Page 71 - Issue 49
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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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