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BMW E30 Bushings and Mounts
 
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Pelican Technical Article:

BMW E30 Bushings and Mounts

Robert Bowen

Applicable Models:

BMW E30 3-Series (1984-93)
The CarTech BMW 3-Series Performance Guide

Whether for on-track duty or simply improved street performance, the E30 series cars have proven to respond to well-chosen upgrades. The CarTech BMW 3-Series Performance Guide offers current and future owners a wealth of important information, including a buyer's guide, year-by-year upgrades and changes, and more. This book is a valuable addition to every BMW owner's library, and Pelican is proud to present you with the excerpt below.

Bushings and Mounts

The next topic to cover is bushing material and its effect on handling. The factory bushings in your car are made from rubber bonded to a metal core, and sometimes a metal shell as well. Although they are low tech, rubber bushings are actually pretty effective. Heavy-duty OEMs and good-quality aftermarket ones are firm. And they last a reasonably long time.

But rubber bushings also have some downsides. They are flexible in more directions than just rotary (twisting). Some of the motion of the suspension is lost in deflecting the rubber rather than moving the shock and spring. This makes handling slightly unpredictable at the limit and, in a racing situation, can make the car hard to drive aggressively.

For years enthusiasts have been quick to replace all the bushings in their suspension with parts made from cast polyurethane. The polyurethane parts are much harder than rubber, reducing deflection in the suspension and tightening handling at the limit. Polyurethane insulators, such as at the top shock mount and upper strut bushing, do not flex nearly as much as rubber and allow the springs and shocks to do their job.

However, polyurethane has its own downsides. It cannot be bonded to metal like rubber can. So, instead of internal twisting, urethane bushings rely on relative motion between the metal sleeve and bushing. This introduces friction, which can make the suspension bind. You can hear the creaking and groaning of all that friction whenever you ride in a car with urethane bushings. Urethane also cold flows, which means that it changes shape from pressure. Urethane bushings subjected to twisting motion can often become sloppy after a few years.

There are two other bearing materials used primarily on competition cars: Delrin/nylon and steel. Delrin bearings are good for locations that see only rotary motion, such as sway bars. Spherical steel bearings can be used to replace any rubber bushings in the suspension, whenever motion needs to occur in more than one direction, such as the semi-trailing arms and the front control arms. Aluminium is often used to replace non-moving isolator bushings such as the subframe and differential mount bushings.

However, these bearing materials transmit significant vibrations to the chassis and to the inside of the car, making them impractical for most street applications. But solid bearings are the only choice for a race car, since they allow the driver to drive at 10/10 without fear of unpredictable suspension movement, and they allow the tuner to set up the chassis without any variables.

The following is a look at the most important bearings in the E30 chassis with some ideas on how to figure out the best ones to use for a street car, for racing, and for dual purpose cars.

The front control arm bushings are best to keep rubber for a street car. Use the offset M3 bushings for any car driven hard, since the added caster improves tracking and steering feel. In some applications (such as five-lug conversions) the offset bushing must be used. For a race car, use the spherical bearing replacements if they are legal in your racing class; use polyurethane or thin Delrin bushings (such as those offered by Treehouse) if they are not legal. For an all-out racing effort, complete control arm assemblies that are adjustable in two axes are also available, but they can't really be recommended for anything else since they won't benefit handling as much as they increase noise and harshness.

Sway-bar bushings can be either rubber or urethane, but make sure that they are well lubricated with waterproof grease to prevent squeaking. End-links are best to keep factory because of their good design.

Upper strut mounts are best kept rubber, either offset or regular, for a street-driven car. Adjustable urethane is a good choice because the benefit of an adjustable mount is worth the small amount of added harshness.

In the rear, I like rubber trailing arm bushings for street cars, again for the noise isolation they provide. If you convert to adjustable pivot bolts, you should stick with rubber bushings even for racing and dual purpose use, because urethane in this location binds and causes added friction from the different pivot axes of the inner and outer mounts. In racing, converting to spherical bearings is the best, but is difficult and expensive.

For the differential mount and the rear subframe mounts, use the hardest material that you can stand for its noise. For most people that means either stock rubber or Group N hard-rubber bushings. Urethane is a good second choice; for racing, go right to solid aluminium to keep the subframe locked down.

Some manufacturers provide altered subframe bushings that actually raise the subframe into the chassis, which helps to compensate for lowering the car. It raises the rear roll center slightly, reducing roll couple in the rear. These types of bushings probably aren't worth using on the street because the benefit is small for the amount of work it takes to install them and because they reduce clearance in the rear. They have to be used with an offset differential mount (or compensating spacers) to avoid introducing problems with the pinion-to-driveshaft angle. Get this angle wrong and you can count on damaging the center support bearing and driveshaft giubos.

Even for street use, avoid the factory rubber rear shock mounts and go right to one of the urethane/aluminium designs. They are superior to the rubber designs, even the heavy-duty ones. For racing, use either the urethane/aluminium style or a spherical bearing mount.

Rubber bushings have low friction, surprisingly, because there is no movement within the bushing itself.
Figure 1

Rubber bushings have low friction, surprisingly, because there is no movement within the bushing itself. The bonded metal core twists the rubber as the suspension part moves, but there is no movement of rubber on metal.

Urethane bushings in this application might work okay for a dual-purpose car
Figure 2

Urethane bushings in this application might work okay for a dual-purpose car, but the noise will drive you crazy on the street and the design of the factory bushing support "eyeball bracket" or "lollipop" can allow the urethane bushing to cold flow and fall out. (Photo Courtesy Grassroots Motorsports/Tim Suddard)

Most E30 drivers are happiest with rubber offset CABs, since they add needed caster yet do not transmit too much vibration to the chassis.
Figure 3

Most E30 drivers are happiest with rubber offset CABs, since they add needed caster yet do not transmit too much vibration to the chassis.

Two-axis, adjustable control arms with spherical bearings are the solution to most E30 front suspension geometry problems
Figure 4

Two-axis, adjustable control arms with spherical bearings are the solution to most E30 front suspension geometry problems. However, the bushings can be too harsh and transmit too much noise for a street car. (Photo Courtesy Ryan Gangemi)

Go with M3-style swaybar attachment (to the struts) if you have a non-M3 and are modifying the struts anyway.
Figure 5

Go with M3-style swaybar attachment (to the struts) if you have a non-M3 and are modifying the struts anyway. You get more roll stiffness with the same bar.

The factory attachments are plenty strong, but if you are moving to an adjustable sway bar you may have to change the end links to the ones that are supplied with the swaybar kit.
Figure 6

The factory attachments are plenty strong, but if you are moving to an adjustable sway bar you may have to change the end links to the ones that are supplied with the swaybar kit. (Photo Courtesy Ireland Engineering)

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Page last updated: Sat 12/3/2016 02:25:25 AM