Wes Duenkel
September 1, 2009

We racers are driven by numbers. How much quicker are the e.t.'s? What's the horsepower improvement? How many g's does it pull? In today's world, no one trusts the 'ol seat-of-the-pants feel or "butt dyno." We want hard facts.

For those who grew up with Fox-body and SN-95 Mustangs, we've learned that adding subframe connectors improves chassis integrity and provides a solid foundation for further performance enhancements. Subframe connectors were often the first or second thing any Mustang owner did, especially if you planned to use sticky tires or make many modifications. After all, subframe connectors make the car "feel" stiffer than before, and they're necessary on those flexible-flyer Foxes.

Frame connectors are a must in any Mustang that will see hard use on the street or the track. Maximum Motorsports' connector kit comes with all the parts needed for a quality install.

Being the fact-driven racers/editors that we are, we wanted numbers. We wanted to quantify the rigidity improvements subframe connectors offer, so we designed a testing plan, found a test car, and sourced one of the best subframe connector kits available.

Subframe connectors stiffen the chassis two ways: They join the front and rear subframes with a solid tube, and they support the floorpan at the rear seat mounts. Tying the subframes together supposedly reduces overall chassis flex, and supporting the floorpan reduces seat-mount wobble, which is a big problem in high-mileage Fox Mustangs. The idea being that you should get the connecters in there before the stress takes its toll. Thus, our test consisted of stressing a Mustang chassis three ways-bending the chassis, twisting the chassis, and flexing the floorpan-all to see if the connectors could quell the flex.

Over time, your Fox Mustang can develop cracks in the floor due to stress of the unibody. Shown is a severe case found in a 10-second drag Mustang.

How exactly can we measure chassis flex? Since we don't have expensive equipment such as a multi-post shaker to simulate driving conditions (it's used by the manufacturers and many NASCAR teams), we had to improvise.

Ever notice when you raise a Mustang with a floor jack that the doors don't open or close well? It's because the chassis flexes when it's not supported at each corner. After installing subframe connectors (especially on Fox Mustangs), the doors often open and close much better-and it's even better when a quality rollbar or cage is installed. With this observation in mind, we decided to compare the change in door misalignment before and after installing subframe connectors, while bending and twisting the chassis. To measure floorpan flex, we would measure the change in distance between the seat headrests while jacking up on the rear seat mount.

We mark our pointer's position with a pen on a strip of tape and the chassis at rest on level ground.

Some of you may be wondering why the chassis flexes. This occurs during many circumstances, such as when the vehicle hits bumps or imperfections in the road, when the engine applies torque to the wheels (and subsequently the suspension), and during cornering, to name a few instances.

With a testing plan finalized, we identified a test subject. Instead of selecting a notoriously flexible subject (such as a Fox convertible), we chose one of the stiffest platforms: an '03 Cobra coupe. Our rationale was that if we found any improvements on the newer, low-mileage SN-95, there would definitely be improvements on older, more flexible Mustangs.

We then selected a Maximum Motorsports subframe connector kit. Never ones to leave well enough alone, the folks at Maximum Motorsports updated its full-length subframe connectors a few years ago for more weld area and rigidity.

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!"

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