With the exception of a couple years in the late '90s and early 2000s, the Mustang has always had a solid rear axle rolling off the showroom floor since 1964. This was partially what made the Mustang in its own pony car class, especially in recent years. Auto journalists for magazines like Motor Trend and Car & Driver have always said they were impressed with how well the Mustang’s solid rear axle kept up with the IRS-equipped Camaro SS in recent years.
Benchmarking the impeccable handling capabilities of the 2012 and 2013 Boss 302, Ford really stepped up their handling game with the unveiling of the 2015 Mustang. 2015 marks a stepping stone for the Mustang because it was the first year that independent rear suspension was standard across the board. Before 2015, the 1999-2004 Mustang SVT Cobra featured IRS, but that was specific to the Cobra.
Many have argued which may be better for the Mustang platform. For comfort, handling and overall capability outside of a straight line, IRS is said to be the better choice. However, from a drag racing perspective, a solid rear axle is the clear winner. So, with different hobbies, tastes, and professions of enthusiasts comes differing opinions on the Mustang's rear suspension.
Don’t worry, though! We’ll break it down for you to figure out which may be better to suit your driving style.
Mustang Rear Suspension: What’s The Big Deal?
You would think that we’re talking about more important engine horsepower and torque numbers with the way some critics rip apart the Mustang’s rear suspension. The reason why the rear suspension of the Mustang is such a big deal is that it's part of the equation that makes the Mustang America’s Pony Car. Ever since the 1960s, the Ford Mustang has been a light, nimble, rear-wheel drive, V8-powered vehicle that always took the crown, even if it may have taken a silver or bronze for horsepower or cubic inch numbers. The Mustang was an underdog, that always clawed its way to the top - and even today we see that with the 2012 Boss 302 Laguna Seca and the powerful 2012 Camaro ZL1.
In a MotorTrend Head2Head, Randy Pobst mentioned that “The power is going right to the ground and turning into acceleration. In fact, it has so much traction, it’s hard to get it sideways under power.” Which is quite impressive coming from a world-renowned driver talking about a solid rear axle against the ZL1 with magnetorheological dampers and 136 more horsepower. He added that he didn’t like hitting the curbs in the Boss, though: “I hit one and stayed off them after. It popped the car loose and made it jump sideways.”
The reasoning behind the car jumping sideways when hitting the curb mid-turn is due to the Mustang’s live rear axle. Due to the nature of a solid rear axle, any disturbance that is experienced on the inside wheel - such as a curb on a road course - is transferred to the outside wheel. In a situation like taking a turn, the outside wheels are holding the majority of the traction through that corner. If the rear outside wheel is disturbed, then it could cause unwanted oversteer, which is what Pobst explained in the Head2Head article.
What made the Boss 302 Laguna Seca so impressive is that even up against competitor vehicles with fancy magnetorheological dampers and more power, it held its own. On paper, the Boss 302 and most Mustangs are #2 to their counterparts, but inch their way towards the top. And that’s why the Mustang is America’s favorite pony car.
What Is a Mustang Solid Rear Axle?
Solid Rear Axle on 2013 GT
A solid rear axle, or SRA, is also known as a live rear axle. What makes a solid rear axle so special is its tried and true design. Ever since pony and muscle cars started making their debut in the '60s, solid rear axles have been helping them put the power to the ground. It’s a simple design consisting of a rear differential, left and right straight axles and that’s it - making it naturally stronger and able to take more abuse in certain situations such as drag racing.
On classic Mustangs all the way up to the 1978 model year, the Mustang solid rear axles used leaf springs to absorb any bumps in the road. From the Fox Body generation forward, Mustangs equipped with a solid rear axle used coil springs providing a steadier, more predictable ride.
The major problem with solid rear axles is in their design. It’s great for straight-line acceleration and when you need to throw a set of drag radials or slicks on there for maximum traction. However, once you start entering some corners, any imperfection in the road that upsets one side of the car will transmit to the other side. You can ask any road course driver: When the car is unsettled or thrown off balance, bad things can happen. Just like any Mustang part, there are pros and cons to each.
What Is Mustang Independent Rear Suspension?
