Suspension - Cautions About Ride Height and Lowering

Nishiboric Rear Suspension and Ride Height Changes
The Nishiboric suspension is a passive, four wheel steering system, integrated into the rear suspension design of all Geo Storms and all 1990-93 Isuzu FWD and AWD sister cars (Isuzu Impulse, Isuzu Stylus, Asuna Sunfire, Isuzu Gemini, and Isuzu Piazza). It is classified as a passive system, because it does not make use of any electric motors, solenoids, or computer system to control it or actuate it. Instead, the system is part of the basic geometry of the rear suspension arms and their attachment. As the rear suspension arm travels through its range of motion, the rear toe angle changes, initially increasing rear toe out, and then decreasing rear toe out and transitioning to rear toe in.
When the driver initiates a turning maneuver, the outside rear wheel suspension will compress slightly, bringing the suspension into the toe-out position. This induces a mild oversteer, which increases the speed of weight transfer and speed of direction change.
As the car continues in a turning action, the body roll increases, and the outside rear suspension compresses further into the toe out position. This induces mild understeer, which improves cornering stability and driver control.
The overall effect is intended to improve both nimbleness and stability.

However, the system has drawbacks. It can make the car feel very twitchy on uneven pavement, sloped pavement, and when driving in a cross wind. Driving the vehicle requires more concentration to maintain a straight line, and long distance interstate travel can cause more driver fatigue.
In addition to this, a driver who is very sensitive to the feel of the car may be irritated by the feeling that the steering seems overly sensitive to mild steering inputs when traveling in a straight line, and then increasingly less responsive to large steering inputs during hard cornering.

The Nishiboric passive four wheel steering system provides several serious challenges if the suspension is modified.
The system is designed so that the static rear toe of 0° at the full, stock ride height (with 3 ˝ inches of air between the top of the tire tread and the rear wheel arch or fender lip). When the rear suspension is compressed 13/32 of an inch, maximum rear toe out is achieved. When the rear suspension is compressed 19/32 of an inch, toe in begins, and continues to increase as the suspension is compressed further.
If the vehicle is lowered 0.4 inches or more, and the alignment is not corrected, the rear suspension will be toed out, making the car unstable and difficult to drive in a straight line. If the rear alignment is corrected to 0° static toe at this new ride height, straight line stability is restored, but the rear suspension will toe in as the suspension is compressed, making the vehicle feel dull and unresponsive during cornering.
The typical fashion or style lowering of these cars is 1 ˝ - 2 ˝ inches (or more for a “tucked” appearance). This moves the suspension arms below the action of the Nishiboric suspension system. Whenever the vehicle is turned, the rear suspension will toe in, causing progressively more understeer as the turning force increases (speed and angle of turn). This is not desirable.

There are two remedies for this situation. If it is desirable to retain the passive steering effect, then the rear suspension inner mounting points must be relocated higher in the vehicle chassis, or the outer mounting points moved lower on the spindles. The change in suspension arm mounting height should be the exact same amount as the vehicle is lowered, in order to compensate for the reduction in ride height and return the geometry to its original condition.
If it is not desirable to retain the passive steering effect, then the rear suspension geometry should be modified to disable the toe in and toe out effect, so that the toe does not change as the suspension arm travels through its range of motion.

Ride Height and Roll Center
The roll center of a suspension is the fulcrum point of the suspension, at which body roll occurs. This fulcrum point is acted upon by the center of gravity, which itself is the center of the mass from which the cornering force acts on the roll center, to cause body roll. The farther apart these two points are, the longer the lever arm, and the easier it is for cornering forces to induce body roll.

Calculating the roll center involves drawing a scale diagram of the pivot points of the suspension, drawing lines through those pivot points, and extending them out until they intersect. Then drawing a line through the left and right side intersections, through the center line of the car. The point at which that line crosses the center line of the car, is the roll center.

The most important factor determining the height of the roll center is the angle of the lower control arms of the suspension. Cars are designed, and factory ride height set, so that the body mounting point of the lower control arm (inboard side) is higher than the ball joint or outer lower control arm mounting point at the base of the spindle. This keeps the roll center high, where it is close to the center of gravity, and keeps body roll at a minimum.

When a car is lowered with lowering springs or coilovers, the body of the vehicle (and the inbound suspension mounting points) are moved closer to the ground. But the outer suspension mounting points at the spindles, remain the same height. With a very minor amount of lowering (a fraction of an inch), the lower control arm pivot points will become parallel with the ground. And with more lowering (an inch or two), the body mounting point will be below the spindle mounting point, and the lower control arm will actually be angled upward. This will move the roll center below ground level. Body roll (noticeable or not) will become excessive, and cornering traction will be dramatically reduced.

