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The 2015 Mustang hit the streets running at its release, and it seems to be another hit for Ford. However, for performance enthusiasts the return of the independent rear suspension was met with justified hesitation.
That’s because every IRS-equipped Mustang ever produced has suffered from excessive wheel hop during aggressive driving, such as what might occur at the drag strip. Early indications are the 2015 model suffers from the same issue. Energy Suspension and Joey Granatelli worked together on this 2015 Mustang GT owned by Granatelli Motor Sports. To establish a baseline, they did a series of drag launches with the stock tires. “One of the biggest things I noticed was the significant amount of wheel hop,” says Energy Suspension’s R&D Engineer Ben LaHatt.
Energy Suspension used a forklift to remove the GT’s IRS subframe. If you have a transmission jack, that will work, too. The IRS subframe is quite heavy so if doing this install yourself, have equipment ready and capable of doing the job.
Wheel Hop
Wheel hop is when the suspension is unable to keep the tires in constant contact with the pavement. The rubber bushings allow the IRS to bounce back and forth within the subframe assembly. Wheel hop can damage your IRS’ half-shafts, and your teeth if not fixed. The Energy Suspension Polyurethane bushings we cover here are designed to provide a more rigid connection between the IRS and the subframe assembly to remedy wheel hop.
This is Energy Suspension’s Polyurethane transmission mount insert. The insert is a two-piece design, and it promises to make for a more rigid feel for both the shifter and the transmission. The insert is designed to enable better shift characteristics for the factory Getrag MT-82 six-speed manual transmission. The insert fills in the space on the transmission, where the mount bolts to the transmission. This is to take the slack out of the mount assembly that is the cause of many missed shifts.
This is a durometer being used to measure a bushing’s hardness. The factory bushing came in with a measured hardness of 62. The Energy Suspension subframe and differential bushings have a durometer rating of 80.
Left: A stock bushing and Energy Suspension bushing side by side. Right: The durometer tests the harndess of a stock bushing.
The 2015 Mustang polyurethane bushings from Energy Suspension that were used in this installation are still in development. We guess you could call them prototype bushings as of right now. The company is still in final testing before these bushings are released to the public. The bushings Energy Suspension is developing are for the subframe kit, differential mount kit, rear suspension kit, rear sway bar kit, shifter kit, front lower control arm kit, and front sway bar kit.
Something Energy Suspension stressed to us is the importance of not using heat to extract the factory bushings. A lot of people don't have access to a press like what is being used here so we get it, but using round steel pieces to force out the factory bushings is the right way to do this. Using a torch, or other source of heat can sometimes damage the component around the bushing, especially those of the thinner metal variety.
“For the subframe and differential mount kit, the goal is to help reduce, or eliminate the wheel hop,” LaHatt says. “Polyurethane helps to stiffen it up. Polyurethane is a performance upgrade. The polyurethane will also last significantly longer than any rubber bushing, to extend the life of the car, and the components.” Lahatt also said the bushings present a more cost-effective way of upgrading the suspension without buying new suspension components.
This piece of the 2015's rear suspension is called the rear camber/toe link. It connects the upper knuckle to the lower lateral link. This is one of the components using thinner material so extra care is taken when pressing out the factory bushing, and installing the Energy Suspension polyurethane bushing.
However, the polyurethane comes at a cost. Most of us already know the polyurethane is stiffer than the rubber bushings they replace. A durometer is the instrument used to measure the hardness of a visco-elastic material such as rubber or polyurethane. The higher durometer reading the stiffer the material, which also means there is less deflection, as well. That increased stiffness can cause an increase in NVH over the factory rubber bushings, but that’s where the performance increase comes from. Remember, the rubber bushings are there to keep NVH levels low, and provide a quiet ride. The OEM bushings are a trade off between high performance and what most consumers consider tolerable NVH levels. They aren’t meant to be the best for performance. They are meant to be the best for NVH standards.
The subframe and differential bushings have the biggest impact on reducing/eliminating wheel hop. The rubber bushings allow the assembly to rock back and forth at a rapid pace, whereas the polyurethane bushings act as a more solid mounting foundation. The higher durometer more polyurethane bushings don't allow the assembly to rock back and forth like the rubber bushings, and that's where the improvement, and the decrease in wheel hop comes into play.
With a more rigid IRS assembly thanks to Energy Suspension’s polyurethane bushings, you’ll be able to use your ’15 Mustang the way it was intended. When the bushings hold up their end of the job, handling is enhanced, drag strip performance is improved, and you won’t lose your teeth in the process.
With the polyurethane bushings installed in place of the factory rubber bushings, it's time for the IRS subframe assembly to go back in the car. The forklift is called into service to assist in reinstallation.
