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Street Car Setup: Solid Rear Axle vs Independent Rear Suspension

Street Car Setup: Solid Rear Axle vs Independent Rear Suspension

I am a big classic car enthusiast, but there’s really no denying that most modern designs and methods reign triumphant in comparison to the ‘old ways’. We can look at evidence that engine blocks are of much higher quality. Computerized systems are more accurate than mechanical. And the structural integrity of a modern vehicle versus a classic vehicle is almost no competition. However, when we look at something like an independent rear suspension (IRS) versus a solid rear axle, we can’t necessarily say that new trumps old. At least, not all the time.

For example, look at modern muscle. The Mustang, Camaro, and even the Challenger all come standard with independent rear suspension. But despite the fact that this technology is being used to improve handling, many builders opt to convert to a solid rear axle. So, what gives? Well, to better understand we have to break down both systems and how they work in real-world scenarios.

What’s the Difference?

For our purposes, we’re going to stick to rear-wheel drive performance vehicles to keep things simple.

Solid Rear Axle

A solid axle is what most of us are used to in the classic car world. The Ford 9-inch, GM 12-Bolt, and Mopar 8 3/4-inch differentials are all examples of solid rear axles. The axle housing itself uses solid tubes that run the full width of the vehicle and both wheels will be mounted to the axle assembly. The axle assembly is then held in place by ether leaf springs or a 4-link system.

A solid axle is an extremely simple setup and was the go-to for muscle cars for a long time. This system is very rigid and very durable, which makes it ideal for cars with high amounts of horsepower that are heading in a straight line.

Independent Rear Suspension

At the heart of an independent rear suspension is an axle that is totally unique to the setup. A solid axle is much simpler and generally uses fewer parts, while the independent rear ‘axle’ is a bit more complex. With an independent rear suspension, the differential housing itself will only hold the ring gear and differential carrier. The axle housing will be mounted into the vehicle with the use of what’s known as a cradle. This cradle is also where the control arms of the rear suspension will mount. Constant velocity axles are used and have no housings covering them; they also feature joints to allow them to articulate. This is truly what allows one wheel to travel up and down without affecting the other and is what defines an independent rear suspension.

These units have been used on modern vehicles because it allows the wheels to move independently of one another, which is ideal for performance in turns or on uneven surfaces.

Real World Results

This freedom, wherein the wheels can move without affecting one another, makes for some major advantages while driving. It also makes it apparent why this setup is being used on modern cars.

A solid rear axle has the tendency to body roll. When the wheel on one side rises or lowers, the corresponding wheel will do the opposite, limiting traction. With an independent rear suspension, if one wheel is moving up and down, it does so without affecting the entire vehicle. This translates to better all-around control over the vehicle, as the weight is under better management. Turns, bumps in the road, and uneven surfaces will have less effect on driving.

What does this all translate to for drivers?

If an independent rear suspension helps with handling, why even visit the topic? (If it ain’t broke then don’t fix it, right?) While there are major drawbacks to a solid rear axle, such as body roll, IRS isn’t perfect either.

When it comes to front engine-mounted cars, a solid axle does bring some major advantages to the table. Even on road courses, it is noted that solid axle front engine-mounted cars will keep up with independent rear suspension because of the weight ratio of the vehicle.

“We often see solid-axle cars keeping up with, or even outrunning, their IRS-equipped competitors,” writes sports car magazine, Grassroots Motorsports. “Why? The reason is simple. An independent rear suspension doesn’t necessarily perform better than a well-designed solid-axle setup. This is especially true for vehicles with a lot of front weight bias, like a Mustang. IRS does work well on mid- and rear-engine supercars since their back tires don’t need as much additional weight to put down the power. Even a Corvette gets pretty squirrelly when it has a 52- to 58-percent rear bias,” the publication explains.

Launching, in particular, can be dramatically improved with the use of a solid rear axle. With an independent rear suspension, the wheels are essentially held in place by the suspension, whereas with a solid axle, the axle housing works as a brace. Because of this, wheel hop can be an issue with an independent rear suspension during high power launches. This isn’t to say that it’s always an issue, but with a dedicated drag car, it is why builders make the switch.

Daily Driver or Dedicated Racer?

Deciding on making the swap is something that needs to be calculated carefully. It really is something that depends on the true intended use of the vehicle. When it comes to a car used primarily for daily driving with occasional track time, it’s better to leave the independent rear suspension under the car, building upon it to improve the characteristics you’re seeking.

However, research of today’s options on the market shows us that not everything comes down to a simple list of pros and cons. As Grassroots mentioned, a rear solid axle setup in a car with a front-mounted engine can be built well enough to handle. And there are independent rear suspension setups in use on cars powerful enough to launch with a wheelie. Ultimately, it does come down to what you have to work with out of the gate. And, of course, how much you are willing to spend.

It’ll Cost Ya

There are a lot of guys who are looking to swap from independent rear suspension to a solid rear axle. We mentioned that the solid axle setup is simple, but this doesn’t make it cheap. A swap kit will run around $5,000. This doesn’t cover labor fees for installation and, when you consider that the vehicle will likely need to be altered because it was designed to mount an independent rear suspension setup, you can wind up spending hundreds, if not thousands, more.

This doesn’t mean that opting for an independent rear suspension is always a cheap option either. One example I found of an extremely beefy rear end setup was for a late-model Mustang with a Ford 9-inch independent rear suspension by Watson Racing. Durability and control in all scenarios is provided with a setup of this caliber, but it sells at over $9,000. This isn’t to say that modifying the factory setup isn’t an option, but it just goes to show the extent of expenses.

Ultimately, as with any modification or upgrade, it’s important to know what you’re getting into. Do the research, talk to the experts, and make an informed decision based on your driving needs. 

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