BMR S550 Rear Suspension: The Kelly Aiken Breakdown

Almost daily we get asked what the most important rear suspension upgrades are for a 2015+ S550. The honest framing of the question is simpler than the answer: what good is horsepower if you cannot put it to the ground?

So Jared and Christy took Project Goldmember, our 2018 Mustang GT, over to BMR Suspension in Seffner, Florida for the Kelly Aiken treatment. Kelly knows these cars in a way very few people do. He has run GoPros underneath his own car to watch what the suspension is actually doing, and BMR parts are under two of the fastest IRS cars in the country.

What follows is what he showed us, including the physical evidence on our own car, and the three packages we built with him.

What We Found Under the Car

This is the part that changes how you think about IRS traction. None of it is theory. It is wear marks on our own car from track use.

The Cradle Is Rocking

The factory cradle bushing has a small lip on top that acts as a limiter. On our 2018 A10 car, Kelly pointed out that when we launched at the track, that limiter was hitting the chassis and leaving a witness mark on the body. The cradle was not flexing slightly. It was moving far enough to bottom out its own travel stop against the body of the car.

The Differential Has Moved

The front differential bushings have small metal alignment nubs built in. On a healthy car those protrusions are barely visible, if at all. On ours, the passenger side was sticking way out and the driver's side slightly out. That tells you the diff moved enough to compromise the clamping force of the bolt, and then re-seated itself diagonally.

The rear diff bushings told the same story from the other end: the bolt on the passenger side was sitting much higher on the bushing than the driver's side.

The Three Packages Compared

Biscuits & Gravy: Level 1

Bread & Butter came first and got popular fast. But it is more than some people need, particularly if you daily the car and just want to harness the power you have at the track occasionally, road race, run HPDEs, or race on weekends.

So we went back to Kelly for the bare essentials. These parts have seen 9-second runs and 1.3 60-foot times. That is a serious result from a starter package, and it is why we point most people here first.

BK081 Spherical Lower Control Arm Bearing

Factory bushings are moulded from soft rubber and contain voids and air pockets. Those voids let the lower control arm move during aggressive launching and driving. A solid spherical setup eliminates the voids, which eliminates the movement, and it also keeps the wheels from moving forward from bushing deflection at wide open throttle.

TCA048 Vertical Links

Light and strong, and they limit the range of cradle movement while still offering bushing compliance and no increased NVH. BMR's are the only vertical link on the market whose bushings can be greased and serviced on the car, which matters more than it sounds three years in.

CB010 Cradle Bushing Lockout

6061-T6 billet aluminum and stainless steel. Billet upper lockout rings capture the inner bushing sleeve front and rear, plus the outer lip of the bushing cup at the front. Lower locating washers capture the bottom of the bushings' inner sleeves. Movement on any plane, fore and aft, lateral and vertical, is nearly eliminated.

Bread & Butter: Level 2

We asked Kelly to pick the components he considered most beneficial for these cars. He called them the bread and butter in the video, which is where the name came from. This set supports cars going well into the 8s, and owners chasing low 1.2 to 1.4 60-foot times.

It builds on Level 1's thinking and then addresses the two problems the evidence above revealed: the cradle rocking, and the differential moving.

CB005 Cradle Bushing Lockout Kit

An 80% to 90% reduction in fore/aft and lateral cradle bushing deflection. It includes spacer rings that lock out the top of the cradle bushing so it cannot deflect at WOT, and lower bushing mounts that stop the lower bushing dipping down and causing the pinion to rise.

BK051 Differential Bushing Lockout Kit

Eliminates the voids in the stock differential bushing setup and locks the differential's movement, which is what keeps you from breaking driveline components.

TR005 Double Adjustable Rear Toe Rods

The factory pieces are stamped steel with an eccentric bolt that slips and pivots under load, which changes your rear alignment while you are driving. BMR's hold geometry better through the suspension arc thanks to an offset spherical bushing, come with lockout plates to defeat the factory eccentric, and offer a wide adjustment range.

Steak & Potatoes: Level 3

Catered to people who are hardcore about racing the car. Steak & Potatoes is Bread & Butter on steroids: a total IRS solution that solidly mounts both the subframe and the differential to the chassis, and adds springs and further components on top of Level 2.

The Hardware Decision

If you are doing the differential bushings, do the hardware while you are in there. Both options fix the same two factory weaknesses; they differ in whether you are willing to drill.

Frequently Asked Questions

Questions on which package suits your car? Email us or call and we will talk it through. Huge thanks to Kelly Aiken and everyone at BMR Suspension.