Let’s talk for a minute about bushings, a bunch of tiny unsung heroes scattered about your car. These little rubber pieces can be found pressed into where your suspension control arms meet your subframe, where your subframe meets your chassis, and even where your engine bolts to your frame rails. They often look like small rubber circles when installed and feature just enough flex to keep things comfortable, maintaining a smooth experience over pockmarked roads and in spite of powertrain vibrations. If you will, imagine how miserable cars would be if every single component were rigidly bolted together. Sounds dreadful, right? All those powertrain and road vibrations would find their way directly into your tailbone. Thankfully, bushings are here to isolate components and offer the flexibility required for a smooth and serene drive. But are stock bushings always the answer? If you’ve ever had a cursory glance at the ads in performance car magazines or the product listings on speed shop websites, you’ve probably encountered an overwhelming variety of bushings, most of which promise a more connected experience.
So what sorts of bushings are available, how do they work, and what impact do they have on vehicle enjoyment? After all, enjoyment and connectedness are hard things to quantify. A bit like trying to explain how good your grandmother’s orange cake is using just the ingredient quantities on the recipe, using numbers to quantify improvements from aftermarket bushings is a bit tricky and often fails to succinctly communicate actual effect from behind the wheel. Speaking from seat-of-the-pants feel, let’s start from the top.
[Editor’s Note: Wow, an article about bushings. Would this be hard to pitch at some publications? Probably. But most publications aren’t run by an enginerd, and you know how much enginerds like bushings. “Engineered rubber isolators,” you might call them. Anyway, have at it, Thomas! -DT]
Automakers typically love inexpensive, effective solutions, and the humble rubber bushing is one of the solutions. Why? Well, let’s take a look at what bushings are supposed to do. Every single project an automaker undertakes comes with targets concerning noise, vibration, and harshness (NVH). Consumers typically like cars to be smooth, quiet, well-isolated from the murmurs exchanged between tire and pavement and from vibrations of components like the engine and rear subframe. The humble rubber bushing is a cheap way of mitigating excess vibration transfer.
However, while rubber bushings mitigate excess vibration transfer, they do that by giving components a little bit of room to move by acting as springs. Now, room to move can be good or bad depending on the construction of the bushing and engineering targets, as our resident suspension engineer Huibert Mees explains.
Bushings come in all different shapes and sizes. The cheap ones are simple rubber donuts pushed into a metal hoop with another smaller metal tube pressed into the middle. These usually squeak over time and wear out because the rubber can slide inside the metal hoop and eventually wear out. The more expensive bushings (and the ones every single car I’ve ever worked on uses) use rubber that is injected into a mold which also contains the outer metal hoop and the inner metal tube. This process bonds the rubber to both metal pieces which makes for a much more durable part. The finished part is then put into a machine which squeezes the outer metal and compresses it into a smaller size. This is called “swaging” and puts the rubber inside the bushing slightly into compression. This is also good for the durability of the part.
Since the rubber is bonded to the metal parts, there is no sliding happening and the life of the part is much better. More sophisticated versions of these bushing can have voids molded into them to give different stiffnesses depending on which direction you push in. They can also contain “rate plates” which are arced metal pieces molded into the rubber. These rate plates have the effect of increasing the stiffness of the bushing when you push on it without increasing the rotational stiffness.
Rotational (or torsional) stiffness is important to control because it adds to the spring rate of the suspension. As the suspension moves up and down, the bushings are deflected rotationally so their rotational stiffness has the effect of resisting the vertical movement of the suspension. All the rotational stiffnesses of all the bushings together comprise what is called the “parasitic rate” which is the spring rate of the suspension which is independent of the actual metal spring and is very hard to tune and control in production. Reducing these rotational stiffnesses as much as possible is an on-going effort of all bushing designers and manufacturers.
So, we’re largely looking at a balancing act between deflection and rotational stiffness. What is deflection? If you’ve ever clutch-dumped a front-wheel-drive car with soft engine mounts, you’ll know exactly what deflection is. It’s the deformation of a flexible part, an attribute that can sometimes add comfort at the expense of feel and performance. As for rotational stiffness, lower is generally better, and it’s really hard to reduce rotational stiffness in bonded rubber bushings. Rubber bushings also have the downside of aging from heat, weather exposure, chemical exposure, and time, eventually requiring replacement. While some rubber bushings are made of cheese, many strike a great balance between stiffness and comfort. For many road car applications, particularly for vehicles made in the past 20 years or so, replacement stock rubber bushings do the job perfectly.
