What Is Unsprung Weight?

What Is Unsprung Weight?

Last Updated October 23, 2019
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There comes a point when any enthusiast is modifying their build where weight comes into play. For some Mustang owners, it’s early on that the weight of their Mustang plays a part in their next modification decision, and for others it’s farther down the line. This especially plays a factor if you enjoy taking your Mustang to the dragstrip or frequent the road course or autocross course on the weekends.

Weight plays a crucial role in how your Mustang will perform against the clock. That’s why you see racers out there with gutted interiors and no passenger or rear seat. Don’t get us wrong, there is a point where that may be necessary given the direction and how serious you’re taking your build, but there are many other areas to focus on before ripping your interior apart: Like unsprung weight.

What Is Unsprung Weight?

"...weight that is not supported by the suspension"

In the simplest of terms, unsprung weight or unsprung mass is the weight that is not supported by the suspension of the vehicle. This would include the mass of components such as the wheels, wheel axle(s), wheel bearings, wheel hubs, tires, portion of the weight of the driveshaft, springs, dampers and brake systems.

The majority of these components are all rotating in order to keep the car moving. If you think about it, power is sent through the crankshaft and transmission, then carried through the driveshaft, to the rear end and ultimately to the rear wheels. As all of these components need to rotate in order to transfer energy, the lighter that component is, the less energy it will take to turn it, meaning more energy is directed to the rear wheels.

The same thing goes from a handling aspect. If you have big, heavy OEM 20” wheels on the front of your Mustang, steering response is going to suffer as a result. This is due to the fact when you go to turn the wheel, the power steering needs to apply energy to turn big heavy brakes, big heavy wheels, and big heavy tires. Notice the emphasis on “big” and “heavy.”

Now, the out-of-the-box handling of today’s S550 Mustang is impressive, to say the least, however, it can be drastically improved upon just by adding a lighter set of wheels to the equation. Why do you think Ford Performance invested all of the R&D into lightweight carbon fiber wheels for the GT350R? In one of MotorTrend’s reviews of the GT350R, Randy Pobst said that the biggest difference he felt between the GT350 and GT350R was the nimbleness behind the carbon fiber wheels on the R.

Unsprung Mass vs Rotational Mass

There is a common misunderstanding between unsprung weight or mass and rotational mass or inertia. Without diving too deep into the physics behind both terms, rotational mass applies to the parts that must be accelerated or decelerated when the speed of the vehicle changes. This includes items like the driveshaft, brake rotors, wheels, and tires.

It is true that most rotating mass is indeed unsprung weight, as these parts aren’t supported by the vehicle’s suspension. In most cases, a reduction in unsprung weight generally implies a reduction in rotating mass, which in turn helps acceleration. To put it in perspective, rotating mass (such as a wheel or brake rotor) is roughly three times harder to accelerate than sprung weight (such as the additional weight of a passenger or rear seat).

This means that dropping just 10 pounds per wheel with the addition of a lighter set of racing-oriented wheels would equate to almost 120 pounds of sprung weight lost on the car, generally speaking. In most drag racing instances, a 100 pounds lost equates to a tenth or car length in the quarter-mile -- that truly adds up if you’re racing competitively!

Drop The Unsprung Weight

Now that you know the importance behind understanding unsprung weight, how it works, and how it’s hurting you much more than the extra weight of a rear seat is, it’s time to find the right parts to upgrade.

The most bang for your buck in dropping unsprung weight will come from a new set of wheels & tires. To give you an idea, many OEM wheels are in excess of 30 pounds each from the factory. Finding the right wheel upgrade can drop 10 pounds or more per corner. Ford Performance has some great wheel options for those looking to cut some weight and get more tire in contact with the asphalt.

On top of that, with the 2015+ Mustang GTs equipped with the Performance Package, you can drop 10 pounds per rotor by upgrading to the Baer Eradispeed-Plus 2-Piece Front Rotors! For those looking to cut more unsprung weight, adding aftermarket wheels with a reduction of 10 pounds per corner along with the Eradispeed-Plus rotors at 10 pounds per front wheel, you’ll then cut roughly the equivalent of 180 pounds of sprung weight from the car. Then, throw an aftermarket one-piece driveshaft dropping an additional 12 pounds. All things being equal, over 200 pounds of sprung weight would approximate to roughly two tenths or two car lengths in the quarter mile - and that’s a lot of time when we’re talking about quarter mile ETs!

What Is Unsprung Weight?

When it comes to modding your Mustang out, many get to the point where they’re happy with how the car performs both in a straight line and on the twisties. However, it doesn’t stop there. Next up is weight reduction, and there are two sides to weight reduction with distinct goals in mind: sprung and unsprung weight or mass. Being a very important key to how a car handles under high load cornering G’s, unsprung weight will become your worst enemy. Getting educated and using that knowledge to figure out what’s next for your build can get you to the top spot at your next road course session.

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