I am a big vintage car enthusiast and especially of the 60s muscle cars, but there is 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 comparison. When looking at suspension systems something like an independent rear suspension (IRS) versus solid rear axles, we cannot necessarily say new beats old. At least, not all the time.
For an 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 suspension handling, many custom car 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 is the Difference?
Solid Rear Axles
Solid rear axles are 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 rear differentials are all examples of solid rear axles. The rear 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.
Solid rear axles are an extremely simple setup and was the go-to for muscle cars and hot rods 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 (IRS)
At the heart of an independent rear suspension is an axle that is 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 is 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 Mean For Drivers?
If an independent rear suspension helps with handling, why even visit the topic? (If it is not breaking then do not 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. The reason is simple. An independent rear suspension does not 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 do not 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.
Launching, in particular, can be dramatically improved with the use of a solid rear axle. With an independent rear suspension, the wheels are 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 is not to say that it is 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 is 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. 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 will Cost You
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 does not make it cheap. A swap kit will run around $5,000. This does not cover labor fees for installation and, when you consider that the vehicle will 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 does not 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 sells at over $9,000. This is not to say that modifying the factory setup is not an option, but it just goes to show the extent of expenses.
Finally
As with any modification or upgrade, it is important to know what you are getting into. Do the research, talk to the experts, and make an informed decision based on your driving needs before making any modification to your rear axles.
