Ford Mustang - IRS vs. Live Axle: My Take

Background

Since its introduction in 1964, the Ford Mustang has been fitted with a live rear axle. The original plan was to have an independent rear suspension (IRS), but Ford discovered that $100 could be saved per car by using a live axle instead. The formula stuck, and up until the current S550 Mustangs, with a few exceptions, the Mustang has been fitted with a live rear axle.

My Knowledge

Having daily driven a 1999 Mustang with a live axle for the past year, I’ve got plenty of experience with live rear axles, from comfort, handling, performance; all the petrolhead criteria. I’ve also driven a 2015 Mustang with IRS, and although I only had a short time behind the wheel, I was able to get a feel for the differences between the cars.

Along with seat time in both, I have an understanding of the mechanical differences between the two, and how they work. By no means am I an engineer, but I have a decent enough understanding to talk briefly about each.

Live Rear Axle

Let’s start with the one more commonly known in the Mustang community: the live rear axle. So, in one sentence, what is a live rear axle? Well, the two rear wheels are essentially connected by an axle, and if one wheel does something, the other will do the same and react accordingly. This makes for some uncomfortable daily driving situations, creating a ‘hop’ effect when hitting potholes and bumps around town.

With the exception of some New-Edge Cobras, all older Mustangs come from the factory with a solid rear end. I’ve got countless hours of seat time in a 1999 Mustang convertible, and as it’s not powerful enough for suspension to make a drastic difference in a straight line, I figured I’d touch more on handling and comfort. The most noticeable thing with the live rear axle is when hitting potholes; one wheel hits, and because they are joined, the other wheel will react the same, even if it hasn’t hit the pothole. This is where the hop effect comes from. It is something that you get used to, but for the first time Mustang driver, it might come as a shock. In terms of handling, I’ve already accepted the fact that the Mustang handles like garbage; there’s body roll, steering is slow, and theres a lack of grip on the standard 205-section tires. However, on a bumpy, twisty road, it’s not hard to get yourself into a sticky situation. Hit one tiny imperfection on the road, and the car can hop, sending you into a slight slide that you really weren’t expecting.

It’s not all bad news when it comes to live rear axles though. They really shine in a straight line, and are much more favourable for a drag-setup car. In fact, when the 2003 Cobra was launched with IRS, some people swapped for a live rear axle, simply because with such a potent motor, there was more time to be made up with the stronger, beefier live rear axle.

Independent Rear Suspension

As a car reviewer, I’ve driven loads of cars with independent rear suspension, but I’ve only had about 20 minutes behind the wheel of a Mustang with IRS. While it was a short amount of time, the differences in my mind were so night and day that it could be felt right away.

The first difference was the comfort, and the absence of the ‘hop’ that you get with a live rear axle. While I’m used to the hops of the live rear axle, I will admit that it isn’t comfortable at all, and getting into something with IRS, the difference can be felt right away. The reason for this is because the wheels are not connected by an axle. Instead, they operate separately, and can take on their own separate functions; when one hits a bump, the other doesn’t take on the same action.

The next big difference is partially down to the IRS, and partially down to 16 years between the two cars I’m comparing. The handling in the new Mustang is leaps and bounds ahead of the 1999. Like I said, this is in part due to 16 years of development in between, but the IRS is also a huge contributor. In the real world, roads are bumpy, and so are tracks. The IRS allows the rear tires to remain in contact with the road, where the live rear axle doesn’t. On a live rear end, if a bump is hit, both wheels can hop, resulting in loss of grip, and more importantly, speed.

The Conclusion

When the S550 was first launched, the inevitable thing to do was to put it against its predecessor, in a ‘Murican style 1/4 mile drag race. To the surprise of many, the 2014 was actually faster than the 2015, for one reason: IRS. While the 2014 did what Mustangs are known to do, go fast in a straight line, IRS turned to new 2015 Mustang into a proper, well-rounded, sports car.

I know I may be a little late to the IRS vs Live axle debate, but this is just my take. I don’t have a preference as I have so little time in an IRS Mustang, but I do lean towards the new setup. What do you think? Whether you’ve driven Mustangs or not, let me know! I’d love to hear what you have to say down below!