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Griggs Racing's Delrin IRS
Lower Control Arm Bushings |
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After much deliberation, I decided to install a set of Griggs Racing's IRS lower control arm bushings made from Delrin (FIG. 1). For those not
familiar with Delrin, it is a polymeric material that is generally less compliant than polyurethane compounds as used in suspension bushings. My only previous experience with Delrin bushings was a rideI was given in a '93 Mustang with Delrin front
control arm bushings. While steering responsiveness was much improved over the stock bushings, the car had a very harsh ride that the owner attributed to the Delrin bushings. It's this prior experience that made me hesitant about installing Delrin IRS
lower control arm bushings.
The factory IRS lower control arm bushings have an inner rubber layer and an outer rubber layer with a metal sleeve between the two rubber layers (FIG. 2). The various attributes of the two rubber layers (e.g.
thickness, durometer, etc) and the metal sleeve (thickness, position, etc) provide for tuning of the bushings. Effectively, the two rubber layers and the metal sleeve are configured for providing desired bushings compliance characteristics.
The
factory bushings absorb and dissipate a fair amount of road noise and impact, improving ride quality and reducing transmitted noise. However, due to the compliance associated with the rubber layers, the factory bushings contribute to adverse toe control
issues during acceleration, heavy braking and cornering. Because Delrin control arm bushings exhibit essentially no deflection under typical loading from the suspension, toe control problems associated with the IRS lower control arm bushings are reduced
significantly.
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FIG. 1 |
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FIG. 2 |
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Pulling the IRS lower control arms off the car revealed that removal of the factory rubber bushings was not going to be a cake walk. Griggs's
didn't provide any instructions so I was on my own. Unlike polyurethane bushings that generally re-use OEM metal shells that are pressed into the control arm, the Delrin IRS lower control arm bushings are designed to be mounted directly in bores in the
IRS lower control arms.
Unfortunately, the bushing extractors that I have access to did not fit the IRS lower control arm bushings. So, I had to get creative. First, I used the bushing extractor (FIG. 3) to put a load on the rubber portion of
the bushing. This entailed installing the bushing extractor and cranking down on the rubber portion of the bushing such that the rubber portion was under a considerable load (trying to pull the rubber portion out the side of the bushing). I then used a
propane torch to heat the perimeter of the control arm around the bushing, thus releasing the rubber portion of the bushing from the metal shell of the bushing (FIG. 4).
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FIG. 3 |
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FIG. 4 |
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With the rubber portion of the bushing removed, I then cut a slot in the shell of each bushing (FIG. 5). A hacksaw or electric reciprocating saw
(e.g. a Saws-All) works great. Cutting the slot relieves the hoop stress in each shell. With the hoop stress in the shells relieved, I used a hammer and punch to tap the shells out of the control arms (FIG. 6). The total time to remove all 4 IRS lower
control arm bushings was about 1 hour. |
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FIG. 5 |
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FIG. 6 |
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After inspecting the bores of the control arms and de-burring them as necessary, I tapped the Delrin bushings into the control arms. Then, after
applying a thin coating of the supplied grease on the bore of the Delrin bushings and on the outer surface of the metal sleeves, I tapped the sleeves into the Delrin bushings (FIG. 7). All in all, installing the Delrin bushings was not too bad once I
got a handle on the installation process.
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FIG. 7 |
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FIG. 8 |
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After re-installing the control arms (FIG. 8) and getting everything buttoned back up, it was time for the moment of truth…how is it going to ride
and handle? As for ride quality, there is essentially no increase in harshness and only the slightest increase in road noise. I'm still in shock! Had I not installed the bushings myself, I would never believe that there are Delrin bushings in the IRS
lower control arms.
The improvement in handling is subtle, yet significant. Prior to the Delrin bushings, changes in the toe that were associated with deflection of the bushings contributed to a slight 'rear-steer' effect during cornering and
braking. With the Delrin bushings installed, the rear-steer effect is for all intensive purposes eliminated. The back end is now even more stable and predictable.
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Copyright 1999-2002 David O. Simmons
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