For the Geo Storm (and its Isuzu sister cars), the front lower control arms go parallel at 7/8 inch below stock ride height. And the rear lower control arms go parallel at 1/2 inch below stock ride height. Lowering the vehicle beyond these points will reducing handling quality and the road holding ability of the car as the roll center approaches ground level and moves farther and farther away from the center of gravity. At 2+ inches of lowering, the roll center goes below the ground level and handling loss increases.

The roll center can be changed and corrected by moving the mounting points of the lower control arms. For the Geo Storm, moving the inboard mounting points on the chassis would be impractical and nearly impossible to relocate without major body work, which would likely be illegal in nearly all racing classes. The easier solution is to move the outer pivot points at the spindle, down. This can be accomplished by either using an extended lower ball joint (front suspension), or fabricating a spindle with the control arm mounting points moved lower (rear suspension). Both of these modifications will change the clearance around the inside of the wheel rim, likely requiring the use of 15 inch or larger diameter wheels. For best results, the increase in front balljoint length, and rear spindle lower mounting point offset, should be close to the same as the amount of ride height reduction.

Ride Height and Tire Clearance
The more a vehicle is lowered, the less space there is between the tires and the structure of the vehicle. The wheel wells of a car were designed by the vehicle manufacturer to provide adequate tire clearance for the stock size wheels and tires, at the stock ride height, with the stock struts. Change tire and wheel sizes, ride height, and install coilovers, and the available space in the wheel wells changes significantly.
Stiffer springs and dampers, and shorter coilovers will limit suspension travel some, but big bumps and potholes in the road will still move the suspension and tire more than a few inches.

A typical safe amount to lower a Geo Storm is about 2 inches. With properly sized wheels and tires, this usually will not cause much contact between the tire and the steel of the fenders and wheel wells for a street driven vehicle. Though the tire tread may contact the plastic fender liners, in which case the tire tread will cut the fender liners.

For racing, it is recommended to leave an inch of space between the peak of the tire’s treaded surface, and the peak of the wheel arch. This is because any use involving high speed cornering on R compound tires, is going to cause lots of tire clearance issues, usually with the fender lips and the front inner wheel wells.

Fender lip clearance can be widened by rolling the fender lips with a fender rolling tool, such as the one from Eastwood. More negative camber will also create more clearance between the tire and the fender lip.
Tire rub on the front inner fender wells is more difficult to resolve. Excessive toe-in will exacerbate the problem, but toe-out will not completely eliminate it. The best solution is to add more shims to the stack of limiting shims under the bellows of the steering rack. This will reduce the angle of the front wheels at the end of the steering column’s swing, and prevent the tire from making contact with the wheel well.
Clearance between the rear inside tire sidewall and the side of the rear strut can also be a problem. This is best resolved by placing a spacer between the brake backing plate and the rear bearing assembly. Alternately, a less desirable solution is to place a wheel spacer between the wheel and the brake drum (or disc), but this will reduce the number of threads holding the wheel to the hub, which can become a serious safety issue with thicker spacers.



So, What Is The Correct Way To Lower A Geo Storm?
For the best handling, the correct way to lower a Geo Storm is:
• Install real coilovers, with quality adjustable dampers and adjustable lower spindle brackets.
• Select a spring rate appropriate for the road surface the vehicle is to be used on, soft enough that the tires will remain in contact with the road surface over bumps.
• Select a spring length that will keep the spring perch above the tire to avoid clearance problems.
• Use a helper spring and spring separator with springs above 275 lb/in spring rate, to prevent the spring from coming unseated when the suspension goes into full droop.
• Adjust the damper to mid stroke position with the adjustable spring perch.
• Adjust the ride height with the adjustable spindle bracket. Set the ride height so that the wheel arch height is approximately one inch above the top edge of the tire tread height.
• Adjust the dampers to match the spring rates.
• Deactivate the Nishiboric Passive Four Wheel Steering mechanism in the rear suspension.
• Install extended ball joints on the front suspension to correct the front roll center.
• Install drop spindles on the rear suspension to correct the rear roll center.
• Use wheels and tires that are the correct and appropriate size for the vehicle.
• Install steering rack limiter shims to prevent tire rub in the front wheel wells.
• Shim the rear bearing assembly to prevent tire rub in the rear wheel wells and outer edge of the damper body.
• Roll the fender lips to prevent tire rub at the wheel arches.

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