Suspension - Bushings, Heim Joints, and Roll Center Correction
Polyurethane Suspension Bushings
The original bushings in the Geo Storm (and most cars) are rubber, cast into and securely attached to sleeves, which
are press fit into the suspension arms and brackets. When bolted into place, the rubber acts as a tiny torsion
spring, resisting movement, adding to the effective spring rate, and increasing its resistance the farther it is
rotated (progressively increasing spring rate). These are cheap to make and provide soft and supple ride
characteristics due to the softness of the material and the progressive resistance action they add to extreme
suspension movement. However, the rubber does not last forever, the progressive resistance degrades as the rubber
ages (or there is no progressive resistance at all when the bushing breaks or rips), they require a press to replace,
many are not sold other than attached to the suspension arms, and most have been discontinued.
Polyurethane bushings are specially made to directly replace the original bushings. These bushings slip into the
arms and then a center sleeve is slipped into the center of the bushing. The outside edges of the bushings have
shoulders to control and restrict side movement of the arm. The polyurethane provides absorption of jarring
movements from the suspension when the tire strikes a bump, while the smooth surface between the outside of the
bushing and the arm or bracket, and the inside of the bushing and the center sleeve, provide for smooth and fluid
movement of the suspension arm. The compromise is an exchange of ride comfort for more accurate suspension movement
and better handling.
Isuzuperformance designed and made polyurethane suspension bushing sets for their own race cars, and frequently
makes them available for purchase via Ebay.
Heim Joint Suspension
Heim jointing all of the suspension connections is the next step beyond polyurethane suspension bushings. Each
bushing is replaced with a spherical bearing, providing smooth and controlled movement of the suspension arm, with
absolutely no vibration or bump absorption. This is considered a rather serious racing modification.
High quality bearings are required to handle the loads generated by racing a car on a track. Teflon coated bearings
are a necessity. The bearings themselves cost in the range $40+ each for the sizes required. This does not include:
One additional concern is the rear longitudinal trailing arm, which is L shaped, and has only two connection points.
The original rubber bushings keep the arm parallel to the ground, and keep the arm from drooping in the middle.
This arm can not be heim jointed on the spindle end, and will require a roller bearing to keep the arm oriented
parallel to the ground.
- The cost of adding bearing cups to the original suspension arms and brackets so that they will accept and hold
the bearings in place.
- The cost of making threaded rod end arms to replace control arms.
- The cost of making shoulder bushings to fit the Heim joints and rod ends to the mounting brackets.
Heim jointing all of the suspension pivot points would likely cost $85+ per pivot point (potentially more expensive
for the roller bearing conversion to the spindle end of the rear longitudinal trailing arm). There are 16 bushings
in the Geo Storm suspension (excluding sway bars). A complete Heim joint replacement would cost upwards of $2,000.
Roll Center Correction
As was mentioned in the previous
section on Ride Height and Roll Center, lowering a Geo Storm from the factory
ride height will cause the lower control arms to go parallel to the ground, or with additional lowering, point
upward. This change in the angle of the lower control arms changes the geometry of the suspension, and has serious
adverse effects on handling and grip.
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.
Reducing ride height moves the roll center farther away from the center of gravity, causing more body roll (even
if that body roll is not perceived by the driver) and a loss of grip.
The roll center can be corrected by 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.
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