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]
Rubber Bushings
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. 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. 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.
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?
Polyurethane Bushings
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.
Delrin Bushings
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.
Spherical Bearings
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.
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.
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.
First polyurethane is generally a terrible a bushing material by itself for three reasons 1. PU takes a compression set – over time it will continue to compress and that’s a problem because unlike rubber bushings which are typically bonded to the inner and outer rings, PU isnt. this leads to the trademark squeakiness of PU, but can also make the joint loose when the inner ring, wallows out the PU bushing material. So you’re probably asking yourself, why not just bond PU to the inner and outer ring that shit is sticky as hell? well, the answer is self generated heat, when say an upper control arm moves, part of the material inside that part compresses and part of it stretches, this generates heat, which brings me to my next point 2. PU hates heat. I saw mention of PU exhaust isolators, never seen those before, usually they are natural rubber/HNBR, EPDM or for very fancy applications silicone, specifically to handle the heat. Heat leads to PU breaking down and taking a compression set faster or failing outright. 3. Durability – PU simply isn’t durable on for most OE application, on a project car that sees 3000 miles a year maybe it will last as long as you have the car, but generally I recommend avoiding PU in most place.
Where PU really shines is actually in a foamed application – BASF, Vibracoustic and Carthane make jounce bumper also known as bump stops out of foamed PU and they are incredibly durable and allow for a progressive rate when the bumper comes into contact with something else (spring cup, axle, frame, etc) They are where lots of modern cars get great mellow impact over large potholes unlike older rubber bump stops that are incompressible.
Also you shouldn’t run PU bushings on a triangulated 4 link like a 71 monte carlo or any A/G body GM, it will cause bind in the suspension, the factory arms are actually designed to twist and deflect for ride quality. The best solution for handling in a triangulated 4 link are rod ends or roto-joints/jonnie joints et. all.
When it comes to Delrin, the biggest issue is contamination. I worked on a project for a high performance American car where we tried to make a delrin bushing work, the original design was a cross-axis ball joint, and while those work well, they are very expensive. What we found was, the delrin bushings would pass all of the abuse and track tests including Nürburgring testing and the ride and handling engineers absolutely loved them, but what killed them was the “car wash from hell” road grit simulator. the issue was, at least on this particular design, that we could not stop the ingress of foreign debris into the area where the inner “race” spun in the delrin bushing material. Despite the fact that the new balance wearing owners of these cars don’t really take them mud bogging, the OEM was intractable in their post mud-slurry test requirement, so the project was killed. The lesson here is that delrin is a fantastic, but typically short lived on a daily driver in anywhere but the most pristine conditions.
Another thing – Hydraulic bushings don’t use “hydraulic fluid” as in oil based hydraulic fluid for rams and pumps, they use a glycol water mixture, oil would degrade the rubber over time. When I worked at Vibracoustic we used blue glycol/water mixture so that if a part leaked we could immediately detect it, as opposed to any green, orange, or red fluids that might come out of a modern car – also it meant quick identification if an OEM said that we had a leak – if it wasn’t blue it wasn’t us.
While everyone seems to dogpile on how bad rubber is for a bushing material, Rubber is typically the best balance of cost/performance/durability. Usually rubber wins out in the eyes of an OEM because of their 100k mile system life tests, pretty amazing it’s just burned tree sap huh?
Rears were that way from the factory. Front is an aftermarket that does so much better to make up for the frankly poor design from BMW
On a more technical note, how much fluid do hydraulic bushings actually have? I was under the impression it was measured in eye droppers. How much of a mess can that make?
I put poly on EVERYTHING I could on this truck and don’t mind the NVH compromise one bit. But on a truck that old, flexible, and loosely put together, the increase in NVH is minimal compared to the increase in handling. So the motor shakes the truck a bit with poly bushings, it should on an old truck!
On my 02.5 GTV 24v VR6 I had, I had a poly lower engine mount (dogbone) and higher durometer engine and transmission mounts. I think because I was conservative with them all and went with the “softer” aftermarket ones that were still stiffer than stock, it was a good compromise. Noticed a little more road and impact noise, and wheel-hop was all but eliminated. The one odd side effect I was not expecting, is that I now got a little gear noise from the transmission. I thought it sounded cool, so it didn’t bother me. It seemed the dogbone acted as a little stethoscope and brought that into the cabin.