Hydraulic Rubber Bushings
While conventional rubber bushings feature low NVH, they’re not exactly the last word in refinement. Why? Well, a rubber bushing acts as a spring, and springs typically function best when paired with matched dampers to prevent excessive motion. How on earth would someone go about damping a bushing? [Editor’s note: I bet there’s some amount of damping inherent to a rubber bushing, due to internal friction. -DT]. By filling it with hydraulic fluid, of course. This hydraulic fluid is pushed through tuned channels in the rubber in order to damp the bushing. Hydraulic bushings have been used in applications like engine and differential mounts for ages now, and for good reason. Since these fluids don’t compress, deflection can be managed to enhance control. Hydraulic bushings are used in a ton of OEM applications, from differential mounts to control arm bushings, so they’re not exactly some new crazy technology. Think of them as the post-graduate alumni cousins of the rubber bushing.
Of course, when a hydraulic bushing fails, it tends to get a bit messy. A typical failure involves the rubber splitting, allowing the hydraulic fluid to leak out and get all over everything in its path. Once that fluid leaks out, not only do you have a mess on your hands, you also have empty cavities inside the bushing that allow for greater deflection. Definitely not an ideal situation. Still, pretty brilliant when it works, yeah?
Alright, what if you need something harder than rubber yet still flexible? Enter the polyurethane bushing. Harder on the durometer scale than a typical rubber bushing, a typical polyurethane bushing trades some NVH dampening for superior durability and road feel. Polyurethane as a material features good chemical resistance, so it’s well-suited to automotive use. Another perk, polyurethane bushings for difficult applications can be made at home fairly easily. Make a mold, pour some polyurethane, and you’ll have replacement bushings for difficult-to-source applications in a weekend or so. It’s also worth noting that polyurethane bushings are fairly cheap and often come in a range of hardness options, typically with Shore durometer ratings like 70A, 80A, and 95A.
Wait a sec, what the hell is a Shore durometer? Don’t worry, a Shore durometer is a machine that typically measures the hardness of flexible materials like rubbers and polymers. A higher number means a harder material, and a lower number means a material is more compliant. In the case of bushings, the A in a 70A rating means the bushing was tested using a type A durometer with a blunt cone-shaped tip. To be precise, a hardened steel tip with a 35 degree cone that features a flat tip 0.79 mm in diameter. Why does this matter? Because the flat type A tip exerts less concentrated pressure than the pointed type D tip. Pretty nifty, right?
So are there any downsides? Polyurethane bushings aren’t exactly quiet. Without proper routine greasing, they often squeak like furious mice due to the lack of a bond between metal and polyurethane parts. Fine for a sketchy turbocharged LS-swapped project with an 800-decibel exhaust, but not exactly brilliant on a flagship luxury sedan, just speaking from experience. In addition, improperly-lubricated polyurethane bushings can bind in certain applications like control arms, which could lead to less predictable handling. Honestly, polyurethane isn’t always the best suspension bushing material because of its maintenance requirements, but it makes a phenomenal isolator material. Think exhaust isolators and engine mounts, particularly engine mounts in high-torque transverse applications. Polyurethane’s also great for anti-roll bar bushings as reduced deflection over rubber bushings helps anti-roll bars do their jobs properly. Honestly, if you’re looking to firm things up a touch and are willing to put up with periodic greasing, strategic use of 70A or 80A polyurethane bushings is cost-effective and pretty swell.
Ah yes, now we’re getting into the good stuff for performance. Its official name is polyoxymethylene, but this wonderful material usually goes by Delrin. It’s a thermoplastic and thus simply too hard to pull a durometer reading from. Hardness of Delrin is measured using the Rockwell scale, a hardness scale typically used for rigid materials like metals and hard plastics. A typical Rockwell scale rating for Delrin clocks in around M89/R122 , so don’t expect much in the way of deflection. However, while Delrin offers the perk of plenty of rigidity, its main attraction is its self-lubricating nature. Expect a larger increase in NVH from Delrin bushings than from polyurethane bushings, but superior smoothness when it comes to motion because its self-lubricating nature means Delrin isn’t prone to binding. Delrin is a phenomenal performance bushing material for anything that moves. Control arms, sure, but shifter bushings are also where Delrin comes into its prime. Minimal deflection, wonderful self-lubrication. Good stuff.
So where do Delrin’s downsides lay? Well, aside from the obvious NVH trade off, making your own Delrin bushings requires access to a lathe and a little bit of practice. In addition, pre-made Delrin bushings can be a bit hard to find for most applications, indicating that the automotive aftermarket hasn’t quite caught on to the magic of this nifty thermoplastic. Still, for the handful of applications where aftermarket Delrin bushings exist, this unusual material can really work a transformation.
