Ever since 18th-Dynasty Egyptian Pharaoh Akhenaten drove one, the Mustang has been known for its use of a solid live axle at the rear. The decision to keep the Mustang with a solid axle instead of independent rear suspension has been causing heated arguments for years. But now, the Mustang will go IRS. So what's it all mean, anyway?
For many of our regular readers, this will be old hat; the live axle/independent rear suspension debate is a motoring classic, especially in reference to the Mustang. In fact, outside of trucks and other cargo vehicles, the current Mustang is likely one of the last mainstream cars you can buy with this setup.
Detractors of the solid live axle usually call it primitive and crude, caveman's suspension setup. There's something to back this up, as it is a very old basic design. Of course, these people are often called "snobs" by live axle supporters, and there's likely a little snobbery going on as well.
Solid rear axle supporters point to the setup's ruggedness and ability to take massive amounts of power, as well as its relative inexpensiveness compared to IRS setups. And there's certainly truth to these claims as well.
So, before we really evaluate what going to IRS means for the Mustang (though, admittedly, they did offer IRS in the 1999 Cobra), let's look at both suspension designs:
The traditional Mustang rear suspension goes by several names — live axle, solid rear axle, beam axle, and so on. The main defining trait about this setup is that there is an actual physical axle connecting both rear wheels. They're joined both out of a common love of spinning around on the road and physically. This means if one wheel hits a pothole on one side, the other wheel will feel it, and move accordingly.
On a hypothetical, super-smooth track surface, this may hardly matter. But in the real world, roads and tracks are imperfect, and you could easily be in a corner, hit a bump on the inside wheel, and then the outside wheel hops as well, potentially breaking contact with the tarmac, loosing grip and, as a result speed.
There's also the issue of unsprung mass. With a solid rear axle, that whole axle (and usually the differential as well) is not sprung — that is, it's not part of the car's mass that the suspension system is supporting. That means that the full mass of all those parts is affected without the benefit of suspension springing when bumps are hit or weight loading is changed.
This can put more stress on the wheels and tires to absorb that energy, which can increase wear and heat, reduce contact with the track surface, and make control and handling harder in general.
Now, a live axle is widely considered less capable for track driving and cornering. In motorsports that don't care about turning — drag racing — live axles have some real advantages.
They're inherently burlier, simpler, and beefier designs, which means an ungodly amount of power can be pushed through a live axle setup. The same goes for load-hauling, which is why cargo vehicles often have a live axle.
There's also a big cost advantage. The story is that the Mustang was orginially supposed to have an IRS setup from the start, but Ford's legume tabulators found they could save around $100/car with a live axle, so in it went. For Mustang owners, this is not always a bad thing, as repairs from accidents or competition are less of a hardship to endure. That can be a big deal.
For the Mustang, the live axle choice helped define what sorts of competition the cars usually were used for — drag racing. Sure, Mustangs certainly did and still do get tracked with often excellent results, but the native sport of the wild Mustang does seem to be the 1/4 mile.
When I tested the GT500 on Road Atlanta, it was clear a lot of very advanced technology had gone into making a live axle that could corner pretty well. But it was also admitted that the overwhelming majority of competition that these cars would ever likely see would be on a drag strip.
There's a vast amount of designs for independent rear suspensions, but the key advantage they all have over beam axles is that what happens to one wheel does not have to affect any other wheel. If you're in a hard turn and the inside wheel runs over an old portable typewriter, that outside wheel doesn't even have to notice anything at all.
This improves both handling and ride quality, and for most companies, the advantages are significant enough to make up for the increased cost, complexity, and (to some degree) fragility. Unsprung weight also drops since the differential can now be connected to the frame of the car and as a result, part of the sprung mass.
The actual IRS setup that Ford will be employing on the 2015 Mustang will likely be this aluminum-heavy unit found by DrivingEnthusiast Blog in a dumpster way back in 2011. It appears to have been a stress-test unit, which is why it was discarded.
It appears to be a pretty well-designed unit, with some compromises made to fit into the modified current-gen platform that the new Mustang is built on. They found evidence of good anti-dive control (to keep the nose up under hard braking) and it all appears to weigh less (overall, not just unsprung) than the outgoing solid beam. We'll have to try it out before passing judgement, but if this is it, it seems promising.
So what's this mean in a more general way for the Mustang's character? There was a certain perverse sense of pride that the Mustang used to carry by having such a seemingly-archaic setup out back. The jeers of Clarkson and other alleged engineering snobs just hardened the resolve of the die-hards and their slightly irrational love of the big iron flagpole under their cars.
And, there were the very real cost and dragstrip advantages. You could run a Mustang with an engine powerful enough to break up God's own kidney stones without having to worry much about the rear axle shattering on you. And even if it did, it didn't mean a second mortgage.
I'm sure the new Mustang will be very capable on the strip or peeling rubber out of a Dairy Queen parking lot, but it won't really be the same sort of Thor's hammer it once was. Even so, the gains it will make for ride and handling should be absolutely worth it.
The new Mustang will undoubtedly corner and handle better in general, and it will open up European markets in ways it's never done before. Previously, a Mustang in Europe meant you were a attention-loving Americanopile. Now, the car may be taken as a serious competitor to sports coupés from almost anywhere in Europe.
Sure, some of the old brutal charm will be lost, but it's probably about time. I do think having an option for an old live axle might be an interesting idea, for the die-hard drag racers and nostalgists out there, but I wouldn't expect that.
Wheels long to be free and independent, after all.
The One Tweak To The 2015 Ford Mustang That Will Change It Forever