However, on my 06 Ford Focus ZX3, the bottom mount had blown out so I needed a new one. Put in a poly bushing and it make the steering column vibrate at idle, made the car noisier, and resonated the shifter and cabin at around 2300rpm. It was unpleasant to say the least. I found out that the stock bushings all failed, and Ford had a new part number to fix the durability issues of the stock part and required a new bracket to install. I bought the new bushing and bracket and installed it, and it acted EXACTLY like the poly bushing did. I learned to live with it, because it would just break again if I put the original part number on it. A guy decided to not buy the car because of the added noise and vibration. I told him that was the new stock part number, so not much I could do about it. sigh I sold it to a guy who most certainly crashed it within the 1st year of owning it, even though it is one of the last unmolested and clean ZX3s left.
When I bought my Valiant Charger, the previous owner put poly bushes in the lower control arm. It certainly made the ride harsher and also had the effect of trying to tear the steering wheel out of my hands when cornering on rough roads (or most of NSW!)
I now have rubber bushes in all suspension components except the strut rods and rear leaf springs, which is a great balance for where I drive.
A friend with a Z32 300ZX has poly in every component, as the previous owner seems to have succumbed to the performance magazine ads of the late 2000s. The effect is the car is really squirmy on rough roads and even worse under brakes. Maybe it would be great on a smooth track, but it’s never raced so it’s just a pain all the time.
Side note, I LOVE that I came from an ‘article’ about magic headunits and talking fishes to this.. with no ads, no pop-ups, no garbage. You guys are doing great work with this site!
Get with PayPal or Patreon, people: I’ll sign up the first time I see a donate button. Already bought an Optima, so that’s covered. Your backer obviously isn’t a Vulture Capitalist, but there needs to be some returns to keep the lights on (and writers paid!) because I am NOT going back to the old hotmess site and this site enriches my life
Launch of Saturn V rocket 204 dB.
Tsar bomba 224 dB, 3,300 times stronger than Hiroshima and 100 times louder than the 204 dB from the Saturn V rocket
The Tunguska event 300 dB flattening an estimated 80 million trees over an area of 2,150 square km
Krakatoa 310 dB. Heard 4,800 km away. Instruments 160 km (100 mi) away from Krakatoa recorded 172 dB, twice as loud as Hiroshima.
Asteroid hits earth and wipes out dinosaurs probably fits in about here.
And now at approximately a trillion times as loud as Krakatoa is the sketchy turbocharged LS-swapped project with an 800-decibel exhaust
I’m assuming that it’s running nitromethane?
Hated it. The turn-in on the front was great but eventually ripped a sway-bar mount off the car. The rear was stiffer and overall the car was more precise. However, the problem was that when I pushed the car laterally, where before the combination of tall sidewall tires and rubber bushings made the rear end drift and break away gradually, with the poly bushings the car would lean a little in a turn and suddenly the rear of the car snapped loose. The car went from being a lot of fun to drive too fast through the mountains to being work.
So where before the poly you could gracefully drift through tight turns after poly you were faster until you had to quit driving and start praying you didn’t oversteer into a wall or something. The suspension of the car had been engineered as a total package of springs, shocks, tires, sway-bars, and apparently bushings, and changing them really upset everything.
What works best for 10/10 on track is often miserable or scary on actual roads. I >want< some compliance as I play on gravel and only go 6 or maybe 7/10 on public roads. Plus, I’m not, you know, like 17 anymore: I want to not jostle kidneys too much.
Rode with several people who put too many FULL RACE components on their cars. Rarely enjoyed the ride much. Few who sink money into this can admit that it isn’t the best way to go for a street car (imho).
I’ve been in plenty of cars that were “improved” by poly bushings that might have handles better than the original, but the ride has been utterly trashed. It takes a lot of time and effort to sort out which specific bushings or combination of bushings will give someone the best compromise for their preferences. For the most part, I think OEM engineers don’t get the respect they deserve for balancing things well for most drivers.
Can confirm. I made the mistake of putting poly bushings in my XJ. They needed WEEKLY lubrication else the mice became furious again.
The worst part was the manufacturer swore up down and sideways the bushings were self lubricating and were inherently silent. HAHA HAHA!!!!! The mice said otherwise.
Unfortunately I had to move out of town soon after installing the bushings and given the Hell that was the leaf spring removal (fuck you Chrysler and your shitty tack welds) I eventually learned to live with the mice.
Some manufacturers have come out with some weird proprietary materials that are neither rubber nor poly, I got some SuperPro engine & subframe mounts that are made from something that’s lighter than poly with very similar hardness to poly, but with rubber-like NVH isolation & durability.
i found the PU bushings to be better than stock for NVH too. It seemed like the factory OEM are a bit quieter up to a point, them the bumps get really hard once the rubber compresses and it feels like it’s actually metal on metal. The poly made the cars more predictable and more engaging to drive.
I prefer a sportier ride on a car as opposed to the fully insulated luxury or dreadful land yachts and American cars of the 80s and 90s, or numb ass Nissan and VW
