The crew at Lethal Performance wasted no time hitting the drag strip in their 2018 Mustang GT project car. After snapping a stock halfshaft while running in Drag Mode, Team Lethal returned to Palm Beach International Raceway and clicked off a best elapsed time of 11.68. Those runs resulted in obvious deformations of the factory differential bushings and telltale marks indicating just how much the IRS cradle moved around under load.

As such, all signs indicated it was time to bolster the factory independent rear suspension with a carefully selected package from BMR Suspension in Seffner, Florida. With that in mind, we met Lethal’s Jared Rosen and Christina High at BMR HQ to document the installation of these upgrades under the rear of the company’s second-gen S550 project.

To fortify the flaccid rear suspension on Lethal Performance’s 2018 Mustang GT project car, BMR Suspension’s Kelly Aiken selected a targeted group of upgrades including adjustable toe links (PN TR005; $229.95), a Level 2 cradle bushing lockout kit (PN CB005; $219.95), Rear Vertical Links, with polyurethane bushings (PN TCA048; $99.95), and rear control arm bushings (PN BK081; $159.95).

“After a pretty decent track outing with drag radials and a Lund Racing E85 tune we decided it was time to prepare the car for the Whipple 3.0-liter kit, which should be showing up shortly,” Jared said. “One of the main concerns we had was the suspension. We really want to see this car perform its best, so by replacing a lot of the factory components with the BMR goodies we know we’ve got a much better shot at quicker 60-foot times, elapsed times, and consistency. So I reached out to Kelly Aiken who has always showed us support over the years and he said to come right up and we’ll get the car all setup.”

By replacing a lot of the factory components with the BMR goodies we know we’ve got a much better shot at quicker 60-foot times. — Jared Rosen, Lethal Performance

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Establishing A Baseline

Those previous runs were the result of just a Lund Racing E85 calibration and a tank full of corn. The car put the power down through a carbon fiber driveshaft and halfshafts from the Driveshaft Shop via Mickey Thompson 19-inch drag radials. With a robust driveline and sticky tires, the car was obviously pushing the limits of the cushy factory suspension.

“I chose to address all of the mounting points which have the most rubber mass, as naturally they will deflect the most. Since I know Lethal will end up with a Whipple supercharger, I did not want to risk them having traction inconsistency issues nor do I want them to have issue breaking IRS and driveline components,” BMR Suspension’s Ford specialist, Kelly Aiken explained. “I also had them upgrade the differential mounting bolts, as those are prone to damage when a car is boosted and seeing high stress loads from achieving traction. Lastly, I chose to do the adjustable toe links to enable them to run the alignment that I spec out for combos such as Lethal’s, where superior performance is the primary goal.”

BMR technician Colt Mills kicks off the project by stripping down the rear suspension to remove the lower control arms. With a jackstand keeping things loaded until the right time, he loosens all the connections, hangs the brake caliper safely out of the way, removes the factory shock, and pulls the coil spring. With the hardware out of the way, he pulls the lower control arm.

For the Lethal crew, choosing the BMR bits was a natural outcome. The company has long sold and utilized those familiar red and hammertone parts under its long line of project cars.

“We’ve been using BMR’s products for many years now on several project cars. Not only have the products they offer performed well for us but the BMR staff are awesome people to deal with as well. Our customers love their products too, so we figured it was the right decision to go with what they have to offer,” Jared explained. “Working with Kelly is very easy. He knows his stuff with suspension as if it’s second nature. His knowledge of cryptocurrency is a bit suspect, however. When it comes down to setting a car up right for the street, track, or road course he’s got it down. He knows the Whipple will be installed shortly, so he chose the parts that were geared toward getting us off the line smoothly and down the track with consistency. He even shared PDF cheat sheets with alignment and shock setting specs that were put together with time and experience. All of this makes for a winning combination.”

You will reuse many of the factory fasteners, so make sure you keep them organized and, if this is your first rodeo, put them in bags or bins marked with their locations. Colt is an old hand, having upgraded 30-plus S550 rear suspensions, so he knows where everything belongs.

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Special Attention

Indeed, BMR’s Ford specialist has dedicated a significant amount of time and energy to refining Ford suspensions. In recent years he concentrated that focus on the S550 platform and its quirks, particularly those lurking in the independent rear suspension.

