While aftermarket wheels and tires can enhance the appearance and performance of almost any vehicle, the additional cornering and stopping traction they provide may be limited by the ability of the vehicle's stock suspension to manage the increased traction. In order to get more out of high performance tires, a suspension upgrade may also be required, and when it comes to suspension upgrades, a set of properly engineered sport springs should be the first upgrade considered.
Sport springs will enhance vehicles in several ways. They reduce the visual tire-to-wheelwell gap to give the vehicle a more aggressive, hunkered down look; and they increase the spring rates and lower the vehicle's center of gravity to enhance the vehicle's responsiveness and handling. For popular vehicles, spring manufacturers offer several types of springs from mild to wild (combining various spring rates and vehicle lowering lengths) and develop their products to emphasize the comfort and performance traits which they feel will best please their customers.
In order to get a better understanding of the performance improvements which can be achieved by upgrading a car's suspension with sport springs, Tire Rack Team conducted a ride & drive comparing a 1999 BMW 328i with a stock suspension to a second 328i equipped with an Eibach Pro-Kit, and a third 328i equipped with an H&R Sport Spring set. All three cars were fitted with a Plus Two wheel and tire package which consisted of Yokohama AVS Sport tires in the 225/40ZR18 88Y size mounted on 18" x 8" Borbet Type TD wheels.
Stock BMW 328i springs and sway bars - For most driving enthusiasts, installing a Plus Size wheel and tire package is the first step taken to personalize their vehicle, so that's where we started.
In the parking lot, the addition of the plus two wheel and tire package on our black BMW 328i test car really caught your eye. The sunlight reflecting off of the 18" wheel's silver painted spokes and machined outer lips was dazzling. However, a second look from the side of the car revealed that with the stock suspension, the 18" wheel and tire package resulted in the car actually appearing "lifted", or just a little higher than stock.
Since this is a Plus Size application, the new tire's overall diameter should remain within ±3% of the car's original tire diameter (it was just +1% in this case). However, while the new and original tires have equivalent overall diameters, the visible sidewall height of the Original Equipment 205/55R16 sized tire is 3 3/4", while the visible sidewall height of the Plus Two 225/40R18 sized tire is only 2 1/2". As a rule of thumb, if a tire's visible sidewall height is only equal to, or is less than the vehicle's wheelwell gap (the distance from the lower edge of the wheelwell to the outer circumference of the tire) it will create an optical illusion that may make the vehicle appear "lifted". We measured the stock car's wheelwell gap and found the front to be about 2 1/2", with the rear to be about 1 1/4". Then we compared the relationship of wheelwell gap to visible tire sidewall height. We confirmed that while the BMW 328i appeared normal with the Original Equipment tire's visible sidewall height of 3 3/4", and that it looked a little "lifted" with the Plus Two tire's reduced visible sidewall height of 2 3/4".
On the road, the stock BMW 328i equipped with the Borbet Type TD wheels and Yokohama AVS Sport tires was praised for its low noise level, ride comfort and responsive handling.
On the track, while the AVS Sport tires gave the BMW more traction and helped it run fast lap times, compared to the other two cars, the stock suspension felt a little "slower" than the AVS Sport tires. The vehicle's motions (squatting during acceleration, diving during braking and leaning in the corners) weren't controlled as precisely or completely as the vehicles equipped with the sport springs. This resulted in less cornering power and slower vehicle responsive during transitions. The car with the stock suspension turned in the slowest lap times of the three combinations tested.
Eibach Pro-Kit - Our next test car was equipped with Eibach Pro-Kit Springs. As in the case of this test, vehicles equipped with higher performance tires which generate more traction than stock tires usually result in additional body lean when the vehicle is cornered at its limit. While Eibach Pro-Kit Spring Sets use progressive spring rates to minimizing disturbances to the vehicle's ride quality, they are designed with overall higher than stock spring rates to help control the additional body roll at the limit to maximize improvements in handling.
Pro-Kit Springs are designed specifically for each vehicle, and are engineered to properly lower the vehicle and its center of gravity which help control the vehicle's movements during quick acceleration, cornering and braking maneuvers. Pro-Kit Springs are also engineered to maintain the vehicle's load carrying capacity with little additional risk of "bottoming" the suspension, and the springs remain in constant contact with the spring perches even when the vehicle is jacked up or lifted off of the ground on a hoist.
In the parking lot, the addition of Eibach Pro-Kit Springs to the Plus Two wheel and tire package on our silver BMW 328i also caught your eye. However this time, the second look revealed that the Eibach Pro-Kit equipped car with the 18" wheel and tire package looked hunkered down on its suspension. The Pro-Kit Springs reduced the front wheelwell gap by about 1 3/4"; resulting in a wheelwell gap of only about 3/4" (which is much less that the 2 3/4" visible sidewall height of the Plus Two wheel and tire package). Rear wheelwell gap was reduced to about 3/8" from the 1 1/4" gap with the stock suspension.
On the road, the Eibach Pro-Kit Spring Set equipped BMW 328i was praised for its real world traits. However, while they provided very good ride comfort, the BMW's ride was just slightly more abrupt than with the stock springs as the vehicle encountered expansion joints and road patches. The Pro-Kit Springs provided noticeably more responsive handling which felt better tuned to the Plus Two wheel and tire package.
On the track, Eibach Pro-Kit Spring Set equipped BMW felt more responsive and better matched to its tires. The handling felt balanced and more of the AVS Sport's traction could be used. The Eibach Pro-Kit Springs allowed the BMW to run lap times which were about 1/2 a second faster than the 328i with the stock suspension. The Eibach Pro-Kit equipped BMW's lower center of gravity and higher than stock progressive spring rate controlled the vehicle's motions more precisely and completely than the stock springs which resulted in more cornering power and faster vehicle response during transitions.
H&R Sport Springs - Our final test car was equipped with H&R Sport Springs. H&R Sport Springs are also designed specifically for each vehicle, and are engineered to lower the vehicle and its center of gravity which helps reduce the speed and degree of the vehicle's movement during quick acceleration, cornering and braking maneuvers. H&R Sport Springs typically use progressive rates to maximize improvements in handling while minimizing disturbances to the vehicle's ride quality. This allows them to offer close-to-stock rates during normal driving for ride comfort, yet increase in stiffness when the vehicle leans as it is pushed to the limit. They are engineered to constantly remain in contact with the vehicle's spring perches (even when the vehicle is lifted off of the ground on a hoist) and maintain the vehicle's load capacity with little additional risk of "bottoming" the suspension.
In the parking lot, the addition of H&R Sport Springs reduced the front wheelwell gap by about 1 1/2"; resulting in a wheelwell gap of only about 1" (which is also much less that the 2 3/4" visible sidewall height of the Plus Two wheel and tire package). Rear wheelwell gap was reduced to about 1/2" from the 1 1/4" gap with the stock suspension.
On the road, the the H&R Sport Spring equipped BMW 328i was also praised for its real world traits. However, while the H&R Sport Springs provided pretty good ride comfort, they produced a ride that was just slightly more abrupt than the vehicle equipped with the Pro-Kit Springs. The H&R Sport Springs provided the most responsive real world handling of the three combinations tested.
On the track, the H&R Sport Spring equipped BMW felt the stiffest of the three. And while they allowed the vehicle to understeer just a little more at the limit and during transitions, they also produce a little more cornering power. The H&R Sport Springs allowed the BMW to run lap times which were a few hundredths of a second faster than the Eibach Pro-Kit equipped car, and the fastest of the three cars tested.
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