After five decades of Mustangs riding on solid rear axles, Ford finally revealed the all-new 2015 pony in early December with a standard independent rear
suspension system. With the griping from IRS fans finally addressed, what is Ford Racing rumored to be up to next? Why, a solid rear axle kit, of course.
Why on earth would Ford be developing a live axle for the Mustang at this point? Mainly to meet the needs of drag racers who spend most of their time
accelerating in quarter-mile bursts in a straight line.
Why a suspension?
The whole point of a suspension system is to enable the wheels and tires to move up and down, following the contours of the road while also keeping as much
of the tire patch on the pavement as possible during acceleration, braking, and cornering. Out in the real world, where cars occasionally have to
change direction, bodies roll and tires tuck under the wheel as a result of the loading.
That's why engineers have developed the crazy, complex linkages that comprise modern suspension systems. As the wheels move through their range of travel,
the camber, caster, and toe angles relative to the vehicle body are constantly changing with the aim of maximizing grip. In typical driving, this is all a
good thing, as it contributes to more stable and responsive handling.
The drag strip is a whole different world, with essentially nonexistent cornering loads but with power levels that can often approach 1000 hp or more,
resulting in significant weight transfer onto the rear axle. With an independent rear suspension, the added loading on the rear wheels will typically
increase the negative camber, meaning the wheels will tip inward at the top as the springs compress.
As the camber angle increases, the outer portion of the tire patch actually lifts up off the pavement, reducing the available traction for
acceleration. In drag racing, even the slightest loss of traction in the first few milliseconds off the line can make the difference between victory and
Back to truck axles
In 2011, when Chevrolet launched the COPO Camaro factory-built drag racer to compete against the extremely successful Mustang Cobra Jet, one of the first
things the engineers did was replace the multi-link independent suspension of the street car with a solid axle. Because the solid axle prevents the wheels
from moving in relation to each other, they remain perpendicular to the road at all times, with the entire tire patch on the track.
Back in 1999, when Ford introduced an SVT Cobra version of the Mustang with an independent rear suspension, the IRS was designed as a bolt-in module that
used the same body attachment points as the normal live axle. This meant that owners of any fourth-generation 1994-2004 Mustang GT could buy the IRS module
from the Ford parts catalog or a salvage yard and install it. Conversely, Cobra owners who wanted to go to the drag strip could also buy a live axle and
swap it for better straight-line grip.
Our sources tell us that's exactly what Ford Racing is preparing for with the new Mustang. Given the success that Ford has had selling turnkey Cobra Jets,
the Mustang body in white, and all the other parts needed to build a Cobra Jet, it would be reasonable to expect the same pattern to continue following the
production launch of the S550.
We expect to see an all-new 2015 Cobra Jet debut at either the SEMA or the Performance Racing Industry show in late 2014, and that car will almost
certainly feature a live axle just like the COPO Camaro. The same axle will probably be available as a bolt-in kit to be applied to existing or new-build
Don't expect many retrofits
Unlike the SN-95 rear suspension swap, the new one will probably be recommended as an off-road (read: as a drag racing) only setup. Aside from what is likely
to be a big step backward for handling and ride, a live axle on the new Mustang will also have the downside of extremely limited travel.
The floor pans of all previous Mustangs were shaped with a pronounced hump at the rear to accommodate the vertical motion of the live axle's center.
Because this car was designed for IRS, the rear area of the pan has actually been shaped for an axle that doesn't move. The upside is that the rear seat
provides more hip room and the trunk has a flat load floor, but there is nowhere for a live axle to move around.
For a straight-up racer like the Cobra Jet or the COPO, this won't be a problem, and drag racers will snap up the new axle kit. For the rest of us who want to
drive our Mustangs daily, this won't come across as an upgrade.