Tech Corner: The Basics of Chassis Stiffening

This isn’t the first time we’ve visited the idea of chassis stiffening. In the past, we discussed US Car Tool and the solutions the company brings to the table for unibody platforms. And while we covered several bases in that piece, we’re revisiting the topic today to highlight when and how you can reinforce the chassis of your vehicle, in both unibody and body-on-frame configurations.

A Brief Introduction

You gotta walk before you can run, so let’s briefly breakdown the differences between unibody and body-on-frame vehicles.

Body-On-Frame

Body-on-frame cars and trucks are technically the more traditional platform, in the sense that this is the original way of constructing vehicles. It consists of a solid frame, where the suspension and driveline live, and a separate body that mounts to that frame with the use of hardware.

Today, this platform is mostly reserved for true SUVs and trucks. But Chevelles, Impalas, and other popular performance cars were also built on frames.

Unibody

Alternately, unibodies use the entire body as a structural point. That means, the body and chassis are considered to be one unit. The unibody stretches to virtually every part of the vehicle from the front inner fenders to the roof and floor to the rear trunk pans. And while there are subframe rails, they are not independent of the unibody construction. Introduced in the 1920s, this style has become the dominant trend in auto manufacturing.

“Unibodies are more rigid due to the forces spreading over the entire vehicle. Because of this, they can also use less material and are lighter. This means better fuel economy, handling, acceleration, and deceleration. Body-on-frame vehicles allow for better NVH, as the body can be isolated separately from the frame. They are also modular, allowing for various bodies to use the same frame.”- Motorports.com

Why and How you Should Stiffen a Unibody Chassis

Now, the unibody may be the long-reigning king of performance vehicles, but this doesn’t mean it’s perfect. Especially, if you’re running a late ‘60s muscle car like the Charger, Camaro, or Mustang. Remember Vin Diesel’s line in the first Fast and Furious? “So much torque, the chassis twisted comin’ off the line.” Yea, well, these twists—along with turns and stopping—all wreak havoc on the unibody.

“Although it may seem unlikely for all that steel framework to flex, examples can be seen in the drag racing world. There, it’s not uncommon to see the chassis flex, lifting the left front wheel off the ground due to the extreme engine torque,” explains Roadkill Customs, an online resource and forum for back yard builders and do-it-yourself hot rodders. This flexing is detrimental to traction and reduces the vehicle’s ability to firmly plant the tires on the ground. Power is lost, the ability to turn is hindered, and even stopping power will suffer.

Now you have an idea of why you should stiffen the chassis. So how do we go about doing it?

Unibody chassis stiffening can be done in a number of ways.

The most common upgrade is to use subframe connectors. You have tons of producers of these units including Competition Engineering, US Car Tool, Hotchkis, and so on. These units tie the front and rear subframes together with a simple and clean installation that instantly stiffens up the entire car.

Another way is to relocate the factory leaf springs, if they are present. Stiffer radiator supports and torque boxes, strut bars and fender ties are also added to help reinforce weaker points of the unibody.

“Different manufacturers deal with the chassis in different ways,” adds Roadkill Customs. “A car with a very stiff chassis and a relatively soft suspension will ride similarly to a car with a flexible chassis and stiffer suspension. In the latter case, though, the chassis is performing a similar function as the springs, albeit undamped with shocks, which can create unpredictable handling in hard cornering.”

This makes it clear that not every car is going to require the same level of detail or upgrades when it comes to reinforcing the chassis. As always, do your research. Find the weak points and stressors of your particular vehicle to determine which mods are necessary for your build.

Why and How you Should Stiffen a Body-on-Frame Chassis

Chassis-flex is not limited to unibody applications. Body-on-frame vehicles suffer too. Now, with rugged trucks and big SUVs sitting on solid frames, it can be easy to assume that frame is more structurally rigid. But this isn’t always true.

A frame has a much smaller profile than a unibody, so flex can actually be more of an issue in the right application. Body-on-frame chassis stiffening in beneficial in all the same ways as it is for a unibody—standing up to engine torque and keeping those tires planted—but, it’s done in a different way.

Frame braces are designed to fit underneath the vehicle and tie the frame together. This gives it more support across multiple planes. While this solution is usually done on cars, truck owners can use these mods to their advantage too.

Frames are built using a C-channel design, which means there is a huge open portion of the frame. This design has its drawbacks, as it still allows for the frame to flex and roll. Therefore another method of stiffening body-on-frame vehicles is to box the frame. Oftentimes, you will find suppliers like Speedtech offering frame stiffening kits with frame braces and boxing in one kit.

A Bit on Roll Cages

For those building an all-out performance vehicle, a roll cage will almost always be a part of the build. But while adding safety, it comes at the cost of adding a significant amount of weight. One hand washes the other. But what many may forget is that the addition of a roll cage is also going to stiffen the chassis. This is true regardless of whether you’re running a unibody or body-on-frame.

“Drive safety overall is the most important thought when it comes to cage design,” says Kent Porter of Precision Chassis Works. “It’s a tricky dance between additional tubing for the sake of safety/chassis rigidity, and adding weight. But don’t just add tubing for the sake of adding tubing. Try to justify every additional tube that is added to the car before it’s welded in. Are you okay with adding a few pounds to the car to make it stiffer in critical areas, or possibly to make it safer in the event of a crash? These are things that each driver must decide for themselves.” -Speedhunters

In some instances, adding a proper roll cage can reduce the want or need for other chassis stiffening upgrades. However, in the case that you are trying to make the car as tight as possible, you will want to go all out. Remember, safety first.

How Stiff is Too Stiff Though?

So, I’ll admit I’m a bit of a fanboy when it comes to reinforcing the chassis of a car—especially older models. I think I get really over-zealous about the concept because it is something that’s left to the wind in many builds. But the truth is, there are some drawbacks when it comes to using reinforcement. And there can be such a thing as “too much” in a given circumstances.

In fact, the growing YouTube Channel, Uncle Tony’s Garage visited this topic for unibodies and how reinforcing the subframes with connectors can actually be harmful to the body of the vehicle. (He explains that body-on-frame vehicles aren’t subjected to the same issues.)

Check out the video below, as his explanation is spot on.

Regardless of how you feel about chassis stiffening, you’ll need to consider comfort at some point when it comes to a daily driver. Yes, performance goes up. But, by reducing flex in any portion of the vehicle, you’re reducing how pleasant the drive is. This might not be a concern for many enthusiasts. However, it’s still something those who like a luxurious ride should be aware of.