While wheels and tires live up to their promise of increased appearance and performance, the additional cornering and stopping traction they provide may be limited by the ability of the vehicle's stock suspension to manage all of the increased traction. So in order to get more performance out of high performance tires, a suspension upgrade may also be required.
In order to get a better understanding of the performance improvements which can be achieved by upgrading a car's suspension, Tire Rack Team conducted a Ride & Drive comparing a stock 1999 BMW 328i to a second 328i equipped with Eibach Pro-Kit Springs, and a third 328i equipped with both Eibach Pro-Kit Springs and Eibach Anti-Roll Kit sway bars. All three cars were equipped 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.
The Yokohama AVS Sport is a member of their Advanced Vehicle System flagship family of performance tires. Developed to emphasize dry traction and handling along with wet traction and comfort, the tread pattern of the AVS Sport has a distinctive Y-shaped design which features a massive, continuous zig-zag center rib for uninterrupted rubber-to-road contact that helps enhance steering response, high speed stability and tread wear. The AVS Sport's tread design also results in an evenly spaced distribution of straight directional tread grooves (which run at angles to the direction of travel) to improve water drainage to enhance hydroplaning resistance. The AVS Sport features Yokohama's Super Performance Silica (SPS) tread compound to further resist wear while it enhances dry and wet traction. Internally, the AVS Sport's twin steel belts are reinforced by jointless nylon construction to provide Y-speed rated (186 mph) capability while minimizing tire weight and maximizing ride comfort. The AVS Sport also features a new casing design which uses steel sidewall inserts to enhance lateral rigidity for improved response and cornering stability.
The Borbet Type TD wheels are light alloy wheels manufactured in Germany which feature a covered lug, five-spoke silver painted center inside a deep, machine finished outer lip. This combination is clearcoated for additional durability, and provides a very aggressive, wide wheel look.
As stated earlier, for most driving enthusiasts, installing a 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 (just +1% in this case). But while both tires have equivalent overall diameters, the visible sidewall height of the Original Equipment 205/55R16 sized tire is 3.75", while the visible sidewall height of Plus Two 225/40R18 sized tire measured only 2.5". 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 2.5", with the rear to be about 1.25". 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 3.75", and that it looked a little "lifted" with the Plus Two tire's reduced visible sidewall height of 2.75".
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, good 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 "slower" than the tires. The stock car's motions (squatting during acceleration, diving during braking and leaning in the corners) weren't controlled as precisely or completely. This resulted in less ultimate 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.
Our next test car was equipped with Eibach Pro-Kit Springs. Pro-Kit Springs are designed specifically for each vehicle, and are engineered to properly lower the vehicle and its center of gravity. This helps reduce how fast and how far the vehicle's body moves during quick acceleration, cornering and braking maneuvers. Eibach Pro-Kit 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.
Vehicles equipped with higher performance wheels and tires than stock can also benefit from the additional suspension performance provided by Eibach Pro-Kit Spring Set. Performance tire and wheel packages usually generate more traction than stock tires which often results in additional body movement when the vehicle is driven hard. The Eibach Pro-Kit is designed with overall higher than stock spring rates to control this additional body movement at the limit.
In the parking lot, the addition of Eibach Pro-Kit Spring Set 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 1.75"; resulting in a wheelwell gap of only about 3/4" (which is much less that the 2.75" visible sidewall height of the Plus Two wheel and tire package). Rear wheelwell gap was reduced to about 3/8" from the .25" gap with the stock suspension.
On the road, the Eibach Pro-Kit Spring Set equipped BMW 328i was also praised for its low noise level, good ride comfort. However, this combination provided 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 suspension felt better "matched" to its tires. More of the AVS Sport tire's traction could be used, and this combination allowed the BMW to run lap times which were about 3/10 of a second faster than the 328i with the stock suspension. The Eibach Pro-Kit Spring Set equipped BMW's lower center of gravity and and higher than stock progressive spring rate controlled the vehicle's motions (squatting during acceleration, diving during braking and leaning in the corners) more precisely and completely. This resulted in more cornering power and faster vehicle response during transitions. The car with the Pro-Kit suspension turned in the middle lap times of the three combinations tested.
Our next test car was equipped with both Eibach Pro-Kit Spring Set and Eibach Anti-Roll Kit sway bars. While springs and sway bars both help control the motions of the car, the job of the sway bars is to help balance the vehicle's handling by distributing the work load between the front and rear tires by resisting body lean when the vehicle is cornering. Eibach Anti-Roll Kits are designed specifically for each vehicle and may contain a front and/or rear sway bar depending on the needs of the vehicle. Eibach Anti-Roll Kit sway bars come complete with all required hardware and many have multiple mounting holes at their ends to allow fine tuning the bar's stiffness and the vehicle's handling.
Sway bars run from one side of the vehicle to the other, and are typically mounted to the vehicle's frame in the center and to its suspension control arms at the ends. One of their advantages is that they don't adversely affect ride quality during most driving conditions. When the vehicle hits a dip or a bump with both tires on an axle at the same time, the sway bar doesn't resist the up and down movement of the suspension. For that matter, the only time sway bars influence ride quality is when only a single wheel hits a pothole or bump.
In the parking lot, since sway bars don't affect ride height, the look of our red test car didn't change from that of the silver Pro-Kit equipped car. It retained an aggressive stance and a hunkered down look.
On the road, the Eibach Pro-Kit Spring Set and Anti-Roll Kit equipped BMW 328i was also praised for its low noise level and good ride comfort. This combination provided even more responsive handling which was quick to implement to the driver's inputs. However, when traveling on our rough, two lane country road, the single wheel bumps were more noticeable.
On the track, this combination completed the transformation of the vehicle. Body roll was reduced allowing faster speeds around the skid pad portion of our track, and driver input was quickly rewarded with changes in the vehicle's direction during the transitions. The Eibach Pro-Kit and Anti-Roll Kit equipped BMW turned in the fastest laps of the day. It was an additional 3/10 of a second faster than the Pro-Kit equipped car and 6/10 of a second faster than the stock car. This car was the easiest to drive at the limit and its tires showed the least wear at the end of the day, a rewarding combination.
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