IRS on 2015 GT
With a different goal in mind, independent rear suspension, or IRS, is best for taking corners and increasing overall ride quality. One of the biggest pitfalls of the Mustang in recent years, according to many automotive journalists, is the fact it doesn’t have IRS like all of its competitors. Typically worded like, “...given that the Mustang is still equipped with that ancient solid rear, it handles really well!” Don’t get us wrong, the 2012-2013 Boss 302 displayed some of the most advanced suspension setups the solid rear axle Mustangs ever saw - and put down the times to show it. But, come the 2015 model year, it was time for a change.
The biggest advantage of an IRS setup is increased cornering ability. If your car goes around a turn and the inside wheel hits a bump in the road, those vibrations won’t be carried to the other side and upset to the vehicle. If the vehicle is able to remain stable throughout the corner, you’re able to push the car harder and put down better track times. On top of all that, ride quality is drastically increased due to the series of bushings required to run an IRS set-up - the bushings minimize driveline noise, vibration and harshness (NVH) from entering the interior of the car, unlike a solid rear axle.
The Mustang featured an independent rear suspension system in select 1999-2004 SVT Cobra models and became standard across the board beginning in 2015.
Mustang IRS vs Solid Rear Axles: Pros & Cons
Depending on what you plan on using your Mustang for, an IRS or Solid Rear Axle could benefit your setup. For example, if you plan on frequenting the autocross or road course with your Mustang, then an independent rear suspension would be the ideal choice for your build. However, if you don’t care much for corners and are all about straight-line power, then a solid rear will help you put the power to ground in the easiest fashion.
The other side of it is whether or not the swap would be cost effective. If you own a 1999-2004 Mustang GT and are looking to make the switch to IRS, then the best possible situation would be to find someone with a 1999-2004 SVT Cobra who would want to swap to a solid rear axle for drag racing. Meetup over a weekend and get the swap done - easy!
On the other hand, with the 2015 Mustangs, most have been keeping the factory IRS intact whether they drag race or carve corners. This is because the S550’s independent rear suspension is much more advanced than any previous factory Mustang IRS system. With the right modifications, you’ll be able to really dial in that IRS.
Mustang IRS Pros & Cons
|Less unsprug weight
||Prone to wheel hop
|Better overall handling
||Harder to launch from a stop
|Keeps both tires on the road in bumpy situations
||Can't take as much abuse as a solid rear
|Ability to adjust camber for increased handling capabilities
||Heavier, requires rear subframe
|Best for street driving
||Sometimes harder to fit drag tires on rear
|More predictable handling
|Mustang Solid Axle Pros & Cons
|Best for drag racing
||Zero camber adjustment in rear
|Easiest to launch from a dead stop
||More unsprung weight
|Less prone to wheel hop
||Transfers vibrations from one side of the car to the other
|Simpler, sturdier design
||Not as good handling capabilities
|Tried and true for Mustang since 1964
||Less predictable handling
|Easy to modify
||Less weight and does not need a rear subframe
With the right amount of fabrication and ingenuity, you can IRS-swap almost any Mustang. There are plenty of Fox Body and even early model Mustangs out there with '99-'04 SVT Cobra Independent Rear Suspension setups. A great example being The Smoking Tire’s Matt Farah and his 1988 LX.
If you already have an SN95 or New Edge Mustang, then an IRS swap would be a bit easier since they’re already made for that particular body style. Typically, however, many people upgrade the bushings and other IRS components when swapping their solid rear out.
If you’re wondering why would one take the solid rear axle out of their Mustang in favor of an independent rear suspension setup, you’re not alone. The age-old debate between the two setups seems to be never-ending. However, the majority of the pros lie with an IRS. They not only ride better, but they also handle much better than a solid rear axle. Don’t get us wrong, though! A solid rear axle with the right modifications can handle quite well, but an IRS is much more capable. Essentially, the only place where a solid rear takes the crown is overall weight and straight-line acceleration.
Mustang IRS & Solid Rear Axle Parts
With the release of the S550 Mustang, the want for a stiffer rear suspension is apparent through many online forums and posts. This could be for various reasons - a couple including the fact that most owners are probably used to a stiffer ride from the previous solid rear axle generations, along with the fact that Ford designs these cars for a bit more comfort than performance. Afterall, the Mustang is a grand touring car which equates to horsepower and comfortable cruising.
Whether you want to upgrade your Mustang’s solid rear axle suspension or independent rear suspension, CJ’s has the parts you need to upgrade any Mustang suspension component on your early model or present model Mustang!
Image Credit: MotorTrend