in most places you will be cleaning. Better to use something that will only remove the surface stuff and not the surface too. When everything is clean, spray the 3M weld-through coating on all bare surfaces and allow it to dry.

    To help you create a plan, here are some pictures of mine after they were finished:

    Please, no laughing at the exhaust welds...:)

    Here are some closeups of the front and back:

    So now you should have an idea where to clean as well as some idea of how the end product will look. By all means, go ahead and do it 'your way' if you think you have a good idea. This is just how I did mine and certainly not the last word in subframe connectors.

    Click below for part two:

    Part 2