“This is not surprising,” notes BimmerWorld’s James Clay while looking at the cracked and worn bushings. “These bushings are under about the most stress of any in the car, especially under hard braking, and the stock ones are way too compliant to begin with, so when you start tracking the car, it just destroys them.”
The bushings being evaluated? The ones that secure our BMW 435i’s thrust arms.
Indeed, our thrust arm bushings were pretty smoked, and our lower control arm bushings weren’t much better, showing some dryness and cracking in the rubber. They had endured nine years and 74,000 miles and were ready to rest.
Ultimately, quality bushings are all about dynamic alignment stability and freedom of articulation. Bushings are placed between suspension components to allow articulation. Bushings also absorb road irregularities, turning that energy into heat, combatting any NVH inside the car.
OEMs tend to focus on the NVH reduction as much or more than the platform stability, though, so stock bushings can be overly compliant, particularly when you slap on a set of sticky tires and grippy brake pads and hit the track.
That compliance can thoroughly isolate vibrations and impacts in the suspension from entering the cabin, but it also allows for a lot of dynamic alignment variance. For example, we could easily stick a big screwdriver in our thrust arm bushings–which fix the front of the arm that locates the fore and aft alignment of the front strut and hub to the chassis–and deflect it several millimeters by hand.
Now, imagine that bushing deflecting under the load of 1g braking–or even 0.3g of acceleration. That compression is going to allow the hub to move forward or rearward relative to the rest of the chassis, which will change the toe under load, which is pretty much the worst time for your toe to change.
The other drawback to overly compliant bushings is a reduction in the ability for the suspension components to articulate freely under load. When the bushing deflects, it can alter the angle of the pivot point created at the interface of the components. This can introduce bind and prevent those arms from freely articulating in their desired arc.
Finally–and this is kind of a minor effect, but still worth thinking about–a soft bushing basically acts a spring. Its compliance means that it absorbs and then releases energy. A lot of this kinetic energy is turned into heat, but some of it is returned as kinetic energy as the bushing springs back into shape after deflection. And this spring action is effectively undamped.
Luckily, bushing manufacturer Powerflex had our backs, offering some options that created additional benefits outside of just a bushing upgrade. We employed its Black Series bushings–Powerflex’s most track-focused option–at the recommendation of BimmerWorld and Powerflex in these key front suspension junctions.
The thrust arm bushings retail for under $150 and the camber-adjustable–more on that cool feature in a minute–control arm bushings are under $175. All of these pieces are also legal for competition in classes, like SCCA Time Trials, that allow bushing substitution but specify that they must retain a non-metallic/monoball construction.
The Powerflex urethane bushings replace the stock units, which must be pressed out with the aid of a hydraulic press. It’s a doable job for the home mechanic with a shop press and a thorough set of press dies. Still, use caution and proper safety, because you’re putting a lot of force into those old shells during removal as well as a fair amount of force to pop in the stiff Powerflex bushings. Things can go wrong in a hurry under tons of hydraulic pressure.
The less compliant urethane mitigates the two biggest issues with the stock bushings. It not only deflects far less than the stock rubber pieces, but its additional stiffness creates a more stable pivot point for suspension arms to articulate around.
The additional bushing resilience in the thrust arm pieces is particularly noticeable under hard braking, where there’s a more stable initial brake bite and less wandering due to dynamic toe changes under hard sustained braking.
Our camber-adjustable lower control arm bushings gave us the benefit of–you probably guessed already–more negative camber. With the previous setup using the Motion Control Systems camber plates on top of our MCS front struts, we maxed out our front negative camber at around 2.5°.
This was good, but not great, and BimmerWorld recommends camber in the high 2s or even 3-plus degrees negative in the front of heavy, tall sedans and coupes like our 3600-pound 4 Series.
The Powerflex adjustable control arm bushings feature the same urethane bushing bits as the normal Black Series bushings, but they also feature a sleeve with an offset hole that moves the pivot point about 8 mm away from the geometric center of the bushing.
This sleeve can be clocked to any point to move that lower control arm pivot in, out, up, down or any position along that circular path. Adjusted to its maximum “out” point, which would give us the most negative camber gain, we picked up about half a degree more camber on each side of the car, now measuring out to 3° to 3.1° negative on both sides.
So, trade-offs. Because there are always trade-offs, right?
Well, in this case, they’re minimal. We expected slightly more NVH, but the reality is it’s barely noticeable. As Clay puts it, “BMW does a LOT of work to kill NVH, so even if you take one part out of the equation, there’s still a bunch of other stuff doing the same job.” Indeed, the car is every bit as delightful on the road as it was before bushing surgery.
The other trade-off comes in the slight offset afforded by the camber-adjustable control arm bushings. Because the lower control arms aren’t mounted perfectly perpendicular to the car–they angle forward slightly from the chassis to the hub–offsetting the control arm pivot not only moves the control arm out, but it also scoots it forward slightly.
With our 18x10-inch BimmerWorld TA5R wheels and 275/35R18 Vitour P1 rubber, we were fairly close to rubbing the front of the wheel well at certain steering angles. Moving that lower control arm pivot out scoots the hub not only out, but slightly forward–not much, but if you’re already close to rubbing, it’s not going to make things any better. Luckily, we still have space, but we’ll have to stick with this tire diameter or risk interference if we go larger.
So, after bolting everything back together and doing an alignment with our Caliper Garage string alignment kit, we’re back on track with a more fitting amount of front camber.
For now, we’re beyond satisfied. The car is noticeably more stable, especially under braking, and the additional camber is paying off in more even tire temps and wear.
