Roll Center Correction for your Lowered BMW
Last week, we talked about correcting the negative effects of lowering your BMW with more adjustability to alignment and eliminating preload on your sway bar for proper handling. What we didn’t tell you is you still have more to do if you want to maximize your traction and handling on the street or the track. This week, we’re going to be a bit more scientific, since our topic is more complex than just some alignment tweaks. We’re headed further down the rabbit hole in suspension tuning with what you need to know about roll center, bump steer, and how to correct them for optimal handling and traction. | |
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As we learned last week, lowering your BMW on coilovers drastically changes the suspension geometry. It does reduce the center of gravity and provide handling benefits, but it also negatively affects your alignment as well as your roll center. If you need a refresher on the alignment changes, read the previous Turner Motorsport Weekly article. After familiarizing yourself with those characteristics, you are ready to dig into roll center and bump steer.
According to our own Mike Day, ‘roll center is the point at which the cornering forces in the suspension are reacted to the vehicle body.’ In normal people speak, it is essentially the pivot point where your car rolls side to side. In a factory suspension setup, your roll center is where it should be for the OEM ride height. Basically, your roll center is an intersection of lines drawn from your control arm ball joints and strut mount and is the center point of the results of that measurement. When you lower a vehicle, your center of gravity is also lowered, but the distance between your roll center and center of gravity lengthens. This means your vehicle’s weight as it rolls in the corners is more pronounced. |
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To correct that change, and reduce the roll moment, Turner Motorsport engineers have developed a solution. The roll center correction race kit raises the center of rotation, which brings the roll center much closer to the car’s center of gravity, and reduces the roll movement during cornering. In effect, you are bringing the point where the car rolls from side to side closer to the center of gravity, which means it takes much more cornering force to cause your BMW’s body to roll. Think of it like a pendulum: the longer the pendulum is, the more force and speed the pendulum carries when it swings. When it is shortened, it has much less force and speed. Your center of gravity is the pendulum, the roll center is the pivot point. Like a metronome for you musicians.
Additionally, lowering your BMW causes bump steer thanks to the change in angle to the tie rods. Bump steer is just what it sounds like: hitting a bump or uneven surface that causes your car to steer on its own. This happens when the tie rod angle is increased when the suspension is lowered and it has to angle upward where the tie rod end ball joint meets the front knuckle. By using spacers and our Turner Motorsport Adjustable Tie Rod Ends, you can return the tie rod angle to that of the factory specs and virtually eliminate bump steer. |
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Lastly, for steering input improvements, the quick-steer kit includes rack limiters, which are essentially spacers installed in the steering rack, to reduce the amount of steering input to achieve the same steering angle. That means less sawing at the wheel and faster steering for improved cornering on the track.
The Turner Motorsport Roll Center Correction Race Kit is all about returning your handling characteristics that are lost and masked by lowered and stiffer suspension while also reducing your steering input at the wheel. This kit is designed to improve your handling, quicken your steering, eliminate bump steer, and ultimately give you optimal performance capabilities on the track with your lowered BMW. Of course, if this all seems a bit complicated, we have a helpful video with Mike Day explaining it all using helpful graphics, which we have linked below. | | | E36 NON-M Quick Steer/Roll Center Correction Race Kit | | | | E36 M3 Quick Steer/Roll Center Correction Race Kit | | | | E36 Drift Steering Angle Kit | | | | E46 NON-M Quick Steer/Roll Center Correction Race Kit | |
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