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Modifying your E30 Suspension
 
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Pelican Technical Article:

Modifying your E30 Suspension

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.

Suspension Modifications

Most enthusiasts start modifying their cars at the suspension; it is where the most "bang for your buck" changes are located. The E30 is a great-handling car, especially for its time, and it doesn't take much to make it a fun weekend track toy, decent autocrosser, or even competitive race car.

Even if your intentions are for nothing faster than an occasional mountain road, a few thousand dollars in suspension modifications can create a car that is arguably more fun than stock without making it any less reliable or any less drivable on the street. But before you start down the path of suspension modification it helps to think of your goals for the car. That means consider your car as a whole, think about what you actually use it for (as opposed to what you would like to do with it), and use Weight transfer is directly related to the height of the center of gravity of the car, the distance between the wheels (track width), and the speed through the corner. So adding wheel spacers or wider wheels with less offset decreases weight transfer. So does lowering the car, since it decreases the height of the center of gravity. When you reduce weight transfer, you directly increase grip by better matching the weight on the tires on each side of the car.

There are a few different ways to lower an E30 and each has its benefits and downsides. By reducing the outside diameter of the tire, the car is lowered without affecting anything in the suspension, as the gap between the fender and tire increases, and the car gets lower to the ground, but there are no other changes in appearance. Since non-M E30s were shipped with rather small 195/65R-14 tires, there isn't much room to decrease tire size without reducing tire footprint significantly and reducing the ultimate grip. The M3 (and later cars with the M-tech suspension package) came with 205/55VR-15 tires.

Springs

     Since reducing tire size isn't really an option, you are left with modifying the suspension. Generally this means replacing the stock springs with shorter, stiffer ones that fit the factory spring perches or changing to smaller-diameter racing springs. The first option, lowering springs, is the cheapest and simplest method. This simply lowers the static ride height of the car and increases the spring rate enough to keep the car off the bump stops (the conical rubber bumper under the strut mount that prevents the strut internals from absorbing the full force of the bump).

The other option for lowering is converting to small-diameter racing springs, also known as coil-overs. A coil-over replaces the factory large diameter front springs with a smaller diameter spring that fits closer to the strut body. When upgrading in this way, the factory lower mount is supplemented by an adjustable perch to change the vehicle ride height.

Ground Clearance

Before choosing a method of lowering (or even choosing to lower your car at all), you need to look at the negative effects of lowering the E30 chassis. The first is simply reduced ground clearance; in the front, the engine's oil pan is the lowest part of the car. If you lower your E30 too much, you run the risk of banging the oil pan on speed bumps or other road irregularities. This can crack the pan and dump all your oil on the ground--not a good thing. In the rear, the exhaust system is usually the limiting factor in lowering, but it is not as big a deal as the oil pan.

Geometry Changes

The biggest issue with lowering the E30 too far is the geometry of the front and rear suspension systems and how that affects the way your car handles. One factor that controls how much your E30 rolls in a corner is roll couple. This is a measure of the rolling force that your car generates in a corner. It's determined by the speed of the corner and the distance between the center of gravity and the roll center of the suspension. The greater this distance, the more your car tends to roll in a corner, and the more the tire's relationship to the ground changes.

Lower the car and you lower the center of gravity, right? Well, it's not quite that straightforward. On the MacPherson front strut suspension, the roll center does not lower as much as the center of gravity when lowering the car. What does this mean? The distance between the center of gravity and the roll center becomes greater as the car is lowered, making it want to roll more when lowered and forcing the suspension to absorb even more force to resist that roll.

Of course you get less weight transfer from the lowering, but the greater roll couple still disturbs the handling at that end of the car, requiring you to use stiffer springs or sway bars to counteract it. Notice that body roll and weight transfer are two separate things. Body roll only affects the relationship of the tire to the road surface by twisting the chassis and suspension in a corner.

Lowering the front suspension also introduces negative camber (the tops of the front tires move inward) but this is not really a bad thing--a little more static negative camber helps with overall grip. High performance driving and sticky tires both prefer more negative camber: 1 to 1.5 degrees of negative camber are great on the street, although race cars on non-DOT rubber can use even more (up to 3 degrees).

The rear suspension has its own set of problems. Although the semitrailing-arm setup works very well to give the E30 excellent factory handling, the geometry of the control arms changes for the worse as the car is lowered. Just as with the front suspension, the lower the rear suspension of your E30, the more negative camber you get. Some negative camber in the rear is good because that gives you grip at that end of the car and reduces the camber loss that naturally occurs in roll with this kind of suspension.

Handling Characteristics

The rear toe also changes as the suspension compresses. Luckily, the wheel toes inboard more, not outward, preventing wild rear suspension behavior. Gaining toe-in upon compression (such as in a corner or during acceleration) is generally a good thing because it adds to rear stability.

Compounding these issues is the fact that the stock E30 suspension is not adjustable. There is no rear alignment adjustment (for camber or toe) at all. The front suspension is adjustable for toe, but not for camber. So if you do not plan to replace any bushings or suspension mounts, stick with a very moderate drop using non-adjustable suspension springs. Do not lower your car more than about 1 to 1.5 inches unless you are willing to swap in adjustable suspension parts and correct the geometry changes that occur with lowering. Sure, being able to "lay rocker" sure looks cool, but it can actually hurt overall handling capability.

Extreme lowering also introduces other problems, including bump steer. This is defined as the front wheels steering in or out as the front suspension compresses. This happens when the front control arm and the tie rods connected to the steering rack move in different arcs. The more the front of the car is lowered, the more bump steer becomes an issue.

Bump steer can be corrected, but it requires a lot of trial and error to space out the outer tie-rod ends far enough to fix the problem without making it worse. Also, the racing-style rod ends that are supplied with bumpsteer correcting kits do not last as long as sealed tie-rod ends on the street.

The last part of the lowering puzzle is suspension travel. When the car is on stock soft springs, it has lots of room for the wheels to move up and down over bumps, allowing the suspension to do its work. As the car goes around a corner and the outside suspension compresses, the travel on that side of the car can become marginal even with the stock suspension.

A mid-corner bump can shove the outside suspension all the way to the bump stop. When this happens, the spring rate basically goes to infinity as travel stops and that tire loses grip. If it happens at the front, near-instant understeer occurs. If it happens to the rear suspension you will find yourself chasing your tail end as it comes around.

To compensate, many people shorten the factory bump stops or replace them with shorter ones. This does help, but the new, shorter bump stop makes for an even less progressive change to fully hard. Other solutions involve modifying the upper suspension mounts or changing the front struts, which are beyond the scope of simply changing coil springs.

Most owners consider modifying their E30 suspension for better handling performance, defined as a better ability to corner predictably and faster in a given corner. For them, especially for a street driven car, an inch or two of lowering is sufficient to improve weight transfer without affecting body roll too much and leads to an overall better handling car.

The E30 just does not have a lot of suspension travel from the factory
Figure 1

The E30 just does not have a lot of suspension travel from the factory. Lower the car and things get worse, particularly in the rear. These raised rear shock mounts help by letting the rear shocks compress more before hitting the bumpstops. (Photo CourtesyGround Control Suspension Systems)

There's nothing wrong with lowering for appearance as long as you are aware that you have to make greater compromises in everyday use
Figure 2

There's nothing wrong with lowering for appearance as long as you are aware that you have to make greater compromises in everyday use. Even if you correct the static alignment of your front and rear wheels, you usually run into other problems with handling. (Photo Courtesy Christian Bouchez)

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Page last updated: Fri 12/9/2016 02:24:26 AM