Bushing Material Types

I originally wrote this when I worked for Turner and have since refined it. Turner Motorsport has deep experience with bushing materials used in BMW suspensions. As a race team we have experimented with varying bushing materials and grades to find a winning setup. And our service department has evaluated, installed, and replaced just about every bushing type out on the market. And our New England climate gives us insight to the durability of these materials. So when it comes to designing our own suspension arms and bushings we have a wealth of experience and knowledge to draw from. We could use any bushing material we want. Here we will explain the four different bushing materials that are common in the BMW tuning world.

RUBBER	Street Use?	Track Use?	Maintenance	Lifespan (with intended use)
	yes	no	none	medium

By far the bushing we like most for street use is a factory-spec rubber material. Rubber is inexpensive, easy to source, maintenance-free, relatively easy to replace, and works very well in everyday use. The downsides are that the factory spec'd the bushing is a compromise between performance and practicality. They injected just enough performance into the design to live up to BMW's reputation while also being compliant enough that non-traditional BMW drivers won't complain.

Rubber is absorbent and flexible by design. As force is applied to it, the rubber absorbs and deflects the force. The amount of allowable motion depends on the stiffness of the rubber material; a soft(er) material will allow more deflection. Deflection in the rubber control arm bushing allows alignment changes at the wheel. On the street you may not be reaching those levels of deflection and will not notice its affects on alignment. But on the track or spirited driving, the imprecision and sponginess will be noticeable especially as the bushing wears. The nice thing about rubber is the built-in elasticity of rubber. It (almost) always returns to its preset position.

URETHANE	Street Use?	Track Use?	Maintenance	Lifespan (with intended use)
	yes	no	yes	short

For the more enthusiastic portion of BMW drivers who want more performance the next step in material is urethane. Urethane comes in multiple grades of stiffness with most of it being stiffer than a stock rubber bushing but still far from a full-race grade stiffness. We are not big fans of urethane and try to avoid using or recommending it. It's always a compromise - it can be too stiff and not very durable for the street but also not stiff enough for the track. It's just not good enough for either task.

There are two great downsides to urethane bushings - maintenance and durability. Combined they are the root causes for urethane's short service life.

Maintenance - The urethane material is dry and does not self-lubricate. Large amounts of grease are required to keep friction levels low and minimize stiction. Dry urethane also squeaks which can be annoying. However, grease attracts and traps dirt and particles and as the debris makes its way to the bushing it will cut and wear the urethane.

Durability - It is not as elastic as rubber so it's slow to return to its preset position. And constant leveraging and motion from the suspension can change urethane's shape. With enough force over a short period of time the tolerances grow larger, introducing slop and play into the bushing.

Urethane remains very popular as people subscribe to them as a performance alternative to rubber. And they do feel pretty good at first. On my E46 M3, I had Rogue hybrid urethane/aluminum front bushings and Powerflex rear trailing arm bushings. For a year and a half they were great (especially the Rogue bushings). I didn't track the car though and only drove it during warmer months (no potholed roads). So it's not all bad but the Turner Service department certainly pulled many urethane bushings that didn't live up to expectations.

DELRIN	Street Use?	Track Use?	Maintenance	Lifespan (with intended use)
	no	yes	none	long

Delrin is a proprietary thermoplastic compound developed by DuPont. Delrin is desired in a lot of applications that require high stiffness, low friction, and excellent directional stability. It has almost zero bushing deflection and, generally, surpasses urethane in stiffness which makes it an improvement for track bushing applications. This is one reason why many racers go to a Delrin bushing when the rules prohibit solid metals like aluminum. Have a look around your next pro racing paddock for urethane bushings. You won't find them.

Delrin is like aluminum in that you start with a solid block and machine as needed. It's a low friction surface and self-lubricates so it never needs maintenance. It also doesn't attract and retain moisture so it has an extremely long lifespan. It's only material weaknesses are heat and certain acids. But through chemical engineering and decades of development, both drawbacks have been minimized. As mentioned, Delrin is used mainly as an alternative for full solid metallic mounts. It satisfies the rules for most sanctioning bodies when a non-metallic bushing is required.

SOLID ALUMINUM	Street Use?	Track Use?	Maintenance	Lifespan (with intended use)
	no	yes	none	long

When zero deflection is required there is no better option than machined billet aluminum. Deflection has no place on most race cars so the rubber or urethane bushings found on street cars is replaced with solid mounts. The solid fixture will have no play or slop so there is no wasted motion. This is key because the twist in rubber or urethane bushings is one of the causes for E36/E46 rear subframe failures. Aluminum will not have any wear and does not require any maintenance. You can even swap aluminum mounts from one car to another (such as replacing a rear subframe). It's a "lifetime" bushing.

Obviously with zero deflection there is no cushion effect against noise and vibration. There are few exceptions but we rarely recommend using solid mounts on a street car. Aluminum mounts should only be used on street vehicles when the owner's threshold for more noise and harshness is extremely high. Exceptions include some diff mounts and rear subframe mounts but only when used with rubber bushings in the rest of the suspension. The other rubber bushings will isolate impacts before reaching the subframe. In fact, BMW began using solid subframe mounts with the F10 M5 so some street applications are becoming acceptable.

ROD ENDS, BALL JOINTS	Street Use?	Track Use?	Maintenance	Lifespan (with intended use)
	yes*	yes	none	long

Rod ends are steel swivel joints pressed into a stainless steel housing. Usually there is no bushing layer which makes movement very precise in all directions. A PTFE coating provides lubrication. There is zero or very little deflection and total freedom of movement. Rod ends or "monoball" bushings are usually pressed into a metallic housing and the whole assembly is then installed on the car. So the "bushing" is just as stiff as a Delrin or aluminum mount with no wasted motion but the bearing allows controlled motion in the intended range and direction.

Rod ends found a strong following in off-road applications where the wheel deflection often meant losing traction. But users found quickly that dirt and debris could easily work their way into the joint. This produced wear on the metal pieces which created slop and too-large tolerances which rendered the precise design useless. The solution was to insulate and cover the ball joint ends with a protective boot. Protect the joint and no debris will get in and no grease or lubricant will escape either.

BMW began using sealed bearings in the early 1990s. They developed a sealed spherical bearing for the outer lower camber arm mount for the E36 M3. It consists of an internal spherical bearing, spacers, and rubber boots that seal the entire assembly from outside debris. The spherical bearing allows much greater movement with almost zero resistance. And the rubber boots make for a very long service life and quiet operation. Technically, these are solid metal bushings but with a small plastic liner. There's almost no downside to these 'bushings' and BMW has expanded their use across their product range, including 1, 3, 5, and 6 series cars. As we develop more and more tuning products for street, track, and mixed use we have also incorporated these sealed bearings into our own components.