2015 Ford Mustang GT: Suspension Walkaround
June 29, 2015
Automotive journalists have griped about it for as long as I can remember. Some worked to retirement as generations of new Mustang came and went with a simple straight rear axle. Along the way Dodge and GM abandoned the pony car segment, leaving the Mustang to go it alone against imported sports coupes with independent rear suspension (IRS).
And then the dormant Dodge Challenger and Chevrolet Camaro were resurrected in 2008 and 2010, respectively, and both reappeared on dealer showroom floors with IRS setups. The market pressure ratcheted up yet another notch.
It eventually became clear that the fully redesigned 2015 Ford Mustang would finally enter the modern age. It would have independent rear suspension, for real, across the board. And this would not be another half-baked 1999-2004 SVT Cobra scenario, a low-volume Mustang special with a compromised IRS system wedged under a car never designed for it.
We wasted no time when the order desk opened and put money down on a 5.0-liter GT with the Performance Pack. We have not been disappointed. Our 2015 Mustang GT is impressively composed on the road, and I for one was thoroughly blown away by its balance, grip and stability when I thrashed it about on a tight, bumpy autocross course and a high-speed racetrack on the same day.
But it's not all down to the long-awaited IRS. The front suspension of the 2015 Mustang has a few tricks of its own. Let's hoist it up on our Rotary Lift and work our way front to back.
It's hard to make out much from here except the top end of a MacPherson strut and a humungous 6-piston Brembo brake caliper.
This flattened area on the outside of the strut tube is there for tire and wheel clearance. But it also tells us something else: this is a twin-tube damper, not a monotube damper.
From below we can see the biggest change to the 2015 Mustang's front suspension. Gone is the one-piece lower wishbone with its single ball joint. Instead, there are two slender links, each with its own ball joint.
Why is this significant? Imagine the more common single ball joint. The joint itself is the lower pivot point for the steering. You want to put it as far out as possible, but you can only go so far because of the brake rotor.
You need two points to define a steering axis, of course. The upper pivot resides under the hood at the center of the upper strut mount.
Now imagine the steering axis line extending through the ball joint to the ground. The offset between the point where this imaginary line strikes the ground and the center of the tire's contact patch is the scrub radius. You don't want this to be very large. We're talking millimeters. Zero would be nice.
But what if you want to fit wide front tires to maximize grip? Maybe you also want powerful brakes with massive calipers and thick rotors. The strut tube is the limiting factor for tire and wheel clearance on the inside, so you can only add the extra width you need outside.
Thing is, your single ball joint is trapped behind the rotor and can't come along. The contact patch moves away from the steering pivot axis, resulting in a larger scrub radius that can worsen stability under heavy braking and screw up your steering.
That's where these dual links come into play. Here the lower steering pivot point is not constrained by the brake rotor because it is a virtual point located where the links would intersect if they were extended.
The wide-angle distortion of this close-up makes it hard to pin down, but that point on the 2015 Mustang exists within the space occupied by the massive brake rotor, a physical impossibility with the single ball joint used on past Mustangs.
Three lobes give away the three outer pistons of the 6-piston Brembo brakes, which are exclusive to the 5.0 GT with the Performance Pack. The standard 5.0 GT and the 2.3-liter EcoBoost with the Performance Pack get 4-piston fixed-caliper brakes instead.
Track-day fans will like the fact that this caliper uses an open window design. The pad extraction opening in 6-piston calipers like this is so tall that a bridge bolt (black arrow) is required to prevent the caliper from flexing during use, but it's easily removed when it's time to swap pads.
The front stabilizer bar (yellow) is hollow in the name of weight savings. The seriously flattened end where it attaches to the link is a good clue, but this isn't always a reliable indicator.
Tubular stabilizer bars don't give up as much roll stiffness as you'd think because torsional stiffness is proportional to the diameter raised to the fourth power. The metal near the outside is doing much more work than the metal near the center, in other words. You don't have to increase the outside diameter much at all to compensate for material that's missing from the center.
The effectiveness of the Mustang's front stabilizer bar is at a maximum here because the connecting link (black) attaches directly to the strut housing for a near 1-to-1 motion ratio with respect to wheel and tire movement.
Also new this time around is a full perimeter subframe that supports and reinforces the lower end of the front suspension.
As ever, the Mustang's longitudinally-mounted engine and rear-drive architecture make it easy to implement front-steer steering, in which the tie-rods and steering arms act on the wheels ahead of the ball joints.
Here it's easy to see the steering shaft (yellow) as is slides past the engine and under the engine mount. We lose sight of it behind the cross-member, but we can nevertheless glimpse the bulge in the steering gearbox housing (green) that contains the pinion it connects to. It's easy to imagine how twirling the steering wheel will rotate the pinion, which in turn advances the rack and tie rods (white) left and right.
We've seen this view before, but I replaced a strategically-shaped air deflector (yellow) that was blocking our view of the works.
Now you can see how cooling air from a duct above the front bumper's splitter is channeled toward the brake rotors.
