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Poly IRS Subframe Bushings |
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The OE IRS subframe bushings are one of the limiting factors in getting the most out of the IRS. The factory bushings are made of a very compliant
elastomer. The OE IRS subframe bushings are compliant enough for loads associated with acceleration, deceleration and cornering to result in the entire IRS assembly being displaced relative to its nominal position. As a result, improvements from other
IRS and front suspension mods cannot be fully realized until displacement of the IRS subframe is reduced, if not eliminated. In fact, higher rate springs, stiffer shocks and higher durometer control arm bushings in the IRS will tend to increase the
displacement of the IRS subframe, as the rate at which loads are applied to the IRS subframe bushings is increased dramatically due to reduced compliance in these other IRS components.
There are two forward IRS subframe bushings and two rear IRS
subframe bushings. This mod involves replacing the OE IRS subframe bushings with poly IRS subframe bushings. In doing so, movement of the IRS subframe is reduced considerably, thereby improving performance under acceleration, braking and cornering.
Specifically, wandering under threshold braking is eliminated and there is a dramatic improvement in mid-corner stability and in corner entry/exit manners.
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Front OE Bushing |
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Rear OE Bushing |
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Energy Suspension (ES) control arm bushings intended for use at the axle end of the rear lower control arm for a Ford 8.8" live axle can be used with minor
modification. The ES part number is 3027. As with most poly bushings that are installed in OE metal shells, the installed length of the bushings is slightly greater than the un-installed length by about 0.10". For the most accurate fitment, each
bushing should be trimmed in place such that the bushing is the same length as the crush sleeve extending through the bushing. No other modification to the ES poly bushings is required. |
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Front Poly Bushing |
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Rear Poly Bushing |
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The rear of the IRS can be lowered and supported on jackstands to provide access to the rear bushings. With the rear of the IRS lowered and supported, each
rear OE bushing can be removed by 'walking' a drill bit around the perimeter of the bushing at the interface between the bushing and the metal shell. After removing the rear OE bushings, any remaining portion of the OE bushings that is still bonded to
the interior surface of the shells must be removed. A die grinder with sanding roll works well. Once the shells are cleaned and lubricated, the poly bushings can be installed and trimmed.
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Prepped OE Bushing Shell |
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The front of the IRS subframe can be lowered and supported on jackstands to provide access to the front bushings. With the front of the IRS lowered and
supported, the OE bushings can be removed and replaced in the same manner as for the rear bushings. It should be noted that with the rear of the IRS in its mounted position relative to the chassis, the front of the IRS cannot be lowered enough to
provide access to the front bushings. Accordingly, the rear of the IRS must be supported in the lowered position on jackstands prior to lowering the front of the IRS subframe.
With this mod, there may be a slight loss in ride quality, a slight
increase in drivetrain noise and a slight increase in road noise, depending on other mods to the IRS and existing vibrations. For example, with the poly IRS subframe bushings, Delrin lower control arm bushings, 650 lb/in coil-over springs, poly diff
bushings and Cobra R shocks, there is a very slight, yet noticable, increase in road noise and harshness over bump. However, with this IRS combo, there is no appreciable difference in ride quality or drivetrain noise under cruise conditions on the open
road. The bulk of the difference with the poly bushings is exhibited in an increase in drivetrain noise under hard acceleration.
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Copyright 1999-2002 David O. Simmons
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