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.
Springs and Shocks
With modern cars, sway bars are among the first things to change and the suspension springs are among the last. However, with the E30, the stock springs are very weak compared to anything offered more recently and you almost certainly need stiffer springs just to keep the suspension from hitting the bump stops over every dip in the road.
Lowering too much with custom springs introduces many other issues with suspension geometry and travel, although their progressive nature means that lowering springs are better riding than constant-ratio coil-overs.
Eibach, H&R, and several others make good E30 lowering springs. Another advantage to buying a name-brand set of springs (especially the H&R cup springs) is that they are easily re-sellable to another E30 owner if you change your mind and want to convert to coil-overs or try a different setup. A common side effect of lowering springs on the E30 is that the rear is lower than the front. An easy way to solve this problem is with varying thicknesses of spring pad inserts (hard rubber pads that sit sandwiched between the spring and the trailing arm).
If replacement springs won't do for your setup, coil-overs are the way to go. A coil-over conversion allows you to use off-the-shelf 60-mm springs (depending on the kit used) to replace the factory springs. Most coilover kits use a threaded spring perch slid over the factory front struts. The OEM lower spring perch can be removed or left in place. This gives you a choice of strut inserts, since any strut that works with the factory strut housing works with your coil-over.
Either way works well. The advantage of replacement strut housings is that the damper/shock can be positioned to increase suspension travel without modifying anything.
In the rear, coil-overs are not practical. The shock mounts, both upper and lower, are not strong enough for coil-over spring use. The BMW body itself is not designed for the loads that a coil-over rear shock puts into it. To use true rear coil-overs requires a full roll cage with shock mounts included or extensive bracing of the rear structure of the body.
The advantage to a coil-over setup is that it allows you to tune the ride height separately from the spring rate and preload (how far the spring is compressed at static ride height). Coil-overs can also be set up to maximize suspension travel even when lowered.
For an all-out competition car, coil-overs are absolutely necessary. There is just no way to achieve the kind of adjustability needed to win races with stock-style springs and shocks. Coil-overs make it possible to adjust the corner weight of the car so that each tire is carrying its fair share of the car's weight. This makes for better, more consistent handling in both left- and right-hand corners.
The same thing can be done more crudely and slowly with lowering springs, however. The same spring pads used to raise a too-low rear can be used to balance the cross weights.
With all this talk about springs it's easy to forget the effect that shocks have on your handling. Shocks/struts/dampers help the springs absorb bump energy and control the release of that energy as the spring expands. In a way they add controlled friction to the suspension.
Without shocks the car bounces all over the place and is impossible to control; every bump sends the wheels bouncing off the road surface, losing traction constantly. Shocks need to be matched to the springs; stiffer springs require stiffer shocks. Stiffer shocks also improve handling and feel at the limit. At the moment of turn in, a stiff shock resists body lean and helps the car turn more quickly. This makes the car easier to drive in transitions, such as on an autocross course.
If you buy a coil-over package from a manufacturer such as Ground Control Suspension Systems, the kit comes with shocks that are matched to the spring rates supplied. The higher-end coil-overs, such as those from AST Suspension with remote reservoirs, have special custom shocks that can be re-valved later to change their behavior if they do not match your expectations as they are delivered out of the box.
If you are the DIY type, there are other strut inserts that can be used in place of the factory front units to increase suspension travel. The stock 325ix inserts are one option, as are aftermarket inserts made for the Volkswagen Corrado. These work with the later-model struts having a 45-mm-insert diameter. Early 318i models came with 51-mm-strut housings. But it's always best to measure carefully before ordering anything.
A set of lowering springs and decent shocks can make your E30 handle better than 90 percent of modern cars, look good, and not cost too much money. If you decide to lower your E30 with replacement springs, pick a name-brand setup that doesn't lower much yet has quite high spring rates for best handling. (Photo Courtesy Ryan Gangemi)
Better coil-overs such as these from Ground Control use modified factory strut housings to increase travel on lowered cars.
With a stock-based strut housing, to increase travel you have to shorten the housing and usually replace the strut retaining nut. The result is a much better setup for high performance street or track use.
On the street, the easiest way to get better handling is to simply replace the soft (and guaranteed to be worn out) factory struts and shocks with Koni adjustable (Koni yellow) street shocks, since you can tweak them to adjust for different track conditions or different springs. (Photo Courtesy Grassroots Motorsports/Tim Suddard)
Rear shocks are so easy to change they can be done in half an hour. (Photo Courtesy Grassroots Motorsports/Tim Suddard)