Tire Rack.com

Spring Testing

Suspension components tested:
BMW Original Equipment Springs, Struts/Shocks & Anti-Roll Bars
Eibach Pro-Kit Spring Set (w/ BMW O.E. Struts/Shocks & Anti-Roll Bars)
Eibach Sportline Spring Set (w/ BMW O.E. Struts/Shocks & Anti-Roll Bars)

Vehicles used:
1999 BMW E46 328i Sedan

People who drive Porsches, BMWs or other upscale performance cars probably don't think of them as being average, but in a sense they are. Regardless of their performance, these cars are designed to meet a broad range of norms—bumper heights, snow chain clearance and family accommodations, to name a few. And their suspensions are tuned to contend with average road conditions.

But, if you are an enthusiast, you may not be satisfied with an average car. You may want it to look more aggressive and purposeful as you cruise the streets and handle better as you attack the corners.

To evaluate the effect of such modifications, Tire Rack conducted a "Real World Road Ride" and "Performance Test Track Drive" using three BMW 325i sedans. The cars were driven by several dozen people whose driving skills ranged from expert to average, a fairly representative cross sample of the driving public. They rated each car on a 1-10 scale and the scores were averaged for each category. One of the BMWs retained its stock suspension while the other two were fitted with Eibach sport springs which are available in two versions:

Pro-Kit springs typically lower vehicles about .75 to 1.25 inches (approximately 1.25 inches front and rear in the case of the BMW 325i) and increases the spring rate just enough to provide a good balance between ride comfort and cornering performance.

Sportline springs offer an increased drop (as low as Eibach engineers say you can safely go on a street car), of about 1.5 to 2 inches (approximately 1.8 inches for the front and 1.5 inches for the rear in the case of the BMW 325i) and again increases the spring rate to help prevent "bottoming out" over bumps and on rough pavement. The Sportline spring's goal is to make the car as low as possible, while again improving handling for the more aggressive driver.

For all of the vehicles, a "Plus Two" wheel and tire package was chosen. It consisted of the Michelin Pilot SX MXX3 Max Performance tires in the 225/45ZR17 size, and BBS RX road wheels in the 17"x8" size. The Michelin Pilot SX MXX3 tires were chosen because their stable internal construction and "rib" style tread design make them one of the better handling Max Performance tires available, and because as one of the firmer riding tires, they would help us accurately evaluate the on-road ride qualities of the BMW sport sedans. The BBS RX wheels were chosen because they share the competition styling of the Super Touring race wheels and have become one of the favorites among enthusiasts.

After the springs were installed, the vehicles went to the alignment rack. The BMW 325i's normal alignment specifications call for the following settings:

Front Rear
Camber -1 Camber -2
Caster +3.5 Caster N/A
Toe 5/32" In Toe 3/16" In

The front camber and caster settings are not adjustable as the 325i comes from the factory, so we were happy to see that our stock 325i had about 1° of negative camber and 3° of positive caster in the front. It's rear wheel camber was set at 2° negative. Toe settings were set to the factory's preferred specs.

One of the traits of MacPherson strut suspensions is that as the vehicle is lowered, the suspension geometry will increase the negative camber. In the case of our Eibach spring equipped vehicles, the front negative camber was found to be more negative (in the 1.75° to 2° negative range). On the plus side, this additional negative camber will help the front tires resist rollover during hard turns and helps them provide more cornering grip. On the minus side, negative camber forces the tire's inside shoulder to carry more of the vehicle's load when traveling in a straight line and usually makes it wear faster. However, we feel that the driver who goes to a plus two fitment and sport springs is probably a more aggressive driver and without the extra negative camber, would have most likely worn out his tire's outside shoulders during spirited cornering. The rear camber was again set at 2° negative, and toe settings were set to the factory's preferred specs.

The first phase of our testing was our 7 mile "Real World Road Ride" driving loop. This allows driver comfort and real everyday handling to be evaluated and compared by each of our representatives over roads selected for the variety of "real world" driving conditions they represent. A combination of expressway, 4-lane, 2-lane, and country roads allows our team to experience tires as you do everyday. Our teams Real World Road Ride evaluations indicated that BMW's Original Equipment springs do a good job of providing comfort, especially on "rough roads", but don't match the extra responsiveness of the Eibach springs. The Eibach Pro-Kit spring's average score exceeded the O.E. springs primarily due to the extra handling response they provided. The Sportline springs fell behind the O.E. springs primarily due to their reduced ability to soak up bumps on rough roads. Our teams combined Real World Road Ride evaluations are attached.

Then we moved to our on-site 5 1/2 acre paved test track. This provides a controlled environment which allows each salesperson to experience and compare the maximum acceleration, cornering and braking performance of the vehicles from behind the wheel. Driving at the performance limits confirmed earlier classroom presentations and provided first hand experiences. On the track the enhanced handling and feel provided by the Eibach springs was readily apparent. On average the Eibach Pro-Kit springs were almost 1.5% faster (just under 1/2 second) and the Sportline springs were 1.7% faster (just over 1/2 second) than the stock springs. And while these percentages don't appear dramatic, just remember that if a driver won the Daytona 500 by 1.5%, he would win by a margin of 7.5 miles or 3 full laps!

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