Wes Duenkel
September 1, 2009
A dial caliper is used to measure the change in door alignment while the chassis is stressed.

Before the brown truck showed up, we made some baseline measurements. We placed two paper clips at the trailing edge of each door to act as pointers, and noted their positions with the car parked on level ground. To bend the chassis, we cantilevered the engine's weight over two jackstands positioned under the front subframe rails. We then measured the change in the pointers' position with a dial caliper to determine the amount of bending before installing the subframe connectors. We measured chassis twist similarly by noting the change in the pointer position, with the front-left wheel and right-rear wheel resting on wooden blocks, and the opposite wheels on the floor.

With the car back on level ground, we measured the distance between two points on the front seat headrests. Then we stressed the floorpan with a floor jack (which lifted the body a half inch), and remeasured the headrest distance with a tape measure. The results were rounded to the nearest millimeter (see Diagram 1).

To twist the chassis, we place the left-front and right-rear wheels on three wood blocks.

With our baseline measurements out of the way, we installed the Maximum Motorsports full-length subframe connectors. The instructions were well written and detailed, and the connectors fit perfectly. After trial-fitting the connectors and prepping the parts, we fired up the welder and got to work. The accompanying photos and captions hit the high points of the installation.

With the welds cool and the paint dry, we went for a test drive. The car certainly felt stiffer, especially when traveling over uneven pavement that rocked the car side-to-side.

Back in the garage, we repeated our chassis stresses and measurements for the "after subframe connectors" portion of the test. Surprisingly, the subframe connectors made little difference in our bending and twist measurements-the results were not significant and well within a reasonable margin of error (see Table 1).

The left rear wheel no longer touches the ground, meaning the suspension is twisting the chassis as much as possible, before we re-measure the door misalignment.

If we couldn't measure a change in chassis rigidity, then why did the car feel so much stiffer after installing subframe connectors? The answer lies in the subframe connector's seat mount reinforcement. A '79-'04 Mustang floorpan is notoriously flimsy. When driving a Mustang over rough pavement or around a fast corner, the driver's weight stresses the seat mounts, causing the floorpan to move, so much so that the floorpans of many Fox cars eventually crack near the rear seat mount.

The change in seat headrest distance from floorpan flex went from 6mm to 1mm after installing the Maximum Motorsports subframe connectors (see Table 2). That's a whopping 500 percent increase in floorpan stiffness! The substantial reduction in floorpan flex explains the "seat of the pants" improvement in stiffness that we felt after installing subframe connectors. Our butts were literally more firmly connected to the chassis of the car, making the car feel stiffer and relieving the stress on the floorpan. Apparently, the numbers agreed with our "butt dyno!"