The Ford Mustang is one of the few vehicles that has used both a solid rear axle and an independent rear suspension in the modern era. For decades, the Mustang stood out as one of the only non-off-roading vehicles to feature a solid rear axle.
So what’s the difference between a solid rear axle and independent rear suspension (IRS)? Each is a viable way to configure the rear suspension, even though many consider solid rear axles to be outdated. In fact, both solid axles and IRS have pros and cons, depending on how you use your Mustang.
How Does a Solid Rear Axle Work?
A solid rear axle, or SRA, is also known as a live rear axle or beam axle. Solid axles are simple in design, consisting of left and right straight axles, plus a rear differential. The differential allows the wheels to spin at different speeds. Apart from that, the two sides of the axles are very much connected. If one wheel dips down, the other will rise up. Vibrations are also transmitted from one side to the other.
This robust construction makes solid rear axles stronger than IRS systems. The straightforward design also makes them inexpensive to produce.
A solid rear axle can use leaf springs or coil springs to absorb bumps in the road. Today, leaf springs are typically reserved for trucks and vans.
Which Mustangs Use a Solid Rear Axle?
Mustangs made from 1964.5 to 2014 use a solid rear axle set-up. The notable exceptions are the SVT Cobra models from 1999-2004, which used an independent rear suspension. These rare performance models are tough to find (and some have even been converted to solid axles). Another exception is the one-off 2018 Cobra Jet Mustang, which uses a solid rear axle despite being an S550.
The Mustang used a rear leaf spring set-up for the initial two generations. When the Fox Body launched, the Mustang swapped out its leaf springs for coil springs. This gave it a steadier and more predictable ride. Still, even with coil springs, the Mustang was often criticized for its “ancient” rear suspension technology.
Apart from trucks, vans, and SUVs, you won’t see many vehicles with solid rear axles these days. Even cheap economy cars today tend to use torsion beams, which are regarded as semi-independent. This makes the pre-2014 Mustang stand out as the go-to performance car for those that want a solid rear axle.
How Does Independent Rear Suspension Work?
Independent rear suspension is more complex than a solid rear axle. It uses disconnected wheels that are able to move independently of one another. This means that they can adapt to road imperfections without transferring motion from one wheel to the other.
An IRS gives a vehicle less unsprung weight, in part because the differential is not bolted directly to the axle. It’s usually attached to the subframe or chassis. This lower unsprung weight is another reason why IRS has better handling characteristics than SRAs.
Which Mustangs Use IRS?
Mustangs that are 2015 model year and newer use independent rear suspension. The aforementioned 1999-2004 Cobra Mustangs use IRS as well. While some enthusiasts decry the move away from a solid rear axle, it represented an important shift for the Mustang moving into the modern era. And besides, the Mustang has always been more of a sports car or pony car than a true muscle car.
Solid Axle vs IRS Mustangs: Pros and Cons
Though IRS is the more modern and advanced system, it does have drawbacks. There are certain applications where a solid rear axle is better.
Mustang Solid Axle Pros and Cons
Solid axles are excellent for straight-line acceleration, where lots of power needs to be transferred to the rear wheels. For drag racers, a solid axle is the better set-up. It will help you get power to the ground, meaning better launches. This is especially true with high horsepower cars.
However, solid rear axle cars aren’t very good through corners. This is because any imperfection on the road that upsets one side of the car will be transferred to the other. With vibrations and bumps transferred throughout the entire rear suspension, cars with live axles can feel unsettled through turns.
A solid axle vehicle also tends to have more body roll. This is because the rear suspension isn’t able to move independently on each side of the vehicle. With a solid axle, this roll also means a loss of traction, as both wheels are not getting equal grip on the road.
Mustang Solid Rear Axle Pros and Cons
|Excellent for drag racing
||Generally worse and less predictable handling, especially over imperfect road surfaces
|Better launches from a stop
||No camber adjustment in rear
|Simpler and more robust design
||More unsprung weight
|Easier to modify
||Transfers vibrations from one side of car to the other
Mustang Independent Rear Suspension Pros and Cons
Independent rear suspensions don’t suffer from the ride quality issues that solid rear axles do. This is aided even more by the series of bushings required to run an IRS set-up. They greatly reduce noise, vibration and harshness (NVH) entering the interior of the car, unlike a solid rear axle.
You also get superior cornering ability because vibrations aren’t transferred from one side of the car to the other. The rear end won’t be upset by bumps that would likely unsettle a solid rear axle Mustang. When the vehicle remains stable throughout the corner, you’re able to push the car harder while maintaining control.
If you’re using your Mustang as a daily driver, autocross car, or track car, you’ll probably favor an IRS. In fact, if you’re modding your Mustang specifically for autocross, you may even want to swap out your solid rear axle for an IRS set-up.
You can still use an IRS Mustang for drag racing. However, even with its well-designed performance suspension, good launches can still be an issue. There are modifications to mitigate wheel hop in your S550 that will make you much more competitive on the drag strip.
Mustang Independent Rear Suspension Pros and Cons
|Better overall handling, with more predictability
||Can be prone to wheel hop
|Adjustable rear camber
||Harder to launch from a stop and worse for drag racing
|Keeps both tires on the road over bumps
||Heavier and requires rear subframe
|Less unsprung weight
||Can’t take as much abuse as solid rear axle
Swapping IRS into Your Mustang
If you want to give your Mustang better on-road manners and handling, you can upfit it with an IRS system. The older, live axle Mustangs from before 2015 can be converted with aftermarket parts.
This offers a lot of benefits, especially for performance driving like autocross and track racing. Plus, it doesn’t have to be too expensive. There are already kits for SN95/New Edge (1994-2004) Mustangs, allowing a direct swap with everything in one box. Converting other solid axle Mustang models is definitely possible, but will require a bit more work.
Swapping a 2015+ Mustang IRS for a rear axle is extremely rare, but not unheard of. For those that are absolutely dedicated to drag racing, this is one way to give your pony the launch characteristics and durability you want.