“The amount of deflection within the S550 IRS under loaded conditions is mind-blowing. The IRS subframe itself will move in any given direction more than 0.500-inch when loaded up…and will oscillate vertically when just driving down the road part throttle,” Kelly said. “Most significantly, past the IRS subframe moving is the IRS rear lower control arm deflection due to the massive rubber bushing on the fore pivot point/mount. The IRS differential mounts are also very soft, and the differential will pivot during normal driving conditions, and of course, when being loaded up.”

The most challenging part of this upgrade is swapping out the front control arm-to-chassis bushing. The easiest way is to use a hydraulic press to remove and replace the bushing. If you don’t have access to one, you can likely grab a simple one at your local discount parts retailer. As you can see by comparing the removed factory bushing with its spherical BMR replacement, the latter is designed to eliminate deflection to transfer the power more directly to the pavement.

That Kelly owns a 2015 Mustang project — which is in the process of a revival with a powerful new engine — certainly helped him gain an intimate knowledge of how those soft factory mounting points can detract from performance under load.

Eliminating the deflection leads to much better consistency. — Kelly Aiken, BMR Suspension

“When the IRS assembly and its components shift from deflection, the alignment and the geometry are impacted negatively,” Kelly added. “From improper and potentially catastrophic pinion rise, to excessive camber and toe changes. Controlling the knuckle assembly that the wheel is connected to pays big dividends in any performance application.”

To get that knuckle under control, he chose BMR’s Adjustable Toe Links (PN TR005; $229.95), Rear Lower Control Arms Vertical Links with polyurethane bushings (PN TCA048; $99.95), and Rear Lower Control Arm Bearing upgrade (PN BK081; $159.95).

After reinstalling the lower arm, Colt completely removes the factory toe link, acquires the proper measurement, and installs the more robust, adjustable BMR unit. These units feature self-lubricating, Teflon-lined chrome-moly rod ends and allow you to customize the rear toe settings to meet your performance driving needs.

“Consistency is the primary concern for my racers,” he added. “Eliminating the deflection leads to much better consistency, which allows the tuner and the car owner to make easier and better progress when shooting for their end goals. Tuners will have their customers call me on a daily basis to sort out their traction issues so they can get better data. Spinning tires and inconsistency issues will make tuners lose their mind!”

Another aspect of the factory suspension movement that adversely influences the car’s performance under loads it the differential mounting setup. The factory bushings allow quite a bit of movement, and they can even deform under shock loads, which will really throw the geometry out of whack. The remedy here was BMR’s Level 2 Cradle Bushing Lockout Kit (PN CB005; $219.95).

Much like the rest of the factory suspension, the stamped steel, rubber-bushed vertical links offer a bit too much leeway of movement for performance applications. The BMR billet units are a direct replacement and help rein in wheelhop thanks to their beefier build and stouter poly bushings. Colt simply tapped the new link into place and attached it with the factory bolts.

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Controlling The Movement

“As you know, I learned early on that hitting the tires hard and having parts move is a bad recipe. This is a big one for me because I see people lose a lot of their time, money, and effort when they break parts. I spend a lot of my spare time coming up with solutions for my customers to help prevent them from having part failures, hence the differential bolt upgrade we performed on the Lethal car. The front bolts we installed are not a BMR catalog part, rather a setup that I came up with for my own cars.”

As we detail in the photos, Kelly suggested drilling out the factory front differential bushings and replacing the factory threaded fasteners with through-bolts featuring beefier shanks.

Before fully reassembling the rear suspension, Colt and the BMR team dropped the factory exhaust from the Lethal Mustang to allow lowering the IRS cradle for full access to the differential bushings. Just the lockouts can be installed with it in place, but to install Kelly’s special upgrade, the cradle has to come down to allow for drilling.

“The factory bushings have voids, and they also place the shear path on the bolt threads which are the weakest part of the fastener system. My method removes the shear loads on the threads and places them on the larger and much stronger shanks of the bolts. Through-bolting also allows to remove the excessive voids within the OE configuration,” Kelly said. “To date I have had approximately 50-60 customers and friends who have broken stock or aftermarket stock style bolts, but I have yet to have a single failure with a through-bolt configuration. I would say the top five fastest IRS S550 Mustangs use this through-bolt arrangement. GForce Engineering uses the BMR through bolt set-up with its 9-inch IRS kits as well.”

