MM News

Make Driving Fun! Just Be Sure Your Chassis Can Take It...

Topic #3 from the "Make Driving Fun" series of MM newsletters.

Welcome to the third installment of our series. Read on for more expert advice on transforming your Mustang into a high-performance handling machine. This week's topic is the importance of chassis stiffness. (If you missed the previous episodes, they're posted on our website in the FAQs & Tech Tips section.)

You might have heard that increasing the stiffness of your Mustang's chassis is the first modification you should do. Is that a myth? No! That's absolutely true for Fox and SN95 Mustangs. The S197 and S550 Mustang chassis are much newer designs and much stiffer than the previous generations.

Quick notes so we're on the same page: the "chassis" is the welded unibody structure that everything else is bolted to. The "suspension" is all the parts that move as they connect the tires to the chassis.

OK, so why is a not-stiff chassis bad?
  • A flexible chassis deflects from suspension loads. In other words, when the chassis flexes, it becomes part of the suspension, but it's uncontrolled; it acts like a fifth suspension spring, but one without a damper to control it. The result is bad ride quality.
  • The suspension is way less effective when mounted to a chassis that deflects. The deflection takes the place of spring movement, so your carefully chosen springs can't do their work, which also messes up handling and ride quality.
  • Chassis deflection under load literally changes the suspension alignment. This means that at exactly the worst possible time, your tires stop pointing in the direction you set up with that expensive precision alignment.
  • Chassis deflection almost always results in more NVH (noise-vibration-harshness) inside the car. When plastic panels are attached to moving parts, they make a racket as they move back and forth relative to each other.
  • Mustangs with tons of power can suffer significant body damage because during a standing-start launch the entire chassis will twist as it reacts to driveline torque. Repeated twisting leads to fatigue cracking of the unibody structure. Ouch.
  • Even Mustangs with near-stock power levels get cracks in the floorpan just from spirited cornering and acceleration on the street, since the front seats' rear mounts don't have good support.

We see this kind of stuff all the time! And that's why you really need to stiffen your Fox or SN95 Mustang chassis. In a nutshell, a stiff chassis provides a firm foundation to support the suspension, which lets the springs and dampers do their job, which leads to better handling and better ride quality. Take it from us, over many years of racing and, er, "spirited" street driving experience, we've learned it's critical to stiffen the chassis when you make the suspension stiffer (i.e., higher rate springs and matching dampers). A stiffer suspension puts higher loads into the chassis so it will flex even more. And as we've discussed, this is bad.

Well, how do I make my Mustang stiffer?
This answer has some parts to it, which we've put in order of importance for you.

Subframe Connectors
Start with installing MM Full-length Subframe Connectors. On Fox and SN95 Mustangs, the unibody has a front subframe and a rear subframe. What connects the two ends of the car is the floorpan under the front seats. This is not a stiff structure! While MM didn't invent the first subframe connectors for Mustangs, we did invent what has been the gold standard for over 20 years: full-length subframe connectors. We even coined the name. MM Full-length Subframe Connectors join the Mustang's front and rear subframes with a strong and stiff rectangular tube, welded in to become a very stiff part of the unibody structure. Braces reinforce the front seat mounting points. Others have copied the MM design, but they're only copiers, not design engineers, so they don't understand the intricacies involved. The result is like any knock-off: it might look decent, but you'll always know it's not the real thing.

The feedback we hear most often from people after they've installed MM Full-length Subframe Connectors is usually something like, "My wife could tell the improvement from the passenger seat!"

The 2005 to current Mustangs have a completely different unibody chassis design. Ford finally stepped up and created a far stiffer chassis for the S197 and S550 generations. The front subframe extends rearward past the front seats and only ends where the mid-mounted fuel tank is located. If you look under the car, you won't see a rear subframe like the 1979-04 Mustangs have. There isn't anything back there for a subframe connector to attach to. In fact, the stock S197 and S550 have chassis stiffness on a par with a Fox/SN95 Mustang after adding all the chassis stiffeners you can imagine!

Strut Tower Brace
Install an MM Strut Tower Brace. Stabilizing the front strut towers to prevent chassis deflection during cornering and braking keeps the front struts in position so they can work correctly. Deflection that moves the strut tower also changes the front suspension alignment, which negates the effects of your great front-end alignment. The tire's contact patch on the road surface is compromised, reducing tire grip, stability, and cornering ability. All of these things, as you may guess, are bad.

MM's Strut Tower Braces for Fox and SN95 Mustangs have the typical brace between the two strut towers. Well, it's actually not typical. The typical strut tower brace has a 1.0"-diameter round tube. We use a 1.25"-diameter round tube. Why? Because it's nearly twice as stiff as a 1" tube. MM strut tower braces also feature braces that connect to the strongest part of the firewall, the pinchweld seam. This improves stability during hard braking, and is especially beneficial in reducing dashboard vibration and noise in convertibles. If you ever want to listen to the radio or minimize squawks from the passenger seat area, this will help a lot.

K-member Brace
Install an MM K-member Brace. The K-member mounts the front suspension, transfers suspension loads from the front control arms into the chassis, and supports the full weight of the engine. So yeah, it's pretty important! If the K-member deflects, it will change the front suspension alignment and thus skew directional stability. A brace from one side of the K-member to the other side stabilizes the mounting points for the front control arms.

Not coincidentally, MM makes braces for the stock Fox and SN95 Mustang K-members. Some SN95 models were originally equipped with a 2-point brace; MM makes a stiffer 4-point version.

Aftermarket K-members
If you read the marketing hype for aftermarket K-members, you'll notice there's a focus on weight savings above all else. We think those manufacturers must have forgotten that the K-member has to support the engine's weight while also carrying front suspension loads—and do so without allowing deflection. So when it comes to supporting the engine and carrying suspension loads, it's true: MM's engineering team knows a certain amount of steel structure is required. But we also know weight matters, too!

Other than needing a lower brace from side to side, stock K-members are reasonably stiff. Their drawbacks are weight and compromised suspension geometry for high-performance driving. When we designed our own K-members, the primary goal was to maintain strength and stiffness while improving suspension geometry. Our second goal was to reduce weight. And while we achieved both goals, we still found that adding a lower K-member brace makes a noticeable improvement in handling. That's why we include the brace. Most aftermarket K-members have no provision for a lower brace, and their performance suffers for it.

Roll Bars
While designed to improve driver safety in a crash, roll bars are also excellent chassis stiffeners. However, they do compromise interior room and daily usefulness for a street-driven Mustang. They're usually only installed when a driver needs either the safety improvement and/or is willing to compromise some comfort for the improved handling that comes with a stiffer chassis.

Thanks for reading. We hope you understand now that when it comes to your Mustang chassis, stiffer is better! Stay tuned for next week's exciting topic about lowering your Mustang, "How Low Can You Go?"