Bump Steer
Steering precision and stability-both of which are affected by bump steer-are the next victims of overlowering. Bump steer is steering input created by the suspension moving through its stroke in response to bumps and roll. It's caused by the suspension's control arms moving in different arcs than the steering linkage as the suspension follows its stroke.
It's fairly easy to design a suspension system that doesn't have bump steer. To do so on a double-wishbone-type suspension, the steering tie rods must lie between two vertical lines drawn between the upper and lower control arm's pivots while pointing at the suspension's instant center.
In order for a MacPherson strut suspension to have no bump steer, the tie rods must lie in line with the lower contol arm with the inner tie rod end in plane with the inner pivot of the control arm (see illustration on this page).
However, most production cars have the steering rack placement compromised by packaging constraints so steering tie rod location is often less than optimal. Lower the car and the problem gets worse.
What can you do to reduce bump steer? Many cars have aftermarket parts available to relocate the tie rod ends of the steering linkage. Tie rod ends with spherical bearings and spacers can be tuned to reduce bump steer by placing the tie rods at a more favorable angle.
SPL and Whiteline make bump steer reduction kits for popular cars like the 240SX, 300ZX and EVO. If these parts are not available for your favorite car, they can be easily fabricated.
By learning what effects changes in suspension geometry have on a car's behavior, you can tune and adjust your suspension to work like you want. Understanding these geometry traits and making them adjustable is a powerful tool when trying to eke out the last bit of cornering performance.
If you're a racer, autocrosser, drifter or just a hard-core canyon carver, these tools will give you a significant edge.
Next month the science continues with damper tuning.
Overlowering:Don't do it
Almost everybody does it. Lowering your car is paramount to improving its handling. The key, however, is to lower it just enough to gain the benefits it creates without suffering the potential drawbacks.
The aftermarket does little to help us in this regard. Nearly every company that makes suspension components, even very reputable ones, spews out thousands of sets of lowering springs that are both too low and too soft for optimal handling. Why do they do this? Are the engineers at these companies incompetent? Is it a conspiracy to make our cars suck? No, the enthusiast is to blame.
The majority of enthusiasts want a low ride height to fill the ugly gap in their stock wheel wells. They also won't accept a ride that, for the most part, is a lot harsher than stock. Macho or not, most enthusiasts don't drive hard enough or well enough to realize that their cars actually handle worse than stock, mistaking reduced roll for better handling.
The original Nissan Sentra SE-R is a typical example of a car with suspension geometry that doesn't allow lowering more than an inch. But the problem isn't limited to the SE-R.
The first problem with lowering the SE-R is that it only has about 2 inches of compression travel at the stock ride height in the front suspension. Let's say you lower the car the typical 1.5 inches. That leaves a half inch of travel before you hit the bump stops. Your typical aftermarket lowering spring might only up the spring rate a paltry 20 percent or so, which isn't nearly enough to keep the car off the bump stops with only a half inch of travel.
The result is poor ride quality and sub-standard handling. As the car leans in a corner, the suspension will settle onto the bump stop. As the bump stop compresses, the spring rate ramps up infinitely, which causes massive weight transfer and relentless understeer.
But it gets better. On the SE-R, the lower control arms are positioned so they begin to point upward as the car is lowered. Now when the car rolls in a corner, the outside tire goes into positive camber. And, if you've been reading this series, you know that is just about the least effective way to corner.
Believe it or not, it gets worse. With the lower control arms pointing upward, the instant center starts to drop rapidly and the roll couple greatly increases. The bigger roll couple causes more weight to transfer to the outside wheels and more body roll.
Finally, the steering tie rods start to point upward more radically because they are shorter than the lower control arm and positioned out of place in the lowered chassis. This causes toe-out when the wheels deflect, making the steering twitchy and the car feel unstable.
The SE-R exhibits just about every problem overlowering can cause and when combined, those problems will ruin its handling. Fortunately, SE-R guys tend to be pretty hard-core and they have taken the issue of making functional drop-in lowering springs into their own hands. This isn't the case with every car. Even worse, this situation is not unique to the Sentra. There are lowering springs available that are capable of causing these or similar problems on just about any car.
What can you do to work with the drawbacks of overlowering or avoid it completely?
SPL makes these tie rod ends to go with its control arms. The tie rod pivot location can b
Make sure your car doesn't use the bump stops under maximum cornering load. The easy way to detect this problem is with a zip-tie telltale on the shock shaft. If the zip-tie is pushed up flush or into the bump stop after a hard turn, then your car is using the bump stops every time you corner hard.
If you must run low, do it racecar style. Get short-bodied high-end coil-over shocks or struts with higher rate springs. Independently adjustable ride height and spring preload are also critical. Suspension components with these features are designed to work at low ride heights. Many popular performance cars have kits to adjust and correct roll centers, camber curves and bump steer.
If you can't get a decent rate drop-in spring for your car, Ground Control makes kits for many cars allowing the use of Eibach 2.5-inch ERS racing springs, which come in nearly an infinite selection of rates and lengths. With Ground Control's threaded spring perches, you can also adjust the ride height.
If you can't do this, run short, soft progressive microcellular urethane bump stops so the wheel rate will ramp up gradually if the bump stops are used. Koni makes excellent bump stops.
If you have a MacPherson strut suspension, be especially aware of short travel and suspension geometry problems. MacPherson strut cars usually have a very small lowering window. It is typically best to run these cars at close to the stock ride height unless you significantly modify many other components.
Other Installments:
Making It Stick Part 1: Four basic steps to better handling
Making It Stick Part 2: Four more steps to better handling
Making It Stick Part 3: It's all in the geometry
Making It Stick Part 4: More lessons in suspension geometry
Making It Stick Part 5: Damper fundamentals
Making It Stick Part 6: More advanced dampers
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SPL Parts
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Global Performance Parts (Whiteline)
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Ground Control Inc.
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By Mike Kojima
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