In the end, Kelly expected some significant improvements from the upgrades. However, they should be even more important as Team Lethal starts to apply more power with a supercharger.

While the Level 2 Cradle Lockout Kit will drastically reduce the movement of both the differential and cradle, that might not be enough for a car putting down 3.0-liter Whipple power. That’s why Kelly recommended drilling out the front diff bushings to replace the factory threaded fasteners with more robust through bolts. Colt did the honors and installed the BMR poly inserts and bushing-locating washers, which drastically firm up the factory bushings while keeping the NVH in check. According to Kelly the shanked bolts offer far fewer shear points than the fully threaded units, which might be the difference between the fastener living or failing on a high-power car. Colt installed the inserts and washers in the traditional fashion on the rear bushings. The final pieces of the puzzle were the cradle braces, which Colt installed after buttoning up the rest of the cradle and suspension. These braces ensure the cradle stays put when you drop the hammer.

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Beyond The 60-Foot

“So many things play a role in achieving a better 60-foot time. Automatic cars with low power levels (500 or less rear-wheel horsepower) are typically tricky to improve 60-foot times by only replacing the factory rubber with more rigid components. On stick-shift cars of any power level, I can usually improve their times with some IRS parts,” he explained. “On average I would say a naturally aspirated car is going to gain around a tenth of a second to the 60-foot, and with boost I have seen results up to 3-4 tenths once the customer reaches out to me for assistance and we throw some mods and settings at the car. What I would like to point out though is, it isn’t “all” about the direct 60-foot improvement from removing the slop. The tuning typically makes progress, which allows for a better 60-foot due to being able to apply more power, sooner.”

Even before heading back to the racetrack, the benefits of the upgrades were apparent. Reining in the IRS delivered a more precise feel and offered the confidence to move forward with those big-power mods.

After an alignment, the Lethal 2018 Mustang was ready to hit the drag strip again and improve upon its naturally aspirated numbers before making the leap to Whipple boost.

“The car drives real nice. You can feel it’s much more solid now especially when you have the pedal to the floor and the A10 is banging through the gears,” Jared elaborated. “I definitely feel the BMR suspension components we added will improve our 60-foot time over what it was for our last outing at the same power level. The upgrades also pave the way to handle the power we plan on putting down here shortly.”

If you have followed the Lethal projects over the years, it shouldn’t be a surprise that this car will soon receive a heaping helping of twin-screw boost.

Next Level

If the mods detailed here are not enough, the next move might be adding some more control via aftermarket dampers. That would be a bold move to ditch the factory active units, but Kelly seems pretty confident there could be a significant difference.

“Honestly? I usually stop at this point and like to get data. This combo paired with the BMR specific alignment settings is going to show the biggest return on investment for performance, and past this point would be springs and rear Viking Shocks. To touch on the shocks briefly, the Viking shocks are by far the most proven and the best product to use. Something that I see often though is, people using the B258 Warrior series due to their low price point,” Kelly added. “Since assisting Viking with the development of the shocks for the S550 over three years ago until the present, I have seen that the S550 simply needs more valving on the rebound side. I have come to the conclusion that the Crusader rear shock should be the only shock used on this IRS system. I have stopped recommending the Warrior line to my customers with S550 Mustangs. In the case of the Lethal car, we wanna see how far we can push the OE MagneRide, but Vikings may be in order soon!”

So, there you have it. The latest platform requires a premium damper to keep things under control.

“It’s no secret that a 3.0-liter Whipple Gen 5 setup is in works for this car. We’ve been getting updates on the kit and testing since it was announced and we’re really excited to know we’ll have one here soon,” Jared added. “We’re definitely excited to see how the car performs with the blower and the 10-speed auto. I know we’re typically banging our heads with the MT-82 setups and have done extremely well with them over the years, but it was time to switch it up and give this new 10R80 a shot.”

And give it a shot they did. Upon returning the PBIR with the newly suspended S550, Jared was able to drop his 60-foot times from the 1.74 range during his previous tune-only (see above) outing to a 1.70 after the suspension, which sets the stage for some impressive results once he adds sticky racing tires and, eventually, a supercharger.

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