Solid Metal Bushings
Right, no more concessions to NVH, we’re jumping face-first into hardcore territory with solid metal bushings. Want something to absolutely not move? You’re in luck. Solid metal bushings are absolutely not something I’d recommend on a car that’s primarily used for street driving unless your daily commute takes you down the Tail of the Dragon or something. They will typically create new rattles, transfer vibrations directly to your spine, and potentially even lead to an increased risk of component damage over stock rubber bushings. In addition to massive NVH increases, the rigidity of solid metal bushings means high transfer of force in case of a shunt. I definitely wouldn’t use solid bushings as engine or gearbox mounts due to vibration transfer, nor would I use them in suspension components.
However, solid bushings still make good sense in some applications. Let’s say that your platform of choice suffers from hideous rear subframe movement and/or wheel hop. In this case, solid subframe and differential mounts are a good choice if you’re frequently hitting up track days as they’ll prevent the rear end of the car from moving around on your unexpectedly and mitigate damaging wheel hop by eliminating the way rubber bushings act as undamped springs. What can I say, sometimes extreme problems call for extreme solutions.
Now we’re getting into a bushing that isn’t a bushing, the spherical bearing (often found in Heim Joints). This one’s a bit weird, but I promise the results are worth it. A spherical bearing is a ball-and-socket design, allowing for fairly free movement and minimal deflection. Perfect for control arms. Because debris making its way between the ball and socket can cause premature wear, binding, and all sorts of nasty stuff, spherical bearings made for street applications are weather-sealed.
So what are the downsides? Well, if the weather sealing ever tears, you typically have to replace the whole spherical bearing. In addition, NVH control definitely isn’t as tight as with a rubber bushing, or even some Delrin bushings depending on application. The upside is that because motion is so fluid and deflection is so little, car manufacturers are starting to use spherical bearings in all sorts of applications. For instance, E90 BMW M3s used spherical bearings in their rear toe control arms because of their massive radial stiffness. Indeed, properly-sealed spherical bearings can hold up to street use and provide reduced deflection, at the expense of some comfort. Spherical bearings are also incredibly popular in the aftermarket, with many good aftermarket suspension arms using them and press-in options available for factory arms.
A Reality Check
Right, now that we’ve covered basically every sort of bushing commonly available either from the dealer parts counter or through the aftermarket, it’s worth considering the reality of vehicle use. While upgraded bushings are awesome for track days and canyon drives, it’s possible to go completely overboard and end up ruining your street car. Upgraded bushings should be considered carefully and strategically in the context of what you’re using your car for. Just as I’d look like a bit of a knob walking into a meeting wearing Nomex boots, you might feel a bit silly if you completely decimated your daily driver’s comfort. Keep in mind that every platform is different, so it’s best to talk to owners who’ve done bushing upgrades and if possible, get a ride around the block in one of their cars. If you find a negligible effect on NVH from certain bushing upgrades, it’s likely worth making the switch if you do a lot of performance driving. Also keep in mind that stiffer bushings change how load is distributed through the suspension and into the body of the car, so you may see wear in areas you never expected if you switch to stiffer bushings. Don’t just take it from me, take it from Huibert.
The function of the suspension depends greatly on the way the bushings work. [Bushings] plus the geometry of the control arms and knuckles determine the way a suspension works and how well it works. They determine if you have a soft and quiet ride or if you have a stiff and well handling car. They also determine the understeer and oversteer characteristics of the vehicle as a whole. The bushings also determine how the forces coming from the tires are distributed throughout the control arms and links. These forces and the way they are distributed are critical to the durability and life expectancy of the suspension parts. Changing the stiffness of these bushings may make the car feel more responsive but unless you fully understand how the changes effect the understeer balance and the durability of the components, you may be getting yourself into trouble down the road.
However, just because aftermarket bushings may lead to unusual consequences in some road cars doesn’t mean that Huibert hasn’t experienced the benefits of aftermarket bushings in some applications.
Older cars (30-40+ years old) will likely benefit because they aren’t so highly optimized with computer modeling like modern cars are. I myself put polyurethane bushings in the rear suspension of my 1971 Monte Carlo and it definitely made a difference for the better.
Unsurprisingly, while polyurethane bushings certainly made a great difference in handling for Huibert’s Monte Carlo, they also came with a very noticeable NVH penalty. He notes, “Just like you say in your article, those bushings I put in my Monte Carlo squeak like crazy! I hate that.” Despite the squeaking, Huibert feels that the polyurethane bushings in his Monte Carlo are an improvement, underscoring the fact that every discerning driver wants a certain feel in their vehicle, and sometimes that feel diverges from OEM targets.
It’s also worth noting that fresh, factory-spec rubber bushings can make an absolute world of difference on an older car. Just because your bushings aren’t severely cracked doesn’t mean they haven’t hardened enough to affect ride, handling, and NVH. The bottom line? Factory-style replacement bushings are almost never a bad idea, and a few strategically-upgraded bushings can really enhance the feel of your car if you’re careful and do your research. Remember, performance numbers and build spec sheets are great and all, but how a car feels is paramount to driver enjoyment.