We're not seeing the tire in these images, but the point of the deflector is to direct air into the void behind the wheel and tire.
Finally we get to the main course of this exercise. But we can't see much from here except the top end of the shock absorber.
The first glance is an overwhelming one. There's a sprawling lower control arm, a toe-link (black) and lots of fiddly bits higher up. And there is an abundance of aluminum.
Clearly this isn't a rehash of the unloved and finicky double-wishbone setup that the SVT crew slapped under the low-volume Cobra some 16 years ago. Let's try and break it down.
Ignore the toe-link, which is frankly in our way. And try not to look for the aforementioned fiddly bits, which are obscured from view here anyway. Just as well — I've marked out the essentials.
Despite its sprawling appearance, this behaves like an A-shaped lower wishbone, complete with two inner mounting points that define the pivot axis and a single outer bushing that locates the lower end of the knuckle in the fore-aft and in-out directions.
Its general hugeness comes from its need to sustain lateral cornering loads, but it also must support the weight of the car and stand up to road impacts, both of which act directly upon it because it's also the lower mounting point for the spring and shock absorber.
Now we can reconsider the black toe link, which prevents the knuckle from steering the way front wheels do. But "prevent" is the wrong word because the differing lengths of the players are designed to induce tiny amounts of bump steer as everything swings through their various arcs.
There isn't much to see up top, just a single camber link that dips down below the edge of the body. As the name implies, this link's sole function is to control the in-out orientation of the top end of the knuckle. In combination with the lower control arm, it defines the camber angle of the wheel.
Being a lone link, it can do nothing to control or define the fore-aft location of the knuckle. It cannot stand up to axle windup when acceleration or braking torque is applied.
That's what the aforementioned fiddly bits are for. The actual name for the hunk at the end of the yellow arrow is an Integral Link. Ford's spec sheet uses the name Integral Link Independent rear suspension to describe the Mustang's rear underpinnings because of this feature.
In the context of acceleration and brake torque, you can see what this does immediately. It keeps the top end of the knuckle from moving fore and aft under even the most immense torque loads. It has no other job. It also frees the camber link to be nothing but a camber link, and it is long enough that it is unaffected by the toe-link. Because each has one job, all three are easier to optimize.
The odd H-shape of the lower control arm is the result of an offshoot (black) that juts out to give the integral link a solid base from which to work. The result is yet another load path into the lower control arm.
From here you see just about everything, and it's all packed close together. What you can't see is any kind of trailing arm. This is a very compact suspension that, as we saw in the first image, exists almost entirely in the shadow of the brake rotor. There's very little intrusion into spaces that could be better used for occupants, cargo or even fuel.
And check out the elevated position of the forward pivot (yellow) of the lower control arm. It's actually higher than the skinny toe-link. The resulting steep incline of the lower control arm's pivot axis is a sign that a large amount of anti-squat is present in the suspension geometry. And it certainly does the trick. Our Mustang GT accelerates hard when asked, but the rear suspension doesn't squat much at all during the most aggressive launches.
Meanwhile, the lower control arm isn't the only large hunk of aluminum down here. The complex rear knuckle (black) is also made of the stuff to save even more unsprung weight.
The spring and shock absorber are separated to maximize cargo and interior space. The stubbier spring fits under the body but the longer, skinnier shock absorber is positioned farther outboard so it can extend higher up into the wheel well.
All Performance Pack-equipped Mustangs are upgraded with monotube rear shocks. They get a thicker rear stabilizer bar, too.
Everything is efficiently packaged back here. Even the attachment point between the stabilizer bar and its drop link is given a second job as a brake hose attachment point.
It's hard to get a good read on the motion ratios because this lower control arm's pivot axis isn't parallel to the long axis of the car. That said, the spring seems to run close to 0.5-to-1 while the damper and stabilizer bar are in the neighborhood of 0.75-to-l.
Our 5.0 GT's rear brakes would be the same whether we'd gotten the Performance Pack or not. These single piston sliding calipers clamp onto inch-thick ventilated rotors.
They come with the EcoBoost Performance Pack, too. Ecoboost Mustangs without that option and V6-powered base cars come with half-inch thick solid rotors instead.
All of this hardware rides on 255/40ZR19 (96Y) front and 275/40ZR19 (101Y) rear rubber. The front rims are 9 inches wide and the rears are 9.5 inches. This only applies to the 5.0 GT Performance Pack, though. The EcoBoost Performance Pack rides on the 255 front tires all around.
Our scales say a mounted front tire weighs 56.2 pounds, while the wider rear assembly registers 60.8 pounds.
I didn't have any expectations before I looked underneath our new Mustang. Well, maybe a few. Going into this I knew our 2015 Ford Mustang GT felt more sophisticated and glued-down than any before, even when driven aggressively on challenging surfaces.
This look underneath suggests Ford has done its homework. They put their A-team on this one. It's going to get really interesting when the more powerful performance versions come on-line in the coming months.
Dan Edmunds, Director of Vehicle Testing