Page 71 - Issue 49
P. 71

styles of IRS suspension which   >>  “Store-bought” Options:   relation to the motor, transmission   on noises & vibrations. These
 each vary slightly in appearance   Heidt’s,team321, roadster   & frame). Comparatively with a   OEM bushings are designed for
 as well: wishbone, single trailing   shop, etc.  straight axle, the axle housing moves   ideal noise/vibration dampening,
 arm, multiple-link trailing arm,                & rotates with the articulation of   but sometimes they allow for
 etc. Selection here is really just a   IMPORTANT NOTE: Many of us rat-  the suspension. On a conventional   too much fl ex, which can cause
 matter of what will fi t best in your   rodders prefer the 15” wheel,   leaf-sprung rear axle, this isn’t as   wheel hop and excessive wear of
 application.  because of cost reasons.          huge of a deal. However if you are   other components. It’s a tradeoff
 I’m not sure about ya’ll, but   Unfortunately, with some modern   on air ride, you will have much more   between performance and comfort.
 I’m not a fan of vehicles that have   IRS setups (2000 & up), wheel   travel, which will relate to more   In most situations, I recommend
 absolutely zero backseat space or   selection will be affected by brake   lateral movement (sagitta) as well as   using atleast some polyurethane
 a pickup truck that has an unusable   caliper/rotor sizing. Most modern   change in the pinion/driveline angle   bushings in substitution of the
 box (due to a raised box fl oor).    axles also have a wider factory   throughout the suspensions travel.   OEM rubber bushings. Poly
 That being said, another important   stance, so it requires a higher   Setting pinion angle is completely   bushings have become readily
 aspect, related to the “appearance”   offset wheel in most cases (or   dependent on your setup and the   available for many different axles.
 of an IRS, is that you will not have   narrowing of axle for conventional   application (It’s also a preferential   Some companies even  offer various
 to “tub” or raise as much of your   offset wheel).   science). In an ideal world a   durometer (hardness) ratings on
 fl oor because the suspension travel   My Application: Because of the   perfectly straight driveline would   their bushings; as the durometer
 is not present in the “pumpkin”   larger OEM brakes on the ’17 mustang   be preferred, but that’s usually not   goes up, so does the relative
 of the axle, but rather in the   IRS, I will be running a set of high   possible. A general rule of thumb   vibration & sound resonance, but
 control arms. So in most cases,   offset Detroit Steel wheels; staggered   for any street driven vehicle is to   also so does the performance of the
 notching of the frame rails is all   20” rear, 18” front (It’s also a good   have a 3 degree difference between   component.
 that is necessary for suspension   excuse to run a set of their bad-a$$   the pinion & driveshaft angle, while   In my opinion, it’s nice to have
 articulation, rather than cutting out   wheels).  However, if you must use a   having the pinion and output shaft   some soft bushings or mounts in
 excessive amounts of fl oor for the   smaller wheel, you will (most likely)   of trans as close to parallel to each   places that may help eliminate the
 pumpkin/axle to slip up inside of.    want to start with an older IRS   other as possible.   most resonant noise & vibration, but
              assembly with smaller brakes.         My Application:  For my builds   if there are large defl ections in the
 The most important factors to consider          I like to use a 3” pipe to mockup a   OEM bushings the effectiveness
 when purchasing an IRS are:  Installation & Technical Tips  driveshaft and  physically measure   of polyurethane replacements is

 >>  Width (wheel fl ange to wheel   While the installation steps of an   the angle of the pinion in relation to   going to be extremely evident. The
 fl ange)      IRS assembly are very similar to   the driveshaft. You can also setup a   most effective use of poly or solid
 >>  Design of arms (Upper arm in   the procedures discussed in the   scale model on the fl oor, if it’s easier   bushings is going to be seen in the
 particular, for fi tment reasons   IFS swap (previous issue #48),   for you to imagine on the fl oor, than   largest of the OEM rubber bushings
 with your full frame vehicle)  there are some very important   it is working under a car. For my   (The larger the OEM bushing the
 >>  Gear ratio, brakes, miles/  differences to consider. Rather   tight fi tment/application I had to   more room for fl ex, in general).
 condition, other factors of   than basically copying & pasting   set my pinion to 3.5* down, with a 1*   My Application: In my case, I
 that model/year  the previously discussed 10 steps, I   downward driveshaft angle (a 2.5*   chose a combination of poly, solid &
 >>  Availability of replacement   have just added the aforementioned   difference at rear driveshaft yoke).   OEM mounts for the main subframe
 parts        important differences below.       Trans is set to 1* up, so that gives a   assembly and the bushings that
 >>  Cost: anywhere from $100-  Reference that article for more of   2* difference between the driveshaft   hold the axle housing to the IRS
 $10,000      the “basics” of a subframe swap.   and the transmission tailhousing.    subframe.  I have had good luck
                                                 It’s not 100% ideal, but given that I   with “Energy Suspension” bushings
 Popular IRS Options:   Driveshaft angle & pinion angle:   didn’t have to tub my fl oor at all, I am   in the past, so I installed a few of
 >>  2000’s Crown Vic  Driveshaft & pinion angle are always   very happy with how it sits. Custom   their bushings in the weakest spots
 >>  70’s Jaguar  important angles to consider when   driveshaft will have to be made.   of the Mustang IRS, as well as in
 >>  80’s corvette  building a rod. While important with                            the front end (Nova Subframe)
 >>  70’s Datsun/Nissan  any axle, it is much easier to set with  Mounting & Upgrades:   which I discussed in the last article,
 >>  2003-2004 & 2015-2017   an IRS because the axle housing is   All OEM IRS assemblies will use   but hadn’t had the new bushings
 Mustang (Which I am using)  mounted stationary in the vehicle (in   some sort of bushing to cut down   for yet.


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          68x72tinman.indd   71                                                                                4/23/18   10:53 AM
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